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Changes in plant debris and carbon stocks across a subalpine forest successional series

Changes in plant debris and carbon stocks across a subalpine forest successional series Background: As a structurally and functionally important component in forest ecosystems, plant debris plays a crucial role in the global carbon cycle. Although it is well known that plant debris stocks vary greatly with tree species composition, forest type, forest origin, and stand age, simultaneous investigation on the changes in woody and non-woody debris biomass and their carbon stock with forest succession has not been reported. Therefore, woody and non-woody debris and carbon stocks were investigated across a subalpine forest successional gradient in Wanglang National Nature Reserve on the eastern Qinghai-Tibet Plateau. − 1 Results: Plant debris ranged from 25.19 to 82.89 Mg∙ha and showed a global increasing tendency across the subalpine forest successional series except for decreasing at the S4 successional stage. Accordingly, the ratios of woody to non-woody debris stocks ranged from 26.58 to 208.89, and the highest and lowest ratios of woody to non-woody debris stocks were respectively observed in mid-successional coniferous forest and shrub forest, implying that woody debris dominates the plant debris. In particular, the ratios of coarse to fine woody debris stocks varied greatly with the successional stage, and the highest and lowest ratios were found in later and earlier successional subalpine forests, respectively. Furthermore, the woody debris stock varied greatly with diameter size, and larger diameter woody debris dominated the plant debris. Correspondingly, the carbon stock of plant debris − 1 ranged from 10.30 to 38.87 Mg∙ha across the successional series, and the highest and lowest values were observed in the mid-coniferous stage and shrub forest stage, respectively. Most importantly, the carbon stored in coarse woody debris in later successional forests was four times higher than in earlier successional forests. Conclusions: The stock and role of woody debris, particularly coarse woody debris, varied greatly with the forest successional stage and dominated the carbon cycle in the subalpine forest ecosystem. Thus, preserving coarse woody debris is a critical strategy for sustainable forest management. Keywords: Coarse woody debris, Fine woody debris, Forest successional series, Later successional stage, Earlier successional stage, Log decay class, Diameter size Introduction et al. 1986), and the latter accounts for ca. 20%–30% of Plant debris consists of woody debris (WD) and non- global woody biomass (Pan et al. 2011). CWD plays cru- woody debris (NWD), both of which play crucial roles cial roles in conserving biodiversity, forest regeneration, not only in nutrient cycling and biodiversity conserva- global carbon sinks, and soil development (Iwashita tion but also in the global carbon cycle (Pan et al. 2011; et al. 2013; Russell et al. 2015;Błońska et al. 2017; Pres- Zhu et al. 2017). In particular, WD includes fine woody cott et al. 2017). Previous investigations have docu- debris (FWD) and coarse woody debris (CWD) (Harmon mented that WD and NWD stocks vary greatly with tree species composition (Raich et al. 2007; Wang et al., * Correspondence: scyangwq@163.com 2019), forest type (Moreira et al. 2019), forest origin School of Life Sciences, Taizhou University, Taizhou 318000, China (Hagemann et al. 2010; Suzuki et al. 2019), and stand 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) 8:40 Page 2 of 14 age (Sefidi 2010; Schilling et al. 2016). Most importantly, species composition is determined mainly by the succes- the rate of plant debris decomposition varies greatly with sion stage (Taylor et al. 2020). Thus, forest succession debris type (Cornwell et al. 2009; Harmon et al. 2013). development may affect the distribution of plant debris FWD and NWD, such as foliar litter, decompose faster decay classes and diameter classes through the following than CWD (Müller-Using and Bartsch 2009; Berbeco aspects. On the one hand, CWD at earlier successional et al. 2012). Additionally, plant debris decomposition dif- stages might decompose faster than those at later suc- fers among tree species, and lower-density WD decom- cession stages due to tree species at earlier successional poses faster (Shorohova and Kapitsa 2016; Guo et al. stages have shorter life cycles and lower CWD quality 2020). The above description implies that the compos- (Puyravaud et al. 2003). For instance, the CWD of ition and proportion of plant debris might control forest broad-leaved species decomposes faster (Yatskov et al. regeneration, biodiversity nursing, soil and water conser- 2003); additionally, the CWD of angiosperms decom- vation, and the cycling of carbon and nutrients. Thereby, poses faster than that of gymnosperms (Herrmann et al. an investigation of the changes in plant debris stock 2015). On the other hand, the CWD in the earlier suc- characteristics with tree species composition determined cessional forests usually have smaller diameters (Ott by forest type is crucial to understand the process and et al. 2006), while in the later succession, the CWD usu- function of forest ecosystems. ally have larger diameters (Vanninen et al. 1996). Gener- In natural or unmanaged forests, forest succession is ally, larger-diameter CWD decomposes slower than an important factor affecting the forest type and species smaller-diameter CWD (Harmon et al. 2013). Thus, diversity (Lebrija-Trejos et al. 2010; West et al. 2012; characterizing the stocks and proportions of different Taylor et al. 2020). Meanwhile, the input of plant debris decay and diameter classes at different successional is determined by the death or falling of aboveground stages is essential for understanding the nutrient dynam- species (Tritton 1981; Sturtevant et al. 1997). As a result, ics of plant debris stock and predicting forest succession forest succession may affect the composition and stock rate. However, little information is available on CWD of plant debris through the following aspects. First, the stock changes with decay and diameter classes across stock and composition of plant debris would vary to forest successional series. some degree with forest succession due to the differ- Plant debris represents an essential carbon pool and ences in tree species composition and tree longevity at strongly influences on the structures and carbon dynam- different succession stages. For instance, litter produc- ics in the forest ecosystems (Harmon et al. 1986). The tion and dynamics significantly vary with overstory tree impacts of various driving forces on CWD carbon stocks species composition in lowland Costa Rica (Raich et al. at the local scale have been widely investigated (e.g., Hall 2007); forest type determines the species composition et al. 2006; Woodall and Liknes 2008; Kurz et al. 2009; and CWD stock (Moreira et al. 2019). Second, tree spe- Jonsson et al. 2011; Woodall et al. 2013). However, only cies in the earlier succession stages may be more suscep- a few studies have evaluated the dependency between tible to disturbance and generate more CWD (Brassard the carbon storages of NWD and the stages of succes- and Chen, 2008). However, higher and lower stocks of sion (Zhang et al. 2011). Furthermore, compared with WD have been observed at the later and earlier succes- carbon stocks in live biomass or soil, the C budgets in sion stages due to differential WD decomposition rates WD and NWD production and turnover at different for- (Carmona et al. 2002). Zhu et al. (2017) have shown that est succession stages have not been simultaneously in- the biomass and C storage of CWD may increase with vestigated. Consequently, a comprehensive assessment forest succession, while a “U-shape” trend of CWD was of the carbon budget and the relative contributions of observed along the forest successional gradient by Sefidi WD and NWD to the carbon budget across forest suc- (2010). Idol et al. (2001) have shown a significant de- cessional series are critically important for understand- crease in volume and mass of woody debris from re- ing the significance of plant debris in carbon sinks in cently harvested to mature stands. Together, the forest ecosystems. changes in plant debris with forest succession remain To understand the changes in the composition, bio- uncertain. Thereby, investigating the changes in plant mass, and carbon stock of plant debris with forest suc- debris stock across forest successional series will help us cession, NWD, FWD, and CWD with different diameter better understand the function of plant debris in forest classes and decay classes were investigated across a sub- ecosystems. alpine forest successional series in Wanglang National In theory, the stock and composition of plant debris Nature Reserve on the eastern Qinghai-Tibet Plateau depend greatly on the WD decomposition, particularly and the upper reaches of the Yangtze River. This region that of CWD (Zell et al. 2009). The decay rates of WD is the main body of China’s second largest forest area and NWD vary with tree species and diameter class and plays paramount roles not only in holding fresh- (Cornwell et al. 2009; Harmon et al. 2013), while tree water, conserving water, and soil and nursing Wang et al. Forest Ecosystems (2021) 8:40 Page 3 of 14 biodiversity but also in the global carbon cycle (Liu dominated by Cacalia palmatisecta, Cyperaceae, Poa 2002; Tan et al. 2014). Subalpine forest communities at pratensis, and others (Taylor et al. 2006). The moss layer different successional stages are observed in the sub- is dominated by Thuidium cymbifolium, Hylocomium alpine forest region on the eastern Qinghai-Tibet Plateau splendens, Mnium heterophyllum, and Phaeoceros laevis due to long-term natural disturbance and the commer- (Li et al. 2012). cial logging of natural forests since the 1950s (Yang et al. 1992). Although the WD stock from the gap center Experimental design to the closed canopy in an over-mature subalpine con- Based on our previous visits and