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Effect of thinning intensity on understory herbaceous diversity and biomass in mixed coniferous and broad-leaved forests of Changbai Mountain

Effect of thinning intensity on understory herbaceous diversity and biomass in mixed coniferous... Background: Herbs are an important part of the forest ecosystem, and their diversity and biomass can reflect the restoration of vegetation after forest thinning disturbances. Based on the near-mature secondary coniferous and broad-leaved mixed forest in Jilin Province Forestry Experimental Zone, this study analyzed seasonal changes of species diversity and biomass of the understory herb layer after different intensities of thinning. Results: The results showed that although the composition of herbaceous species and the ranking of importance values were affected by thinning intensity, they were mainly determined by seasonal changes. Across the entire growing season, the species with the highest importance values in thinning treatments included Carex pilosa, Aegopodium alpestre, Meehania urticifolia, and Filipendula palmata, which dominated the herb layer of the coniferous and broad-leaved mixed forest. The number of species, Margalef index, Shannon-Wiener index and Simpson index all had their highest values in May, and gradually decreased with months. Pielou index was roughly inverted “N” throughout the growing season. Thinning did not increase the species diversity. Thinning can promote the total biomass, above- and below-ground biomass. The number of plants per unit area and coverage were related to the total biomass, above- and below-ground biomass. The average height had a significantly positive correlation with herb biomass in May but not in July. However, it exerted a significantly negative correlation with herb biomass in September. The biomass in the same month increased with increasing thinning intensity. Total herb biomass, above- and below-ground biomass showed positive correlations with Shannon-Winner index, Simpson index and Pielou evenness index in May. Conclusions: Thinning mainly changed the light environment in the forest, which would improve the plant diversity and biomass of herb layer in a short time. And different thinning intensity had different effects on the diversity of understory herb layer. The findings provide theoretical basis and reference for reasonable thinning and tending in coniferous and broad-leaved mixed forests. Keywords: Selective thinning, Mixed coniferous and broad-leaved forests, Herbaceous plants, Diversity, Biomass * Correspondence: xfc0707@163.com Gerong Wang and Yue Sun are authors contributed equally to this work and should be considered as co-first authors. Key Laboratory of State Forestry Administration on Conservation and Efficient Utilization of Precious and Rare Forest Resource in Changbai Mountain, Forestry College, Beihua University, 132013 Jilin, China Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Wang et al. Forest Ecosystems (2021) 8:53 Page 2 of 13 Background thinning for secondary coniferous and broad-leaved Understory herbaceous plants are an important part of the mixed forests in Northeast China. species diversity of forest ecosystems (Han et al. 2016). In temperate forests, more than 80 % of vascular plants are Materials and methods herbaceous plants (Whigham 2004), and herbaceous Study area plants are of great significance for maintaining the species The study site was located at Dapo Forest Farm (43°57′ N, diversity of forest ecosystems. Compared with trees and 127°43′ E), the state-owned forest protection center of Jilin shrubs, understory herbs are more sensitive to distur- Province Forestry Experimental Zone. In July 2011, 4 per- bances and are the most adaptable to the environment. If manent monitoring plots with an area of 1 ha (100 m × the disturbance severely destroys the original forest eco- 100 m) were set up in the near-mature forest. Each plot was system, herbaceous plants will usually recover fast, thereby divided into 25 plots of 20 m × 20 m and the forest stand quickly improving the soil site conditions, reducing the in- survey was conducted. In December 2012, 4 plots were tensity of solar radiation on the ground, and providing im- thinned with actual intensity of 0, 17.2 %, 32.7 %, and 50.8 % portant protection for the regeneration of trees and respectively, which were defined as unthinning (CK), light shrubs and the growth of seedlings (Huang et al. 2016). thinning (LT), moderate thinning (MT) and heavy thinning Therefore, the species diversity and biomass of the herb (HT) (Liu et al. 2019). layer can reflect the vegetation restoration status after for- est disturbance. Herbaceous plant diversity Forest thinning is an important disturbance in the forest At the beginning of May 2015, 4 herbal plots of 1 m × 1 m managements. Thinning can have an important impact on were randomly set up in each 20 m × 20 m plot, and a total the stability of the forest ecosystem, and even destroy the of 100 herbal plots were set up. From early May to early balance of the forest ecosystem (Lei et al. 2005). As an im- October 2015, the species name, abundance, average height, portant disturbance in the forest managements, thinning has and coverage of the herb plots in the four plots were investi- long attracted people’s attention (Zhou et al. 2012). Reason- gated, and the total coverage was also recorded. The relative able thinning can maintain the stability and structure of the abundance (RA), relative coverage (RC), relative height (RH), forest ecosystem while obtaining timber, and achieve sustain- and importance value (IV) of herbaceous plants based on the able forest management (Zhou et al. 2015). Thinning can obtained data were calculated. Furthermore, the Margalef affect forest growth, understory vegetation regeneration, for- index (R), Shannon-Wiener index (H), Simpson index (D) est spatial structure, forest species diversity (Rong et al. 2014; and Pielou index (Jsw) were also calculated as follows: You et al. 2015;Gonget al. 2015;Huang 2016). Some studies have reported that thinning can not only promote the ðRA þ RC þ RHÞ growth of tree height and diameter at breast height, increase Importance value: IV ¼ ð1Þ forest stock and improve wood quality (An et al. 2012), but also improve the understory environment, reduce understory competition among species, and promote the increase in the ðS  1Þ growth and diversity of understory species, especially the Margalef index: R ¼ ð2Þ lnN understory herb layer (Li et al. 2012). Due to long-term human disturbance, the original broad-leaved Korean pine forests in Northeast China have been severely damaged, and now there are a large Shannon  Wiener index: H ¼ ðP lnPÞð3Þ i i i¼1 number of secondary coniferous and broad-leaved mixed forests (Zhang et al. 2009). Reasonable thinning can ac- celerate the succession rate of this forest type, realize its Simpson index: D ¼ 1  P ð4Þ sustainable management, and play an important role in i¼1 maintaining the stability of timber yield and species di- versity (Liu et al. 2014). Through thinning of different intensities in the near-mature secondary coniferous and ðP lnP Þ i i i¼1 broad-leaved mixed forest in the State-owned Forest Pielou index: Jsw ¼ ð5Þ lnS Protection Center of the Jilin Province Forestry Experi- mental Zone, this study investigated the species diversity where IV is the importance value, RA is the relative and the seasonal changes of biomass of the understory abundance, RC is the relative coverage, RH is the relative herb layer after thinning, and evaluated the influence of height, S is the total number of species in the sample thinning on herbaceous plants. The study aims to pro- plot, N is the total individuals of species in the sample vide a theoretical basis for reasonable tending and plot, and N is the individual of the i-th species Number, i Wang et al. Forest Ecosystems (2021) 8:53 Page 3 of 13 P is the relative value of i, and the calculation formula 2 species, accounting for 13.5 % of the total number of of P is: genera. Among them, there are 7 genera (Lilium, Viola, Geranium, Hippochaete, Cimicifuga, Arisaema and Anemone) containing 2 species and 3 genera P ¼ ð6Þ (Rubia, Carex,and Urtica) containing 3 species. In summary, a single family and a single genus had an Herbaceous plant biomass absolute advantage in the survey plots, and the In early May, mid-July, and early September 2015, the phenomenon of few families and few genera was ob- understory herbaceous biomass was sampled and sur- vious, which was a typical species distribution of tem- veyed three times. Specifically, two 1 m × 1 m subplots perate forest herbs. were randomly set up in each 20 m × 20 m plot. We in- Compared with the control, thinning increased the vestigated all species names, abundance, height, cover- number of species and genera of herbaceous plants. age, total coverage, and finally harvested the whole plant Light thinning, moderate thinning and heavy thinning for all the herbs in the subplot. The samples were taken increased 2 species and 3 genera, 8 species and 9 genera, back to the laboratory to clean the soil at the roots with 1 species and 1 genus, respectively. For the family, mod- water. The above- and belowground parts were cut with erate thinning increased 1 family, while light thinning scissors, and weighed their fresh weight (MA for above- and heavy thinning both decreased by 1 family com- ground part and MB for belowground part). Then the pared with the control (Table 1). sample was first quenched at 105 °C for 30 min, adjusted The number of species, genera and families of herb- to 85 °C and dried to a constant weight. After drying, aceous plants in each thinning treatment showed sea- weigh the dry weight of the aboveground and below- sonal variations. In May, the number of species, genera ground parts and record them as WA and WB. The herb and families in the thinning were higher than those of biomass of the entire subplot was calculated based on the control, and the number of species, genera and fam- the coverage, the dry weight of the herbaceous plants, ilies in the heavy thinning plot was the largest. In June, and the area of the entire subplot (Sun et al. 2017). the number of species, genera and families in the thin- ning were all low compared with the control, and light Data analysis thinning had the smallest number of species, genera and Correlation analysis and one-way analysis of variance families. In July, August and September, moderate thin- (one-way ANOVA) were performed to test thinning ef- ning had the largest number of species, genera and fam- fects on herbaceous diversity and biomass. A total of ilies while heavy thinning was opposite. In October, the 100 1 m × 1 m herbal plots in each thinning treatment number of herbaceous species in each plot decreased was defined as replicates. Multiple comparisons were sharply, with little difference among thinning treatments. performed using the least significant difference method The number of species in the control and moderate (LSD) once the thinning effects were significant. All data thinning was slightly higher than that of light thinning analysis was performed using SPSS18.0 software. The and heavy thinning. significance level was set at α < 0.05. Seasonal variation of importance values Results The importance values of herbaceous plants in different Effect of thinning on plant diversity thinning in each month are shown in Table 2. In May, Species composition the top 5 importance values of herbaceous plants in the During the whole growing season, a total of 87 species control were Anemone raddeana, Hylomecon japonica, of herbaceous plants were investigated in 4 plots, Aegopodium alpestre, Meehania urticifolia and Corydalis belonging to 37 families and 74 genera. From the distri- ambigua. The sum of the importance values of these bution of each family, the number of species belonging plants was 51.38 %, dominating in the plot. Among to Ranunculaceae, Liliaceae, Umbelliferae and Composi- them, there were 15 species with importance values tae was the largest, with 11, 7, 7 and 5 species respect- greater than 1 %, and the sum of their importance values ively. There were 19 families containing only one species accounted for 93.54 % of all herb species. Carex pilosa, and 8 families including 2 species. Only 2 families (Rosa- Anemone raddeana, Filipendula palmata, Meehania ceae and Papaveraceae) included 3 species. Four families urticifolia, and Cardamine leucantha were the top 5 (Cyperaceae, Rubiaceae, Caryophyllaceae and Urticaceae) importance values of herbaceous plants in the light included 4 species, accounting for 10.8 % of all families. thinning, with the sum of the importance values of From the distribution of the genera, there are 64 51.21 %. There are 14 species of herbs with importance genera with 1 species, accounting for 86.5 % of the total