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Plant community structure and possible vegetation changes after drought on a granite catena in the Kruger National Park, South Africa

Plant community structure and possible vegetation changes after drought on a granite catena in... KOEDOE - African Protected Area Conservation and Science ISSN: (Online) 2071-0771, (Print) 0075-6458 Page 1 of 11 Review Article Plant community structure and possible vegetation changes aer dr ft ought on a granite catena in the Kruger National Park, South Africa Authors: A preliminary study investigated the associations between vegetation communities along Andri C. van Aardt catenary soil gradients in 2015. The severe drought of 2016 in South Africa presented the Daryl Codron opportunity to study post-drought savanna vegetation changes. This hillslope transect was Ettienne J. Theron 1 surveyed for five successive seasons. The Braun-Blanquet method was used, and the data were Pieter J. du Preez† analysed by means of the TWINSPAN algorithm, which resulted in the classification of Affiliations: different communities on the crest, sodic site and riparian area. Change in herbaceous and Department of Plant grassy vegetation composition and diversity in the transect is compared between rainfall Sciences, Faculty of Natural years, wet and dry seasons, and three different zones (crest, sodic site and riparian areas). and Agricultural Sciences, Spatial and temporal autocorrelation of the woody component shifted the focus to variance University of the Free State, within the graminoid and herbaceous layers. Clear vegetation changes were observed on the Bloemfontein, South Africa crest and the sodic sites, whereas changes in the riparian area were less obvious. In all three Department of Zoology and habitats, species richness decreased after the drought and did not reach pre-drought levels Entomology, Faculty of even after two years. However, plant species diversity was maintained as climax species were Natural and Agricultural replaced by pioneer and sub-climax species. These changes in community structure, which Sciences, University of the had reverted to systems dominated by climax species by the end of the sampling period, might Free State, Bloemfontein, be an indication of the savanna ecosystem’s resilience to drought conditions. South Africa Conservation implications: Although clear vegetation changes were observed in the five Corresponding author: successive seasons after the drought, this study showed that the savanna ecosystem is relatively Andri C. van Aardt, vanaardtac@ufs.ac.za resistant to drought and that human intervention is not needed. Keywords: Drought; Vegetation classification; Savanna; Diversity; Catena. Dates: Received: 04 Sept. 2019 Accepted: 14 Apr. 2020 Published: 29 Oct. 2020 Introduction How to cite this article: The Earth’s environment is dominated by three great natural components, namely, climate, Van Aardt, A.C., Codron, D., vegetation and soil. Climate is considered the most important factor influencing the distribution Theron, E.J. & Du Preez, P.J., and composition of vegetation on a micro and sub-continental scale (Campbell et al. 2008; Furley 2020, ‘Plant community structure and possible 2010; Scholes 1997; Schulze 1997). Vegetation development is controlled largely by light, vegetation changes ae ft r temperature and moisture (Bond, Midgley & Woorward 2003; Schulze 1997). Topography and drought on a granite catena in the chemical and physical compositions of the soil also influence vegetation and, in conjunction the Kruger National Park, with climate, are responsible for the intricate interactions that govern the worldwide distribution South Africa’, Koedoe 62(2), a1585. https://doi.org/ of vegetation (Campbell et al. 2008; Furley 2010; Scholes 1997). Understanding how these 10.4102/koedoe.v62i2.1585 interactions regulate the ecology of plant communities is critical for characterising the impacts of global change on biodiversity at local and regional scales. The savanna biome is unique because it consists of both woody vegetation and a grass layer. Climate and other regulating factors likely affect these two components differently, resulting in spatio-temporal heterogeneity of tree:grass compositions. Severe droughts, for example, may remove trees, leading to negative effects on woody plant diversity (Swemmer 2016; Walker et al. 1987; Zambatis & Biggs 1995). By reducing tree densities, droughts in savanna provide opportunities for drought-adapted flora to thrive, for instance, by promoting seedling recruitment of fast-growing, palatable shrub species and the re-establishment of a grassy layer (Swemmer et al. 2018; Vetter 2009). In this way, drought can help maintain the balance between trees and grasses (Swemmer 2016). Grasses, on the other hand, can take decades to recover their R Read online: ead online: Copyright: © 2020. The Authors. Licensee: AOSIS. This work is licensed under the Creative Commons Attribution License. Sc Scan this QR an this QR Note: Additional supporting information may be found in the online version of this article. Online Appendix 1; Online Appendix 2; and code with your code with your Online Appendix 3. smart phone or smart phone or Note: Special Issue: Connections between abiotic and biotic components of a granite catena ecosystem in Kruger National Park, mobile de mobile device vice sub-edited by Beanelri Janecke and Johan van Tol. t to r o read online. ead online. †, 1960-2019. http://www.koedoe.co.za Open Access Page 2 of 11 Review Article productive potential or might recover comfortably before long-term research is needed to establish baselines for the next drought (Swemmer et al. 2018). The herbaceous monitoring and understanding ecological change (Smit et al. 2013). We describe taxonomic community changes, as well layer thus also regularly experiences negative responses to as testing for shifts in diversity, over two wet and two dry drought (Zambatis & Biggs 1995); however, Abbas, Bond seasons through the drought period and compare these with and Midgley (2019) indicated that grasses can resprout pre-drought conditions (April 2015) described elsewhere vigorously after the onset of rainfall events. In fact, this (Theron, Van Aardt & Du Preez 2020). We focused only on layer usually responds to droughts and other climate changes the herbaceous and grassy components of the vegetation first, primarily because of the shallow depth of root because we were interested in resolving short-term penetration. Upper soil layers are more susceptible to responses in savanna plant resilience to drought. desiccation than the deeper strata penetrated by many woody plants. Furthermore, the extensive root structures of trees Study area increase their access to subterranean reserves of ground water. Shorter term responses of grassy and herbaceous The study site is in the southern parts of KNP south of vegetation were highlighted by Buitenwerf, Swemmer and Skukuza (see study area figure in Theron et al. 2020) at Peel (2011), who showed that dynamics of this savanna 25.111ºS and 31.579ºE. Kruger National Park falls within the component are mainly controlled by interannual changes arid ‘BSh’ (hot semi-arid climate) climate type according to in rainfall. The response of the grass layer to climate is of the Köppen–Geiger classification system (Venter, Scholes & importance for conservation planning and application, Eckhardt 2003). ‘BSh’ is one of the four climate types within because it is an important food source for grazer populations this category. The main features of ‘BSh’ climate are distinct (Staver, Wigley-Coetzee & Botha 2018). seasonal rainfall and temperature variations. Mean annual precipitation in KNP is generally in the range of 650 mm The savanna regions of South Africa are considered semi- annually (Smit et al. 2013). On a local scale, MAP of the arid, receiving rainfall mostly during the summer months Granite Lowveld varies between 450 and 900 mm along the between October and April (Walker et al. 1987). Fluctuations eastern plains and the western escarpment, respectively in annual rainfall, including droughts, are a regular and (eds. Mucina & Rutherford 2006). However, the average recurrent feature of the climate (Rouault & Richard 2003). annual total rainfall as recorded at the Skukuza In more than half of the 80 summer rainfall districts Meteorological Station is 553 mm (Zambatis 2006). The identified by Rouault and Richard (2003), droughts were mean annual temperature in the vicinity of the study area recorded during 1926, 1933, 1945, 1949, 1952 1970, 1983 and varies between 21ºC and 22ºC (Khomo et al. 2011; Scholes, 1992 (Fauchereau et al. 2003; Rouault & Richard 2003; Bond & Eckhardt 2003). This area experiences an insignificant Gommes & Petrassi 1996). Rouault and Richard (2003) and seasonal and diurnal temperature variation with extreme Staver et al. (2018) indicated that the 1982–1983 drought periods of inundation and aridity (Kruger, Makamo & was the worst drought recorded since 1922; however, Shongwe 2002). The study site is underlain by the Nelspruit Swemmer (2016) indicated that the drought of 2015–2016 Suite geological formation and consists of granite and gneiss was the worst drought that the Lowveld experienced in the mostly occurring in the eastern parts of KNP (Alard 2009; past 33 years. In the savanna areas of KwaZulu-Natal, this Smit et al. 2013; Van Zijl & Le Roux 2014). Granite gneiss is drought was shown to be the worst in 50 years by Abbas et widespread in the eastern regions of KNP and results in al. (2019). Research by Hu and Fedorov (2019) indicated that shallow, nutrient-poor soils that vary from grey to red to the drought of 2015–2016 was worse than the droughts of brown in colour (Venter 1990). Descriptions of the different 1982 and 1997. These studies show that, since the 1960s, soil forms found along the catena at the site were provided drought is more often associated with El Niño events; in Figure 2 within the article by Theron et al. (2020). The notably, however, annual rainfall during wet years has also increased since the 1970s. vegetation type at the study site is mostly Granite Lowveld (SVI3), characterised by a ground layer of tall grasses with South African savannas experienced drought conditions during the rainfall seasons of 2014–2015 and 2015–2016. In the Kruger National Park (KNP), and the surrounding areas December 2015 2016 2017 of the Lowveld, below average rainfall occurred at annual (255 mm) and monthly scales (Swemmer 2016). This resulted in devastating effects on vegetation, animal and human April 2015 2016 2017 2018 welfare in certain areas. These years were also marked by unusually high temperatures, resulting in higher evaporation st nd rd th th 1 Severe drought 2 3 4 5 rates, further reducing water availability (Swemmer 2016). Season Season Season Season Season condions The severity of these conditions provided us with the - no vegetaon surveys opportunity to study their effects on short-term responses of vegetation, specifically on the grassy and herbaceous FIGURE 1: Survey events timeline: First season represented by April 2015; component. We conducted a study of seasonal and annual December 2015 and April 2016 no sampling because of lack of vegetation; second season represented by December 2016; third season represented by plant community dynamics along a granitic catenal gradient. April 2017; fourth season represented by December 2017; and fifth season This catena forms part of a research supersite, where represented by April 2018. http://www.koedoe.co.za Open Access Page 3 of 11 Review Article intermittent trees and other woody species (eds. Mucina & frequency thresholds were set at 75, 60 and 50 for the Rutherford 2006). respective diagnostic, constant and dominant species. An asterisk indicates alien invasive species. Methods Diversity and richness Data collection In addition to descriptions of community composition and The same hillslope transect was surveyed for five seasons; the how this changed over time, we evaluated changes in first survey was conducted prior to the onset of severe drought diversity and compared these across time for each of the conditions (Theron et al. 2020) during December 2015 and three communities. We compared changes in species richness April 2016 (Figure 1). The second and fourth surveys represent as well as changes in alpha-diversity. We used the Chao the start of the rainy summer season, while the third and fifth estimator as an indicator of species richness, as this index surveys reflect the end thereof ( Figure 1). Relevés of 10 m accounts for the occurrences of singletons and doubletons, were aligned along a 500 m transect. Cover abundance was and the Shannon index was used to quantify alpha-diversity. recorded