investigations on forest iferous forest (Xiao et al. 2016), the water storage poten- vegetation and plant debris in Wanglang National Na- tial of WD (Wang et al. 2016), and the changes in ture Reserve, we divided the different forest types into microbial biomass and epixylic plant diversity with decay six succession stages according to Zhang et al. (2011) classes (Wang et al. 2017; Chang et al. 2019) have been and through interviews with local supervisors. S1 succes- widely investigated in this region, little attention has sional stage is dominated by shrubs (e.g., H. rham- been given to the changes in the composition, biomass noides), S2 successional stage is dominated by deciduous stock and carbon storage of plant debris with forest suc- broadleaved species (e.g., Betula), S3 successional stage cession. Therefore, we hypothesized that (1) plant debris is dominated by deciduous broadleaved (Betula) and stocks would increase from the earlier successional stage coniferous (A. faxoniana) mixed forest and later succes- to the later succession stage in the subalpine forest re- sional stages (S4, S5, and S6) are all dominated by con- gion, (2) the stocks and proportions of different decay iferous species. Here S4 represents the earlier coniferous and diameter classes should differ at different succes- successional stage (A. faxoniana and P. purpurea); S5 sional stages, and (3) the C stock of plant debris would represents the middle coniferous successional stage (A. increase across the forest succession. The objectives faxoniana); S6 represents the mature coniferous succes- were to (1) investigate the changes in plant debris stocks sional stage (P. purpurea). The six successional forests across the forest successional series located in the sub- are widely distributed in the subalpine forest region (Fig. alpine forest region, (2) elucidate the stocks and propor- A1). Site information and specific characteristics of each tion of different decay and diameter classes at different successional stage are recorded in Table A1. successional stages, and (3) assess the carbon budget and the relative contributions of CWD with different Sampling method decay classes and diameter classes to the carbon budget Woody debris was classified into CWD with a diam- across the forest successional series. These results could eter ≥ 10 cm and FWD with a diameter of 2 cm ≤ d< 10 help us better understand the function and significance cm (Ward and Aumen 1986; Harmon and Hua 1991). of plant debris in carbon sinks in forest ecosystems and CWD includes fallen logs, snags (dead standing trees), provide clear insight into the preservation of CWD, stumps, and large branches (Harmon et al. 1986). Snags which is a critical strategy for sustainable forest manage- refer to CWD which inclination is not more than 45°, ment in the subalpine forest on the eastern Qinghai- base diameter ≥ 10 cm, and length > 1 m. Stumps with a Tibet Plateau. height < 1 m were defined as CWD, which includes coarse roots above the soil surface (Harmon and Sexton Materials and methods 1996; Currie and Nadelhoffer 2002). Among these, dead Field description coarse roots of fallen logs and snags were measured and The study region is located in the Wanglang National recorded, and of which stumps were neglected due to Nature Reserve (32°49′–33°02′ N, 103°55′–104°10′ E; they were rarely observed. Additionally, we reclassified 2300–4983 m a.s.l.), which is situating in Pingwu County, foliar litter, dead twig, dead fine bark, and epiphytes as Sichuan, Southwest China (Fig. A1). The region is a NWD. Three plots of 10 m × 20 m of each successional transitional area between the Tibetan Plateau and the Si- forest were established for CWD and FWD investigation, chuan Basin. The annual precipitation is approximately and nine subplots of 1 m × 1 m were established for 859.9 mm, and the annual mean temperature is approxi- NWD investigation. Five decay class (I-V) systems opti- mately 2.9 °C, with maximum and minimum tempera- mized according to Błońska et al. (2018) were used to tures of 26.2 °C and − 17.8 °C, respectively (Zhang et al. classify the decomposition degree of WD based on the 2011). The tree canopy is dominated by Abies faxoniana, morphology and hardness observed in the field (Table Picea purpurea, Sabina saltuaria, Betula platyphylla, A2). Five diameter sizes (D1–D5) were applied to classify and B. albo-sinensis. The understory shrubs are domi- the WD diameter according to Harmon et al. (1986) and nated by Salix wallichiana, Hippophae rhamnoides, Xiong et al. (2016), that is, 2 cm ≤ d<5cm (D1), 5cm ≤ Rhododendron lapponicum, Lonicera spp., Sorbus rufopi- d < 10 cm (D2), 10 cm ≤ d < 20 cm (D3), 20 cm ≤ d<40 losa,and Rosa sweginzowii. The herbaceous layer is cm (D4) and d ≥ 40 cm (D5). Then, the decay class, base Wang et al. Forest Ecosystems (2021) 8:40 Page 4 of 14 diameter, end diameter, and length (fallen logs and large π  l  d π  l  d  d 2 2 3 V ¼ þ branches) or height (snags and stumps) of the WD were 12 12 recorded and measured. For CWD, the length beyond π  l  d þ ð3Þ the plot, only the part within the plot be recorded. Next, for the CWD with decay classes I to III, three dish sam- where d , d and l are the base diameter, end diameter, 2 3, ples of each decay class with a thickness of approxi- and length or height of the WD in the field (cm), mately 5 cm were obtained by chain saw. For the CWD respectively. with decay classes IV and V and FWD, three samples − 1 The WD stock (G ,Mg∙ha ) was calculated using the with a weight of approximately 500 g were harvested. following formula: For NWD, all samples within the subplot of 1 m × 1 m were harvested. The samples were taken back to V  ρ G ¼ ð4Þ the laboratory for stock and C concentration 200  cosθ  100 measurements. where θ is the slope of each plot (°), 200 is the area of each plot (m ), and 100 is the unit conversion factor. Measurement of plant debris stock and C stock The NWD stock (G ) was calculated using the follow- After the dish samples of CWD from decay classes I to ing formula: III were delivered to the laboratory, their diameter (d ) 1 P and thickness (h) were measured. Then, the volume (V ) G ¼ ð5Þ 1  cosθ  100 and density (ρ) were calculated (Bonan 2008). After the CWD samples from decay classes IV and V and the where m is the dry weight of the NWD sample (g), θ is FWD samples were delivered to the laboratory, we used the slope of each subplot (°), 1 is the area of each subplot the drainage method to determine the volume (V ) (m ), and 100 is the unit conversion factor. − 1 (Jonsson 2000). Then, we weighed these samples after The carbon stocks (Mg∙ha of C) of WD and NWD oven drying at 85 °C to a constant weight (m ). For were calculated using the following formula: NWD, the sample was weighed after oven drying at Carbon stock ¼ G  carbon concentration ð6Þ 1=2 85 °C to a constant weight (m ). The volume (V)of WD in the field was calculated according to Zhu et al. (2017), where G and G are the stocks of WD and NWD, 1 2 and the stock (G) of WD in the field was converted by respectively. the volume (V) and the sample density (ρ). For C con- centration, the most important factor determining C Statistical analysis concentration (%) of plant debris were decay class and One-way analysis of variance (ANOVA) and least signifi- geographical location, and there was little difference be- cant difference (LSD) tests were applied to examine the tween plant species or other factors (Zhu et al. 2017). different significance of WD stocks and C stocks among So, for WD from the different decay classes, carbon con- different decay classes, among different diameter sizes, centrations of 50.5%, 50.0%, 49.7%, 43.3%, and 37.9% and among these successional forests. The significance refer to Chang et al. (2015), who have investigated the was selected at the 0.05 level. The interaction effects of changes in the C concentrations with WD decay classes successional stages, decay classes, and diameter sizes on in a similar subalpine forest; for NWD, a carbon concen- the WD stock and C stock were analyzed by Multivariate tration of 41.9% refer to Zhu et al. (2017). analysis of variance (MANOVA). All statistical analyses − 3 The WD density (ρ,g∙cm ) was calculated as follows: were carried out by IBM SPSS Statistics v. 20 (IBM Cor- poration, New York, USA). π  h  d V ¼ ð1Þ Results Plant debris stock across the successional series − 1 Plant debris stock ranged from 25.19 to 82.89 Mg∙ha ρ ¼ ð2Þ across the subalpine forest successional series, and the 1=2 highest and lowest stocks of plant debris were observed in the later successional stage (S5) and earlier forest where m is the dry weight of the CWD and FWD sam- stage (S1), respectively (Table 1). When all plant debris ples (g), and d and h are the fresh dish sample diameter components were considered together, the stocks of and thickness (cm), respectively. plant debris represented a total tendency of increasing The volume of WD (V,cm ) in the field was calculated from the S1 to S6 stands except for the sudden decrease as follows: observed in S4 medium-aged earlier coniferous forest Wang et al. Forest Ecosystems (2021) 8:40 Page 5 of 14 Table 1 The stocks and ratios of plant debris across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau Successional series S1 S2 S3 S4 S5 S6 − 1 WD (Mg∙ha ) 31.73c 24.49c 51.49bc 36.07c 82.46a 77.27ab −1 NWD (Mg∙ha ) 1.31a 0.70bc 0.82bc 1.07ab 0.43c 0.80bc −1 Plant debris (Mg∙ha ) 33.04c 25.19c 52.31bc 37.14c 82.89a 78.08ab CWD/FWD 39.75 10.57 79.02 32.33 84.55 435.63 WD/NWD 26.58 37.98 65.15 34.93 208.89 110.69 Stock is shown as means (n = 3). One-way ANOVA tests were applied among six successional series in WD, NWD, Plant debris, CWD/FWD and WD/NWD. Values followed by different lowercase letters mean significant difference among six successional series at P = 0.05 level. WD Woody debris, NWD Non-woody debris, CWD Coarse woody debris, FWD Fine woody debris Fig. 1 Stock of plant debris component (a) and their allocation (b) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Different lowercase letters indicate that the total stock of plant debris differed