value greater than 1 %, and the sum of their importance number of genera. There are 10 genera with more than values accounted for 88.35 % of all herb species. Carex Wang et al. Forest Ecosystems (2021) 8:53 Page 4 of 13 Table 1 Changes in the number of families, genera and species of herbaceous plants in each thinning treatment in different months Month CK LT MT HT Species Genus Family Species Genus Family Species Genus Family Species Genus Family May 36 ± 2 34 ± 0 18 ± 0 43 ± 3 39 ± 1 24 ± 0 44 ± 3 40 ± 1 21 ± 0 44 ± 3 41 ± 1 25 ± 0 June 37 ± 2 36 ± 1 25 ± 0 32 ± 3 30 ± 0 21 ± 0 35 ± 2 32 ± 1 22 ± 0 33 ± 2 32 ± 1 23 ± 0 July 38 ± 3 36 ± 1 22 ± 0 39 ± 2 36 ± 1 25 ± 0 46 ± 3 40 ± 1 27 ± 0 36 ± 3 31 ± 1 21 ± 0 August 33 ± 2 27 ± 0 20 ± 0 29 ± 1 25 ± 0 22 ± 0 36 ± 3 30 ± 0 20 ± 0 30 ± 1 26 ± 0 19 ± 0 September 23 ± 1 21 ± 0 15 ± 0 21 ± 0 19 ± 0 15 ± 0 25 ± 1 22 ± 0 15 ± 0 14 ± 0 13 ± 0 13 ± 0 October 10 ± 0 10 ± 0 8 ± 0 8 ± 0 7 ± 0 7 ± 0 10 ± 0 8 ± 0 7 ± 0 9 ± 0 8 ± 0 7 ± 0 Total 65 ± 2 53 ± 1 31 ± 0 67 ± 2 56 ± 1 30 ± 0 73 ± 3 62 ± 1 32 ± 0 66 ± 2 54 ± 1 30 ± 0 pilosa, Anemone raddeana, Filipendula palmata, Mee- moderate thinning and heavy thinning plots were Carex hania urticifolia and Poa annua were the top 5 import- pilosa, Meehania urticifolia, Carex remotiuscula, Aego- ance values of herbaceous plants in the moderate podium alpestre and Filipendula palmata, but their im- thinning. The sum of the importance values of these portance values were different. The order of importance plants was 49.36 %, and there were 18 species with the values in the moderate thinning was Carex pilosa > Mee- importance value greater than 1 % and the sum of their hania urticifolia > Carex remotiuscula > Aegopodium importance values accounted for 89.56 % of all herb- alpestre > Filipendula palmata. The importance values aceous species. Filipendula palmata, Aegopodium alpes- of the heavy thinning were Carex pilosa > Filipendula tre, Anemone raddeana, Anemone amurensis, and palmata > Aegopodium alpestre > Carex remotiuscula > Meehania urticifolia were the top 5 herbaceous plants in Meehania urticifolia. In the moderate thinning and the heavy thinning plots. The sum of the important heavy thinning, the sum of the importance values of values of these plants was 46.09 %. There were 17 these plants was 58.10 % and 56.22 %, respectively. Herbs species of herbaceous plants with the importance value with importance values greater than 1 % were 20 and 17, greater than 1 %, and the sum of their importance values and accounted for 91.30 % and 91.49 % of all herb species, accounted for 89.93 % of all herb species. The propor- respectively. The number of species in the control was tion of important value of early spring herbs (including large, because the poor light conditions in the control led Anemone raddeana, Anemone amurensis, Hylomecon to late plant phenology, and there were more early spring japonica, Gagea lutea, Corydalis ambigua, Adonis herbs in the control, which led to the existence of early amurensis, etc.) in the control was relatively high summer herbs and early spring herbs at the same time, (42.64 %), while that of light thinning, moderate thinning resulting in a large number of species; while in the and heavy thinning were 24.90 %, 18.64 % and 22.09 % re- thinning plot, due to the better light conditions caused by spectively, which was significantly lower than that of thinning, most of the early spring herbs had died, resulting the control, indicating that the phenological period of in the number of species has declined. herbs after thinning was advanced. Some early spring In July, the top 5 importance values of herbaceous herbs in early May had completed the whole life cycle plants in the control were Carex pilosa, Laportea bulbi- and died. fera, Filipendula palmata, Cardamine leucantha, and In June, the top 5 important values of herbaceous Meehania urticifolia. The sum of the importance values plants in the control were Carex pilosa, Poa annua, of these plants was 56.33 %, there were 16 species with Meehania urticifolia, Aegopodium alpestre and Filipen- the importance value greater than 1 %, and the sum of dula palmata. The sum of the importance values of their importance values accounted for 90.20 % of all herb these plants was 50.88 %. There were 18 species with the species. The top 5 important values of herbaceous plants importance value greater than 1, and the sum of their in the light thinning were Carex pilosa, Filipendula pal- importance values accounted for 92.57 % of all herb- mata, Aegopodium alpestre, Matteuccia struthiopteris aceous species. The top 5 important values of herb- and Anemone amurensis. The sum of the importance aceous plants in the light thinning were Carex pilosa, values of these plants was 60.67 %, and there were 19 Athyrium multidentatum, Aegopodium alpestre, Filipen- species with the importance value greater than 1 %. The dula palmata and Brachybotrys paridiformis. The sum sum of their importance values accounted for 90.52 % of of the importance values was 59.92 %, and there were 16 all herb species. The top 5 important values of herb- species with the importance value greater than 1. The aceous plants in the moderate thinning and heavy thin- top 5 importance values of herbaceous plants in the ning plots were Carex pilosa, Meehania urticifolia, Wang et al. Forest Ecosystems (2021) 8:53 Page 5 of 13 Table 2 The top 5 herbs with relative importance value in different months Month CK LT MT HT Species Importance Species Importance Species Importance Species Importance value (%) value (%) value (%) value (%) May Anemone 18.11 Carex pilosa 18.93 Carex pilosa 17.53 Filipendula 12.92 raddeana palmata Hylomecon 9.96 Anemone 14.47 Anemone 12.04 Aegopodium 12.32 japonica raddeana raddeana alpestre Aegopodium 9.47 Filipendula 8.89 Filipendula 8.35 Anemone 9.38 alpestre palmata palmata raddeana Meehania 7.23 Meehania 4.48 Meehania 7.35 Anemone 6.84 urticifolia urticifolia urticifolia amurensis Corydalis 6.61 Cardamine 4.44 Poa annua 4.09 Meehania 4.63 ambigua leucantha urticifolia Others 48.62 Others 48.79 Others 50.64 Others 53.91 June Carex pilosa 14.39 Carex pilosa 28.14 Carex pilosa 26.63 Carex pilosa 19.29 Poa annua 9.87 Athyrium 8.92 Meehania 9.26 Filipendula 10.94 multidentatum urticifolia palmata Meehania 9.87 Aegopodium 8.90 Carex 8.68 Aegopodium 10.33 urticifolia alpestre remotiuscula alpestre Aegopodium 8.73 Filipendula 8.07 Aegopodium 8.18 Carex 8.42 alpestre palmata alpestre remotiuscula Filipendula 8.02 Brachybotrys 5.89 Filipendula 5.35 Meehania 7.24 palmata paridiformis palmata urticifolia Others 49.12 Others 40.08 Others 41.90 Others 43.78 July Carex pilosa 16.27 Carex pilosa 35.84 Carex pilosa 27.52 Carex pilosa 27.76 Laportea 10.98 Filipendula 9.67 Filipendula 8.42 Filipendula 15.34 bulbifera palmata palmata palmata Filipendula 9.77 Aegopodium 5.62 Carex 7.35 Aegopodium 7.96 palmata alpestre remotiuscula alpestre Cardamine 9.74 Matteuccia 5.21 Meehania 5.92 Meehania 5.55 leucantha struthiopteris urticifolia urticifolia Meehania 9.57 Anemone 4.33 Aegopodium 5.73 Carex 3.64 urticifolia amurensis alpestre remotiuscula Others 43.67 Others 39.33 Others 45.06 Others 39.75 August Milium 14.43 Carex pilosa 24.45 Carex pilosa 22.99 Carex pilosa 16.56 effusum Meehania 13.26 Aegopodium 10.54 Meehania 10.69 Aegopodium 10.99 urticifolia alpestre urticifolia alpestre Carex pilosa 11.93 Filipendula 10.20 Aegopodium 10.26 Filipendula 10.59 palmata alpestre palmata Aegopodium 10.30 Brachybotrys 7.34 Filipendula 9.67 Carex 9.64 alpestre paridiformis palmata callitrichos Filipendula 10.25 Meehania 5.73 Brachybotrys 5.81 Meehania 6.38 palmata urticifolia paridiformis urticifolia Others 39.83 Others 41.74 Others 40.58 Others 45.84 September Carex pilosa 17.66 Carex pilosa 33.06 Carex pilosa 27.94 Carex pilosa 22.89 Aegopodium 16.34 Aegopodium 11.01 Meehania 11.90 Filipendula 16.76 alpestre alpestre urticifolia palmata Milium 12.75 Filipendula 10.79 Aegopodium 10.64 Aegopodium 16.64 effusum palmata alpestre alpestre Meehania 12.46 Athyrium 7.93 Filipendula 9.67 Urtica 16.28 urticifolia multidentatum palmata angustifolia Wang et al. Forest Ecosystems (2021) 8:53 Page 6 of 13 Table 2 The top 5 herbs with relative importance value in different months (Continued) Month CK LT MT HT Species Importance Species Importance Species Importance Species Importance value (%) value (%) value (%) value (%) Cardamine 8.76 Carex 6.88 Athyrium 5.61 Meehania 5.67 leucantha remotiuscula multidentatum urticifolia Others 32.03 Others 30.33 Others 34.24 Others 21.76 October Carex pilosa 40.12 Carex pilosa 58.27 Carex pilosa 47.63 Carex pilosa 39.57 Meehania 21.20 Aegopodium 15.78 Aegopodium 19.20 Aegopodium 17.47 urticifolia alpestre alpestre alpestre Aegopodium 17.57 Meehania 10.89 Carex 14.91 Carex 13.60 alpestre urticifolia remotiuscula callitrichos Milium 8.08 Carex callitrichos 5.38 Meehania 10.24 Meehania 8.86 effusum urticifolia urticifolia Cardamine 5.68 Filipendula 4.20 Carex callitrichos 2.93 Carex 8.60 leucantha palmata remotiuscula Others 7.35 Others 5.48 Others 5.09 Others 11.90 Carex remotiuscula, Aegopodium alpestre and Filipen- paridiformis and Meehania urticifolia, with the sum of dula palmata. The order of importance values in the the important values of 58.26 %. There were 19 species moderate thinning was Carex pilosa > Filipendula with the importance value greater than 1 %, accounting palmata > Carex remotiuscula > Meehania urticifolia > for 94.23 % of all herb species. The top 5 important Aegopodium alpestre, and the importance values of the values of herbaceous plants in the moderate thinning plots with heavy thinning were Carex pilosa > Filipen- were Carex pilosa, Meehania urticifolia, Aegopodium dula palmata > Aegopodium alpestre > Meehania urtici- alpestre, Filipendula palmata and Brachybotrys paridi- folia > Carex remotiuscula. In the moderate and heavy formis. The sum of the important values was 59.42 %, thinning, the sum of the importance values of these and there were 16 species with the importance value plants was 54.94 % and 60.25 %, respectively. The herb greater than 1 %, accounting for 87.55 % of all herb spe- plants with importance values greater than 1 % were 21 cies. The top 5 important values of herbaceous plants in and 22 species and accounted for 92.47 % and 93.50 % of the heavy thinning were Carex pilosa, Aegopodium all herb species, respectively. 38, 39, 46 and 36 species of alpestre, Filipendula palmata, Carex callitrichos and herbs were found in control, light thinning, moderate Meehania urticifolia. The sum of the importance values thinning and heavy thinning respectively. Compared was 54.16 %, and there were 21 species with the import- with the control, moderate thinning significantly in- ance value greater than 1 %, accounting for 95.51 % of all creased the number of herbs. This is due to the good herb species. Among the top 5 species of herbs in differ- light under the forest and the invasion of light loving ent thinning plots, there are 4 common species, namely herbs species after moderate thinning, and there are Carex pilosa, Meehania urticifolia, Aegopodium alpestre more shade environment under the forest and more and Filipendula palmata. From the important values of shade loving herbs are retained, which makes the light these 4 species, the important values of Carex pilosa loving and shade loving species co-exist, resulting in a increased significantly after thinning, while that of relatively large number of species. After heavy thinning, Meehania urticifolia decreased. Aegopodium alpestre the shade environment under the forest is less and the and Filipendula palmata were relatively consistent in light condition is excellent, which is especially suitable different plots, and there was no significant change. for the survival of photophilous herbs, but the loss of In September, the top 5 importance values of herb- photophilous species. aceous plants in the control were Carex pilosa, Aegopo- In August, the top 5 importance values of herbaceous dium alpestre, Milium effusum, Meehania urticifolia, plants in the control were Milium effusum, Meehania and Cardamine leucantha, the sum of the important urticifolia, Carex pilosa, Aegopodium alpestre and values was 67.97 %. There were 11 species of herbs with Filipendula palmata. The sum of the importance values importance values more than 1 %, accounting for was 60.17 %, and there were 15 species with the import- 94.00 % of all herb species. The top 5 important values ance value greater than 1, accounting for 90.59 % of all of herbaceous plants in the light thinning were Carex herb species. The top 5 importance values of herbaceous pilosa, Aegopodium alpestre, Filipendula palmata, plants in the light thinning were Carex pilosa, Aegopo- Athyrium multidentatum and Carex remotiuscula. The dium alpestre, Filipendula palmata, Brachybotrys sum of the important values was 69.67 %. There were 15 Wang et al. Forest Ecosystems (2021) 8:53 Page 7 of 13 species with importance values greater than 1 %, ac- number of species in different plots, and the species counting for 96.65 % of all herb species. The top 5 im- in different plots were almost the same. portance values of herbaceous plants in the moderate thinning were Carex pilosa, Meehania urticifolia, Aego- Seasonal changes in diversity index podium alpestre, Filipendula palmata and Athyrium Calculation of diversity index showed that there were multidentatum. The sum of the important values was significant variations (P < 0.05) in different thinning. Ex- 65.76 %. There were 16 species with importance value cept for the control, which showed an increasing trend more than 1 %, accounting for 92.82 % of all herb spe- from May to June, the Margalef index showed a similar cies. The top 5 importance values of herbaceous plants trend in all plots in other months (Fig. 1a). It gradually in the heavy thinning were Carex pilosa, Filipendula decreased from May to July, increased from July to palmata, Aegopodium alpestre, Urtica angustifolia and August, and then fell rapidly from August to Octo- Meehania urticifolia. The sum of the important values ber. The Margalef index of different treatments was 78.24 %. There were 13 species with the importance showed various patterns in different months with value greater than 1 %, accounting for 99.14 % of all herb moderate thinning > heavy thinning > light thinning > species. 