per species according to the modified Braun-Blanquet For each sample (i.e. per season and per habitat), ordinal scale (Kent 2012; Kent & Coker 1992; Van der Maarel & abundance data as scored by the Braun-Blanquet system Franklin 2013; Theron et al. 2020). were converted to abundance cover data, rounded to integer values, following Van der Maarel (2007): r = 1; + = 2; 1 = 3; Classification, richness and diversity analysis 2a = 8; 2b = 18; 3 = 38; 4 = 63; 5 = 88. Diversity estimates were computed using the iNext package (Hsieh, Ma & Chao 2016) The analysis done by Theron et al. (2020) indicated that the for R (R Core Team 2015). The iNext function was used for catenal vegetation communities can be divided into crest, extrapolation and prediction of diversity indices based on sodic site and riparian areas. Each of these habitat types rarefaction procedures, with the expected means and contains different plant communities that are bound by standard errors extrapolated from the asymptotes of the different soil forms. Thus, the analysis of data for the seasons fitted accumulation curves (see Figure 2). In all cases, after the drought (December 2016–April 2018) was guided by accumulation curves approached or reached an asymptote, these differentiations. Each topographical unit was thus and observed data represented between 80 and 100% of analysed separately to look at the vegetation composition or extrapolated estimates (in the case of species richness), and change over the period of December 2016–April 2018. During between 94% and 100% of extrapolated estimates (for this study, December samples were regarded as wet seasons, Shannon diversity), depending on the sample. Thus, and April samples were regarded as dry seasons, irrespective sampling effort is considered sufficient for reliable estimations of the delayed effect, because most summer rainfall usually of diversity in these communities. occurred during December. Ethical considerations Classification Ethical approval was obtained from the Interfaculty Animal VegCap (unpublished database tool designed by N. Collins) Ethics Committee of the University of the Free State (UFS- was used to capture vegetation data into a macro-enabled AED2019/0121). Excel spreadsheet. From there, the data were imported into JUICE© (Tichý & Holt 2006) where a Modified TWINSPAN Classification (Role ček et al. 2009) analysis was carried out. Results and discussion Parameters for this analysis included the following: pseudo- Classification species cut level (5); analysis was constrained to a minimum Different plant communities were classified for each group size of 3–54 clusters; and division reached an topographical unit as defined by Theron et al. (2020). In this endpoint if dissimilarity went lower than 0.3 based on article, the data for 2015 were not included in the classification average Sorensen dissimilarity. The resulting clusters were in order to prevent a repetition of information. then arranged within both JUICE© and Excel to form the final vegetation communities. Although all the species were Crest communities (December 2016–April 2018) recorded during the field surveys, woody species were removed from the data in order to look at the change in These communities located on the crest zone and upslope graminoids and herbaceous species after the drought. This beyond the sodic site occur on the Clovelly, Pinedene, follows, for example, Rouault and Richard (2003), who Fernwood, Estcourt, Mispha and Sterkspruit soil forms indicated that trees and other vegetation with extensive (Theron et al. 2020). The soil depth varies from 533 to 620 mm root structures have access to subterranean reserves of deep, with an average pH of 5.95–6.08. Soil texture is mostly H₂O groundwater and will thus not be immediately affected loamy sand to coarse loamy sand (Theron et al. 2020). by the drought. The naming of communities and sub- Vegetation classification resulted in three communities and communities was carried out according to the guidelines two sub-communities that perfectly align with the different presented in Brown et al. (2013). In order to obtain sampling seasons, showing a clear change in vegetation diagnostic, constant and dominant species, we made use of composition since the onset of the rainy season in the Analysis of Columns of a Synoptic Table in JUICE. The December 2016 (Online Appendix 1). Although there are only http://www.koedoe.co.za Open Access Page 4 of 11 Review Article Interpolated Observed Extrapolated a Interpolated Observed Extrapolated Interpolated Observed Extrapolated c Cr 2015 S 2015 R 2015 60 60 60 50 50 50 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 0 1000 2000 3000 0 1000 2000 3000 4000 0 1000 2000 3000 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated Interpolated Observed Extrapolated e Interpolated Observed Extrapolated f Cr Dec. 2016 S Dec. 2016 R Dec. 2016 60 60 60 50 50 40 40 30 30 30 20 20 20 10 10 10 0 0 0 200 400 600 0 150 300 450 600 0 350 700 1050 1400 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated g Interpolated Observed Extrapolated Interpolated Observed Extrapolated h i Cr Apr. 2017 S Apr. 2017 R Apr. 2017 60 60 50 50 50 40 40 40 30 30 30 20 20 10 10 0 0 0 600 1200 1800 2400 0 250 500 750 1000 0 350 700 1050 1400 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated j Interpolated Observed Extrapolated Interpolated Observed Extrapolated k l Cr Dec. 2017 S Dec. 2017 R Dec. 2017 60 60 60 50 50 50 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 0 200 400 600 800 0 200 400 600 800 0 400 800 1200 1600 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated m Interpolated Observed Extrapolated n Interpolated Observed Extrapolated o Cr Apr. 2018 S Apr. 2018 R Apr. 2018 60 60 60 50 50 50 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 0 350 700 1050 1400 0 250 500 750 1000 0 250 500 750 1000 Cover abundance Cover abundance Cover abundance FIGURE 2: Rarefied species accumulation curves for herbaceous vegetation from the three habitats ( Cr = crest, S = sodic, R = riparian) sampled at five time intervals from a single catenal transect (10 m neighbouring relevés forming 500 m long belt transect) in the southern KNP. Error bars depict 95% confidence intervals of the richness estimates. http://www.koedoe.co.za Open Access Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Page 5 of 11 Review Article three communities, the sub-communities of community community 3. A possible explanation for that might be the 3 distinguish between season 4 (S4) (December 2017) and increase in rainfall after the severe drought experienced in season 5 (S5) (April 2018), although their composition was 2015–2016. From Species Group H, it is also clear that the very similar. The vegetation of the crest communities can be grasses Eragrostis cylindriflora, Aristida adscensionis and Melinis compared to community 3 (Vachellia excuvialis–Pogonarthria repens start to establish with average cover-abundance values: squarrosa) from Theron et al. (2020). As indicated in the 3. Aristida congesta subsp. barbicollis–Bulbostylis *barbata ‘Materials and methods’ section, data on the woody species Community were removed as it obscured the focus of this study: Diagnostic species: Aristida congesta s. barbicollis 77.1, 1. Heliotropium ciliatum–Cleome monophylla Community Bulbostylis barbata 79.4 2. Zornia glochidiata–Crotalaria sphaerocarpa subsp. sphaerocarpa Constant species: Aristida congesta s. barbicollis 75, Bare soil 81, Community Bulbostylis barbata 78, Schmidtia pappophoroides 66 3. Aristida congesta subsp. barbicollis–Bulbostylis *barbata Dominant species: None Community 3.1. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– This community represents sampling seasons 4 and 5 Melhania acuminata Sub-community (December 2017 and April 2018), which is more or less one 3.2. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– year after rainfall occurred that terminated the 2015–2016 Schmidtia pappophoroides Sub-community drought. Species from Species Group F distinguishes this community from the other communities. Plants from Species Crest community descriptions: Group H also started to occur in more relevés during these 1. Heliotropium ciliatum – Cleome monophylla Community seasons, which might indicate that the veld was starting to Diagnostic species: Cleome monophylla 84.3, Heliotropium improve after the drought conditions: ciliatum 93.4 3.1. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– Constant species: Bare soil 77, Bulbostylis hispidula 77, Melhania acuminata Sub-community Chlorophytum recurvifolium 62, Cleome monophylla 92, Dipcadi Diagnostic species: Melhania acuminata 79.4, Panicum papillatum 62, Heliotropium ciliatum 100, Kyllinga alba 85, coloratum 78.5 Phyllanthus maderaspatensis 62, Tragus berteronianus 77, Urochloa Constant species: Aristida congesta s. barbicollis 94, Bare soil mosambicensis 85 100, Bulbostylis barbata 81, Eragrostis cylindriflora 69, Hibiscus Dominant species: None micranthus v. micranthus 62, Melhania acuminata 88, Panicum coloratum 75, Panicum maximum 69, Perotis patens 88, This community mostly represents vegetation sampled Pogonarthria squarrosa 94, Tricholaena monachne 75 during the December 2016 (S2) season. Species from Species Dominant species: None Group A (Online Appendix 1) define this community. These species are mostly absent or occur with very low cover- Sub-community 3.1 mostly represents vegetation sampled abundance values in the other communities. From a growth- during April 2018 (S5). This sub-community is distinguished form perspective, it is notable that this community contains by the presence of species from Species Group E, which are the most geophytic plants. There is also a strong presence of either absent from other communities or occur with very low species from Species Group B and Species Group I and the cover-abundance values. When looking at Species Group D, ‘pseudo-species’ indicated as bare soil (Species Group J): it is clear that the graminoids (Pogonarthria squarrosa, 2. Zornia glochidiata–Crotalaria sphaerocarpa subsp. sphaerocarpa Tricholaena monachne, Eragrostis superba and Digitaria eriantha) Community mostly occur during the December sampling seasons (S3 and Diagnostic species: Crotalaria sphaerocarpa s. sphaerocarpa S5). Although these grasses do occur in other communities, it 82.3, Zornia glochidiata 84.8 is with very low cover abundance: Constant species: Aristida congesta s. congesta 79, Crotalaria 3.2. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– sphaerocarpa s. sphaerocarpa 74, Eragrostis superba 63, Pogonarthria Schmidtia pappophoroides Sub-community squarrosa 95, Setaria sphacelata v. sericea 63, Tricholaena monachne Diagnostic species: None 68, Vernonia fastigiata 79, Zornia glochidiata 89 Constant species: Aristida adscensionis 62, Bare soil 62, Bulbostylis Dominant species: None barbata 75, Schmidtia pappophoroides 75, Urochloa panicoides 62 Dominant species: None Vegetation found in this community represents the sampling during April 2017, which is mostly dominated by species from This sub-community represents crest vegetation during Species Group C (Online Appendix 1). Again, the species found April 2017 (S4). Species Group G distinguishes this sub- here do not occur in other communities. Notable is the high community from the Aristida congesta subsp. barbicollis– cover abundance of species found in this community when Bulbostylis *barbata–Melhania acuminate sub-community 3.1. compared to that of community 1. Furthermore, species from Furthermore, the absence of species from Species Group E Species Group B are shared between community 1 and is also very prominent in this sub-community. However, community 2; however, Aristida congesta subsp. congesta occur from Species Group F it is clear that the cover-abundance with much higher cover abundance in community 2 than in http://www.koedoe.co.za Open Access Page 6 of 11 Review Article values of Aristida congesta subsp. barbicollis and Panicum This community is defined by species from Species Group C, maximum decreased from sub-community 3.1 to 3.2. which occur here and are absent from other communities or occur with low cover-abundance values. Cynodon dactylon is A possible explanation might be that during December 2017 (S4; sub-community 3.2), the species only started to establish known as a pioneer grass (Van Oudtshoorn 2018) and Tribilus at the site and favourable environmental conditions such as terrestris is known to occur in disturbed areas (Van Wyk & an increase in rainfall allowed the improvement of cover in Malan 1998). Portulaca *oleracea is a creeping succulent that April 2018 (S5; sub-community 3.1). grows vigorously under warm conditions covering the soil surface (Bromilow 2018). This community was mostly restricted