significantly among successional gradient (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 6 of 14 (F = 6.34, P< 0.01; Fig. 1a). The stock was significantly In particular, the ratios of CWD to FWD stocks varied higher in CWD than in FWD and NWD from all six greatly with successional stages, ranging from 10.57 in successional stages (Fig. 1b). The proportion of CWD the broadleaved forest (S2) to 435.63 in the natural con- stock ranged from 88% to 98% along the forest succes- iferous forest (Table 1). Furthermore, the coefficient of sion gradient. Among these, only 1%–9% and 1%–4% of variation (CV) of plant debris stocks along the forest the stocks were stored in FWD and NWD, respectively successional series reached 48.93% (Fig. 2d). Meanwhile, (Fig. 1b). In detail, the stocks of different plant debris the CV of the FWD stocks was significantly higher than − 1 components were 22.20–80.37 Mg∙ha for CWD, 0.63– the CV of the NWD stocks (P< 0.05; Fig. 2d). − 1 − 1 2.36 Mg∙ha for FWD, and 0.40–1.31 Mg∙ha for NWD (Fig. 2). CWD displayed a similar trend as that of Changes in WD stocks with decay class across the plant debris (Fig. 1a), and the stock of CWD was signifi- successional series cantly higher in S5 and S6 than in the S1, S2, and S4 As shown in Table 2, the highest and lowest stocks of − 1 successional stages (F = 6.28, P < 0.01; Fig. 2a). In con- WD were observed in decay class IV (13.67 Mg∙ha ) − 1 trast, the stocks of FWD and NWD decreased from S1 and decay class II (8.77 Mg∙ha ), respectively. However, to S6, and the NWD stock was significantly lower in S5 a slight variation pattern of WD stock was observed than in S1 (F = 5.23, P < 0.01; Fig. 2c). However, slight across the five decay classes (F = 0.542, P> 0.05; Table 3). differences in FWD stocks were observed across the for- A higher proportion of WD stock was observed in est successional series (F = 0.61, P> 0.05; Fig. 2b). higher decay classes in the later succession stage S6, al- The ratios of WD to NWD stocks ranged from 26.58 though there was no significant difference (Fig. 3). Actu- to 208.89, and the highest and lowest ratios of WD to ally, among the different successional forests, a NWD stocks were observed in the mid-coniferous forest significant difference between decay classes was ob- (S5) and shrub forest stages (S1), respectively (Table 1). served only in the earlier succession stage (S1) but was Fig. 2 Stock of coarse woody debris CWD (a), fine woody debris FWD (b) and non-woody debris NWD (c) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Figure d indicates the coefficients of variation (CV) of stock of plant debris, CWD, FWD and NWD at different succession ages. Different lowercase letters mean significant difference among different succession stages or different plant debris components (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 7 of 14 Table 2 Changes in woody debris stock with decay classes and the WD (F = 3.576, P < 0.001; Table 3), although the WD diameter sizes in the subalpine forest on eastern Qinghai-Tibet stock was not significantly affected by decay class (F = Plateau 0.542, P> 0.05). −1 −1 Decay classes Stock (Mg∙ha ) Diameter sizes Stock (Mg∙ha ) I 11.66a D1 0.57c The C stocks of plant debris across the successional series II 8.77a D2 1.59c The plant debris C stocks ranged from 10.30 to 38.87 III 13.56a D3 12.95b − 1 Mg∙ha across the subalpine forest succession series, IV 13.67a D4 13.24b and the highest and lowest stocks of plant debris were V 11.39a D5 35.38a observed in the later succession stage (S5) and the earl- Stock is shown as means (n = 3). One-way ANOVA tests were applied among ier forest stage (S2), respectively (Table 4), showing a five decay classes and five diameter sizes. Values followed by different global tendency of increasing from the S1 to S6 stands lowercase letters mean significant difference among five decay classes or five diameter sizes at P = 0.05 level. D1–D5 indicated woody debris in diameter of except for the sudden decrease observed in S4 medium- 2cm ≤ d < 5 cm, 5 cm ≤ d < 10 cm, 10 cm ≤ d < 20 cm, 20 cm ≤ d < 40 cm, aged earlier coniferous forest (Fig. 5a). Furthermore, the d ≥ 40 cm, respectively stock was significantly higher in CWD than in FWD and NWD at each successional stage (Fig. 5). Compared with not observed in the other forests (Fig. 3). Similarly, the the mean proportions of C stocks of NWD (2%) and WD stock of each decay class varied slightly among the FWD (4.5%), the minimum proportion of CWD C stor- six plant communities (Fig. 3). age is 87% in the subalpine forest ecosystem (Fig. 5b). Especially in the later stages of succession, the total plant Changes in WD stocks with diameter size across the debris C pool was dominated mainly by CWD successional series (P< 0.001; Fig. 5b). The WD stock was significantly higher in larger- Among plant debris components, the changes in the diameter (D5) than finer-diameter WD (D1 and D2) plant debris C stock with plant debris component and (F = 25.97, P< 0.001; Tables 2 and 3). Consistently, the successional stages showed a similar pattern of plant stock of the coarsest diameter (D5) was remarkably debris component stock (Fig. 2). The C stock of CWD higher than that of the other diameter sizes in the older − 1 ranged from 8.99 to 37.66 Mg∙ha and showed a global successional series S5 and S6, and presented a significant increase trend (Fig. 6a), which was significantly higher in increasing tendency with diameter size (P< 0.05). The the later successional stages S5 and S6 than in the earlier highest stock at S3 and S4 stages were observed in the successional stages S1 and S2 (P< 0.05; Fig. 6a). Most coarser diameters of D4 and D3, respectively (P< 0.05). importantly, the C stored in CWD was more than four The stocks of diameters D3 and D5 were higher than times higher in the highest forest (S5) than in the lowest that of the finer WD in S1. However, a slight difference forest (S2) (Fig. 6a). The C stock ranged from 0.30 to in WD stock was observed between diameter sizes at the − 1 1.17 Mg∙ha for FWD (Fig. 6b) and 0.18 to 0.55 S2 stage (P> 0.05; Fig. 4). Meanwhile, the WD stock of − 1 Mg∙ha for NWD (Fig. 6c), respectively, and showed D3, D4, and D5 diameters varied significantly with suc- the opposite tendency to CWD. C stored in FWD and cessional stages (Fig. 4). Furthermore, the WD stock of NWD (P< 0.05) was lower in the S6 stage than in the diameter D5 at S5 or S6 stage was obviously higher than S1 stage, but this difference was nonsignificant for FWD this of the earlier succession stands (P< 0.05), and the (P> 0.05; Figs. 6b and c). Besides, the CVs of the C highest WD stocks of diameters D4 and D3 were ob- stocks varied significantly with the different components served in the mid-successional forests S3 and S4, re- of plant debris. The CV of the FWD C stock was signifi- spectively (P< 0.05; Fig. 4). Additionally, an interaction cantly higher than the CV of the CWD and NWD stocks effect was observed among successional series, decay in the six successional stages (P< 0.05; Fig. 6d). classes, and diameter classes for the stock measured in Table 3 Multivariate analysis of variance (MANOVA) for the effects of succession series, decay classes and diameter sizes on the stocks of woody debris and carbon Source variance df Stock of woody debris C stock of woody debris F-value P-value F-value P-value *** *** Succession series (S) 5 5.377 < 0.001 7.149 < 0.001 Decay classes (DC) 4 0.542 0.705 1.564 0.185 *** *** Diameter sizes (D) 4 25.973 < 0.001 27.185 < 0.001 *** *** S × DC × D 5 3.576 < 0.001 4.072 < 0.001 *** Significant effect: P < 0.001 Wang et al. Forest Ecosystems (2021) 8:40 Page 8 of 14 Fig. 3 Stock of woody debris in different decay classes among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Different lowercase letters mean significant difference among different decay classes (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Both succession stage (F = 7.147, P< 0.001) and diam- significantly at different diameter sizes among the six eter class (F = 27.185, P < 0.001) had significant effects plant communities, which was partly consistent with our on the C concentration of WD, but decay class had a second hypothesis. Additionally, our results still demon- slight effect on the C stock (F = 1.564, P> 0.05). Further- strated a global tendency of increase from the S1 to S6 more, the C stock of WD was significantly influenced by stands except for the decrease observed in S4 medium- the interaction among succession age, diameter size, and aged earlier coniferous forest, which partly agreed with decay class (F = 4.072, P< 0.001; Table 3). the third hypothesis. This result confirmed that CWD changed greatly with forest succession and demonstrated that CWD dominated the plant debris in natural forests Discussion and that the ratios and components of plant debris fluctu- Our results indicated that plant debris stocks increased ated sharply with forest successional series. from the earlier successional stage to the later successional stage except for decreasing at the S4 successional stage in the subalpine forest region, which partly supported our Changes in WD and NWD stocks with forest succession first hypothesis. Meanwhile, the results showed that the Forest succession is an important factor that affects the WD stock varied slightly at different decay classes but stocks of CWD or litter in different forest ecosystems Fig. 4 Stock of woody debris in different diameter sizes among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. D1–D5 indicated woody debris in diameter of 2 cm ≤ d < 5 cm, 5 cm ≤ d < 10 cm, 10 cm ≤ d < 20 cm, 20 cm ≤ d < 40 cm, d ≥ 40 cm, respectively. Different lowercase letters mean significant difference among different diameter sizes (P < 0.05). Different uppercase letters mean significant difference among different succession stages (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 9 of 14 Table 4 The carbon stock of plant debris across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau Successional series S1 S2 S3 S4 S5 S6 −1 WD (Mg∙ha ) 14.97c 10.00c 23.73bc 17.37c 38.69a 33.14ab −1 NWD (Mg∙ha ) 0.55a 0.29bc 0.35bc 0.45ab 0.18c 0.34bc −1 Plant debris (Mg∙ha ) 15.53c 10.30c 24.07bc 17.82c 38.87a 33.48ab Carbon stock is shown as means (n = 3). One-way ANOVA tests were applied six successional series in WD, NWD, and Plant debris. Values followed by different lowercase letters mean significant difference among six successional series at P = 0.05 level. WD Woody debris, NWD Non-woody debris, C Carbon (Idol et al. 2001; Sefidi 2010; Zhang et al. 2011; Aryal A greater amount of CWD was found in earlier- or et al. 2015). However, the tendency of the plant debris older-growth forests than in other stands (presenting a biomass and C stocks, particularly in CWD components, ‘U-shaped’ pattern) (Eaton and Lawrence 2006; Sefidi differed greatly from those in previous reports (Table 5). 