23, 21, 25 and 14 species of herbs were found in control in May, and control > moderate thinning > the control, light thinning, moderate thinning and heavy light thinning > heavy thinning from June to August. thinning, respectively. This is due to the fact that more From August to October, the Margalef index of the photophilous species, such as Impatiens noli-tangere, four plots all showed a significant decreasing trend have invaded the plots after heavy thinning. The earlier with control > light thinning > moderate thinning > invasive species will occupy a wide area in the sample heavy thinning. plot, and prevent the invasion of other species. Conse- In the four treatments, the Shannon-Wiener index and quently, the number of species will decrease after heavy Simpson index had similar patterns (Fig. 1b and c). The thinning. With the coming of autumn, the number of Shannon-Wiener index and Simpson index gradually de- photophilous species in the heavy thinning plots in Sep- creased from May to July, increased gradually from July tember was lower because of their higher individuals to August, and then decreased sharply. In June, July and and more easily withered and died. September, the Shannon-Wiener index and Simpson In October, the top 5 importance values of herbaceous index in the control were higher than the other three plants in the control were Carex pilosa, Meehania urtici- thinning treatments. In May, August and October, the folia, Aegopodium alpestre, Milium effusum and Carda- Shannon-Wiener index and Simpson index of the four mine leucantha, the sum of the importance value was plots were relatively the same. 92.65 %. There are 8 species with importance value Throughout the growing season, the Pielou index was greater than 1 %, accounting for 98.60 % of all herb spe- roughly in the shape of an inverted “N” (Fig. 1d), namely cies. The top 5 importance values of herbaceous plants it had a high value at the beginning of growth, a low in the light thinning were Carex pilosa, Aegopodium value in June or July, and the second peak in August, alpestre, Meehania urticifolia, Carex callitrichos and and then it decreased sharply. The Pielou index of the Filipendula palmata. The sum of the important values control was generally higher than that of the other thin- was 94.52 %, and there were 7 species with the import- ning treatments. The control plot and the heavy thin- ance value greater than 1 %, accounting for 99.32 % of all ning showed the first decrease in June, while the light herb species. The top 5 importance values of herbaceous and moderate thinning showed the first decrease in July. plants in the moderate thinning were Carex pilosa, Aego- podium alpestre, Carex remotiuscula, Meehania urticifo- Effect of thinning on biomass lia and Carex callitrichos. The sum of the important Seasonal changes in biomass values of was 94.91 %. There were 6 species with im- In May, thinning can promote the increase of total bio- portance value greater than 1 %, accounting for mass, above-ground biomass and belowground biomass 97.38 % of all herb species. The top 5 importance of herbaceous plants, but there were no significant dif- values of herbaceous plants in the heavy thinning ferences among thinning treatments. Total biomass and were Carex pilosa, Aegopodium alpestre, Carex calli- above-ground biomass were the largest in light thinning, trichos, Meehania urticifolia,and Carex remotiuscula. and the belowground biomass was the largest in moder- The sum of the important values was 88.10 %. There ate thinning. In terms of root-to-shoot ratio, the moder- were 8 species with important value greater than 1 %, ate thinning was significantly higher than the other three accounting for 99.34 % of all herb species. There were treatments, and the light and heavy thinning were not 10, 8, 10 and 9 species of herbs in the control, light significantly different from the control. Compared with thinning, moderate thinning and heavy thinning, re- the control, the number of herbaceous plants decreased spectively. There was no significant difference in the after light thinning, but the difference was not obvious. Wang et al. Forest Ecosystems (2021) 8:53 Page 8 of 13 Fig. 1 Monthly change of herbaceous plant diversity index Moderate and heavy thinning significantly increased the biomass in September. Heavy thinning rather than light number of herbaceous plants. In terms of average height thinning and moderate thinning significantly increased and total coverage, moderate thinning was significantly above-ground biomass. After thinning, the root-shoot higher than that of light thinning. ratio increased significantly, but the difference in the The biomass of herbaceous plants increased in July root-shoot ratio was not significant among light, moder- compared with May. Compared with the control, thin- ate and heavy thinning, indicating that thinning changed ning promoted the increase of total biomass, above- and the distribution of above- and belowground biomass of belowground biomass. Among them, the total biomass, herbaceous plants. The nutrients were mostly allocated above- and belowground biomass were significantly to the belowground part, but the thinning intensity did higher in heavy thinning than those of the control. The not change the distribution of plant biomass between difference between moderate thinning and the control above- and belowground parts. Thinning increased the was not significant. In terms of root-shoot ratio, the dif- number of herbaceous plants, and the number of herb- ference between the four treatments was not significant, aceous plants in the moderate and the heavy thinning indicating that thinning intensity did not change the bio- was significantly higher than that in the control. The mass allocation between the above- and belowground average height of herbs decreased after thinning, which parts. Thinning increased the number of plants, and the was evident in light thinning. Thinning significantly in- number increased with increasing thinning intensity. creased the total coverage of herbs, but there were no The average height of herbaceous plants after thinning significant differences among light, moderate and heavy decreased, but the difference was not significant. In thinning. addition, thinning increased the total coverage of herb- The correlation analysis of the biomass in different aceous plants. The coverage was significantly higher in months is shown in Fig. 2. In May, the total biomass sig- the heavy thinning than that in the control, while the nificantly correlated with the number, average height, light and moderate thinning were not significantly differ- and the total coverage, but it did not significantly correl- ent from the control. ate with the root-shoot ratio. Above-ground biomass Compared with the control, thinning significantly in- was positively correlated with the number, average creased the total herb biomass and belowground height and total coverage, but not correlated with the Wang et al. Forest Ecosystems (2021) 8:53 Page 9 of 13 root-shoot ratio. Belowground biomass was positively correlated with root-shoot ratio, the number and average height, but did not change with the total coverage. In July, the number and total coverage of herbaceous plants were positively correlated with total biomass, above- and belowground biomass. The average height of herbaceous plants did not correlate with total biomass, above- and belowground biomass. The root-shoot ratio significantly correlated with total biomass and below- ground biomass. In September, the root-shoot ratio positively correlated with total biomass and belowground bio- mass. The number of herb plants had a significantly and positively correlation with total biomass and be- lowground biomass. The average height of herb plants was significantly and negatively correlated with the total biomass and above-ground biomass. The total coverage of herb plants was positively re- lated to the total biomass, above- and belowground biomass. It was found that the herb biomass was different over time (Table 3). In the control, the total biomass, above- and belowground biomass all showed their maximum in July. After thinning, the total biomass, above- and be- lowground biomass all gradually increased over time and reached their maximum in September. In the thinning treatments, the total biomass, above- and belowground biomass in September were significantly higher than those in May. Correlations between diversity and biomass The correlation analysis between herb biomass and diversity index in different months is shown in Fig. 3. In May, total herb biomass, above- and below ground biomass were significantly and positively cor- related with Shannon-Wiener index, Simpson index, and Pielou index. The correlation between herb bio- mass and diversity index in July and September was not significant. Compared with May, the relationship between species diversity and biomass were changed, indicating that seasonal changes will change the relationships. Discussion Importance values variation Forest thinning, as an important forest management practice, is an important way to change the composition and structure of the forest, adjust forest density, and im- prove the understory environmental conditions. It will directly affect the composition and diversity of under- Fig. 2 Correlation of various biomasses of herbaceous plants in story vegetation, and influence tree growth and commu- different months nity stability (Zhang and Tang 2008). The importance values of each species varied significantly among Wang et al. Forest Ecosystems (2021) 8:53 Page 10 of 13 Table 3 Biomass of herbaceous plants in different months Month Treatments Total Aboveground Belowground Root-shoot Number per Mean height Total coverage biomass biomass biomass ratio m (cm) (%) − 2 − 2 − 2 (g·m ) (g·m ) (g·m ) May CK 113 ± 10a 53 ± 4a 59 ± 7a 1.11 ± 0.10a 216 ± 12a 11.95 ± 0.36ab 68.75 ± 3.70ab LT 194 ± 12b 86 ± 5b 109 ± 8bc 1.29 ± 0.06a 198 ± 11a 11.14 ± 0.36a 59.60 ± 2.77a MT 188 ± 21b 71 ± 10ab 117 ± 12b 1.86 ± 0.17b 275 ± 18b 12.44 ± 0.48b 72.75 ± 3.55b HT 163 ± 15b 76 ± 8b 88 ± 8c 1.18 ± 0.09a 265 ± 19b 12.09 ± 0.41ab 67.53 ± 4.46ab July CK 207 ± 16a 104 ± 11a 104 ± 8a 1.05 ± 0.08a 122 ± 14a 30.14 ± 1.18a 65.63 ± 3.13a LT 240 ± 13a 108 ± 6a 132 ± 9ab 1.24 ± 0.08a 175 ± 20ab 26.89 ± 1.88a 70.20 ± 3.70a MT 236 ± 19a 107 ± 9a 128 ± 12a 1.22 ± 0.09a 191 ± 18b 28.07 ± 1.20a 70.65 ± 2.98a HT 339 ± 53b 154 ± 16b 185 ± 39b 1.12 ± 0.13a 219 ± 29b 28.79 ± 1.34a 81.67 ± 3.89b September CK 161 ± 8a 85 ± 3a 76 ± 5a 0.89 ± 0.05a 109 ± 10a 25.12 ± 0.75a 58.30 ± 2.24a LT 273 ± 22b 112 ± 7a 161 ± 17b 1.43 ± 0.12b 134 ± 26ab 21.20 ± 1.90b 70.06 ± 3.96b MT 289 ± 30b 113 ± 8a 176 ± 23bc 1.51 ± 0.13b 181 ± 16b 24.62 ± 0.98ab 76.80 ± 3.28b HT 418 ± 43c 192 ± 28b 225 ± 31c 1.41 ± 0.25b 180 ± 43b 21.47 ± 1.36ab 78.13 ± 4.93b different thinning treatments. The reason may be that species (Hulík and Douda 2017). The overstory reaches the growth of overstorey changes with season, which in its growth peak in August, and the canopy closure con- turn causes changes in the understory environment. The tinues to increase, resulting in a continuous decrease in importance values of species in different treatments understory light intensity. Early spring plants and some showed similar patterns with seasonal changes. The im- early summer plants withered and died, and the number portance values of early spring