to April 2017 (S2) and April 2018 (S4). Thus, again The above community descriptions cannot be directly as seen in the Crest communities, there are certain species that compared to what was found in 2015 (Theron et al. 2020) show preferences for certain sampling seasons: because of the removal of the woody species which then dominated the community. There are, however, species such 1.1. Tribulus terrestris–Portulaca *oleracea–Urochloa panicoides as Aristida congesta, Tricholaena monachne, Melhania acuminata, Sub-community Panicum maximum and Perotis patens that occur on the site Diagnostic species: Urochloa panicoides 91.7 during most of the sampling seasons. It is, nevertheless, clear Constant species: Alternanthera pungens 62, Bare soil 94, that the grass Pogonarthria squarrosa (Species Group D) only Portulaca oleracea 81, Schkuhria pinnata 100, Sporobolus nitens started to reappear in the vegetation in growing season 3 69, Tribulus terrestris 81, Urochloa panicoides 88 (April 2017), then diminished and reappeared again in Dominant species: None growing season 5 (April 2018). This might indicate that this grass is also restricted to certain sampling seasons and does The vegetation found in this sub-community mostly represents not occur on the crest sites throughout the year. Van species from growing season 4 with a single occurrence of Oudtshoorn (2018) indicated that P. squarrosa is a weak season 2. Species from Species Group A (Online Appendix 2) perennial tufted grass that can grow for two to five seasons. define this sub-community. These species are completely Diminishing of this grass during season 4 (December 2017) absent or occur with very low cover-abundance values in other might therefore still be due to the effects of the drought; communities and sub-community on the sodic site. Urochloa indicating that the drought still affected vegetation composition panicoides, which defines this sub-community, is known as a one year after the onset of the rainy season. pioneer annual tufted grass and will thus only be present for one season (Van Oudtshoorn 2018). In this subcommunity, this Sodic site communities (December 2016–April 2018) grass co-occurs with Sporobolus nitens, which defined the The communities occur between the crest and the riparian area communities found in 2015 before the drought: on the mid-slope of the hill, and are also sodic sites. Soils are 1.2. Tribulus terrestris–Portulaca *oleracea–Heliotropium ciliatum mostly of the Sterkspruit form; however, there were also instances Sub-community of Mispah soil forms present. The depth varies between 180 mm Diagnostic species: None and 500 mm with an average pH of 6.20–6.43. Soil texture is H2O Constant species: Bare soil 100, Cynodon dactylon 75, Gomphrena coarse sandy loam. The vegetation classification resulted in two celosioides 67, Heliotropium ciliatum 92, Ledebouria luteola 67, communities and four sub-communities (Online Appendix 2). In Portulaca oleracea 92, Schkuhria pinnata 100, Tribulus terrestris terms of vegetation composition, these communities can be 100, Urochloa mosambicensis 92 compared to the Dactyloctenium aegyptium–Sporobolus nitens Dominant species: None (community 4) of Theron et al. (2020): 1. Tribulus terrestris–Portulaca *oleracea Community This sub-community is mostly represented by growing a. T ribulus terrestris–Portulaca *oleracea–Urochloa season 2 (December 2016) at the onset of the rainy season panicoides Sub-community after the severe drought. Furthermore, this sub-community is b. T ribulus terrestris–Portulaca *oleracea–Heliotropium defined by the presence of species from Species Group B, ciliatum Sub-community which include two geophytic species. There is also a complete 2. Chloris virgata–Eragrostis cylindriflora Community absence of the species from Species Group A in this sub- a. Chloris vir gata–Eragrostis cylindriflora–Sporobolus community. Very notable in this sub-community is the almost nitens Sub-community complete absence of Sporobolus nitens (Species Group G) and b. Chloris vir gata–Eragrostis cylindriflora–Chloris gayana Dactyloctenium aegyptium (Species Group F), which Sub-community completely dominated the vegetation during 2015 (growing season 1) (Theron et al. 2020): Sodic site community descriptions: 2. Chloris virgata–Eragrostis cylindriflora Community 1. Tribulus terrestris–Portulaca *oleracea Community Diagnostic species: Chloris virgata 83.5 Diagnostic species: Portulaca oleracea 78.5, Tribulus terrestris 89.8 Constant species: Alternanthera pungens 78, Bare soil 85, Chloris Constant species: Bare soil 96, Cynodon dactylon 64, Portulaca virgata 100, Dactyloctenium aegyptium 67, Eragrostis cylindriflora *oleracea 86, Schkuhria pinnata 100, Tribulus terrestris 89, 74, Schkuhria pinnata 96, Sporobolus nitens 93, Urochloa Urochloa mosambicensis 68 mosambicensis 70 Dominant species: None Dominant species: Bare soil 4 http://www.koedoe.co.za Open Access Page 7 of 11 Review Article This community is defined by the presence of species from Riparian area communities (December 2016–April 2018) Species Group F. Although some of the species that occur in this The communities occur between the sodic site on the lower Species Group were also present in community 1, they occur midslope of the hill and the drainage line. Soil forms found with much higher cover-abundance values in community 2: in this area include Dundee, Mispah, Bonheim and Sterkspruit. The depth of these soils varies from 100 mm to 2.1. Chloris virgata–Eragrostis cylindriflora–Sporobolus nitens 600 mm with an average pH of between 6.21–6.73. Soil Sub-community H20 texture also varies from sandy loam to loamy to sandy clay Diagnostic species: None loam. In contrast to the other terrain units depicted along Constant species: Alternanthera pungens 92, Bare soil 100, the catena, the riparian area’s classification did not result Chloris virgata 100, Dactyloctenium aegyptium 62, Gomphrena in communities that could depict the different seasons of celosioides 92, Schkuhria pinnata 92, Sporobolus nitens 100, sampling. The vegetation classification resulted in five Urochloa mosambicensis 92 communities (Online Appendix 3). The vegetation of the Dominant species: None riparian communities can be compared to communities 1 (Panicum maximum–Pupalia lappacea) and 2 (Themeda Vegetation in this sub-community is mostly from growing triandra–Flueggea virosa) from Theron et al.’s (2020) 2015 season 5 (April 2018) with a single occurrence of vegetation from study: growing season 3 (April 2017). Although S. nitens is the diagnostic 1. Eragrostis cylindriflora–Urochloa mosambicensis Community species for this sub-community, the presence of species from 2. Themeda triandra–Panicum maximum Community Species Group D defines this sub-community. These species are 3. Eragrostis superba–Bothriochloa insculpta Community completely absent from sub-community 2.2. Season 5 marks the 4. Eragrostis rigidior–Urochloa mosambicensis Community return of S. nitens (with high cover abundance) and Dactyloctenium 5. Bothriochloa radicans–Eragrostis superba Community aegyptium (with low cover-abundance and only in some relevés) which dominated the communities found on the sodic site by Riparian area community descriptions: Theron et al. (2020) in 2015: 1. Eragrostis cylindriflora–Urochloa mosambicensis Community 2.2. Chloris virgata–Eragrostis cylindriflora–Chloris gayana Diagnostic species: None Sub-community Constant species: Bare soil 67, Eragrostis cylindriflora 83, Diagnostic species: None Panicum maximum 75, Urochloa mosambicensis 83 Constant species: Alternanthera pungens 64, Bare soil 71, Chloris Dominant species: None gayana 71, Chloris virgata 100, Dactyloctenium aegyptium 71, Eragrostis cylindriflora 93, Schkuhria pinnata 100, Sporobolus nitens 86 Eragrostis cylindriflora (Species Group G) and Urochloa mosambicensis (Species Group H) define this community. Dominant species: Bare soil 7 Species from Species Group A are mostly present in community 1 and absent or occur with low cover-abundance Sub-community 2.2 is defined by the presence of perennial value in other communities in the riparian areas. This grasses from Species Group E, which are absent from sub- community represents sampling seasons 2, 4 and 5. It is community 2.1. Although having low cover abundances and notable that none of the relevés done during season 2 (just at not occurring in all relevés, this is the only season in which the onset of the rainy season) is present in this community. these grass species were found. All three of these grass species Community 1 also share a lot of species from Species Group (Chloris gayana, Eragrostis gummiflua and Aristida stipitata) are B with community 2: regarded by Van Oudtshoorn (2018) as sub-climax species, which might indicate that after the third season, the sodic site 2. Themeda triandra–Panicum maximum Community started to recover from the severe drought of 2015–2016. Diagnostic species: None Constant species: Cymbopogon caesius 68, Panicum maximum 95, Species such as Schkuhria pinnata, Urochloa mosambicensis Themeda triandra 95, Urochloa mosambicensis 74 and Chloris virgata occurred on the site through most of the Dominant species: None sampling seasons since 2015. Sporobolus nitens that formed part of the diagnostic species that defined the sodic site Community 2 is defined by the presence of species from communities in 2015 (Theron et al. 2020) only started to Species Group C, which are mostly restricted to this community appear in April 2017 (S3) with low cover-abundance values. although they occur with low cover-abundance values. The high cover-abundance values of this diagnostic species Notable in this community is the strong presence of Themeda only started returning in December 2017 (S4) and increased triandra (Species Group D) and Panicum maximum (Species in April 2018 (S5). From Online Appendix 2, it is clear that Group H), which were also present as diagnostic species certain species on the sodic site are restricted to certain defining the riparian areas in Theron et al. (2020). It seems as if sampling seasons such as April or December. However, it is Themeda triandra is mostly limited to this community with high also clear that the vegetation composition on the sodic cover-abundance values. However, Panicum maximum occurs site changed from December 2016 until April 2018. The throughout all the communities present in the riparian area mentioned changes can possibly be assigned to the recovery throughout all the sampling seasons. This community is also of the site after the drought of 2015–2016. http://www.koedoe.co.za Open Access Page 8 of 11 Review Article mostly represented by sampling seasons 3 and 5 with some water collects (Van Oudtshoorn 2018). A possible explanation instances of sampling season 4: for this might be that after rains, water can remain close to the surface in the vicinity of the riparian area, which 3. Eragrostis superba–Bothriochloa insculpta Community contributes to the additional moisture that is favourable for Diagnostic species: None these grasses. Constant species: Bare soil 100, Bothriochloa insculpta 67, Eragrostis superba 100, Panicum maximum 67, Urochloa Although there is no distinction to be made between the mosambicensis 67 sampling seasons in the riparian area of the study site, there Dominant species: None are differences in the vegetation composition over the study period. When comparing the vegetation of the riparian area Community 3 is the community with the lowest number of with communities 1 and 2 (Theron et al. 2020), it is clear that species in all the communities found in the riparian area, and Panicum maximum, Urochloa mosambicenis and Themeda there are no species that clearly distinguish this community triandra remained an important part of the vegetation from all the other communities in the riparian area. The cover composition over all the different sampling seasons. abundance of species in this community is also low, and species do not occur in all the relevés found in this community. Richness and diversity of plant communities It is only the grass Eragrostis superba (Species Group H), From Figure 3a, it seems as if the species richness decreased known to grow in disturbed areas (Van Oudtshoorn 2018), at all the sites during the drought and subsequently that occurs in all three relevés that make up the community. increased more-or-less progressively through time as the Vegetation in this community mostly represents sampling communities recovered from the drought between 2015 and seasons 2 and 5. The reason for the low number of species the onset of the current sampling period. However, might be that the vegetation still needed to recover after the pre- versus post-drought richness estimates are only drought. significantly