2010). A monotonic increase in CWD amount or C Fig. 5 Carbon stock of plant debris component (a) and their allocation (b) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Different lowercase letters indicate that the total carbon stock of plant debris differed significantly among successional gradient (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 10 of 14 Fig. 6 Carbon stock of coarse woody debris CWD (a), fine woody debris FWD (b) and non-woody debris NWD (c) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Figure d indicates the coefficients of variation (CV) of carbon stock of plant debris, CWD, FWD and NWD at different succession ages. Different lowercase letters mean significant difference among different succession stages or different plant debris components (P < 0.05). Horizontal bars indicate standard errors of means (n =3) − 1 − 1 Table 5 Comparisons of plant debris stock (Mg∙ha ) and plant debris carbon stock (Mg∙ha ) among different regions Area Earlier succession Mid-succession Later succession Tendency Reference CWD Carbon CWD Carbon CWD Carbon Subalpine forests of western Sichuan 25.73 11.40 50.45 23.24 63.98 29.10 Increasing This study Primary temperate forests of southern 18.00 – 64.50 – 173.75 – Increasing Carmona et al. (2002) Chile Gorazbon forests of Northern Iran 37.05 – 25.95 – 51.25 –‘U-shaped’ Sefidi (2010) 3 −1 3 − 1 3 − 1 (m ∙ha ) (m ∙ha ) (m ∙ha ) Dry tropical forest of Southern Mexico 51.62 – 11.38 – 37.46 –‘U-shaped’ Eaton and Lawrence (2006) Upland oak-hickory forests of southern 137.2 69.0 39.8 19.9 59.0 29.7 Decreasing Idol et al. (2001) Indiana NWD Carbon NWD Carbon NWD Carbon Subalpine forests of western Sichuan 1.01 0.42 0.82 0.35 0.77 0.32 Decreasing This study Semi-evergreen tropical forests of 5.20 2.34 7.10 3.20 6.29 2.83 Increasing than Aryal et al. (2015) south-eastern Mexico decreasing Subtropical forests of eastern China 6.38 0.60 8.15 0.65 11.69 0.75 Increasing Yan et al. (2009) (%) (%) (%) Wang et al. Forest Ecosystems (2021) 8:40 Page 11 of 14 density was found following secondary forest succession The effect of decay class and diameter on WD stock in Chiloe (Carmona et al. 2002) and Chinese forests across the succession series (Tang and Zhou 2005; Zeng et al. 2015; Zhu et al. 2017). Interestingly, decay class did not affect the storage of Idol et al. (2001) have shown a large decrease in volume WD in five successional stages of forest ecosystems in and mass of woody debris from recently harvested to our study (Fig. 3). However, it has been reported that mature stands (Table 5). However, our results suggested decay class IV was the most abundant decay class in a a global increasing curve for CWD with succession ex- Northern Iran forest (Sefidi 2010). Yan et al. (2007) have cept for the sudden decrease observed in the medium- reported that the evergreen broadleaved forests of later aged earlier coniferous forest (S4) (Figs. 2a and 6a). In successional stages have a higher proportion of WD with general, the accumulation of CWD depends not only on decay classes IV and V in the subtropical region of east- plant debris production but also on the rate of decom- ern China. In contrast, Carmona et al. (2002) have re- position (Tritton 1981; Sturtevant et al. 1997). Since ported that CWD at advanced decay levels was more CWD breaks down persistently and slowly, its produc- abundant in earlier-stage stands, while most CWD was tion rates ultimately outpace its decomposition rates. in the intermediate decay classes in older forests. These Therefore, the later succession stages with more above- contradictions can be partly attributed to the difference ground biomass commonly have higher CWD stocks in the vegetation composition. Additionally, all study (Smith et al. 2006), while the earlier successional stands sites were located within a nature reserve with little dis- with less live biomass have lower CWD stocks. Most im- turbance and management practice, which is another portantly, the difference is likely that their analyses were crucial reason for the average distribution of WD at dif- simply based on the data of general successional gradi- ferent decay classes. ents that lacked consideration of earlier coniferous for- In addition to the significant effect of the succession ests. Although the earlier coniferous forest has a higher stage on the stock of plant debris, diameter size influ- succession stage than that of mixed forest, it has a rapid enced the storage of WD (Fig. 4). Our results illustrated growth period for trunk biomass. Meanwhile, the CWD that larger-diameter WD dominated the plant debris as is almost decomposed in the earlier stage, and there is the successional stage increased. Our current estimates relatively little WD, especially CWD. were also confirmed by several studies investigating Compared with the biomass and C stocks of CWD, whether CWD was dominant in plant debris across for- the stocks of NWD had an opposite tendency (Figs. 2c est succession stage (Eaton and Lawrence 2006; Van and 6c). First, we estimated that the biomass and C Mantgem et al. 2009; Sefidi 2010; Yang et al. 2010; Zeng stocks reached a peak in the earlier succession stage (S1) et al. 2015). Tree species at different successional stages and then decreased with the forest succession stage but produced different biomasses of CWD. For instance, tree suddenly increased in the medium stage earlier conifer- species with a cohort of old and large-diameter fir and ous forest (S4). However, in other forest ecosystems spruce coniferous species in an old-growth stand could (Table 5), the production of litter increased through the lead to more biomass of CWD, while most were small succession process (Yan et al. 2009; Zhang et al. 2013) and thin willow shrub and birch in the earlier succes- or increased rapidly and then decreased with the forest sional forest. However, as suggested by other reports, stage (Aryal et al. 2015). The discrepancy of rates of in- the relative contribution of CWD to the plant debris put (aboveground biomass) and output (decomposition) biomass declined with forest succession (Brown and may be one of the possible reasons (Dent et al. 2006; Lugo 1990; Krankina and Harmon 1995; Delaney et al. Wang et al. 2007). The tree species of litter and climate 1998). The difference may be due to different standards of different forest ecosystems also determined the de- of classification of diameter sizes and climate conditions composition rates of litter (Müller-Using and Bartsch used at different study sites. 2009; Berbeco et al. 2012). Second, in contrast to the CWD stock, the higher stock in the young shrub forest Plant debris carbon stock across the succession series (S1) was contributed by shrub twigs. Meanwhile, the Countrywide, the estimated plant debris C stock was − 1 later successional stage of the mid- and mature conifer- 5.88 ± 0.35 Mg C∙ha in a temperate forest (Zhu et al. ous forests (S5 and S6) had lower stocks, which was 2017). In our study, the plant debris C stock ranged − 1 likely related to the lower inputs of needles and the fas- from 10.30 to 38.87 Mg∙ha (Table 3), much higher ter losses of thinner litter, which decelerated the than the average of Chinese forests but similar to some amounts of remains and sped up the rate of results from local forests in China (Yang et al. 2010) and decomposition. southern Indiana (Idol et al. 2001) (Table 5). The higher C stock in our ecosystem is likely a result of natural for- ests with little disturbance and management practices. The source of nationwide data, including many young- Wang et al. Forest Ecosystems (2021) 8:40 Page 12 of 14 growth plantations and areas of excessive harvest, oc- Availability of data and materials All data generated or analyzed during this study are included in this curred in planted forests (Guo et al. 2013) and resulted published article. in a low average C stock value. In addition, the plant debris investigation in our study tended to select forests Declarations with a greater distribution of WD (Pan et al. 2011), Ethics approval and consent to participate which caused the overestimates of plant debris amount Not applicable. and C stock. Consent for publication Not applicable. Conclusions Competing interests The change in the biomass and C stock of WD repre- The authors declare that they have no competing interests. sented a total tendency of increasing from the S1 to S6 stands, while the changes of FWD and NWD were de- Author details School of Life Sciences, Taizhou University, Taizhou 318000, China. creased across the subalpine forest succession series. Wanglang National Nature Reserve Authority, Pingwu 622550, Sichuan, CWD dominated the plant debris regardless of the forest China. successional stages. 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Soil Biol Biochem 57:803–813. https://doi.org/10.1 016/j.soilbio.2012.08.005 Zhu J, Hu H, Tao S, Chi X, Li P, Jiang L, Ji C, Zhu J, Tang Z, Pan Y (2017) Carbon stocks and changes of dead organic matter in China’s forests. Nat Commun 8(1):1–10. https://doi.org/10.1038/s41467-017-00207-1 Ott L, Mann P, Van Cleve K (2006) Successional processes in the Alaskan boreal forest. In: Chapin FS, Oswood MW, van Cleve K, Viereck LA, Verbyla DL (eds) Alaska’s changing boreal forest. Oxford: Oxford University Press, pp 100–119 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Forest Ecosystems" Springer Journals