plant were greater in of understory herbaceous species declined significantly, May, and the top 5 importance values of various treat- but the dominant species did not change (Gong et al. ments were Anemone amurensis, Hylomecon japonica, 2016). The temperature drops significantly in September, Anemone raddeana,and Corydalis ambigua. Further- and the overstory stops growing, some tall trees begin to more, these plants constitute the main body of herb- defoliate, and the canopy closure is reduced. At this aceous plants under the broad-leaved Korean pine forest time, most of the herbaceous plants also stop the growth in early spring. At this time, the overstorey has not yet of the above-ground parts and enter the nutrient reserve reached canopy closure, the understory has sufficient period, which causes a significant decrease in the num- sunlight, and the understory temperature continues to ber of species. Understory herbaceous plants still consist rise. In addition to early spring plants, other herbaceous of dominant species such as Carex pilosa, Aegopodium plants have also sprouted in large numbers, so there are alpestre and Meehania urticifolia (Wu et al. 2018). In more species in May (Chen and Matter 2017; Kleinn October, the Changbai Mountain area has entered the et al. 2020). In June, the trees grow fast, but the over- end of autumn. Although most of the trees in the upper storey is not completely closed. The light intensity of layer of the canopy have fallen leaves, there is sufficient understory gradually decreases, and the plants wither sunlight understory. But due to the low temperature, and die in early spring. Except for the control, the num- most of the herbaceous plants have withered and died, ber of species significantly decreased in other thinning causing the number of species to drop to the lowest treatments. At this time, Carex pilosa, Aegopodium value in the growing season (Dai et al. 2013). Based on alpestre, Meehania urticifolia, and Filipendula palmata the data of the entire growing season, the top herb- replaced early spring plants as the dominant species of aceous plants in the various treatments are mainly com- understory herbs, and ferns also showed greater advan- posed of Carex pilosa, Aegopodium alpestre, Meehania tages due to their vigorous growth (Gao et al. 2019). The urticifolia and Filipendula palmata, indicating that these overstory grows fast in July. The canopy density is rela- species have strong adaptability and a wide ecological tively high, and the understory light intensity is low. At range. this time, the understory is mainly covered by shade- tolerant species such as Carex pilosa, Carex remotius- Herbaceous plant species variation cula, Carex callitrichos, Carex siderosticta, Meehania The monthly changes in the number of herbaceous plant urticifolia, Laportea bulbifera, Urtica angustifolia.As species in different thinning treatments indicate that the precipitation begins to increase, understory wet plants number of species in the thinning in May is higher than grow quickly, leading to an increase in the number of that in the control. This may be due to the increased Wang et al. Forest Ecosystems (2021) 8:53 Page 11 of 13 understory light intensity after thinning, which leads to the advance of plant phenology (Ye et al. 2015). The number of species in the thinning treatments decreased in June. This is due to the advance of the phenological period after thinning that causes some early spring short-lived plants to wither and die, resulting in a de- crease in the number of species in the thinned treat- ments (Li et al. 2006). From July to September, the number of species in the four treatments gradually de- creased. This is because July is the rainy season in this area. The soil under the forest is moist and the temperature is high, which is most suitable for plant growth. Therefore, the number of species in July reached the maximum. Although the enhancement of understory light intensity might have caused the death of some shade-tolerant species in moderate thinning, heliophile will quickly invade the site, leading to species reaching the maximum in the moderate thinning during this period. Although light thinning may not significantly in- crease light intensity, the thinning will form gaps, which will lead to the death of some rare shade-tolerant herb- aceous plants, and the heliophile cannot invade the plot, resulting in the decrease of species numbers. In the heavy thinning treatment, the understory sunlight is sig- nificantly enhanced and there are more gaps, causing the death of most of the shade-tolerant herbaceous plants. Although the heliophile can invade the site, they cannot compensate for the death number of shade-tolerant plant, resulting in a significant decrease in species num- bers (Ding et al. 2016). The understory temperature and light intensity of the four treatments are very low in October, and the understory environmental conditions are roughly the same. Most of the herbaceous plants withered and died, and only a few herbs with strong cold tolerance could survive (Small and McCarthy 2002). Herbaceous plant individuals number variation Throughout the growing season, the number of individ- ual plants in the heavy thinning is the largest, indicating that thinning can promote the increase in the number of individual plants. This is due to the increase in the intensity of thinning, which leads to the increase of understory light intensity, subsequently promoting the increase of understory herb plants (Westerband and Horvitz 2017; Didion 2020). The number of individuals in the moderate thinning was higher than that in the heavy thinning in July. This is due to the presence of large herb plants in the heavy thinning, occupying a lar- ger space under the forest, resulting in the death of some small one (Zhang et al. 2019). The number of indi- vidual plants in the control showed a decreasing trend. Fig. 3 Correlation between herb biomass and diversity index in This was because most of the control in May were early different months spring plants. Although the early spring plants are small, there are more individuals on the same stem, so the Wang et al. Forest Ecosystems (2021) 8:53 Page 12 of 13 number of individuals was the largest. Early spring other plants in the moderate and heavy thinning were plants gradually die from June to August, early summer significantly higher than those in the light thinning and plants and late summer plants dominant the herbaceous the control (Sun et al. 2017). plants. These plants are relatively large, occupying more The diversity of herbaceous plants is an important in- space understory, resulting in a decrease in the number dicator of the succession and development of forests. of individual plants. The overstory defoliate in Septem- This study only studied four-intensity thinning in near- ber, the increase in understory light intensity increases mature forests. In order to better reflect the effects of the ground temperature, and the secondary growth of thinning on the understory environment, herbaceous some plants leads to an increase in the number of indi- plant diversity and biomass, therefore, it is necessary to viduals. The temperature gradually decreases in October, further carry out relevant experiments in forest with dif- and most understory herbs are not cold-resistant, and ferent development stages and stand characteristics. It is will gradually wither and die, leading to a decrease in also a necessity to analyze the impact of thinning on for- the number of individuals (Grant et al. 2019). The in- est species diversity and biomass, guide forest manage- crease in the number of individual plants in May and ment and accelerate the development of secondary June after thinning is due to the increase in understory forests to primary forests. In addition, this study also light and the increase in understory temperature due to demonstrates that the species diversity and biomass of thinning, which advances the phenological period of shrubs will also be affected by thinning. In the future, re- plants. There are two types of plants including early search can be conducted on the impact of thinning on spring plants and early summer plants in June (especially the species diversity and biomass of shrubs. in heavy thinning). At this time, the plant individuals are small, and the understory space can meet the needs of Conclusions plant growth. Therefore, the number of individual plants The changes of biodiversity in the herb layer under the tends to increase. After June, the size of individual plants coniferous and broad-leaved mixed forest in Changbai gradually increases after June, resulting in insufficient Mountain were mainly affected by seasons. However, understory space and the competition causing the death thinning changed the light environment in the forest, of some individuals, which led to a decrease in the num- which could improve the plant diversity and biomass of ber of individuals (Li et al. 2017). herb layer in a short time. Moderate thinning had the most significant effect on the number of species, and the Herbaceous plant biomass variation number of individual plants, average height and total The total biomass, aboveground biomass and below- coverage of plants increased significantly in all months ground biomass of the thinning treatments were greater of the growing season. With the increase of thinning in- than those in the control, indicating that thinning can tensity, the biomass of herb layer increased accordingly. promote the biomass growth of understory herbaceous Total herb biomass, above- and belowground biomass plants. The greater the thinning intensity, the faster the showed positive correlations with Shannon-Winner plant biomass will grow. This is because thinning can index, Simpson index and Pielou index in May. The significantly improve the understory light environment, findings provide theoretical basis and reference for rea- thereby promoting biomass growth of understory herbs sonable thinning and tending in coniferous and broad- (Tinya and Ódor 2016). The plant heights of the light leaved mixed forests. thinning and the control showed a peak curve, and the plant heights of the heavy and moderate thinning Acknowledgements showed a double-peak curve. This is because July is the We would like to thank the two anonymous reviewers and the associate editor for their constructive comments that greatly helped improving the rainy season and the temperature is high, and the envir- manuscript. onmental conditions are moderate. Both overstory and understory entered the vigorous growth period, and the Authors’ contributions average height of the herbs reached the maximum. In Gerong Wang designed the study and developed the main hypotheses, Mo August, as the stand canopy density increase, the under- Zhou and Mengjia Wu collected and prepared the data, Naiqian Guan, Yuwen Wang, Runhua Jiang and Zhiyu Liu analyzed the data, Fucai Xia and story light of the light thinning and the control was rela- Yue Sun drafted the manuscript. All authors commented preliminary versions tively poor, some tall herbs gradually withered and died of the manuscript and contributed to improve the final version. The due to insufficient growth. The light environment of the author(s) read and approved the final manuscript. moderate and the heavy thinning improved significantly after thinning. After the highest canopy closure time in Funding This work was financially supported by a grant from the National Key August, understory herbaceous plants have a significant Research and Development Program of China (2017YFC0504102) and the high growth. For example, the heights of Carex pilosa, Science and Technology Program of Jilin Provincial Education Department ferns, and Impatiens noli-tangere, Veratrum nigrum and (JJKH20180349KJ). Wang et al. Forest Ecosystems (2021) 8:53 Page 13 of 13 Availability of data and materials Lei XD, Lu YC, Zhang HR, Zhang ZL, Chen XG (2005) Effects of thinning on mixed The datasets used and/or analysed during the current study are available stands of Larix olgensis, Abies nephrolepis and Picea jazoensis. Sci Silv Sin 41(4): from the corresponding author on reasonable request. 78–85 (in Chinese) Li CY, Ma LY, Xu X (2006) Reviews on research progress of effect of tending on forest biodiversity. World For Res 19(6):27–32 (in Chinese) Declarations Li LP, Mohammat A, Abdusalih N, Sarbay N, Wan HW (2017) Plant body size patterns of mountainous trees and grassland herbs in Xinjiang region, China. Ethics approval and consent to participate Biodivers Sci 25(11):1202–1212 (in Chinese) Not applicable. Li RX, Ma HJ, Min JG, Hao JP, Guan QW (2012) Short-term and Long-term effects of thinning on the undergrowth diversity in the Pinus massoniana plantation. Ecol Environ Sci 21(5):807–812 (in Chinese) Consent for publication Liu HY, Chen DL, Liu J, Zhao Y, Zhang YX, Guo JP (2014) Research on Not applicable. undergrowth plant diversity in Larix principis-rupprechtii plantation with different thinning levels-taking Longxing forest farm of Guandishan in Shanxi Competing interests Province as an example. For Resour Manag (1):50–56 (in Chinese) The authors declare that they have no competing interests. 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Effect of thinning intensity on understory herbaceous diversity and biomass in mixed coniferous and broad-leaved forests of Changbai Mountain