different for the sodic and riparian habitats 4. Eragrostis rigidior–Urochloa mosambicensis Community (non-overlapping 95% confidence intervals between Diagnostic species: None groups); variance in estimates for the crest communities is Constant species: Bare soil 100, Eragrostis rigidior 77, Eragrostis high and overlaps with the pre-drought estimate. superba 62, Urochloa mosambicensis 92 Interestingly, however, the recovery in species richness in Dominant species: None Crest Sodic Vegetation in this community is dominated by species from Riparian Species Group E, which are mostly absent from the other communities in the riparian area. Furthermore, Urochloa mosambicensis (Species Group H) also occurs more frequently and with a higher cover abundance in this community. According to Van Oudtshoorn (2018), U. mosambicensis grows in disturbed or overgrazed and trampled areas. The high occurrence of this species in the riparian area might indicate that animals were seeking shade in order to evade the heat of the day during the drought (2015–2016). He further also indicated that Eragrostis rigidior is known to occur in disturbed 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 soil. It is also important to note that most of the relevés Sampling events over me present in this community represent sampling season 2, Crest which was just after the 2015–2016 drought: Sodic Riparian 5. Bothriochloa radicans–Eragrostis superba Community Diagnostic species: None Constant species: Bare soil 62, Bothriochloa radicans 77, Dicoma tomentosa 62, Eragrostis superba 69 Dominant species: Bare soil 8 This is the only community that is solely represented by vegetation sampled during sampling season 4. The vegetation 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 is mostly dominated by the presence of species from Species Sampling events over me Group F, which is mostly absent or occurs with low cover- abundance values in other communities of the riparian area. FIGURE 3: Bar chart representing the species richness (Chao estimate) and diversity (Shannon index) for the different sampling seasons at the different The grasses Bothriochloa radicans and Eragrostis trichophora are topographical units. The height of each column represents the mean, and the known to occur in areas with additional moisture or where error bar represents the upper portion of the 95 % confidence interval. http://www.koedoe.co.za Open Access Diversity Richness Page 9 of 11 Review Article sodic and riparian habitats appeared to slow or even reverse Climax Subclimax Pioneer Perennial by the end of the study period (April 2018), although this Annual Deciduous woody plants Bare soil could be because the final sample was taken in the dry season. Overall, species richness in crest habitats was greater than in both sodic and riparian habitats. Figure 3b represents the changes that took place in diversity over the different sampling seasons. In contrast to richness, species diversity did not differ between pre- and post-drought periods. However, a more cyclic seasonal shift is apparent, in 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 that diversity was often highest in the wet seasons (December Crest samples), compared with both dry season samples (April). Climax Subclimax Pioneer Perennial The sodic and riparian habitats are an exception to this trend, Annual Deciduous woody plants Bare soil because diversity in these areas was low in December 2016, perhaps because of a lag in recovery from the drought. As with species richness, diversity was also consistently greater 80 in the crest, compared with the other two habitats. While these indices of diversity provide some indication about changes in the studied communities, their overall function might be better represented in terms of changes in 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 plant functional groups. Indeed, in all three habitats, the Sodic proportional representation of plant functional groups differed between 2015 and 2016, with climax and subclimax Climax Subclimax Pioneer Perennial species being replaced by pioneers, perennials, annuals and Annual Deciduous woody plants Bare soil – in some cases, especially in the sodic habitat – bare soil (Figure 4). By the end of the sampling period, however, the frequency distribution of functional groups at each habitat was qualitatively similar to pre-drought conditions. General discussion With this study, we aimed to determine how savanna plant 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 communities along a catenal gradient changed over time Riparian following a severe drought. The catenal gradient studied could be divided into three plant communities – crest and FIGURE 4: Graphs showing the different percentage covers of different growth forms ([a] crest, [b], sodic site and [c] riparian) during the different sampling midslope with the highest diversity; sodic site, and riparian seasons. areas. The crest and sodic sites further showed a definite change in species composition among the different sampling is expected that grasses inhabiting the sandy crest and valley seasons. There was also an association between April bottoms would have a higher mortality rate than those sampling seasons for the crest as well as associations inhabiting the clay-rich sodic sites and downslopes. The between the December and April sampling sites for the physical properties of sandy soils would compound the sodic site. Vegetation in the riparian section of the study effects of droughts because they retain less water than do revealed no clear distinction between different sampling clay soils, and also through exasperating water infiltration seasons or any correlation between April and December. In and percolation of any available surface water. The effect of a study by Scholes (1985), he investigated the drought of soil properties was shown to also affect this catena complex 1981–1983 and found that the grasses were more adversely (Theron et al. 2020). This is also comparable to this study affected by the drought than the trees. Although we because most of the grass species dominating the climax excluded data for woody plants from this study, it is clear community (sampling S1; 2015) returned to the vegetation that vegetation changes took place in the ground layer composition of communities during sampling season 3 (graminoids, forbs, herbs and geophytes), especially in the (April 2017). We furthermore found that richness and crest and sodic site communities (see Janecke 2020). diversity declined and that recovery was not complete two years after the drought, especially in the sodic and Previous studies have indicated that the physical and riparian habitats, which have maintained a low level of chemical properties of soils would affect grass mortality species richness throughout the sampling period. These rates during drought conditions (Khomo & Rogers 2005; Khomo et al. 2011; Scholes 1985). Specifically referring to the shifts coincided with changes in functional group characteristics of the study site and its catenary properties, it representation following the drought. http://www.koedoe.co.za Open Access % Cover % Cover % Cover Page 10 of 11 Review Article Kruger National Park is placed within the SANParks repository Conclusion (not for free, open access). Definite changes in plant community composition were seen in the crest, midslope and sodic sites during the different Disclaimer sampling seasons. Shifts were also seen in terms of species The views and opinions expressed in this article are those of composition at certain times of the year. This was not always the authors and do not necessarily reflect the official policy or clear in terms of richness and diversity of plant species. We position of any affiliated agency of the authors. would, however, be cautious to extrapolate these findings to all vegetation successions along a catena. References In the riparian area, no distinctions were clear between the Abbas, H.A., Bond, W.J. & Midgley, J.J., 2019, ‘The worst drought in 50 years in a South different sampling seasons and no cyclic correspondence was African savannah: Limited impact on vegetation’, African Journal of Ecology 57(4), 1–10. https://doi.org/10.1111/aje.12640 observed between April and December. This phenomenon Alard, G.F., 2009, ‘A comparison of grass production and utilization in sodic and crest might be ascribed to water movement through the process of patches on a semi-arid granitic savanna catena in the southern Kruger National Park, South Africa’, MSc thesis, Faculty of Science, University of the Witwatersrand. hydraulic lift from deeper soil layers which lessen the impact Bond, W.J., Midgley, G.F. & Woorward, F.I., 2003, ‘What controls South African of drought on the vegetation. vegetation – Climate or fire?’ South African Journal of Botany 69(1), 79–91. https://doi.org/10.1016/S0254-6299(15)30362-8 Bromilow, C., 2018, Problem plants and alien weeds of Southern Africa, 4th edn., Briza We recommend that future studies following droughts Publications, Pretoria. should be done over more sampling seasons than reported Brown, L.R., Du Preez, P.J., Bezuidenhoudt, H., Bredenkamp, G.J, Mostert, T.H.C. & Collins, N.B., 2013, ‘Guidelines for phytosociological classifications and here to better relate seasons to plant assemblages. Lastly, the descriptions of vegetation in southern Africa’, Koedoe 55(1), Art. #1103, 10. https://doi.org/10.4102/koedoe.v55i1.1103 recovery of the plant growth forms from 2015 to 2018 might Buitenwerf, R., Swemmer, A.M. & Peel, M.J.S., 2011, ‘Long-term dynamics of be an indication of the resilience of the savanna ecosystem, in herbaceous vegetation structure and composition in two African savanna reserves’, Journal of Applied Ecology 48(1), 238–246. https://doi.org/10.1111/ spite of the recovery not being complete. j.1365-2664.2010.01895.x Campbell, N.A., Reece, J.B, Urry, L.A., Cain, M.L., Wasserman, S.A., Minorsky, P.V., 2008, Biology, 8th edn., Pearson Benjamin Cummings, San Francisco, CA. Acknowledgements Fauchereau, N., Trzaska, S., Rouault, M. & Richard, Y., 2003, ‘Rainfall variability and changes in Southern Africa during the 20th century in the global warming context’, The authors thank the South African National Parks for Natural Hazards 29(2), 139–154. https://doi.org/10.1023/A:1023630924100 providing them with access to the research sites within Furley, P., 2010, ‘Tropical savannas: Biomass, plant ecology, and the role of fire and soil on vegetation’, Progress in Physical Geography 34(4), 563–585. https://doi. Kruger National Park. A special thanks goes to the field org/10.1177/0309133310364934 rangers who accompanied them during the surveys. The Gommes, R. & Petrassi, F., 1996, Rainfall variability and drought in sub-Saharan authors also thank Louis Scott and Leslie Brown for Africa, Food and Agriculture Organization of the United Nations, Rome. Hsieh, T.C., Ma, K.H. & Chao, A., 2016, ‘iNext: An R package for rarefaction and suggestions on the writing of the manuscript. extrapolation of species diversity (Hill numbers)’, Methods in Ecology and Evolution 7(12), 1451–1456. https://doi.org/10.1111/2041-210X.12613 Hu, S. & Fedorov, A.V., 2019, ‘The extreme El Niño of 2015–2016: The role of westerly Competing interests and easterly wind bursts, and preconditioning by the failed 2014 event’, Climate Dynamics 48(1), 1–19. https://doi.org/10.1007/s00382-017-3531-2 The authors declare that they have no financial or personal Janecke, B.B., 2020, ‘Vegetation structure and spatial heterogeneity in the Granite relationships that may have inappropriately influenced them Supersite, Kruger National Park’, Koedoe 62(2), a1591. https://doi.org/10.4102/ koedoe.v62i2.1591 in writing this article. Kent, M., 2012, Vegetation description and data analysis: A practical approach , 2nd edn., Wiley-Blackwell Publishers, West Sussex. Kent, M. & Coker, C., 1992, Vegetation description and analysis: A practical approach , Authors’ contributions Bellhaven Press, West Sussex. Khomo, L.M., Hartshorn, A.S., Rogers, K.H. & Chadwick, O.A., 2011, ‘Impact of rainfall E.J.T. and A.C.v.A. (partially) were responsible for the and topography on the distribution of clays and major cations in granitic catenas of fieldwork and data collection during field surveys. A.C.v.A. Southern Africa’, Catena 87(1), 119–128. https://doi.org/10.1016/j.catena.2011. 