Changes in plant debris and carbon stocks across a subalpine forest successional series

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10.1186/s40663-021-00320-0
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Abstract

Background: As a structurally and functionally important component in forest ecosystems, plant debris plays a crucial role in the global carbon cycle. Although it is well known that plant debris stocks vary greatly with tree species composition, forest type, forest origin, and stand age, simultaneous investigation on the changes in woody and non-woody debris biomass and their carbon stock with forest succession has not been reported. Therefore, woody and non-woody debris and carbon stocks were investigated across a subalpine forest successional gradient in Wanglang National Nature Reserve on the eastern Qinghai-Tibet Plateau. − 1 Results: Plant debris ranged from 25.19 to 82.89 Mg∙ha and showed a global increasing tendency across the subalpine forest successional series except for decreasing at the S4 successional stage. Accordingly, the ratios of woody to non-woody debris stocks ranged from 26.58 to 208.89, and the highest and lowest ratios of woody to non-woody debris stocks were respectively observed in mid-successional coniferous forest and shrub forest, implying that woody debris dominates the plant debris. In particular, the ratios of coarse to fine woody debris stocks varied greatly with the successional stage, and the highest and lowest ratios were found in later and earlier successional subalpine forests, respectively. Furthermore, the woody debris stock varied greatly with diameter size, and larger diameter woody debris dominated the plant debris. Correspondingly, the carbon stock of plant debris − 1 ranged from 10.30 to 38.87 Mg∙ha across the successional series, and the highest and lowest values were observed in the mid-coniferous stage and shrub forest stage, respectively. Most importantly, the carbon stored in coarse woody debris in later successional forests was four times higher than in earlier successional forests. Conclusions: The stock and role of woody debris, particularly coarse woody debris, varied greatly with the forest successional stage and dominated the carbon cycle in the subalpine forest ecosystem. Thus, preserving coarse woody debris is a critical strategy for sustainable forest management. Keywords: Coarse woody debris, Fine woody debris, Forest successional series, Later successional stage, Earlier successional stage, Log decay class, Diameter size Introduction et al. 1986), and the latter accounts for ca. 20%–30% of Plant debris consists of woody debris (WD) and non- global woody biomass (Pan et al. 2011). CWD plays cru- woody debris (NWD), both of which play crucial roles cial roles in conserving biodiversity, forest regeneration, not only in nutrient cycling and biodiversity conserva- global carbon sinks, and soil development (Iwashita tion but also in the global carbon cycle (Pan et al. 2011; et al. 2013; Russell et al. 2015;Błońska et al. 2017; Pres- Zhu et al. 2017). In particular, WD includes fine woody cott et al. 2017). Previous investigations have docu- debris (FWD) and coarse woody debris (CWD) (Harmon mented that WD and NWD stocks vary greatly with tree species composition (Raich et al. 2007; Wang et al., * Correspondence: scyangwq@163.com 2019), forest type (Moreira et al. 2019), forest origin School of Life Sciences, Taizhou University, Taizhou 318000, China (Hagemann et al. 2010; Suzuki et al. 2019), and stand 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) 8:40 Page 2 of 14 age (Sefidi 2010; Schilling et al. 2016). Most importantly, species composition is determined mainly by the succes- the rate of plant debris decomposition varies greatly with sion stage (Taylor et al. 2020). Thus, forest succession debris type (Cornwell et al. 2009; Harmon et al. 2013). development may affect the distribution of plant debris FWD and NWD, such as foliar litter, decompose faster decay classes and diameter classes through the following than CWD (Müller-Using and Bartsch 2009; Berbeco aspects. On the one hand, CWD at earlier successional et al. 2012). Additionally, plant debris decomposition dif- stages might decompose faster than those at later suc- fers among tree species, and lower-density WD decom- cession stages due to tree species at earlier successional poses faster (Shorohova and Kapitsa 2016; Guo et al. stages have shorter life cycles and lower CWD quality 2020). The above description implies that the compos- (Puyravaud et al. 2003). For instance, the CWD of ition and proportion of plant debris might control forest broad-leaved species decomposes faster (Yatskov et al. regeneration, biodiversity nursing, soil and water conser- 2003); additionally, the CWD of angiosperms decom- vation, and the cycling of carbon and nutrients. Thereby, poses faster than that of gymnosperms (Herrmann et al. an investigation of the changes in plant debris stock 2015). On the other hand, the CWD in the earlier suc- characteristics with tree species composition determined cessional forests usually have smaller diameters (Ott by forest type is crucial to understand the process and et al. 2006), while in the later succession, the CWD usu- function of forest ecosystems. ally have larger diameters (Vanninen et al. 1996). Gener- In natural or unmanaged forests, forest succession is ally, larger-diameter CWD decomposes slower than an important factor affecting the forest type and species smaller-diameter CWD (Harmon et al. 2013). Thus, diversity (Lebrija-Trejos et al. 2010; West et al. 2012; characterizing the stocks and proportions of different Taylor et al. 2020). Meanwhile, the input of plant debris decay and diameter classes at different successional is determined by the death or falling of aboveground stages is essential for understanding the nutrient dynam- species (Tritton 1981; Sturtevant et al. 1997). As a result, ics of plant debris stock and predicting forest succession forest succession may affect the composition and stock rate. However, little information is available on CWD of plant debris through the following aspects. First, the stock changes with decay and diameter classes across stock and composition of plant debris would vary to forest successional series. some degree with forest succession due to the differ- Plant debris represents an essential carbon pool and ences in tree species composition and tree longevity at strongly influences on the structures and carbon dynam- different succession stages. For instance, litter produc- ics in the forest ecosystems (Harmon et al. 1986). The tion and dynamics significantly vary with overstory tree impacts of various driving forces on CWD carbon stocks species composition in lowland Costa Rica (Raich et al. at the local scale have been widely investigated (e.g., Hall 2007); forest type determines the species composition et al. 2006; Woodall and Liknes 2008; Kurz et al. 2009; and CWD stock (Moreira et al. 2019). Second, tree spe- Jonsson et al. 2011; Woodall et al. 2013). However, only cies in the earlier succession stages may be more suscep- a few studies have evaluated the dependency between tible to disturbance and generate more CWD (Brassard the carbon storages of NWD and the stages of succes- and Chen, 2008). However, higher and lower stocks of sion (Zhang et al. 2011). Furthermore, compared with WD have been observed at the later and earlier succes- carbon stocks in live biomass or soil, the C budgets in sion stages due to differential WD decomposition rates WD and NWD production and turnover at different for- (Carmona et al. 2002). Zhu et al. (2017) have shown that est succession stages have not been simultaneously in- the biomass and C storage of CWD may increase with vestigated. Consequently, a comprehensive assessment forest succession, while a “U-shape” trend of CWD was of the carbon budget and the relative contributions of observed along the forest successional gradient by Sefidi WD and NWD to the carbon budget across forest suc- (2010). Idol et al. (2001) have shown a significant de- cessional series are critically important for understand- crease in volume and mass of woody debris from re- ing the significance of plant debris in carbon sinks in cently harvested to mature stands. Together, the forest ecosystems. changes in plant debris with forest succession remain To understand the changes in the composition, bio- uncertain. Thereby, investigating the changes in plant mass, and carbon stock of plant debris with forest suc- debris stock across forest successional series will help us cession, NWD, FWD, and CWD with different diameter better understand the function of plant debris in forest classes and decay classes were investigated across a sub- ecosystems. alpine forest successional series in Wanglang National In theory, the stock and composition of plant debris Nature Reserve on the eastern Qinghai-Tibet Plateau depend greatly on the WD decomposition, particularly and the upper reaches of the Yangtze River. This region that of CWD (Zell et al. 2009). The decay rates of WD is the main body of China’s second largest forest area and NWD vary with tree species and diameter class and plays paramount roles not only in holding fresh- (Cornwell et al. 2009; Harmon et al. 2013), while tree water, conserving water, and soil and nursing Wang et al. Forest Ecosystems (2021) 8:40 Page 3 of 14 biodiversity but also in the global carbon cycle (Liu dominated by Cacalia palmatisecta, Cyperaceae, Poa 2002; Tan et al. 2014). Subalpine forest communities at pratensis, and others (Taylor et al. 2006). The moss layer different successional stages are observed in the sub- is dominated by Thuidium cymbifolium, Hylocomium alpine forest region on the eastern Qinghai-Tibet Plateau splendens, Mnium heterophyllum, and Phaeoceros laevis due to long-term natural disturbance and the commer- (Li et al. 2012). cial logging of natural forests since the 1950s (Yang et al. 1992). Although the WD stock from the gap center Experimental design to the closed canopy in