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Abstract

Background: Herbs are an important part of the forest ecosystem, and their diversity and biomass can reflect the restoration of vegetation after forest thinning disturbances. Based on the near-mature secondary coniferous and broad-leaved mixed forest in Jilin Province Forestry Experimental Zone, this study analyzed seasonal changes of species diversity and biomass of the understory herb layer after different intensities of thinning. Results: The results showed that although the composition of herbaceous species and the ranking of importance values were affected by thinning intensity, they were mainly determined by seasonal changes. Across the entire growing season, the species with the highest importance values in thinning treatments included Carex pilosa, Aegopodium alpestre, Meehania urticifolia, and Filipendula palmata, which dominated the herb layer of the coniferous and broad-leaved mixed forest. The number of species, Margalef index, Shannon-Wiener index and Simpson index all had their highest values in May, and gradually decreased with months. Pielou index was roughly inverted “N” throughout the growing season. Thinning did not increase the species diversity. Thinning can promote the total biomass, above- and below-ground biomass. The number of plants per unit area and coverage were related to the total biomass, above- and below-ground biomass. The average height had a significantly positive correlation with herb biomass in May but not in July. However, it exerted a significantly negative correlation with herb biomass in September. The biomass in the same month increased with increasing thinning intensity. Total herb biomass, above- and below-ground biomass showed positive correlations with Shannon-Winner index, Simpson index and Pielou evenness index in May. Conclusions: Thinning mainly changed the light environment in the forest, which would improve the plant diversity and biomass of herb layer in a short time. And different thinning intensity had different effects on the diversity of understory herb layer. The findings provide theoretical basis and reference for reasonable thinning and tending in coniferous and broad-leaved mixed forests. Keywords: Selective thinning, Mixed coniferous and broad-leaved forests, Herbaceous plants, Diversity, Biomass * Correspondence: xfc0707@163.com Gerong Wang and Yue Sun are authors contributed equally to this work and should be considered as co-first authors. Key Laboratory of State Forestry Administration on Conservation and Efficient Utilization of Precious and Rare Forest Resource in Changbai Mountain, Forestry College, Beihua University, 132013 Jilin, China Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Wang et al. Forest Ecosystems (2021) 8:53 Page 2 of 13 Background thinning for secondary coniferous and broad-leaved Understory herbaceous plants are an important part of the mixed forests in Northeast China. species diversity of forest ecosystems (Han et al. 2016). In temperate forests, more than 80 % of vascular plants are Materials and methods herbaceous plants (Whigham 2004), and herbaceous Study area plants are of great significance for maintaining the species The study site was located at Dapo Forest Farm (43°57′ N, diversity of forest ecosystems. Compared with trees and 127°43′ E), the state-owned forest protection center of Jilin shrubs, understory herbs are more sensitive to distur- Province Forestry Experimental Zone. In July 2011, 4 per- bances and are the most adaptable to the environment. If manent monitoring plots with an area of 1 ha (100 m × the disturbance severely destroys the original forest eco- 100 m) were set up in the near-mature forest. Each plot was system, herbaceous plants will usually recover fast, thereby divided into 25 plots of 20 m × 20 m and the forest stand quickly improving the soil site conditions, reducing the in- survey was conducted. In December 2012, 4 plots were tensity of solar radiation on the ground, and providing im- thinned with actual intensity of 0, 17.2 %, 32.7 %, and 50.8 % portant protection for the regeneration of trees and respectively, which were defined as unthinning (CK), light shrubs and the growth of seedlings (Huang et al. 2016). thinning (LT), moderate thinning (MT) and heavy thinning Therefore, the species diversity and biomass of the herb (HT) (Liu et al. 2019). layer can reflect the vegetation restoration status after for- est disturbance. Herbaceous plant diversity Forest thinning is an important disturbance in the forest At the beginning of May 2015, 4 herbal plots of 1 m × 1 m managements. Thinning can have an important impact on were randomly set up in each 20 m × 20 m plot, and a total the stability of the forest ecosystem, and even destroy the of 100 herbal plots were set up. From early May to early balance of the forest ecosystem (Lei et al. 2005). As an im- October 2015, the species name, abundance, average height, portant disturbance in the forest managements, thinning has and coverage of the herb plots in the four plots were investi- long attracted people’s attention (Zhou et al. 2012). Reason- gated, and the total coverage was also recorded. The relative able thinning can maintain the stability and structure of the abundance (RA), relative coverage (RC), relative height (RH), forest ecosystem while obtaining timber, and achieve sustain- and importance value (IV) of herbaceous plants based on the able forest management (Zhou et al. 2015). Thinning can obtained data were calculated. Furthermore, the Margalef affect forest growth, understory vegetation regeneration, for- index (R), Shannon-Wiener index (H), Simpson index (D) est spatial structure, forest species diversity (Rong et al. 2014; and Pielou index (Jsw) were also calculated as follows: You et al. 2015;Gonget al. 2015;Huang 2016). Some studies have reported that thinning can not only promote the ðRA þ RC þ RHÞ growth of tree height and diameter at breast height, increase Importance value: IV ¼ ð1Þ forest stock and improve wood quality (An et al. 2012), but also improve the understory environment, reduce understory competition among species, and promote the increase in the ðS  1Þ growth and diversity of understory species, especially the Margalef index: R ¼ ð2Þ lnN understory herb layer (Li et al. 2012). Due to long-term human disturbance, the original broad-leaved Korean pine forests in Northeast China have been severely damaged, and now there are a large Shannon  Wiener index: H ¼ ðP lnPÞð3Þ i i i¼1 number of secondary coniferous and broad-leaved mixed forests (Zhang et al. 2009). Reasonable thinning can ac- celerate the succession rate of this forest type, realize its Simpson index: D ¼ 1  P ð4Þ sustainable management, and play an important role in i¼1 maintaining the stability of timber yield and species di- versity (Liu et al. 2014). Through thinning of different intensities in the near-mature secondary coniferous and ðP lnP Þ i i i¼1 broad-leaved mixed forest in the State-owned Forest Pielou index: Jsw ¼ ð5Þ lnS Protection Center of the Jilin Province Forestry Experi- mental Zone, this study investigated the species diversity where IV is the importance value, RA is the relative and the seasonal changes of biomass of the understory abundance, RC is the relative coverage, RH is the relative herb layer after thinning, and evaluated the influence of height, S is the total number of species in the sample thinning on herbaceous plants. The study aims to pro- plot, N is the total individuals of species in the sample vide a theoretical basis for reasonable tending and plot, and N is the individual of the i-th species Number, i Wang et al. Forest Ecosystems (2021) 8:53 Page 3 of 13 P is the relative value of i, and the calculation formula 2 species, accounting for 13.5 % of the total number of of P is: genera. Among them, there are 7 genera (Lilium, Viola, Geranium, Hippochaete, Cimicifuga, Arisaema and Anemone) containing 2 species and 3 genera P ¼ ð6Þ (Rubia, Carex,and Urtica) containing 3 species. In summary, a single family and a single genus had an Herbaceous plant biomass absolute advantage in the survey plots, and the In early May, mid-July, and early September 2015, the phenomenon of few families and few genera was ob- understory herbaceous biomass was sampled and sur- vious, which was a typical species distribution of tem- veyed three times. Specifically, two 1 m × 1 m subplots perate forest herbs. were randomly set up in each 20 m × 20 m plot. We in- Compared with the control, thinning increased the vestigated all species names, abundance, height, cover- number of species and genera of herbaceous plants. age, total coverage, and finally harvested the whole plant Light thinning, moderate thinning and heavy thinning for all the herbs in the subplot. The samples were taken increased 2 species and 3 genera, 8 species and 9 genera, back to the laboratory to clean the soil at the roots with 1 species and 1 genus, respectively. For the family, mod- water. The above- and belowground parts were cut with erate thinning increased 1 family, while light thinning scissors, and weighed their fresh weight (MA for above- and heavy thinning both decreased by 1 family com- ground part and MB for belowground part). Then the pared with the control (Table 1). sample was first quenched at 105 °C for 30 min, adjusted The number of species, genera and families of herb- to 85 °C and dried to a constant weight. After drying, aceous plants in each thinning treatment showed sea- weigh the dry weight of the aboveground and below- sonal variations. In May, the number of species, genera ground parts and record them as WA and WB. The herb and families in the thinning were higher than those of biomass of the entire subplot was calculated based on the control, and the number of species, genera and fam- the coverage, the dry weight of the herbaceous plants, ilies in the heavy thinning plot was the largest. In June, and the area of the entire subplot (Sun et al. 2017). the number of species, genera and families in the thin- ning were all low compared with the control, and light Data analysis thinning had the smallest number of species, genera and Correlation analysis and one-way analysis of variance families. In July, August and September, moderate thin- (one-way ANOVA) were performed to test thinning ef- ning had the largest number of species, genera and fam- fects on herbaceous diversity and biomass. A total of ilies while heavy thinning was opposite. In October, the 100 1 m × 1 m herbal plots in each thinning treatment number of herbaceous species in each plot decreased was defined as replicates. Multiple comparisons were sharply, with little difference among thinning treatments. performed using the least significant difference method The number of species in the control and moderate (LSD) once the thinning effects were significant. All data thinning was slightly higher than that of light thinning analysis was performed using SPSS18.0 software. The and heavy thinning. significance level was set at α < 0.05. Seasonal variation of importance values Results The importance values of herbaceous plants in different Effect of thinning on plant diversity thinning in each month are shown in Table 2. In May, Species composition the top 5 importance values of herbaceous plants in the During the whole growing season, a total of 87 species control were Anemone raddeana, Hylomecon japonica, of herbaceous plants were investigated in 4 plots, Aegopodium alpestre, Meehania urticifolia and Corydalis belonging to 37 families and 74 genera. From the distri- ambigua. The sum of the importance values of these bution of each family, the number of species belonging plants was 51.38 %, dominating in the plot. Among to Ranunculaceae, Liliaceae, Umbelliferae and Composi- them, there were 15 species with importance values tae was the largest, with 11, 7, 7 and 5 species respect- greater than 1 %, and the sum of their importance values ively. There were 19 families containing only one species accounted for 93.54 % of all herb species. Carex pilosa, and 8 families including 2 species. Only 2 families (Rosa- Anemone raddeana, Filipendula palmata, Meehania ceae and Papaveraceae) included 3 species. Four families urticifolia, and Cardamine leucantha were the top 5 (Cyperaceae, Rubiaceae, Caryophyllaceae and Urticaceae) importance values of herbaceous