05.017 and P.J.d.P. contributed towards the analysis and Khomo, L.M. & Rogers, K.H., 2005, ‘Proposed mechanism for the origin of sodic interpretation of the plant communities. D.C. contributed patches in Kruger National Park, South Africa’, African Journal of Ecology 43(1), 29–34. https://doi.org/10.1111/j.1365-2028.2004.00532.x towards the analysis and interpretation of the statistical Kruger, A.C., Makamo, L.B. & Shongwe, S., 2002, ‘An analysis of Skukuza climate data’, elements of the article. All authors contributed to the writing Koedoe 45(1), 1–7. https://doi.org/10.4102/koedoe.v45i1.16 of the manuscript. Mucina, L. & Rutherford, M.C. (eds.), 2006, The vegetation of South Africa, Lesotho and Swaziland, Strelizia 19, South African National Biodiversity Institute, Pretoria. R Core Team, 2015, computer software, R: A language and environment for statistical computing , R Foundation for Statistical Computing, Vienna. Funding information Roleček, J., Lubomír, T., David, Z., & Milan, C., 2009, ‘Modified TWINSPAN classification in which the hierarchy respects cluster heterogeneity’, Journal of Vegetation The authors are grateful to the University of the Free State Science 20(4), 596–602. https://doi.org/10.1111/j.1654-1103.2009.01062.x (UFS) Strategic Research Fund for partially funding this Rouault, M. & Richard, Y., 2003, ‘Intensity and spatial extension of drought in South multidisciplinary research. Africa at different time scales’, Water SA 29(4), 489–500. https://doi.org/10.4314/ wsa.v29i4.5057 Scholes, R.J., 1985, ‘Drought related grass, tree and herbivore mortality in a southern African savanna’, in J.C. Tothill & J.J. Mott (ed s.), Ecology and management of the Data availability world’s savannas, pp. 350–353, Australian Academy of Science, Washington, DC. Scholes, R.J., 1997, ‘Savanna’, in R.M. Cowling, D.M. Richardson & S.M. Pierce (eds.), Study data are available and may be provided, on request, by Vegetation of Southern Africa , pp. 258–277, Cambridge University Press, the corresponding author. Data from all research done within Cambridge. http://www.koedoe.co.za Open Access Page 11 of 11 Review Article Scholes, R.J., Bond, W.J. & Eckhardt, H.C., 2003, ‘Vegetation dynamics in the Kruger Van der Maarel, E. & Franklin, J., 2013, Vegetation ecology , 2nd edn., John Wiley & ecosystem’, in J.T. Du Toit, K.H. Rogers & H.C. Biggs (eds.), The Kruger experience: Sons, Chichester. Ecology and management of Savanna heterogeneity, pp. 242–262, Island Press, Van Oudtshoorn, F., 2018, Guide to grasses of southern Africa, Briza Publications, Pretoria. London. Van Wyk, B. & Malan, S., 1998, Field guide to the wild flowers of the Highveld , Struik Schulze, R.E., 1997, ‘Climate’, in R.M. Cowling, D.M. Richardson & S.M. Pierce (eds.), Nature, Cape Town. Vegetation of Southern Africa , pp. 21–42, Cambridge University Press, Cambridge. Van Zijl, G. & Le Roux, P., 2014, ‘Creating a conceptual hydrological soil response map Smit, I.P.J., Riddell, E.S., Cullum, C. & Petersen, R., 2013, ‘Kruger National Park for the Stevenson Hamilton Research Supersite, Kruger National Park, South research supersites: Establishing long-term research site for cross-disciplinary, Africa’, Water SA 40(2), 331–336. multiscaled learning’, Koedoe 55(1), Art. #1107, 7, https://doi.org/10.4102/ koedoe.v55i1.1107 Venter, F.J., 1990, ‘A classification of land for management planning in the Kruger Staver, A.C., Wigley-Coetsee, C. & Botha, J., 2018, ‘Grazer movements exacerbate grass National Park’, PhD thesis, University of South Africa, Pretoria. declines during drought in an African savanna’, Journal of Ecology 107(1), 1482–1491. Venter, F.J., Scholes, R.J. & Eckhardt, H.C., 2003, ‘The abiotic template and its associated Swemmer, A.M., Bond, W.J., Donaldson, J., Hempson, G.P., Malherbe, J. & Smit, I.P.J., vegetation paern’ tt , in J.T. Du Toit, K.H. Rogers & H.C. Biggs (eds.), The Kruger 2018, ‘The ecology of drought – A workshop report’, South African Journal of experience: Ecology and management of savanna heterogeneity, pp. 83–129, Science 114(9–10) Art.#5098, 3. https://doi.org/10.17159/sajs.2018/5098 Island Press, London. Swemmer, T., 2016, The Lowveld’s worst drought in 33 years? 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National Park , Scientific Services, Kruger National Park, Cape Town. Van der Maarel, E., 2007, ‘Transformation of cover-abundance values for appropriate Zambatis, N. & Biggs, H.C., 1995, ‘Rainfall and temperatures during the 1991/92 numerical treatment – Alternatives to the proposals by Podani’, Journal of Vegetation Science 18(5), 767–770. https://doi.org/10.1111/j.1654-1103.2007. drought in the Kruger National Park’, Koedoe 38(1), 1–16. https://doi.org/10.4102/ tb02592.x koedoe.v38i1.301 http://www.koedoe.co.za Open Access http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png KOEDOE - African Protected Area Conservation and Science Unpaywall

Plant community structure and possible vegetation changes after drought on a granite catena in the Kruger National Park, South Africa

KOEDOE - African Protected Area Conservation and ScienceOct 29, 2020

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KOEDOE - African Protected Area Conservation and Science ISSN: (Online) 2071-0771, (Print) 0075-6458 Page 1 of 11 Review Article Plant community structure and possible vegetation changes aer dr ft ought on a granite catena in the Kruger National Park, South Africa Authors: A preliminary study investigated the associations between vegetation communities along Andri C. van Aardt catenary soil gradients in 2015. The severe drought of 2016 in South Africa presented the Daryl Codron opportunity to study post-drought savanna vegetation changes. This hillslope transect was Ettienne J. Theron 1 surveyed for five successive seasons. The Braun-Blanquet method was used, and the data were Pieter J. du Preez† analysed by means of the TWINSPAN algorithm, which resulted in the classification of Affiliations: different communities on the crest, sodic site and riparian area. Change in herbaceous and Department of Plant grassy vegetation composition and diversity in the transect is compared between rainfall Sciences, Faculty of Natural years, wet and dry seasons, and three different zones (crest, sodic site and riparian areas). and Agricultural Sciences, Spatial and temporal autocorrelation of the woody component shifted the focus to variance University of the Free State, within the graminoid and herbaceous layers. Clear vegetation changes were observed on the Bloemfontein, South Africa crest and the sodic sites, whereas changes in the riparian area were less obvious. In all three Department of Zoology and habitats, species richness decreased after the drought and did not reach pre-drought levels Entomology, Faculty of even after two years. However, plant species diversity was maintained as climax species were Natural and Agricultural replaced by pioneer and sub-climax species. These changes in community structure, which Sciences, University of the had reverted to systems dominated by climax species by the end of the sampling period, might Free State, Bloemfontein, be an indication of the savanna ecosystem’s resilience to drought conditions. South Africa Conservation implications: Although clear vegetation changes were observed in the five Corresponding author: successive seasons after the drought, this study showed that the savanna ecosystem is relatively Andri C. van Aardt, vanaardtac@ufs.ac.za resistant to drought and that human intervention is not needed. Keywords: Drought; Vegetation classification; Savanna; Diversity; Catena. Dates: Received: 04 Sept. 2019 Accepted: 14 Apr. 2020 Published: 29 Oct. 2020 Introduction How to cite this article: The Earth’s environment is dominated by three great natural components, namely, climate, Van Aardt, A.C., Codron, D., vegetation and soil. Climate is considered the most important factor influencing the distribution Theron, E.J. & Du Preez, P.J., and composition of vegetation on a micro and sub-continental scale (Campbell et al. 2008; Furley 2020, ‘Plant community structure and possible 2010; Scholes 1997; Schulze 1997). Vegetation development is controlled largely by light, vegetation changes ae ft r temperature and moisture (Bond, Midgley & Woorward 2003; Schulze 1997). Topography and drought on a granite catena in the chemical and physical compositions of the soil also influence vegetation and, in conjunction the Kruger National Park, with climate, are responsible for the intricate interactions that govern the worldwide distribution South Africa’, Koedoe 62(2), a1585. https://doi.org/ of vegetation (Campbell et al. 2008; Furley 2010; Scholes 1997). Understanding how these 10.4102/koedoe.v62i2.1585 interactions regulate the ecology of plant communities is critical for characterising the impacts of global change on biodiversity at local and regional scales. The savanna biome is unique because it consists of both woody vegetation and a grass layer. Climate and other regulating factors likely affect these two components differently, resulting in spatio-temporal heterogeneity of tree:grass compositions. Severe droughts, for example, may remove trees, leading to negative effects on woody plant diversity (Swemmer 2016; Walker et al. 1987; Zambatis & Biggs 1995). By reducing tree densities, droughts in savanna provide opportunities for drought-adapted flora to thrive, for instance, by promoting seedling recruitment of fast-growing, palatable shrub species and the re-establishment of a grassy layer (Swemmer et al. 2018; Vetter 2009). In this way, drought can help maintain the balance between trees and grasses (Swemmer 2016). Grasses, on the other hand, can take decades to recover their R Read online: ead online: Copyright: © 2020. The Authors. Licensee: AOSIS. This work is licensed under the Creative Commons Attribution License. Sc Scan this QR an this QR Note: Additional supporting information may be found in the online version of this article. Online Appendix 1; Online Appendix 2; and code with your code with your Online Appendix 3. smart phone or smart phone or Note: Special Issue: Connections between abiotic and biotic components of a granite catena ecosystem in Kruger National Park, mobile de mobile device vice sub-edited by Beanelri Janecke and Johan van Tol. t to r o read online. ead online. †, 1960-2019. http://www.koedoe.co.za Open Access Page 2 of 11 Review Article productive potential or might recover comfortably before long-term research is needed to establish baselines for the next drought (Swemmer et al. 2018). The herbaceous monitoring and understanding ecological change (Smit et al. 2013). We describe taxonomic community changes, as well layer thus also regularly experiences negative responses to as testing for shifts in diversity, over two wet and two dry drought (Zambatis & Biggs 1995); however, Abbas, Bond seasons through the drought period and compare these with and Midgley (2019) indicated that grasses can resprout pre-drought conditions (April 2015) described elsewhere vigorously after the onset of rainfall events. In fact, this (Theron, Van Aardt & Du Preez 2020). We focused only on layer usually responds to droughts and other climate changes the herbaceous and grassy components of the vegetation first, primarily because of the shallow depth of root because we were interested in resolving short-term penetration. Upper soil layers are more susceptible to responses in savanna plant resilience to drought. desiccation than the deeper strata penetrated by many woody plants. Furthermore, the extensive root structures of trees Study area increase their access to subterranean reserves of ground water. Shorter term responses of grassy and herbaceous The study site is in the southern parts of KNP south of vegetation were highlighted by Buitenwerf, Swemmer and Skukuza (see study area figure in Theron et al. 2020) at Peel (2011), who showed that dynamics of this savanna 25.111ºS and 31.579ºE. Kruger National Park falls within the component are mainly controlled by interannual changes arid ‘BSh’ (hot semi-arid climate) climate type according to in rainfall. The response of the grass layer to climate is of the Köppen–Geiger classification system (Venter, Scholes & importance for conservation planning and application, Eckhardt 2003). ‘BSh’ is one of the four climate types within because it is an important food source for grazer populations this category. The main features of ‘BSh’ climate are distinct (Staver, Wigley-Coetzee & Botha 2018). seasonal rainfall and temperature variations. Mean annual precipitation in KNP is generally in the range of 650 mm The savanna regions of South Africa are considered semi- annually (Smit et al. 2013). On a local scale, MAP of the arid, receiving rainfall mostly during the summer months Granite Lowveld varies between 450 and 900 mm along the between October and April (Walker et al. 1987). Fluctuations eastern plains and the western escarpment, respectively in annual rainfall, including droughts, are a regular and (eds. Mucina & Rutherford 2006). However, the average recurrent feature of the climate (Rouault & Richard 2003). annual total rainfall as recorded at the Skukuza In more than half of