an over-mature subalpine con- Based on our previous visits and investigations on forest iferous forest (Xiao et al. 2016), the water storage poten- vegetation and plant debris in Wanglang National Na- tial of WD (Wang et al. 2016), and the changes in ture Reserve, we divided the different forest types into microbial biomass and epixylic plant diversity with decay six succession stages according to Zhang et al. (2011) classes (Wang et al. 2017; Chang et al. 2019) have been and through interviews with local supervisors. S1 succes- widely investigated in this region, little attention has sional stage is dominated by shrubs (e.g., H. rham- been given to the changes in the composition, biomass noides), S2 successional stage is dominated by deciduous stock and carbon storage of plant debris with forest suc- broadleaved species (e.g., Betula), S3 successional stage cession. Therefore, we hypothesized that (1) plant debris is dominated by deciduous broadleaved (Betula) and stocks would increase from the earlier successional stage coniferous (A. faxoniana) mixed forest and later succes- to the later succession stage in the subalpine forest re- sional stages (S4, S5, and S6) are all dominated by con- gion, (2) the stocks and proportions of different decay iferous species. Here S4 represents the earlier coniferous and diameter classes should differ at different succes- successional stage (A. faxoniana and P. purpurea); S5 sional stages, and (3) the C stock of plant debris would represents the middle coniferous successional stage (A. increase across the forest succession. The objectives faxoniana); S6 represents the mature coniferous succes- were to (1) investigate the changes in plant debris stocks sional stage (P. purpurea). The six successional forests across the forest successional series located in the sub- are widely distributed in the subalpine forest region (Fig. alpine forest region, (2) elucidate the stocks and propor- A1). Site information and specific characteristics of each tion of different decay and diameter classes at different successional stage are recorded in Table A1. successional stages, and (3) assess the carbon budget and the relative contributions of CWD with different Sampling method decay classes and diameter classes to the carbon budget Woody debris was classified into CWD with a diam- across the forest successional series. These results could eter ≥ 10 cm and FWD with a diameter of 2 cm ≤ d< 10 help us better understand the function and significance cm (Ward and Aumen 1986; Harmon and Hua 1991). of plant debris in carbon sinks in forest ecosystems and CWD includes fallen logs, snags (dead standing trees), provide clear insight into the preservation of CWD, stumps, and large branches (Harmon et al. 1986). Snags which is a critical strategy for sustainable forest manage- refer to CWD which inclination is not more than 45°, ment in the subalpine forest on the eastern Qinghai- base diameter ≥ 10 cm, and length > 1 m. Stumps with a Tibet Plateau. height < 1 m were defined as CWD, which includes coarse roots above the soil surface (Harmon and Sexton Materials and methods 1996; Currie and Nadelhoffer 2002). Among these, dead Field description coarse roots of fallen logs and snags were measured and The study region is located in the Wanglang National recorded, and of which stumps were neglected due to Nature Reserve (32°49′–33°02′ N, 103°55′–104°10′ E; they were rarely observed. Additionally, we reclassified 2300–4983 m a.s.l.), which is situating in Pingwu County, foliar litter, dead twig, dead fine bark, and epiphytes as Sichuan, Southwest China (Fig. A1). The region is a NWD. Three plots of 10 m × 20 m of each successional transitional area between the Tibetan Plateau and the Si- forest were established for CWD and FWD investigation, chuan Basin. The annual precipitation is approximately and nine subplots of 1 m × 1 m were established for 859.9 mm, and the annual mean temperature is approxi- NWD investigation. Five decay class (I-V) systems opti- mately 2.9 °C, with maximum and minimum tempera- mized according to Błońska et al. (2018) were used to tures of 26.2 °C and − 17.8 °C, respectively (Zhang et al. classify the decomposition degree of WD based on the 2011). The tree canopy is dominated by Abies faxoniana, morphology and hardness observed in the field (Table Picea purpurea, Sabina saltuaria, Betula platyphylla, A2). Five diameter sizes (D1–D5) were applied to classify and B. albo-sinensis. The understory shrubs are domi- the WD diameter according to Harmon et al. (1986) and nated by Salix wallichiana, Hippophae rhamnoides, Xiong et al. (2016), that is, 2 cm ≤ d<5cm (D1), 5cm ≤ Rhododendron lapponicum, Lonicera spp., Sorbus rufopi- d < 10 cm (D2), 10 cm ≤ d < 20 cm (D3), 20 cm ≤ d<40 losa,and Rosa sweginzowii. The herbaceous layer is cm (D4) and d ≥ 40 cm (D5). Then, the decay class, base Wang et al. Forest Ecosystems (2021) 8:40 Page 4 of 14 diameter, end diameter, and length (fallen logs and large π  l  d π  l  d  d 2 2 3 V ¼ þ branches) or height (snags and stumps) of the WD were 12 12 recorded and measured. For CWD, the length beyond π  l  d þ ð3Þ the plot, only the part within the plot be recorded. Next, for the CWD with decay classes I to III, three dish sam- where d , d and l are the base diameter, end diameter, 2 3, ples of each decay class with a thickness of approxi- and length or height of the WD in the field (cm), mately 5 cm were obtained by chain saw. For the CWD respectively. with decay classes IV and V and FWD, three samples − 1 The WD stock (G ,Mg∙ha ) was calculated using the with a weight of approximately 500 g were harvested. following formula: For NWD, all samples within the subplot of 1 m × 1 m were harvested. The samples were taken back to V  ρ G ¼ ð4Þ the laboratory for stock and C concentration 200  cosθ  100 measurements. where θ is the slope of each plot (°), 200 is the area of each plot (m ), and 100 is the unit conversion factor. Measurement of plant debris stock and C stock The NWD stock (G ) was calculated using the follow- After the dish samples of CWD from decay classes I to ing formula: III were delivered to the laboratory, their diameter (d ) 1 P and thickness (h) were measured. Then, the volume (V ) G ¼ ð5Þ 1  cosθ  100 and density (ρ) were calculated (Bonan 2008). After the CWD samples from decay classes IV and V and the where m is the dry weight of the NWD sample (g), θ is FWD samples were delivered to the laboratory, we used the slope of each subplot (°), 1 is the area of each subplot the drainage method to determine the volume (V ) (m ), and 100 is the unit conversion factor. − 1 (Jonsson 2000). Then, we weighed these samples after The carbon stocks (Mg∙ha of C) of WD and NWD oven drying at 85 °C to a constant weight (m ). For were calculated using the following formula: NWD, the sample was weighed after oven drying at Carbon stock ¼ G  carbon concentration ð6Þ 1=2 85 °C to a constant weight (m ). The volume (V)of WD in the field was calculated according to Zhu et al. (2017), where G and G are the stocks of WD and NWD, 1 2 and the stock (G) of WD in the field was converted by respectively. the volume (V) and the sample density (ρ). For C con- centration, the most important factor determining C Statistical analysis concentration (%) of plant debris were decay class and One-way analysis of variance (ANOVA) and least signifi- geographical location, and there was little difference be- cant difference (LSD) tests were applied to examine the tween plant species or other factors (Zhu et al. 2017). different significance of WD stocks and C stocks among So, for WD from the different decay classes, carbon con- different decay classes, among different diameter sizes, centrations of 50.5%, 50.0%, 49.7%, 43.3%, and 37.9% and among these successional forests. The significance refer to Chang et al. (2015), who have investigated the was selected at the 0.05 level. The interaction effects of changes in the C concentrations with WD decay classes successional stages, decay classes, and diameter sizes on in a similar subalpine forest; for NWD, a carbon concen- the WD stock and C stock were analyzed by Multivariate tration of 41.9% refer to Zhu et al. (2017). analysis of variance (MANOVA). All statistical analyses − 3 The WD density (ρ,g∙cm ) was calculated as follows: were carried out by IBM SPSS Statistics v. 20 (IBM Cor- poration, New York, USA). π  h  d V ¼ ð1Þ Results Plant debris stock across the successional series − 1 Plant debris stock ranged from 25.19 to 82.89 Mg∙ha ρ ¼ ð2Þ across the subalpine forest successional series, and the 1=2 highest and lowest stocks of plant debris were observed in the later successional stage (S5) and earlier forest where m is the dry weight of the CWD and FWD sam- stage (S1), respectively (Table 1). When all plant debris ples (g), and d and h are the fresh dish sample diameter components were considered together, the stocks of and thickness (cm), respectively. plant debris represented a total tendency of increasing The volume of WD (V,cm ) in the field was calculated from the S1 to S6 stands except for the sudden decrease as follows: observed in S4 medium-aged earlier coniferous forest Wang et al. Forest Ecosystems (2021) 8:40 Page 5 of 14 Table 1 The stocks and ratios of plant debris across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau Successional series S1 S2 S3 S4 S5 S6 − 1 WD (Mg∙ha ) 31.73c 24.49c 51.49bc 36.07c 82.46a 77.27ab −1 NWD (Mg∙ha ) 1.31a 0.70bc 0.82bc 1.07ab 0.43c 0.80bc −1 Plant debris (Mg∙ha ) 33.04c 25.19c 52.31bc 37.14c 82.89a 78.08ab CWD/FWD 39.75 10.57 79.02 32.33 84.55 435.63 WD/NWD 26.58 37.98 65.15 34.93 208.89 110.69 Stock is shown as means (n = 3). One-way ANOVA tests were applied among six successional series in WD, NWD, Plant debris, CWD/FWD and WD/NWD. Values followed by different lowercase letters mean significant difference among six successional series at P = 0.05 level. WD Woody debris, NWD Non-woody debris, CWD Coarse woody debris, FWD Fine woody debris Fig. 1 Stock of plant debris component (a) and their allocation (b) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Different lowercase