plants in the light included 4 species, accounting for 10.8 % of all families. thinning, with the sum of the importance values of From the distribution of the genera, there are 64 51.21 %. There are 14 species of herbs with importance genera with 1 species, accounting for 86.5 % of the total value greater than 1 %, and the sum of their importance number of genera. There are 10 genera with more than values accounted for 88.35 % of all herb species. Carex Wang et al. Forest Ecosystems (2021) 8:53 Page 4 of 13 Table 1 Changes in the number of families, genera and species of herbaceous plants in each thinning treatment in different months Month CK LT MT HT Species Genus Family Species Genus Family Species Genus Family Species Genus Family May 36 ± 2 34 ± 0 18 ± 0 43 ± 3 39 ± 1 24 ± 0 44 ± 3 40 ± 1 21 ± 0 44 ± 3 41 ± 1 25 ± 0 June 37 ± 2 36 ± 1 25 ± 0 32 ± 3 30 ± 0 21 ± 0 35 ± 2 32 ± 1 22 ± 0 33 ± 2 32 ± 1 23 ± 0 July 38 ± 3 36 ± 1 22 ± 0 39 ± 2 36 ± 1 25 ± 0 46 ± 3 40 ± 1 27 ± 0 36 ± 3 31 ± 1 21 ± 0 August 33 ± 2 27 ± 0 20 ± 0 29 ± 1 25 ± 0 22 ± 0 36 ± 3 30 ± 0 20 ± 0 30 ± 1 26 ± 0 19 ± 0 September 23 ± 1 21 ± 0 15 ± 0 21 ± 0 19 ± 0 15 ± 0 25 ± 1 22 ± 0 15 ± 0 14 ± 0 13 ± 0 13 ± 0 October 10 ± 0 10 ± 0 8 ± 0 8 ± 0 7 ± 0 7 ± 0 10 ± 0 8 ± 0 7 ± 0 9 ± 0 8 ± 0 7 ± 0 Total 65 ± 2 53 ± 1 31 ± 0 67 ± 2 56 ± 1 30 ± 0 73 ± 3 62 ± 1 32 ± 0 66 ± 2 54 ± 1 30 ± 0 pilosa, Anemone raddeana, Filipendula palmata, Mee- moderate thinning and heavy thinning plots were Carex hania urticifolia and Poa annua were the top 5 import- pilosa, Meehania urticifolia, Carex remotiuscula, Aego- ance values of herbaceous plants in the moderate podium alpestre and Filipendula palmata, but their im- thinning. The sum of the importance values of these portance values were different. The order of importance plants was 49.36 %, and there were 18 species with the values in the moderate thinning was Carex pilosa > Mee- importance value greater than 1 % and the sum of their hania urticifolia > Carex remotiuscula > Aegopodium importance values accounted for 89.56 % of all herb- alpestre > Filipendula palmata. The importance values aceous species. Filipendula palmata, Aegopodium alpes- of the heavy thinning were Carex pilosa > Filipendula tre, Anemone raddeana, Anemone amurensis, and palmata > Aegopodium alpestre > Carex remotiuscula > Meehania urticifolia were the top 5 herbaceous plants in Meehania urticifolia. In the moderate thinning and the heavy thinning plots. The sum of the important heavy thinning, the sum of the importance values of values of these plants was 46.09 %. There were 17 these plants was 58.10 % and 56.22 %, respectively. Herbs species of herbaceous plants with the importance value with importance values greater than 1 % were 20 and 17, greater than 1 %, and the sum of their importance values and accounted for 91.30 % and 91.49 % of all herb species, accounted for 89.93 % of all herb species. The propor- respectively. The number of species in the control was tion of important value of early spring herbs (including large, because the poor light conditions in the control led Anemone raddeana, Anemone amurensis, Hylomecon to late plant phenology, and there were more early spring japonica, Gagea lutea, Corydalis ambigua, Adonis herbs in the control, which led to the existence of early amurensis, etc.) in the control was relatively high summer herbs and early spring herbs at the same time, (42.64 %), while that of light thinning, moderate thinning resulting in a large number of species; while in the and heavy thinning were 24.90 %, 18.64 % and 22.09 % re- thinning plot, due to the better light conditions caused by spectively, which was significantly lower than that of thinning, most of the early spring herbs had died, resulting the control, indicating that the phenological period of in the number of species has declined. herbs after thinning was advanced. Some early spring In July, the top 5 importance values of herbaceous herbs in early May had completed the whole life cycle plants in the control were Carex pilosa, Laportea bulbi- and died. fera, Filipendula palmata, Cardamine leucantha, and In June, the top 5 important values of herbaceous Meehania urticifolia. The sum of the importance values plants in the control were Carex pilosa, Poa annua, of these plants was 56.33 %, there were 16 species with Meehania urticifolia, Aegopodium alpestre and Filipen- the importance value greater than 1 %, and the sum of dula palmata. The sum of the importance values of their importance values accounted for 90.20 % of all herb these plants was 50.88 %. There were 18 species with the species. The top 5 important values of herbaceous plants importance value greater than 1, and the sum of their in the light thinning were Carex pilosa, Filipendula pal- importance values accounted for 92.57 % of all herb- mata, Aegopodium alpestre, Matteuccia struthiopteris aceous species. The top 5 important values of herb- and Anemone amurensis. The sum of the importance aceous plants in the light thinning were Carex pilosa, values of these plants was 60.67 %, and there were 19 Athyrium multidentatum, Aegopodium alpestre, Filipen- species with the importance value greater than 1 %. The dula palmata and Brachybotrys paridiformis. The sum sum of their importance values accounted for 90.52 % of of the importance values was 59.92 %, and there were 16 all herb species. The top 5 important values of herb- species with the importance value greater than 1. The aceous plants in the moderate thinning and heavy thin- top 5 importance values of herbaceous plants in the ning plots were Carex pilosa, Meehania urticifolia, Wang et al. Forest Ecosystems (2021) 8:53 Page 5 of 13 Table 2 The top 5 herbs with relative importance value in different months Month CK LT MT HT Species Importance Species Importance Species Importance Species Importance value (%) value (%) value (%) value (%) May Anemone 18.11 Carex pilosa 18.93 Carex pilosa 17.53 Filipendula 12.92 raddeana palmata Hylomecon 9.96 Anemone 14.47 Anemone 12.04 Aegopodium 12.32 japonica raddeana raddeana alpestre Aegopodium 9.47 Filipendula 8.89 Filipendula 8.35 Anemone 9.38 alpestre palmata palmata raddeana Meehania 7.23 Meehania 4.48 Meehania 7.35 Anemone 6.84 urticifolia urticifolia urticifolia amurensis Corydalis 6.61 Cardamine 4.44 Poa annua 4.09 Meehania 4.63 ambigua leucantha urticifolia Others 48.62 Others 48.79 Others 50.64 Others 53.91 June Carex pilosa 14.39 Carex pilosa 28.14 Carex pilosa 26.63 Carex pilosa 19.29 Poa annua 9.87 Athyrium 8.92 Meehania 9.26 Filipendula 10.94 multidentatum urticifolia palmata Meehania 9.87 Aegopodium 8.90 Carex 8.68 Aegopodium 10.33 urticifolia alpestre remotiuscula alpestre Aegopodium 8.73 Filipendula 8.07 Aegopodium 8.18 Carex 8.42 alpestre palmata alpestre remotiuscula Filipendula 8.02 Brachybotrys 5.89 Filipendula 5.35 Meehania 7.24 palmata paridiformis palmata urticifolia Others 49.12 Others 40.08 Others 41.90 Others 43.78 July Carex pilosa 16.27 Carex pilosa 35.84 Carex pilosa 27.52 Carex pilosa 27.76 Laportea 10.98 Filipendula 9.67 Filipendula 8.42 Filipendula 15.34 bulbifera palmata palmata palmata Filipendula 9.77 Aegopodium 5.62 Carex 7.35 Aegopodium 7.96 palmata alpestre remotiuscula alpestre Cardamine 9.74 Matteuccia 5.21 Meehania 5.92 Meehania 5.55 leucantha struthiopteris urticifolia urticifolia Meehania 9.57 Anemone 4.33 Aegopodium 5.73 Carex 3.64 urticifolia amurensis alpestre remotiuscula Others 43.67 Others 39.33 Others 45.06 Others 39.75 August Milium 14.43 Carex pilosa 24.45 Carex pilosa 22.99 Carex pilosa 16.56 effusum Meehania 13.26 Aegopodium 10.54 Meehania 10.69 Aegopodium 10.99 urticifolia alpestre urticifolia alpestre Carex pilosa 11.93 Filipendula 10.20 Aegopodium 10.26 Filipendula 10.59 palmata alpestre palmata Aegopodium 10.30 Brachybotrys 7.34 Filipendula 9.67 Carex 9.64 alpestre paridiformis palmata callitrichos Filipendula 10.25 Meehania 5.73 Brachybotrys 5.81 Meehania 6.38 palmata urticifolia paridiformis urticifolia Others 39.83 Others 41.74 Others 40.58 Others 45.84 September Carex pilosa 17.66 Carex pilosa 33.06 Carex pilosa 27.94 Carex pilosa 22.89 Aegopodium 16.34 Aegopodium 11.01 Meehania 11.90 Filipendula 16.76 alpestre alpestre urticifolia palmata Milium 12.75 Filipendula 10.79 Aegopodium 10.64 Aegopodium 16.64 effusum palmata alpestre alpestre Meehania 12.46 Athyrium 7.93 Filipendula 9.67 Urtica 16.28 urticifolia multidentatum palmata angustifolia Wang et al. Forest Ecosystems (2021) 8:53 Page 6 of 13 Table 2 The top 5 herbs with relative importance value in different months (Continued) Month CK LT MT HT Species Importance Species Importance Species Importance Species Importance value (%) value (%) value (%) value (%) Cardamine 8.76 Carex 6.88 Athyrium 5.61 Meehania 5.67 leucantha remotiuscula multidentatum urticifolia Others 32.03 Others 30.33 Others 34.24 Others 21.76 October Carex pilosa 40.12 Carex pilosa 58.27 Carex pilosa 47.63 Carex pilosa 39.57 Meehania 21.20 Aegopodium 15.78 Aegopodium 19.20 Aegopodium 17.47 urticifolia alpestre alpestre alpestre Aegopodium 17.57 Meehania 10.89 Carex 14.91 Carex 13.60 alpestre urticifolia remotiuscula callitrichos Milium 8.08 Carex callitrichos 5.38 Meehania 10.24 Meehania 8.86 effusum urticifolia urticifolia Cardamine 5.68 Filipendula 4.20 Carex callitrichos 2.93 Carex 8.60 leucantha palmata remotiuscula Others 7.35 Others 5.48 Others 5.09 Others 11.90 Carex remotiuscula, Aegopodium alpestre and Filipen- paridiformis and Meehania urticifolia, with the sum of dula palmata. The order of importance values in the the important values of 58.26 %. There were 19 species moderate thinning was Carex pilosa > Filipendula with the importance value greater than 1 %, accounting palmata > Carex remotiuscula > Meehania urticifolia > for 94.23 % of all herb species. The top 5 important Aegopodium alpestre, and the importance values of the values of herbaceous plants in the moderate thinning plots with heavy thinning were Carex pilosa > Filipen- were Carex pilosa, Meehania urticifolia, Aegopodium dula palmata > Aegopodium alpestre > Meehania urtici- alpestre, Filipendula palmata and Brachybotrys paridi- folia > Carex remotiuscula. In the moderate and heavy formis. The sum of the important values was 59.42 %, thinning, the sum of the importance values of these and there were 16 species with the importance value plants was 54.94 % and 60.25 %, respectively. The herb greater than 1 %, accounting for 87.55 % of all herb spe- plants with importance values greater than 1 % were 21 cies. The top 5 important values of herbaceous plants in and 22 species and accounted for 92.47 % and 93.50 % of the heavy thinning were Carex pilosa, Aegopodium all herb species, respectively. 38, 39, 46 and 36 species of alpestre, Filipendula palmata, Carex callitrichos and herbs were found in control, light thinning, moderate Meehania urticifolia. The sum of the importance values thinning and heavy thinning respectively. Compared was 54.16 %, and there were 21 species with the import- with the control, moderate thinning significantly in- ance value greater than 1 %, accounting for 95.51 % of all creased the number of herbs. This is due to the good herb species. Among the top 5 species of herbs in differ- light under the forest and the invasion of light loving ent thinning plots, there are 4 common species, namely herbs species after moderate thinning, and there are Carex pilosa, Meehania urticifolia, Aegopodium alpestre more shade environment under the forest and more and Filipendula palmata. From the important values of shade loving herbs are retained, which makes the light these 4 species, the important values of Carex pilosa loving and shade loving species co-exist, resulting in a increased significantly after thinning, while that of relatively large number of species. After heavy thinning, Meehania urticifolia decreased. Aegopodium alpestre the shade environment under the forest is less and the and Filipendula palmata were relatively consistent in light condition is excellent, which is especially suitable different plots, and there was no significant change. for the survival of photophilous herbs, but the loss of In September, the top 5 importance values of herb- photophilous species. aceous plants in the control were Carex pilosa, Aegopo- In August, the top 5 importance values of herbaceous dium alpestre, Milium effusum, Meehania urticifolia, plants in the control were Milium effusum, Meehania and Cardamine leucantha, the sum of the important urticifolia, Carex pilosa, Aegopodium alpestre and values was 67.97 %. There were 11 species of herbs with Filipendula palmata. The sum of the importance values importance values more than 1 %, accounting for was 60.17 %, and there were 15 species with the import- 94.00 % of all herb species. The top 5 important values ance value greater than 1, accounting for 90.59 % of all of herbaceous plants in the light thinning were Carex herb species. The top 5 importance values of herbaceous pilosa, Aegopodium alpestre, Filipendula palmata, plants in the light thinning were Carex pilosa, Aegopo- Athyrium multidentatum and Carex remotiuscula. The dium alpestre, Filipendula palmata, Brachybotrys sum of the important values was 69.67 %. There were 15 Wang et al. Forest Ecosystems (2021) 8:53 Page 7 of 13 species with importance values greater than 1 %, ac- number of species in different plots, and the species counting for 96.65 % of all herb species. The top 5 im- in different plots were almost the same. portance values of herbaceous plants in the moderate thinning were Carex pilosa, Meehania urticifolia, Aego- Seasonal changes in diversity index podium alpestre, Filipendula palmata and Athyrium Calculation of diversity index showed that there were multidentatum. The sum of the important values was significant variations (P < 0.05) in different thinning. Ex- 65.76 %. There were 16 species with importance value cept for the control, which showed an increasing trend more than 1 %, accounting for 92.82 % of all herb spe- from May to June, the Margalef index showed a similar cies. The top 5 importance values of herbaceous plants trend in all plots in other months (Fig. 1a). It gradually in the heavy thinning were Carex pilosa, Filipendula decreased from May to July, increased from July to palmata, Aegopodium alpestre, Urtica angustifolia and August, and then fell rapidly from August to Octo- Meehania urticifolia. The sum of the important values ber. The Margalef index of different treatments was 78.24 %. There were 13 species with the importance showed various patterns in different months with value greater than 1 %, accounting for 99.14 % of all herb moderate thinning > heavy thinning > light thinning > species. 