the 80 summer rainfall districts Meteorological Station is 553 mm (Zambatis 2006). The identified by Rouault and Richard (2003), droughts were mean annual temperature in the vicinity of the study area recorded during 1926, 1933, 1945, 1949, 1952 1970, 1983 and varies between 21ºC and 22ºC (Khomo et al. 2011; Scholes, 1992 (Fauchereau et al. 2003; Rouault & Richard 2003; Bond & Eckhardt 2003). This area experiences an insignificant Gommes & Petrassi 1996). Rouault and Richard (2003) and seasonal and diurnal temperature variation with extreme Staver et al. (2018) indicated that the 1982–1983 drought periods of inundation and aridity (Kruger, Makamo & was the worst drought recorded since 1922; however, Shongwe 2002). The study site is underlain by the Nelspruit Swemmer (2016) indicated that the drought of 2015–2016 Suite geological formation and consists of granite and gneiss was the worst drought that the Lowveld experienced in the mostly occurring in the eastern parts of KNP (Alard 2009; past 33 years. In the savanna areas of KwaZulu-Natal, this Smit et al. 2013; Van Zijl & Le Roux 2014). Granite gneiss is drought was shown to be the worst in 50 years by Abbas et widespread in the eastern regions of KNP and results in al. (2019). Research by Hu and Fedorov (2019) indicated that shallow, nutrient-poor soils that vary from grey to red to the drought of 2015–2016 was worse than the droughts of brown in colour (Venter 1990). Descriptions of the different 1982 and 1997. These studies show that, since the 1960s, soil forms found along the catena at the site were provided drought is more often associated with El Niño events; in Figure 2 within the article by Theron et al. (2020). The notably, however, annual rainfall during wet years has also increased since the 1970s. vegetation type at the study site is mostly Granite Lowveld (SVI3), characterised by a ground layer of tall grasses with South African savannas experienced drought conditions during the rainfall seasons of 2014–2015 and 2015–2016. In the Kruger National Park (KNP), and the surrounding areas December 2015 2016 2017 of the Lowveld, below average rainfall occurred at annual (255 mm) and monthly scales (Swemmer 2016). This resulted in devastating effects on vegetation, animal and human April 2015 2016 2017 2018 welfare in certain areas. These years were also marked by unusually high temperatures, resulting in higher evaporation st nd rd th th 1 Severe drought 2 3 4 5 rates, further reducing water availability (Swemmer 2016). Season Season Season Season Season condions The severity of these conditions provided us with the - no vegetaon surveys opportunity to study their effects on short-term responses of vegetation, specifically on the grassy and herbaceous FIGURE 1: Survey events timeline: First season represented by April 2015; component. We conducted a study of seasonal and annual December 2015 and April 2016 no sampling because of lack of vegetation; second season represented by December 2016; third season represented by plant community dynamics along a granitic catenal gradient. April 2017; fourth season represented by December 2017; and fifth season This catena forms part of a research supersite, where represented by April 2018. http://www.koedoe.co.za Open Access Page 3 of 11 Review Article intermittent trees and other woody species (eds. Mucina & frequency thresholds were set at 75, 60 and 50 for the Rutherford 2006). respective diagnostic, constant and dominant species. An asterisk indicates alien invasive species. Methods Diversity and richness Data collection In addition to descriptions of community composition and The same hillslope transect was surveyed for five seasons; the how this changed over time, we evaluated changes in first survey was conducted prior to the onset of severe drought diversity and compared these across time for each of the conditions (Theron et al. 2020) during December 2015 and three communities. We compared changes in species richness April 2016 (Figure 1). The second and fourth surveys represent as well as changes in alpha-diversity. We used the Chao the start of the rainy summer season, while the third and fifth estimator as an indicator of species richness, as this index surveys reflect the end thereof ( Figure 1). Relevés of 10 m accounts for the occurrences of singletons and doubletons, were aligned along a 500 m transect. Cover abundance was and the Shannon index was used to quantify alpha-diversity. recorded per species according to the modified Braun-Blanquet For each sample (i.e. per season and per habitat), ordinal scale (Kent 2012; Kent & Coker 1992; Van der Maarel & abundance data as scored by the Braun-Blanquet system Franklin 2013; Theron et al. 2020). were converted to abundance cover data, rounded to integer values, following Van der Maarel (2007): r = 1; + = 2; 1 = 3; Classification, richness and diversity analysis 2a = 8; 2b = 18; 3 = 38; 4 = 63; 5 = 88. Diversity estimates were computed using the iNext package (Hsieh, Ma & Chao 2016) The analysis done by Theron et al. (2020) indicated that the for R (R Core Team 2015). The iNext function was used for catenal vegetation communities can be divided into crest, extrapolation and prediction of diversity indices based on sodic site and riparian areas. Each of these habitat types rarefaction procedures, with the expected means and contains different plant communities that are bound by standard errors extrapolated from the asymptotes of the different soil forms. Thus, the analysis of data for the seasons fitted accumulation curves (see Figure 2). In all cases, after the drought (December 2016–April 2018) was guided by accumulation curves approached or reached an asymptote, these differentiations. Each topographical unit was thus and observed data represented between 80 and 100% of analysed separately to look at the vegetation composition or extrapolated estimates (in the case of species richness), and change over the period of December 2016–April 2018. During between 94% and 100% of extrapolated estimates (for this study, December samples were regarded as wet seasons, Shannon diversity), depending on the sample. Thus, and April samples were regarded as dry seasons, irrespective sampling effort is considered sufficient for reliable estimations of the delayed effect, because most summer rainfall usually of diversity in these communities. occurred during December. Ethical considerations Classification Ethical approval was obtained from the Interfaculty Animal VegCap (unpublished database tool designed by N. Collins) Ethics Committee of the University of the Free State (UFS- was used to capture vegetation data into a macro-enabled AED2019/0121). Excel spreadsheet. From there, the data were imported into JUICE© (Tichý & Holt 2006) where a Modified TWINSPAN Classification (Role ček et al. 2009) analysis was carried out. Results and discussion Parameters for this analysis included the following: pseudo- Classification species cut level (5); analysis was constrained to a minimum Different plant communities were classified for each group size of 3–54 clusters; and division reached an topographical unit as defined by Theron et al. (2020). In this endpoint if dissimilarity went lower than 0.3 based on article, the data for 2015 were not included in the classification average Sorensen dissimilarity. The resulting clusters were in order to prevent a repetition of information. then arranged within both JUICE© and Excel to form the final vegetation communities. Although all the species were Crest communities (December 2016–April 2018) recorded during the field surveys, woody species were removed from the data in order to look at the change in These communities located on the crest zone and upslope graminoids and herbaceous species after the drought. This beyond the sodic site occur on the Clovelly, Pinedene, follows, for example, Rouault and Richard (2003), who Fernwood, Estcourt, Mispha and Sterkspruit soil forms indicated that trees and other vegetation with extensive (Theron et al. 2020). The soil depth varies from 533 to 620 mm root structures have access to subterranean reserves of deep, with an average pH of 5.95–6.08. Soil texture is mostly H₂O groundwater and will thus not be immediately affected loamy sand to coarse loamy sand (Theron et al. 2020). by the drought. The naming of communities and sub- Vegetation classification resulted in three communities and communities was carried out according to the guidelines two sub-communities that perfectly align with the different presented in Brown et al. (2013). In order to obtain sampling seasons, showing a clear change in vegetation diagnostic, constant and dominant species, we made use of composition since the onset of the rainy season in the Analysis of Columns of a Synoptic Table in JUICE. The December 2016 (Online Appendix 1). Although there are only http://www.koedoe.co.za Open Access Page 4 of 11 Review Article Interpolated Observed Extrapolated a Interpolated Observed Extrapolated Interpolated Observed Extrapolated c Cr 2015 S 2015 R 2015 60 60 60 50 50 50 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 0 1000 2000 3000 0 1000 2000 3000 4000 0 1000 2000 3000 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated Interpolated Observed Extrapolated e Interpolated Observed Extrapolated f Cr Dec. 2016 S Dec. 2016 R Dec. 2016 60 60 60 50 50 40 40 30 30 30 20 20 20 10 10 10 0 0 0 200 400 600 0 150 300 450 600 0 350 700 1050 1400 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated g Interpolated Observed Extrapolated Interpolated Observed Extrapolated h i Cr Apr. 2017 S Apr. 2017 R Apr. 2017 60 60 50 50 50 40 40 40 30 30 30 20 20 10 10 0 0 0 600 1200 1800 2400 0 250 500 750 1000 0 350 700 1050 1400 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated j Interpolated Observed Extrapolated Interpolated Observed Extrapolated k l Cr Dec. 2017 S Dec. 2017 R Dec. 2017 60 60 60 50 50 50 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 0 200 400 600 800 0 200 400 600 800 0 400 800 1200 1600 Cover abundance Cover abundance Cover abundance Interpolated Observed Extrapolated m Interpolated Observed Extrapolated n Interpolated Observed Extrapolated o Cr Apr. 2018 S Apr. 2018 R Apr. 2018 60 60 60 50 50 50 40 40 40 30 30 30 20 20 20 10 10 10 0 0 0 0 350 700 1050 1400 0 250 500 750 1000 0 250 500 750 1000 Cover abundance Cover abundance Cover abundance FIGURE 2: Rarefied species accumulation curves for herbaceous vegetation from the three habitats ( Cr = crest, S = sodic, R = riparian) sampled at five time intervals from a single catenal transect (10 m neighbouring relevés forming 500 m long belt transect) in the southern KNP. Error bars depict 95% confidence intervals of the richness estimates. http://www.koedoe.co.za Open Access Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Richness Page 5 of 11 Review Article three communities, the sub-communities of community community 3. A possible explanation for that might be the 3 distinguish between season 4 (S4) (December 2017) and increase in rainfall after the severe drought experienced in season 5 (S5) (April 2018), although their composition was 2015–2016. From Species Group H, it is also clear that the very similar. The vegetation of the crest communities can be grasses Eragrostis cylindriflora, Aristida adscensionis and Melinis compared to community 3 (Vachellia excuvialis–Pogonarthria repens start to establish with average cover-abundance values: squarrosa) from Theron et al. (2020). As indicated in the 3. Aristida congesta subsp. barbicollis–Bulbostylis *barbata ‘Materials and methods’ section, data on the woody species Community were removed as it obscured the focus of this study: Diagnostic species: Aristida congesta s. barbicollis 77.1, 1. Heliotropium ciliatum–Cleome monophylla Community Bulbostylis barbata 79.4 2. Zornia glochidiata–Crotalaria sphaerocarpa subsp. sphaerocarpa Constant species: Aristida congesta s. barbicollis 75, Bare soil 81, Community Bulbostylis barbata 78, Schmidtia pappophoroides 66 3. Aristida congesta subsp. barbicollis–Bulbostylis *barbata Dominant species: None Community 3.1. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– This community represents sampling seasons 4 and 5 Melhania acuminata Sub-community (December 2017 and April 2018), which is more or less one 3.2. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– year after rainfall occurred that terminated the 2015–2016 Schmidtia pappophoroides Sub-community drought. Species from Species Group F distinguishes this community from the other communities. Plants from Species Crest community descriptions: Group H also started to occur in more relevés during these 1. Heliotropium ciliatum – Cleome monophylla Community seasons, which might indicate that the veld was starting to Diagnostic species: Cleome monophylla 84.3, Heliotropium improve after the drought conditions: ciliatum 93.4 3.1. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– Constant species: Bare soil 77, Bulbostylis hispidula 77, Melhania acuminata Sub-community Chlorophytum recurvifolium 62, Cleome monophylla 92, Dipcadi Diagnostic species: Melhania acuminata 79.4, Panicum papillatum 62, Heliotropium ciliatum 100, Kyllinga alba 85, coloratum 78.5 Phyllanthus maderaspatensis 62, Tragus berteronianus 77, Urochloa Constant species: Aristida congesta s. barbicollis 94, Bare soil mosambicensis 85 100, Bulbostylis barbata 81, Eragrostis cylindriflora 69, Hibiscus Dominant species: None micranthus v. micranthus 62, Melhania acuminata 88, Panicum coloratum 75, Panicum maximum 69, Perotis patens 88, This community mostly represents vegetation sampled Pogonarthria squarrosa 94, Tricholaena monachne 75 during the December 2016 (S2) season. Species from Species Dominant species: None Group A (Online Appendix 1) define this community. These species are mostly absent or occur with very low cover- Sub-community 3.1 mostly represents vegetation sampled abundance values in the other communities. From a growth- during April 2018 (S5). This sub-community is distinguished form perspective, it is notable that this community contains by the presence of species from Species Group E, which are the most geophytic plants. There is also a strong presence of either absent from other communities or occur with very low species from Species Group B and Species Group I and the cover-abundance values. When looking at Species Group D, ‘pseudo-species’ indicated as bare soil (Species Group J): it is clear that the graminoids (Pogonarthria squarrosa, 2. Zornia glochidiata–Crotalaria sphaerocarpa subsp. sphaerocarpa Tricholaena monachne, Eragrostis superba and Digitaria eriantha) Community mostly occur during the December sampling seasons (S3 and Diagnostic species: Crotalaria sphaerocarpa s. sphaerocarpa S5). Although these grasses do occur in other communities, it 82.3, Zornia glochidiata 84.8 is with very low cover abundance: Constant species: Aristida congesta s. congesta 79, Crotalaria 3.2. Aristida congesta subsp. barbicollis–Bulbostylis *barbata– sphaerocarpa s. sphaerocarpa 74, Eragrostis superba 63, Pogonarthria Schmidtia pappophoroides Sub-community squarrosa 95, Setaria sphacelata v. sericea 63, Tricholaena monachne Diagnostic species: None 68, Vernonia fastigiata 79, Zornia glochidiata 89 Constant species: Aristida adscensionis 62, Bare soil 62, Bulbostylis Dominant species: None barbata 75, Schmidtia pappophoroides 75, Urochloa panicoides 62 Dominant species: None Vegetation found in this community represents the sampling during April 2017, which is mostly dominated by species from This sub-community represents crest vegetation during Species Group C (Online Appendix 1). Again, the species found April 2017 (S4). Species Group G distinguishes this sub- here do not occur in other communities. Notable is the high community from the Aristida congesta subsp. barbicollis– cover abundance of species found in this community when Bulbostylis *barbata–Melhania acuminate sub-community 3.1. compared to that of community 1. Furthermore, species from Furthermore, the absence of species from Species Group E Species Group B are shared between community 1 and is also very prominent in this sub-community. However, community 2; however, Aristida congesta subsp. congesta occur from Species Group F it is clear that the cover-abundance with much higher cover abundance in community 2 than in http://www.koedoe.co.za Open Access Page 6 of 11 Review Article values of Aristida congesta subsp. barbicollis and Panicum This community is defined by species from Species Group C, maximum decreased from sub-community 3.1 to 3.2. which occur here and are absent from other communities or occur with low cover-abundance values. Cynodon dactylon is A possible explanation might be that during December 2017 (S4; sub-community 3.2), the species only started to establish known as a pioneer grass (Van Oudtshoorn 2018) and Tribilus at the site and favourable environmental conditions such as terrestris is known to occur in disturbed areas (Van Wyk & an increase in rainfall allowed the improvement of cover in Malan 1998). Portulaca *oleracea is a creeping succulent that April 2018 (S5; sub-community 3.1). grows vigorously under warm conditions covering the soil surface (Bromilow 2018). This community was mostly restricted to April 2017 (S2) and April 2018 (S4). Thus, again The above community descriptions cannot be directly as seen in the Crest communities, there are certain species that compared to what was found in 2015 (Theron et al. 2020) show preferences for certain sampling seasons: because of the removal of the woody species which then dominated the community. There are, however, species such 1.1. Tribulus terrestris–Portulaca *oleracea–Urochloa panicoides as Aristida congesta, Tricholaena monachne, Melhania acuminata, Sub-community Panicum maximum and Perotis patens that occur on the site Diagnostic species: Urochloa panicoides 91.7 during most of the sampling seasons. It is, nevertheless, clear Constant species: Alternanthera pungens 62, Bare soil 94, that the grass Pogonarthria squarrosa (Species Group D) only Portulaca oleracea 81, Schkuhria pinnata 100, Sporobolus nitens started to reappear in the vegetation in growing season 3 69, Tribulus terrestris 81, Urochloa panicoides 88 (April 2017), then diminished and reappeared again in Dominant species: None growing season 5 (April 2018). This might indicate that this grass is also restricted to certain sampling seasons and does The vegetation found in this sub-community mostly represents not occur on the crest sites throughout the year. Van species from growing season 4 with a single occurrence of Oudtshoorn (2018) indicated that P. squarrosa is a weak season 2. Species from Species Group A (Online Appendix 2) perennial tufted grass that can grow for two to five seasons. define this sub-community. These species are completely Diminishing of this grass during season 4 (December 2017) absent or occur with very low cover-abundance values in other might therefore still be due to the effects of the drought; communities and sub-community on the sodic site. Urochloa indicating that the drought still affected vegetation composition panicoides, which defines this sub-community, is known as a one year after the onset of the rainy season. pioneer annual tufted grass and will thus only be present for one season (Van Oudtshoorn 2018). In this subcommunity, this Sodic site communities (December 2016–April 2018) grass co-occurs with Sporobolus nitens, which defined the The communities occur between the crest and the riparian area communities found in 2015 before the drought: on the mid-slope of the hill, and are also sodic sites. Soils are 1.2. Tribulus terrestris–Portulaca *oleracea–Heliotropium ciliatum mostly of the Sterkspruit form; however, there were also instances Sub-community of Mispah soil forms present. The depth varies between 180 mm Diagnostic species: None and 500 mm with an average pH of 6.20–6.43. Soil texture is H2O Constant species: Bare soil 100, Cynodon dactylon 75, Gomphrena coarse sandy loam. The vegetation classification resulted in two celosioides 67, Heliotropium ciliatum 92, Ledebouria luteola 67, communities and four sub-communities (Online Appendix 2). In Portulaca oleracea 92, Schkuhria pinnata 100, Tribulus terrestris terms of vegetation composition, these communities can be 100, Urochloa mosambicensis 92 compared to the Dactyloctenium aegyptium–Sporobolus nitens Dominant species: None (community 4) of Theron et al. (2020): 1. Tribulus terrestris–Portulaca *oleracea Community This sub-community is mostly represented by growing a. T ribulus terrestris–Portulaca *oleracea–Urochloa season 2 (December 2016) at the onset of the rainy season panicoides Sub-community after the severe drought. Furthermore, this sub-community is b. T ribulus terrestris–Portulaca *oleracea–Heliotropium defined by the presence of species from Species Group B, ciliatum Sub-community which include two geophytic species. There is also a complete 2. Chloris virgata–Eragrostis cylindriflora Community absence of the species from Species Group A in this sub- a. Chloris vir gata–Eragrostis cylindriflora–Sporobolus community. Very notable in this sub-community is the almost nitens Sub-community complete absence of Sporobolus nitens (Species Group G) and b. Chloris vir gata–Eragrostis cylindriflora–Chloris gayana Dactyloctenium aegyptium (Species Group F), which Sub-community completely dominated the vegetation during 2015 (growing season 1) (Theron et al. 2020): Sodic site community descriptions: 2. Chloris virgata–Eragrostis cylindriflora Community 1. Tribulus terrestris–Portulaca *oleracea Community Diagnostic species: Chloris virgata 83.5 Diagnostic species: Portulaca oleracea 78.5, Tribulus terrestris 89.8 Constant species: Alternanthera pungens 78, Bare soil 85, Chloris Constant species: Bare soil 96, Cynodon dactylon 64, Portulaca virgata 100, Dactyloctenium aegyptium 67, Eragrostis cylindriflora *oleracea 86, Schkuhria pinnata 100, Tribulus terrestris 89, 74, Schkuhria pinnata 96, Sporobolus nitens 93, Urochloa Urochloa mosambicensis 68 mosambicensis 70 Dominant species: None Dominant species: Bare soil 4 http://www.koedoe.co.za Open Access Page 7 of 11 Review Article This community is defined by the presence of species from Riparian area communities (December 2016–April 2018) Species Group F. Although some of the species that occur in this The communities occur between the sodic site on the lower Species Group were also present in community 1, they occur midslope of the hill and the drainage line. Soil forms found with much higher cover-abundance values in community 2: in this area include Dundee, Mispah, Bonheim and Sterkspruit. The depth of these soils varies from 100 mm to 2.1. Chloris virgata–Eragrostis cylindriflora–Sporobolus nitens 600 mm with an average pH of between 6.21–6.73. Soil Sub-community H20 texture also varies from sandy loam to loamy to sandy clay Diagnostic species: None loam. In contrast to the other terrain units depicted along Constant species: Alternanthera pungens 92, Bare soil 100, the catena, the riparian area’s classification did not result Chloris virgata 100, Dactyloctenium aegyptium 62, Gomphrena in communities that could depict the different seasons of celosioides 92, Schkuhria pinnata 92, Sporobolus nitens 100, sampling. The vegetation classification resulted in five Urochloa mosambicensis 92 communities (Online Appendix 3). The vegetation of the Dominant species: None riparian communities can be compared to communities 1 (Panicum maximum–Pupalia lappacea) and 2 (Themeda Vegetation in this sub-community is mostly from growing triandra–Flueggea virosa) from Theron et al.’s (2020) 2015 season 5 (April 2018) with a single occurrence of vegetation from study: growing season 3 (April 2017). Although S. nitens is the diagnostic 1. Eragrostis cylindriflora–Urochloa mosambicensis Community species for this sub-community, the presence of species from 2. Themeda triandra–Panicum maximum Community Species Group D defines this sub-community. These species are 3. Eragrostis superba–Bothriochloa insculpta Community completely absent from sub-community 2.2. Season 5 marks the 4. Eragrostis rigidior–Urochloa mosambicensis Community return of S. nitens (with high cover abundance) and Dactyloctenium 5. Bothriochloa radicans–Eragrostis superba Community aegyptium (with low cover-abundance and only in some relevés) which dominated the communities found on the sodic site by Riparian area community descriptions: Theron et al. (2020) in 2015: 1. Eragrostis cylindriflora–Urochloa mosambicensis Community 2.2. Chloris virgata–Eragrostis cylindriflora–Chloris gayana Diagnostic species: None Sub-community Constant species: Bare soil 67, Eragrostis cylindriflora 83, Diagnostic species: None Panicum maximum 75, Urochloa mosambicensis 83 Constant species: Alternanthera pungens 64, Bare soil 71, Chloris Dominant species: None gayana 71, Chloris virgata 100, Dactyloctenium aegyptium 71, Eragrostis cylindriflora 93, Schkuhria pinnata 100, Sporobolus nitens 86 Eragrostis cylindriflora (Species Group G) and Urochloa mosambicensis (Species Group H) define this community. Dominant species: Bare soil 7 Species from Species Group A are mostly present in community 1 and absent or occur with low cover-abundance Sub-community 2.2 is defined by the presence of perennial value in other communities in the riparian areas. This grasses from Species Group E, which are absent from sub- community represents sampling seasons 2, 4 and 5. It is community 2.1. Although having low cover abundances