letters indicate that the total stock of plant debris differed significantly among successional gradient (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 6 of 14 (F = 6.34, P< 0.01; Fig. 1a). The stock was significantly In particular, the ratios of CWD to FWD stocks varied higher in CWD than in FWD and NWD from all six greatly with successional stages, ranging from 10.57 in successional stages (Fig. 1b). The proportion of CWD the broadleaved forest (S2) to 435.63 in the natural con- stock ranged from 88% to 98% along the forest succes- iferous forest (Table 1). Furthermore, the coefficient of sion gradient. Among these, only 1%–9% and 1%–4% of variation (CV) of plant debris stocks along the forest the stocks were stored in FWD and NWD, respectively successional series reached 48.93% (Fig. 2d). Meanwhile, (Fig. 1b). In detail, the stocks of different plant debris the CV of the FWD stocks was significantly higher than − 1 components were 22.20–80.37 Mg∙ha for CWD, 0.63– the CV of the NWD stocks (P< 0.05; Fig. 2d). − 1 − 1 2.36 Mg∙ha for FWD, and 0.40–1.31 Mg∙ha for NWD (Fig. 2). CWD displayed a similar trend as that of Changes in WD stocks with decay class across the plant debris (Fig. 1a), and the stock of CWD was signifi- successional series cantly higher in S5 and S6 than in the S1, S2, and S4 As shown in Table 2, the highest and lowest stocks of − 1 successional stages (F = 6.28, P < 0.01; Fig. 2a). In con- WD were observed in decay class IV (13.67 Mg∙ha ) − 1 trast, the stocks of FWD and NWD decreased from S1 and decay class II (8.77 Mg∙ha ), respectively. However, to S6, and the NWD stock was significantly lower in S5 a slight variation pattern of WD stock was observed than in S1 (F = 5.23, P < 0.01; Fig. 2c). However, slight across the five decay classes (F = 0.542, P> 0.05; Table 3). differences in FWD stocks were observed across the for- A higher proportion of WD stock was observed in est successional series (F = 0.61, P> 0.05; Fig. 2b). higher decay classes in the later succession stage S6, al- The ratios of WD to NWD stocks ranged from 26.58 though there was no significant difference (Fig. 3). Actu- to 208.89, and the highest and lowest ratios of WD to ally, among the different successional forests, a NWD stocks were observed in the mid-coniferous forest significant difference between decay classes was ob- (S5) and shrub forest stages (S1), respectively (Table 1). served only in the earlier succession stage (S1) but was Fig. 2 Stock of coarse woody debris CWD (a), fine woody debris FWD (b) and non-woody debris NWD (c) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Figure d indicates the coefficients of variation (CV) of stock of plant debris, CWD, FWD and NWD at different succession ages. Different lowercase letters mean significant difference among different succession stages or different plant debris components (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 7 of 14 Table 2 Changes in woody debris stock with decay classes and the WD (F = 3.576, P < 0.001; Table 3), although the WD diameter sizes in the subalpine forest on eastern Qinghai-Tibet stock was not significantly affected by decay class (F = Plateau 0.542, P> 0.05). −1 −1 Decay classes Stock (Mg∙ha ) Diameter sizes Stock (Mg∙ha ) I 11.66a D1 0.57c The C stocks of plant debris across the successional series II 8.77a D2 1.59c The plant debris C stocks ranged from 10.30 to 38.87 III 13.56a D3 12.95b − 1 Mg∙ha across the subalpine forest succession series, IV 13.67a D4 13.24b and the highest and lowest stocks of plant debris were V 11.39a D5 35.38a observed in the later succession stage (S5) and the earl- Stock is shown as means (n = 3). One-way ANOVA tests were applied among ier forest stage (S2), respectively (Table 4), showing a five decay classes and five diameter sizes. Values followed by different global tendency of increasing from the S1 to S6 stands lowercase letters mean significant difference among five decay classes or five diameter sizes at P = 0.05 level. D1–D5 indicated woody debris in diameter of except for the sudden decrease observed in S4 medium- 2cm ≤ d < 5 cm, 5 cm ≤ d < 10 cm, 10 cm ≤ d < 20 cm, 20 cm ≤ d < 40 cm, aged earlier coniferous forest (Fig. 5a). Furthermore, the d ≥ 40 cm, respectively stock was significantly higher in CWD than in FWD and NWD at each successional stage (Fig. 5). Compared with not observed in the other forests (Fig. 3). Similarly, the the mean proportions of C stocks of NWD (2%) and WD stock of each decay class varied slightly among the FWD (4.5%), the minimum proportion of CWD C stor- six plant communities (Fig. 3). age is 87% in the subalpine forest ecosystem (Fig. 5b). Especially in the later stages of succession, the total plant Changes in WD stocks with diameter size across the debris C pool was dominated mainly by CWD successional series (P< 0.001; Fig. 5b). The WD stock was significantly higher in larger- Among plant debris components, the changes in the diameter (D5) than finer-diameter WD (D1 and D2) plant debris C stock with plant debris component and (F = 25.97, P< 0.001; Tables 2 and 3). Consistently, the successional stages showed a similar pattern of plant stock of the coarsest diameter (D5) was remarkably debris component stock (Fig. 2). The C stock of CWD higher than that of the other diameter sizes in the older − 1 ranged from 8.99 to 37.66 Mg∙ha and showed a global successional series S5 and S6, and presented a significant increase trend (Fig. 6a), which was significantly higher in increasing tendency with diameter size (P< 0.05). The the later successional stages S5 and S6 than in the earlier highest stock at S3 and S4 stages were observed in the successional stages S1 and S2 (P< 0.05; Fig. 6a). Most coarser diameters of D4 and D3, respectively (P< 0.05). importantly, the C stored in CWD was more than four The stocks of diameters D3 and D5 were higher than times higher in the highest forest (S5) than in the lowest that of the finer WD in S1. However, a slight difference forest (S2) (Fig. 6a). The C stock ranged from 0.30 to in WD stock was observed between diameter sizes at the − 1 1.17 Mg∙ha for FWD (Fig. 6b) and 0.18 to 0.55 S2 stage (P> 0.05; Fig. 4). Meanwhile, the WD stock of − 1 Mg∙ha for NWD (Fig. 6c), respectively, and showed D3, D4, and D5 diameters varied significantly with suc- the opposite tendency to CWD. C stored in FWD and cessional stages (Fig. 4). Furthermore, the WD stock of NWD (P< 0.05) was lower in the S6 stage than in the diameter D5 at S5 or S6 stage was obviously higher than S1 stage, but this difference was nonsignificant for FWD this of the earlier succession stands (P< 0.05), and the (P> 0.05; Figs. 6b and c). Besides, the CVs of the C highest WD stocks of diameters D4 and D3 were ob- stocks varied significantly with the different components served in the mid-successional forests S3 and S4, re- of plant debris. The CV of the FWD C stock was signifi- spectively (P< 0.05; Fig. 4). Additionally, an interaction cantly higher than the CV of the CWD and NWD stocks effect was observed among successional series, decay in the six successional stages (P< 0.05; Fig. 6d). classes, and diameter classes for the stock measured in Table 3 Multivariate analysis of variance (MANOVA) for the effects of succession series, decay classes and diameter sizes on the stocks of woody debris and carbon Source variance df Stock of woody debris C stock of woody debris F-value P-value F-value P-value *** *** Succession series (S) 5 5.377 < 0.001 7.149 < 0.001 Decay classes (DC) 4 0.542 0.705 1.564 0.185 *** *** Diameter sizes (D) 4 25.973 < 0.001 27.185 < 0.001 *** *** S × DC × D 5 3.576 < 0.001 4.072 < 0.001 *** Significant effect: P < 0.001 Wang et al. Forest Ecosystems (2021) 8:40 Page 8 of 14 Fig. 3 Stock of woody debris in different decay classes among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Different lowercase letters mean significant difference among different decay classes (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Both succession stage (F = 7.147, P< 0.001) and diam- significantly at different diameter sizes among the six eter class (F = 27.185, P < 0.001) had significant effects plant communities, which was partly consistent with our on the C concentration of WD, but decay class had a second hypothesis. Additionally, our results still demon- slight effect on the C stock (F = 1.564, P> 0.05). Further- strated a global tendency of increase from the S1 to S6 more, the C stock of WD was significantly influenced by stands except for the decrease observed in S4 medium- the interaction among succession age, diameter size, and aged earlier coniferous forest, which partly agreed with decay class (F = 4.072, P< 0.001; Table 3). the third hypothesis. This result confirmed that CWD changed greatly with forest succession and demonstrated that CWD dominated the plant debris in natural forests Discussion and that the ratios and components of plant debris fluctu- Our results indicated that plant debris stocks increased ated sharply with forest successional series. from the earlier successional stage to the later successional stage except for decreasing at the S4 successional stage in the subalpine forest region, which partly supported our Changes in WD and NWD stocks with forest succession first hypothesis. Meanwhile, the results showed that the Forest succession is an important factor that affects the WD stock varied slightly at different decay classes but stocks of CWD or litter in different forest ecosystems Fig. 4 Stock of woody debris in different diameter sizes among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. D1–D5 indicated woody debris in diameter of 2 cm ≤ d < 5 cm, 5 cm ≤ d < 10 cm, 10 cm ≤ d < 20 cm, 20 cm ≤ d < 40 cm, d ≥ 40 cm, respectively. Different lowercase letters mean significant difference among different diameter sizes (P < 0.05). Different uppercase letters mean significant difference among different succession stages (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 9 of 14 Table 4 The carbon stock of plant debris across a successional gradient in the subalpine forest on eastern Qinghai-Tibet Plateau Successional series S1 S2 S3 S4 S5 S6 −1 WD (Mg∙ha ) 14.97c 10.00c 23.73bc 17.37c 38.69a 33.14ab −1 NWD (Mg∙ha ) 0.55a 0.29bc 0.35bc 0.45ab 0.18c 0.34bc −1 Plant debris (Mg∙ha ) 15.53c 10.30c 24.07bc 17.82c 38.87a 33.48ab Carbon stock is shown as means (n = 3). One-way ANOVA tests were applied six successional series in WD, NWD, and Plant debris. Values followed by different lowercase letters mean significant difference among six successional series at P = 0.05 level. WD Woody debris, NWD Non-woody debris, C Carbon (Idol et al. 2001; Sefidi 2010; Zhang et al. 2011; Aryal A greater amount of CWD was found in earlier- or et al. 2015). However, the tendency of the plant debris older-growth forests than in other stands (presenting a biomass and C stocks, particularly in CWD components, ‘U-shaped’ pattern) (Eaton and Lawrence 2006; Sefidi differed greatly from those in previous reports (Table 5). 