23, 21, 25 and 14 species of herbs were found in control in May, and control > moderate thinning > the control, light thinning, moderate thinning and heavy light thinning > heavy thinning from June to August. thinning, respectively. This is due to the fact that more From August to October, the Margalef index of the photophilous species, such as Impatiens noli-tangere, four plots all showed a significant decreasing trend have invaded the plots after heavy thinning. The earlier with control > light thinning > moderate thinning > invasive species will occupy a wide area in the sample heavy thinning. plot, and prevent the invasion of other species. Conse- In the four treatments, the Shannon-Wiener index and quently, the number of species will decrease after heavy Simpson index had similar patterns (Fig. 1b and c). The thinning. With the coming of autumn, the number of Shannon-Wiener index and Simpson index gradually de- photophilous species in the heavy thinning plots in Sep- creased from May to July, increased gradually from July tember was lower because of their higher individuals to August, and then decreased sharply. In June, July and and more easily withered and died. September, the Shannon-Wiener index and Simpson In October, the top 5 importance values of herbaceous index in the control were higher than the other three plants in the control were Carex pilosa, Meehania urtici- thinning treatments. In May, August and October, the folia, Aegopodium alpestre, Milium effusum and Carda- Shannon-Wiener index and Simpson index of the four mine leucantha, the sum of the importance value was plots were relatively the same. 92.65 %. There are 8 species with importance value Throughout the growing season, the Pielou index was greater than 1 %, accounting for 98.60 % of all herb spe- roughly in the shape of an inverted “N” (Fig. 1d), namely cies. The top 5 importance values of herbaceous plants it had a high value at the beginning of growth, a low in the light thinning were Carex pilosa, Aegopodium value in June or July, and the second peak in August, alpestre, Meehania urticifolia, Carex callitrichos and and then it decreased sharply. The Pielou index of the Filipendula palmata. The sum of the important values control was generally higher than that of the other thin- was 94.52 %, and there were 7 species with the import- ning treatments. The control plot and the heavy thin- ance value greater than 1 %, accounting for 99.32 % of all ning showed the first decrease in June, while the light herb species. The top 5 importance values of herbaceous and moderate thinning showed the first decrease in July. plants in the moderate thinning were Carex pilosa, Aego- podium alpestre, Carex remotiuscula, Meehania urticifo- Effect of thinning on biomass lia and Carex callitrichos. The sum of the important Seasonal changes in biomass values of was 94.91 %. There were 6 species with im- In May, thinning can promote the increase of total bio- portance value greater than 1 %, accounting for mass, above-ground biomass and belowground biomass 97.38 % of all herb species. The top 5 importance of herbaceous plants, but there were no significant dif- values of herbaceous plants in the heavy thinning ferences among thinning treatments. Total biomass and were Carex pilosa, Aegopodium alpestre, Carex calli- above-ground biomass were the largest in light thinning, trichos, Meehania urticifolia,and Carex remotiuscula. and the belowground biomass was the largest in moder- The sum of the important values was 88.10 %. There ate thinning. In terms of root-to-shoot ratio, the moder- were 8 species with important value greater than 1 %, ate thinning was significantly higher than the other three accounting for 99.34 % of all herb species. There were treatments, and the light and heavy thinning were not 10, 8, 10 and 9 species of herbs in the control, light significantly different from the control. Compared with thinning, moderate thinning and heavy thinning, re- the control, the number of herbaceous plants decreased spectively. There was no significant difference in the after light thinning, but the difference was not obvious. Wang et al. Forest Ecosystems (2021) 8:53 Page 8 of 13 Fig. 1 Monthly change of herbaceous plant diversity index Moderate and heavy thinning significantly increased the biomass in September. Heavy thinning rather than light number of herbaceous plants. In terms of average height thinning and moderate thinning significantly increased and total coverage, moderate thinning was significantly above-ground biomass. After thinning, the root-shoot higher than that of light thinning. ratio increased significantly, but the difference in the The biomass of herbaceous plants increased in July root-shoot ratio was not significant among light, moder- compared with May. Compared with the control, thin- ate and heavy thinning, indicating that thinning changed ning promoted the increase of total biomass, above- and the distribution of above- and belowground biomass of belowground biomass. Among them, the total biomass, herbaceous plants. The nutrients were mostly allocated above- and belowground biomass were significantly to the belowground part, but the thinning intensity did higher in heavy thinning than those of the control. The not change the distribution of plant biomass between difference between moderate thinning and the control above- and belowground parts. Thinning increased the was not significant. In terms of root-shoot ratio, the dif- number of herbaceous plants, and the number of herb- ference between the four treatments was not significant, aceous plants in the moderate and the heavy thinning indicating that thinning intensity did not change the bio- was significantly higher than that in the control. The mass allocation between the above- and belowground average height of herbs decreased after thinning, which parts. Thinning increased the number of plants, and the was evident in light thinning. Thinning significantly in- number increased with increasing thinning intensity. creased the total coverage of herbs, but there were no The average height of herbaceous plants after thinning significant differences among light, moderate and heavy decreased, but the difference was not significant. In thinning. addition, thinning increased the total coverage of herb- The correlation analysis of the biomass in different aceous plants. The coverage was significantly higher in months is shown in Fig. 2. In May, the total biomass sig- the heavy thinning than that in the control, while the nificantly correlated with the number, average height, light and moderate thinning were not significantly differ- and the total coverage, but it did not significantly correl- ent from the control. ate with the root-shoot ratio. Above-ground biomass Compared with the control, thinning significantly in- was positively correlated with the number, average creased the total herb biomass and belowground height and total coverage, but not correlated with the Wang et al. Forest Ecosystems (2021) 8:53 Page 9 of 13 root-shoot ratio. Belowground biomass was positively correlated with root-shoot ratio, the number and average height, but did not change with the total coverage. In July, the number and total coverage of herbaceous plants were positively correlated with total biomass, above- and belowground biomass. The average height of herbaceous plants did not correlate with total biomass, above- and belowground biomass. The root-shoot ratio significantly correlated with total biomass and below- ground biomass. In September, the root-shoot ratio positively correlated with total biomass and belowground bio- mass. The number of herb plants had a significantly and positively correlation with total biomass and be- lowground biomass. The average height of herb plants was significantly and negatively correlated with the total biomass and above-ground biomass. The total coverage of herb plants was positively re- lated to the total biomass, above- and belowground biomass. It was found that the herb biomass was different over time (Table 3). In the control, the total biomass, above- and belowground biomass all showed their maximum in July. After thinning, the total biomass, above- and be- lowground biomass all gradually increased over time and reached their maximum in September. In the thinning treatments, the total biomass, above- and belowground biomass in September were significantly higher than those in May. Correlations between diversity and biomass The correlation analysis between herb biomass and diversity index in different months is shown in Fig. 3. In May, total herb biomass, above- and below ground biomass were significantly and positively cor- related with Shannon-Wiener index, Simpson index, and Pielou index. The correlation between herb bio- mass and diversity index in July and September was not significant. Compared with May, the relationship between species diversity and biomass were changed, indicating that seasonal changes will change the relationships. Discussion Importance values variation Forest thinning, as an important forest management practice, is an important way to change the composition and structure of the forest, adjust forest density, and im- prove the understory environmental conditions. It will directly affect the composition and diversity of under- Fig. 2 Correlation of various biomasses of herbaceous plants in story vegetation, and influence tree growth and commu- different months nity stability (Zhang and Tang 2008). The importance values of each species varied significantly among Wang et al. Forest Ecosystems (2021) 8:53 Page 10 of 13 Table 3 Biomass of herbaceous plants in different months Month Treatments Total Aboveground Belowground Root-shoot Number per Mean height Total coverage biomass biomass biomass ratio m (cm) (%) − 2 − 2 − 2 (g·m ) (g·m ) (g·m ) May CK 113 ± 10a 53 ± 4a 59 ± 7a 1.11 ± 0.10a 216 ± 12a 11.95 ± 0.36ab 68.75 ± 3.70ab LT 194 ± 12b 86 ± 5b 109 ± 8bc 1.29 ± 0.06a 198 ± 11a 11.14 ± 0.36a 59.60 ± 2.77a MT 188 ± 21b 71 ± 10ab 117 ± 12b 1.86 ± 0.17b 275 ± 18b 12.44 ± 0.48b 72.75 ± 3.55b HT 163 ± 15b 76 ± 8b 88 ± 8c 1.18 ± 0.09a 265 ± 19b 12.09 ± 0.41ab 67.53 ± 4.46ab July CK 207 ± 16a 104 ± 11a 104 ± 8a 1.05 ± 0.08a 122 ± 14a 30.14 ± 1.18a 65.63 ± 3.13a LT 240 ± 13a 108 ± 6a 132 ± 9ab 1.24 ± 0.08a 175 ± 20ab 26.89 ± 1.88a 70.20 ± 3.70a MT 236 ± 19a 107 ± 9a 128 ± 12a 1.22 ± 0.09a 191 ± 18b 28.07 ± 1.20a 70.65 ± 2.98a HT 339 ± 53b 154 ± 16b 185 ± 39b 1.12 ± 0.13a 219 ± 29b 28.79 ± 1.34a 81.67 ± 3.89b September CK 161 ± 8a 85 ± 3a 76 ± 5a 0.89 ± 0.05a 109 ± 10a 25.12 ± 0.75a 58.30 ± 2.24a LT 273 ± 22b 112 ± 7a 161 ± 17b 1.43 ± 0.12b 134 ± 26ab 21.20 ± 1.90b 70.06 ± 3.96b MT 289 ± 30b 113 ± 8a 176 ± 23bc 1.51 ± 0.13b 181 ± 16b 24.62 ± 0.98ab 76.80 ± 3.28b HT 418 ± 43c 192 ± 28b 225 ± 31c 1.41 ± 0.25b 180 ± 43b 21.47 ± 1.36ab 78.13 ± 4.93b different thinning treatments. The reason may be that species (Hulík and Douda 2017). The overstory reaches the growth of overstorey changes with season, which in its growth peak in August, and the canopy closure con- turn causes changes in the understory environment. The tinues to increase, resulting