and notable that none of the relevés done during season 2 (just at not occurring in all relevés, this is the only season in which the onset of the rainy season) is present in this community. these grass species were found. All three of these grass species Community 1 also share a lot of species from Species Group (Chloris gayana, Eragrostis gummiflua and Aristida stipitata) are B with community 2: regarded by Van Oudtshoorn (2018) as sub-climax species, which might indicate that after the third season, the sodic site 2. Themeda triandra–Panicum maximum Community started to recover from the severe drought of 2015–2016. Diagnostic species: None Constant species: Cymbopogon caesius 68, Panicum maximum 95, Species such as Schkuhria pinnata, Urochloa mosambicensis Themeda triandra 95, Urochloa mosambicensis 74 and Chloris virgata occurred on the site through most of the Dominant species: None sampling seasons since 2015. Sporobolus nitens that formed part of the diagnostic species that defined the sodic site Community 2 is defined by the presence of species from communities in 2015 (Theron et al. 2020) only started to Species Group C, which are mostly restricted to this community appear in April 2017 (S3) with low cover-abundance values. although they occur with low cover-abundance values. The high cover-abundance values of this diagnostic species Notable in this community is the strong presence of Themeda only started returning in December 2017 (S4) and increased triandra (Species Group D) and Panicum maximum (Species in April 2018 (S5). From Online Appendix 2, it is clear that Group H), which were also present as diagnostic species certain species on the sodic site are restricted to certain defining the riparian areas in Theron et al. (2020). It seems as if sampling seasons such as April or December. However, it is Themeda triandra is mostly limited to this community with high also clear that the vegetation composition on the sodic cover-abundance values. However, Panicum maximum occurs site changed from December 2016 until April 2018. The throughout all the communities present in the riparian area mentioned changes can possibly be assigned to the recovery throughout all the sampling seasons. This community is also of the site after the drought of 2015–2016. http://www.koedoe.co.za Open Access Page 8 of 11 Review Article mostly represented by sampling seasons 3 and 5 with some water collects (Van Oudtshoorn 2018). A possible explanation instances of sampling season 4: for this might be that after rains, water can remain close to the surface in the vicinity of the riparian area, which 3. Eragrostis superba–Bothriochloa insculpta Community contributes to the additional moisture that is favourable for Diagnostic species: None these grasses. Constant species: Bare soil 100, Bothriochloa insculpta 67, Eragrostis superba 100, Panicum maximum 67, Urochloa Although there is no distinction to be made between the mosambicensis 67 sampling seasons in the riparian area of the study site, there Dominant species: None are differences in the vegetation composition over the study period. When comparing the vegetation of the riparian area Community 3 is the community with the lowest number of with communities 1 and 2 (Theron et al. 2020), it is clear that species in all the communities found in the riparian area, and Panicum maximum, Urochloa mosambicenis and Themeda there are no species that clearly distinguish this community triandra remained an important part of the vegetation from all the other communities in the riparian area. The cover composition over all the different sampling seasons. abundance of species in this community is also low, and species do not occur in all the relevés found in this community. Richness and diversity of plant communities It is only the grass Eragrostis superba (Species Group H), From Figure 3a, it seems as if the species richness decreased known to grow in disturbed areas (Van Oudtshoorn 2018), at all the sites during the drought and subsequently that occurs in all three relevés that make up the community. increased more-or-less progressively through time as the Vegetation in this community mostly represents sampling communities recovered from the drought between 2015 and seasons 2 and 5. The reason for the low number of species the onset of the current sampling period. However, might be that the vegetation still needed to recover after the pre- versus post-drought richness estimates are only drought. significantly different for the sodic and riparian habitats 4. Eragrostis rigidior–Urochloa mosambicensis Community (non-overlapping 95% confidence intervals between Diagnostic species: None groups); variance in estimates for the crest communities is Constant species: Bare soil 100, Eragrostis rigidior 77, Eragrostis high and overlaps with the pre-drought estimate. superba 62, Urochloa mosambicensis 92 Interestingly, however, the recovery in species richness in Dominant species: None Crest Sodic Vegetation in this community is dominated by species from Riparian Species Group E, which are mostly absent from the other communities in the riparian area. Furthermore, Urochloa mosambicensis (Species Group H) also occurs more frequently and with a higher cover abundance in this community. According to Van Oudtshoorn (2018), U. mosambicensis grows in disturbed or overgrazed and trampled areas. The high occurrence of this species in the riparian area might indicate that animals were seeking shade in order to evade the heat of the day during the drought (2015–2016). He further also indicated that Eragrostis rigidior is known to occur in disturbed 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 soil. It is also important to note that most of the relevés Sampling events over me present in this community represent sampling season 2, Crest which was just after the 2015–2016 drought: Sodic Riparian 5. Bothriochloa radicans–Eragrostis superba Community Diagnostic species: None Constant species: Bare soil 62, Bothriochloa radicans 77, Dicoma tomentosa 62, Eragrostis superba 69 Dominant species: Bare soil 8 This is the only community that is solely represented by vegetation sampled during sampling season 4. The vegetation 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 is mostly dominated by the presence of species from Species Sampling events over me Group F, which is mostly absent or occurs with low cover- abundance values in other communities of the riparian area. FIGURE 3: Bar chart representing the species richness (Chao estimate) and diversity (Shannon index) for the different sampling seasons at the different The grasses Bothriochloa radicans and Eragrostis trichophora are topographical units. The height of each column represents the mean, and the known to occur in areas with additional moisture or where error bar represents the upper portion of the 95 % confidence interval. http://www.koedoe.co.za Open Access Diversity Richness Page 9 of 11 Review Article sodic and riparian habitats appeared to slow or even reverse Climax Subclimax Pioneer Perennial by the end of the study period (April 2018), although this Annual Deciduous woody plants Bare soil could be because the final sample was taken in the dry season. Overall, species richness in crest habitats was greater than in both sodic and riparian habitats. Figure 3b represents the changes that took place in diversity over the different sampling seasons. In contrast to richness, species diversity did not differ between pre- and post-drought periods. However, a more cyclic seasonal shift is apparent, in 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 that diversity was often highest in the wet seasons (December Crest samples), compared with both dry season samples (April). Climax Subclimax Pioneer Perennial The sodic and riparian habitats are an exception to this trend, Annual Deciduous woody plants Bare soil because diversity in these areas was low in December 2016, perhaps because of a lag in recovery from the drought. As with species richness, diversity was also consistently greater 80 in the crest, compared with the other two habitats. While these indices of diversity provide some indication about changes in the studied communities, their overall function might be better represented in terms of changes in 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 plant functional groups. Indeed, in all three habitats, the Sodic proportional representation of plant functional groups differed between 2015 and 2016, with climax and subclimax Climax Subclimax Pioneer Perennial species being replaced by pioneers, perennials, annuals and Annual Deciduous woody plants Bare soil – in some cases, especially in the sodic habitat – bare soil (Figure 4). By the end of the sampling period, however, the frequency distribution of functional groups at each habitat was qualitatively similar to pre-drought conditions. General discussion With this study, we aimed to determine how savanna plant 2015 Dec. 16 Apr. 17 Dec. 17 Apr. 18 communities along a catenal gradient changed over time Riparian following a severe drought. The catenal gradient studied could be divided into three plant communities – crest and FIGURE 4: Graphs showing the different percentage covers of different growth forms ([a] crest, [b], sodic site and [c] riparian) during the different sampling midslope with the highest diversity; sodic site, and riparian seasons. areas. The crest and sodic sites further showed a definite change in species composition among the different sampling is expected that grasses inhabiting the sandy crest and valley seasons. There was also an association between April bottoms would have a higher mortality rate than those sampling seasons for the crest as well as associations inhabiting the clay-rich sodic sites and downslopes. The between the December and April sampling sites for the physical properties of sandy soils would compound the sodic site. Vegetation in the riparian section of the study effects of droughts because they retain less water than do revealed no clear distinction between different sampling clay soils, and also through exasperating water infiltration seasons or any correlation between April and December. In and percolation of any available surface water. The effect of a study by Scholes (1985), he investigated the drought of soil properties was shown to also affect this catena complex 1981–1983 and found that the grasses were more adversely (Theron et al. 2020). This is also comparable to this study affected by the drought than the trees. Although we because most of the grass species dominating the climax excluded data for woody plants from this study, it is clear community (sampling S1; 2015) returned to the vegetation that vegetation changes took place in the ground layer composition of communities during sampling season 3 (graminoids, forbs, herbs and geophytes), especially in the (April 2017). We furthermore found that richness and crest and sodic site communities (see Janecke 2020). diversity declined and that recovery was not complete two years after the drought, especially in the sodic and Previous studies have indicated that the physical and riparian habitats, which have maintained a low level of chemical properties of soils would affect grass mortality species richness throughout the sampling period. These rates during drought conditions (Khomo & Rogers 2005; Khomo et al. 2011; Scholes 1985). Specifically referring to the shifts coincided with changes in functional group characteristics of the study site and its catenary properties, it representation following the drought. http://www.koedoe.co.za Open Access % Cover % Cover % Cover Page 10 of 11 Review Article Kruger National Park is placed within the SANParks repository Conclusion (not for free, open access). Definite changes in plant community composition were seen in the crest, midslope and sodic sites during the different Disclaimer sampling seasons. Shifts were also seen in terms of species The views and opinions expressed in this article are those of composition at certain times of the year. This was not always the authors and do not necessarily reflect the official policy or clear in terms of richness and diversity of plant species. We position of any affiliated agency of the authors. would, however, be cautious to extrapolate these findings to all vegetation successions along a catena. References In the riparian area, no distinctions were clear between the Abbas, H.A., Bond, W.J. & Midgley, J.J., 2019, ‘The worst drought in 50 years in a South different sampling seasons and no cyclic correspondence was African savannah: Limited impact on vegetation’, African Journal of Ecology 57(4), 1–10. https://doi.org/10.1111/aje.12640 observed between April and December. 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