2010). A monotonic increase in CWD amount or C Fig. 5 Carbon stock of plant debris component (a) and their allocation (b) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Different lowercase letters indicate that the total carbon stock of plant debris differed significantly among successional gradient (P < 0.05). Horizontal bars indicate standard errors of means (n =3) Wang et al. Forest Ecosystems (2021) 8:40 Page 10 of 14 Fig. 6 Carbon stock of coarse woody debris CWD (a), fine woody debris FWD (b) and non-woody debris NWD (c) in six plant communities among different stages of succession in a subalpine forest in Wanglang National Nature Reserve on eastern Qinghai-Tibet Plateau. Figure d indicates the coefficients of variation (CV) of carbon stock of plant debris, CWD, FWD and NWD at different succession ages. Different lowercase letters mean significant difference among different succession stages or different plant debris components (P < 0.05). Horizontal bars indicate standard errors of means (n =3) − 1 − 1 Table 5 Comparisons of plant debris stock (Mg∙ha ) and plant debris carbon stock (Mg∙ha ) among different regions Area Earlier succession Mid-succession Later succession Tendency Reference CWD Carbon CWD Carbon CWD Carbon Subalpine forests of western Sichuan 25.73 11.40 50.45 23.24 63.98 29.10 Increasing This study Primary temperate forests of southern 18.00 – 64.50 – 173.75 – Increasing Carmona et al. (2002) Chile Gorazbon forests of Northern Iran 37.05 – 25.95 – 51.25 –‘U-shaped’ Sefidi (2010) 3 −1 3 − 1 3 − 1 (m ∙ha ) (m ∙ha ) (m ∙ha ) Dry tropical forest of Southern Mexico 51.62 – 11.38 – 37.46 –‘U-shaped’ Eaton and Lawrence (2006) Upland oak-hickory forests of southern 137.2 69.0 39.8 19.9 59.0 29.7 Decreasing Idol et al. (2001) Indiana NWD Carbon NWD Carbon NWD Carbon Subalpine forests of western Sichuan 1.01 0.42 0.82 0.35 0.77 0.32 Decreasing This study Semi-evergreen tropical forests of 5.20 2.34 7.10 3.20 6.29 2.83 Increasing than Aryal et al. (2015) south-eastern Mexico decreasing Subtropical forests of eastern China 6.38 0.60 8.15 0.65 11.69 0.75 Increasing Yan et al. (2009) (%) (%) (%) Wang et al. Forest Ecosystems (2021) 8:40 Page 11 of 14 density was found following secondary forest succession The effect of decay class and diameter on WD stock in Chiloe (Carmona et al. 2002) and Chinese forests across the succession series (Tang and Zhou 2005; Zeng et al. 2015; Zhu et al. 2017). Interestingly, decay class did not affect the storage of Idol et al. (2001) have shown a large decrease in volume WD in five successional stages of forest ecosystems in and mass of woody debris from recently harvested to our study (Fig. 3). However, it has been reported that mature stands (Table 5). However, our results suggested decay class IV was the most abundant decay class in a a global increasing curve for CWD with succession ex- Northern Iran forest (Sefidi 2010). Yan et al. (2007) have cept for the sudden decrease observed in the medium- reported that the evergreen broadleaved forests of later aged earlier coniferous forest (S4) (Figs. 2a and 6a). In successional stages have a higher proportion of WD with general, the accumulation of CWD depends not only on decay classes IV and V in the subtropical region of east- plant debris production but also on the rate of decom- ern China. In contrast, Carmona et al. (2002) have re- position (Tritton 1981; Sturtevant et al. 1997). Since ported that CWD at advanced decay levels was more CWD breaks down persistently and slowly, its produc- abundant in earlier-stage stands, while most CWD was tion rates ultimately outpace its decomposition rates. in the intermediate decay classes in older forests. These Therefore, the later succession stages with more above- contradictions can be partly attributed to the difference ground biomass commonly have higher CWD stocks in the vegetation composition. Additionally, all study (Smith et al. 2006), while the earlier successional stands sites were located within a nature reserve with little dis- with less live biomass have lower CWD stocks. Most im- turbance and management practice, which is another portantly, the difference is likely that their analyses were crucial reason for the average distribution of WD at dif- simply based on the data of general successional gradi- ferent decay classes. ents that lacked consideration of earlier coniferous for- In addition to the significant effect of the succession ests. Although the earlier coniferous forest has a higher stage on the stock of plant debris, diameter size influ- succession stage than that of mixed forest, it has a rapid enced the storage of WD (Fig. 4). Our results illustrated growth period for trunk biomass. Meanwhile, the CWD that larger-diameter WD dominated the plant debris as is almost decomposed in the earlier stage, and there is the successional stage increased. Our current estimates relatively little WD, especially CWD. were also confirmed by several studies investigating Compared with the biomass and C stocks of CWD, whether CWD was dominant in plant debris across for- the stocks of NWD had an opposite tendency (Figs. 2c est succession stage (Eaton and Lawrence 2006; Van and 6c). First, we estimated that the biomass and C Mantgem et al. 2009; Sefidi 2010; Yang et al. 2010; Zeng stocks reached a peak in the earlier succession stage (S1) et al. 2015). Tree species at different successional stages and then decreased with the forest succession stage but produced different biomasses of CWD. For instance, tree suddenly increased in the medium stage earlier conifer- species with a cohort of old and large-diameter fir and ous forest (S4). However, in other forest ecosystems spruce coniferous species in an old-growth stand could (Table 5), the production of litter increased through the lead to more biomass of CWD, while most were small succession process (Yan et al. 2009; Zhang et al. 2013) and thin willow shrub and birch in the earlier succes- or increased rapidly and then decreased with the forest sional forest. However, as suggested by other reports, stage (Aryal et al. 2015). The discrepancy of rates of in- the relative contribution of CWD to the plant debris put (aboveground biomass) and output (decomposition) biomass declined with forest succession (Brown and may be one of the possible reasons (Dent et al. 2006; Lugo 1990; Krankina and Harmon 1995; Delaney et al. Wang et al. 2007). The tree species of litter and climate 1998). The difference may be due to different standards of different forest ecosystems also determined the de- of classification of diameter sizes and climate conditions composition rates of litter (Müller-Using and Bartsch used at different study sites. 2009; Berbeco et al. 2012). Second, in contrast to the CWD stock, the higher stock in the young shrub forest Plant debris carbon stock across the succession series (S1) was contributed by shrub twigs. Meanwhile, the Countrywide, the estimated plant debris C stock was − 1 later successional stage of the mid- and mature conifer- 5.88 ± 0.35 Mg C∙ha in a temperate forest (Zhu et al. ous forests (S5 and S6) had lower stocks, which was 2017). In our study, the plant debris C stock ranged − 1 likely related to the lower inputs of needles and the fas- from 10.30 to 38.87 Mg∙ha (Table 3), much higher ter losses of thinner litter, which decelerated the than the average of Chinese forests but similar to some amounts of remains and sped up the rate of results from local forests in China (Yang et al. 2010) and decomposition. southern Indiana (Idol et al. 2001) (Table 5). The higher C stock in our ecosystem is likely a result of natural for- ests with little disturbance and management practices. The source of nationwide data, including many young- Wang et al. Forest Ecosystems (2021) 8:40 Page 12 of 14 growth plantations and areas of excessive harvest, oc- Availability of data and materials All data generated or analyzed during this study are included in this curred in planted forests (Guo et al. 2013) and resulted published article. in a low average C stock value. In addition, the plant debris investigation in our study tended to select forests Declarations with a greater distribution of WD (Pan et al. 2011), Ethics approval and consent to participate which caused the overestimates of plant debris amount Not applicable. and C stock. Consent for publication Not applicable. Conclusions Competing interests The change in the biomass and C stock of WD repre- The authors declare that they have no competing interests. sented a total tendency of increasing from the S1 to S6 stands, while the changes of FWD and NWD were de- Author details School of Life Sciences, Taizhou University, Taizhou 318000, China. creased across the subalpine forest succession series. Wanglang National Nature Reserve Authority, Pingwu 622550, Sichuan, CWD dominated the plant debris regardless of the forest China. successional stages. 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Journal

"Forest Ecosystems"Springer Journals

Published: Jul 8, 2021

Keywords: Coarse woody debris; Fine woody debris; Forest successional series; Later successional stage; Earlier successional stage; Log decay class; Diameter size

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