in a continuous decrease in importance values of species in different treatments understory light intensity. Early spring plants and some showed similar patterns with seasonal changes. The im- early summer plants withered and died, and the number portance values of early spring plant were greater in of understory herbaceous species declined significantly, May, and the top 5 importance values of various treat- but the dominant species did not change (Gong et al. ments were Anemone amurensis, Hylomecon japonica, 2016). The temperature drops significantly in September, Anemone raddeana,and Corydalis ambigua. Further- and the overstory stops growing, some tall trees begin to more, these plants constitute the main body of herb- defoliate, and the canopy closure is reduced. At this aceous plants under the broad-leaved Korean pine forest time, most of the herbaceous plants also stop the growth in early spring. At this time, the overstorey has not yet of the above-ground parts and enter the nutrient reserve reached canopy closure, the understory has sufficient period, which causes a significant decrease in the num- sunlight, and the understory temperature continues to ber of species. Understory herbaceous plants still consist rise. In addition to early spring plants, other herbaceous of dominant species such as Carex pilosa, Aegopodium plants have also sprouted in large numbers, so there are alpestre and Meehania urticifolia (Wu et al. 2018). In more species in May (Chen and Matter 2017; Kleinn October, the Changbai Mountain area has entered the et al. 2020). In June, the trees grow fast, but the over- end of autumn. Although most of the trees in the upper storey is not completely closed. The light intensity of layer of the canopy have fallen leaves, there is sufficient understory gradually decreases, and the plants wither sunlight understory. But due to the low temperature, and die in early spring. Except for the control, the num- most of the herbaceous plants have withered and died, ber of species significantly decreased in other thinning causing the number of species to drop to the lowest treatments. At this time, Carex pilosa, Aegopodium value in the growing season (Dai et al. 2013). Based on alpestre, Meehania urticifolia, and Filipendula palmata the data of the entire growing season, the top herb- replaced early spring plants as the dominant species of aceous plants in the various treatments are mainly com- understory herbs, and ferns also showed greater advan- posed of Carex pilosa, Aegopodium alpestre, Meehania tages due to their vigorous growth (Gao et al. 2019). The urticifolia and Filipendula palmata, indicating that these overstory grows fast in July. The canopy density is rela- species have strong adaptability and a wide ecological tively high, and the understory light intensity is low. At range. this time, the understory is mainly covered by shade- tolerant species such as Carex pilosa, Carex remotius- Herbaceous plant species variation cula, Carex callitrichos, Carex siderosticta, Meehania The monthly changes in the number of herbaceous plant urticifolia, Laportea bulbifera, Urtica angustifolia.As species in different thinning treatments indicate that the precipitation begins to increase, understory wet plants number of species in the thinning in May is higher than grow quickly, leading to an increase in the number of that in the control. This may be due to the increased Wang et al. Forest Ecosystems (2021) 8:53 Page 11 of 13 understory light intensity after thinning, which leads to the advance of plant phenology (Ye et al. 2015). The number of species in the thinning treatments decreased in June. This is due to the advance of the phenological period after thinning that causes some early spring short-lived plants to wither and die, resulting in a de- crease in the number of species in the thinned treat- ments (Li et al. 2006). From July to September, the number of species in the four treatments gradually de- creased. This is because July is the rainy season in this area. The soil under the forest is moist and the temperature is high, which is most suitable for plant growth. Therefore, the number of species in July reached the maximum. Although the enhancement of understory light intensity might have caused the death of some shade-tolerant species in moderate thinning, heliophile will quickly invade the site, leading to species reaching the maximum in the moderate thinning during this period. Although light thinning may not significantly in- crease light intensity, the thinning will form gaps, which will lead to the death of some rare shade-tolerant herb- aceous plants, and the heliophile cannot invade the plot, resulting in the decrease of species numbers. In the heavy thinning treatment, the understory sunlight is sig- nificantly enhanced and there are more gaps, causing the death of most of the shade-tolerant herbaceous plants. Although the heliophile can invade the site, they cannot compensate for the death number of shade-tolerant plant, resulting in a significant decrease in species num- bers (Ding et al. 2016). The understory temperature and light intensity of the four treatments are very low in October, and the understory environmental conditions are roughly the same. Most of the herbaceous plants withered and died, and only a few herbs with strong cold tolerance could survive (Small and McCarthy 2002). Herbaceous plant individuals number variation Throughout the growing season, the number of individ- ual plants in the heavy thinning is the largest, indicating that thinning can promote the increase in the number of individual plants. This is due to the increase in the intensity of thinning, which leads to the increase of understory light intensity, subsequently promoting the increase of understory herb plants (Westerband and Horvitz 2017; Didion 2020). The number of individuals in the moderate thinning was higher than that in the heavy thinning in July. This is due to the presence of large herb plants in the heavy thinning, occupying a lar- ger space under the forest, resulting in the death of some small one (Zhang et al. 2019). The number of indi- vidual plants in the control showed a decreasing trend. Fig. 3 Correlation between herb biomass and diversity index in This was because most of the control in May were early different months spring plants. Although the early spring plants are small, there are more individuals on the same stem, so the Wang et al. Forest Ecosystems (2021) 8:53 Page 12 of 13 number of individuals was the largest. Early spring other plants in the moderate and heavy thinning were plants gradually die from June to August, early summer significantly higher than those in the light thinning and plants and late summer plants dominant the herbaceous the control (Sun et al. 2017). plants. These plants are relatively large, occupying more The diversity of herbaceous plants is an important in- space understory, resulting in a decrease in the number dicator of the succession and development of forests. of individual plants. The overstory defoliate in Septem- This study only studied four-intensity thinning in near- ber, the increase in understory light intensity increases mature forests. In order to better reflect the effects of the ground temperature, and the secondary growth of thinning on the understory environment, herbaceous some plants leads to an increase in the number of indi- plant diversity and biomass, therefore, it is necessary to viduals. The temperature gradually decreases in October, further carry out relevant experiments in forest with dif- and most understory herbs are not cold-resistant, and ferent development stages and stand characteristics. It is will gradually wither and die, leading to a decrease in also a necessity to analyze the impact of thinning on for- the number of individuals (Grant et al. 2019). The in- est species diversity and biomass, guide forest manage- crease in the number of individual plants in May and ment and accelerate the development of secondary June after thinning is due to the increase in understory forests to primary forests. In addition, this study also light and the increase in understory temperature due to demonstrates that the species diversity and biomass of thinning, which advances the phenological period of shrubs will also be affected by thinning. In the future, re- plants. There are two types of plants including early search can be conducted on the impact of thinning on spring plants and early summer plants in June (especially the species diversity and biomass of shrubs. in heavy thinning). At this time, the plant individuals are small, and the understory space can meet the needs of Conclusions plant growth. Therefore, the number of individual plants The changes of biodiversity in the herb layer under the tends to increase. After June, the size of individual plants coniferous and broad-leaved mixed forest in Changbai gradually increases after June, resulting in insufficient Mountain were mainly affected by seasons. However, understory space and the competition causing the death thinning changed the light environment in the forest, of some individuals, which led to a decrease in the num- which could improve the plant diversity and biomass of ber of individuals (Li et al. 2017). herb layer in a short time. Moderate thinning had the most significant effect on the number of species, and the Herbaceous plant biomass variation number of individual plants, average height and total The total biomass, aboveground biomass and below- coverage of plants increased significantly in all months ground biomass of the thinning treatments were greater of the growing season. With the increase of thinning in- than those in the control, indicating that thinning can tensity, the biomass of herb layer increased accordingly. promote the biomass growth of understory herbaceous Total herb biomass, above- and belowground biomass plants. The greater the thinning intensity, the faster the showed positive correlations with Shannon-Winner plant biomass will grow. This is because thinning can index, Simpson index and Pielou index in May. The significantly improve the understory light environment, findings provide theoretical basis and reference for rea- thereby promoting biomass growth of understory herbs sonable thinning and tending in coniferous and broad- (Tinya and Ódor 2016). The plant heights of the light leaved mixed forests. thinning and the control showed a peak curve, and the plant heights of the heavy and moderate thinning Acknowledgements showed a double-peak curve. This is because July is the We would like to thank the two anonymous reviewers and the associate editor for their constructive comments that greatly helped improving the rainy season and the temperature is high, and the envir- manuscript. onmental conditions are moderate. Both overstory and understory entered the vigorous growth period, and the Authors’ contributions average height of the herbs reached the maximum. In Gerong Wang designed the study and developed the main hypotheses, Mo August, as the stand canopy density increase, the under- Zhou and Mengjia Wu collected and prepared the data, Naiqian Guan, Yuwen Wang, Runhua Jiang and Zhiyu Liu analyzed the data, Fucai Xia and story light of the light thinning and the control was rela- Yue Sun drafted the manuscript. All authors commented preliminary versions tively poor, some tall herbs gradually withered and died of the manuscript and contributed to improve the final version. The due to insufficient growth. The light environment of the author(s) read and approved the final manuscript. moderate and the heavy thinning improved significantly after thinning. After the highest canopy closure time in Funding This work was financially supported by a grant from the National Key August, understory herbaceous plants have a significant Research and Development Program of China (2017YFC0504102) and the high growth. For example, the heights of Carex pilosa, Science and Technology Program of Jilin Provincial Education Department ferns, and Impatiens noli-tangere, Veratrum nigrum and (JJKH20180349KJ). Wang et al. Forest Ecosystems (2021) 8:53 Page 13 of 13 Availability of data and materials Lei XD, Lu YC, Zhang HR, Zhang ZL, Chen XG (2005) Effects of thinning on mixed The datasets used and/or analysed during the current study are available stands of Larix olgensis, Abies nephrolepis and Picea jazoensis. Sci Silv Sin 41(4): from the corresponding author on reasonable request. 78–85 (in Chinese) Li CY, Ma LY, Xu X (2006) Reviews on research progress of effect of tending on forest biodiversity. 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Journal

"Forest Ecosystems" – Springer Journals

Published: Aug 2, 2021

Keywords: Selective thinning; Mixed coniferous and broad-leaved forests; Herbaceous plants; Diversity; Biomass

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