A preliminary study investigated the associations between vegetation communities along catenary soil gradients in 2015. The severe drought of 2016 in South Africa presented the opportunity to study post-drought savanna vegetation changes. This hillslope transect was surveyed for five successive seasons. The Braun-Blanquet method was used, and the data were analysed by means of the TWINSPAN algorithm, which resulted in the classification of different communities on the crest, sodic site and riparian area. Change in herbaceous and grassy vegetation composition and diversity in the transect is compared between rainfall years, wet and dry seasons, and three different zones (crest, sodic site and riparian areas). Spatial and temporal autocorrelation of the woody component shifted the focus to variance within the graminoid and herbaceous layers. Clear vegetation changes were observed on the crest and the sodic sites, whereas changes in the riparian area were less obvious. In all three habitats, species richness decreased after the drought and did not reach pre-drought levels even after two years. However, plant species diversity was maintained as climax species were replaced by pioneer and sub-climax species. These changes in community structure, which had reverted to systems dominated by climax species by the end of the sampling period, might be an indication of the savanna ecosystem’s resilience to drought conditions.
Although clear vegetation changes were observed in the five successive seasons after the drought, this study showed that the savanna ecosystem is relatively resistant to drought and that human intervention is not needed.
The Earth’s environment is dominated by three great natural components, namely, climate, vegetation and soil. Climate is considered the most important factor influencing the distribution and composition of vegetation on a micro and sub-continental scale (Campbell et al.
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
The savanna regions of South Africa are considered semi-arid, receiving rainfall mostly during the summer months between October and April (Walker et al.
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 of the Lowveld, below average rainfall occurred at annual (255 mm) and monthly scales (Swemmer
The study site is in the southern parts of KNP south of Skukuza (see study area figure in Theron et al.
The same hillslope transect was surveyed for five seasons; the first survey was conducted prior to the onset of severe drought conditions (Theron et al.
Survey events timeline: First season represented by April 2015; December 2015 and April 2016 no sampling because of lack of vegetation; second season represented by December 2016; third season represented by April 2017; fourth season represented by December 2017; and fifth season represented by April 2018.
The analysis done by Theron et al. (
VegCap (unpublished database tool designed by N. Collins) was used to capture vegetation data into a macro-enabled Excel spreadsheet. From there, the data were imported into JUICE© (Tichý & Holt
In addition to descriptions of community composition and how this changed over time, we evaluated changes in diversity and compared these across time for each of the three communities. We compared changes in species richness as well as changes in alpha-diversity. We used the Chao estimator as an indicator of species richness, as this index accounts for the occurrences of singletons and doubletons, and the Shannon index was used to quantify alpha-diversity. For each sample (i.e. per season and per habitat), ordinal abundance data as scored by the Braun-Blanquet system were converted to abundance cover data, rounded to integer values, following Van der Maarel (
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 m2 neighbouring relevés forming 500 m long belt transect) in the southern KNP. Error bars depict 95% confidence intervals of the richness estimates.
Ethical approval was obtained from the Interfaculty Animal Ethics Committee of the University of the Free State (UFS-AED2019/0121).
Different plant communities were classified for each topographical unit as defined by Theron et al. (
These communities located on the crest zone and upslope beyond the sodic site occur on the Clovelly, Pinedene, Fernwood, Estcourt, Mispha and Sterkspruit soil forms (Theron et al.
Crest community descriptions:
Diagnostic species:
Constant species: Bare soil 77,
Dominant species:
This community mostly represents vegetation sampled during the December 2016 (S2) season. Species from Species Group A (Online Appendix 1) define this community. These species are mostly absent or occur with very low cover-abundance values in the other communities. From a growth-form perspective, it is notable that this community contains the most geophytic plants. There is also a strong presence of species from Species Group B and Species Group I and the ‘pseudo-species’ indicated as bare soil (Species Group J):
Vegetation found in this community represents the sampling during April 2017, which is mostly dominated by species from Species Group C (Online Appendix 1). Again, the species found here do not occur in other communities. Notable is the high cover abundance of species found in this community when compared to that of community 1. Furthermore, species from Species Group B are shared between community 1 and community 2; however,
This community represents sampling seasons 4 and 5 (December 2017 and April 2018), which is more or less one year after rainfall occurred that terminated the 2015–2016 drought. Species from Species Group F distinguishes this community from the other communities. Plants from Species Group H also started to occur in more relevés during these seasons, which might indicate that the veld was starting to improve after the drought conditions:
Diagnostic species:
Constant species:
Dominant species:
Sub-community 3.1 mostly represents vegetation sampled during April 2018 (S5). This sub-community is distinguished by the presence of species from Species Group E, which are either absent from other communities or occur with very low cover-abundance values. When looking at Species Group D, it is clear that the graminoids (
Diagnostic species:
Constant species:
Dominant species:
This sub-community represents crest vegetation during April 2017 (S4). Species Group G distinguishes this sub-community from the
The above community descriptions cannot be directly compared to what was found in 2015 (Theron et al.
The communities occur between the crest and the riparian area on the mid-slope of the hill, and are also sodic sites. Soils are mostly of the Sterkspruit form; however, there were also instances of Mispah soil forms present. The depth varies between 180 mm and 500 mm with an average pHH2O of 6.20–6.43. Soil texture is coarse sandy loam. The vegetation classification resulted in two communities and four sub-communities (Online Appendix 2). In terms of vegetation composition, these communities can be compared to the
Sodic site community descriptions:
Diagnostic species:
Constant species:
Dominant species:
This community is defined by species from Species Group C, which occur here and are absent from other communities or occur with low cover-abundance values.
Diagnostic species:
Constant species:
Dominant species:
The vegetation found in this sub-community mostly represents species from growing season 4 with a single occurrence of season 2. Species from Species Group A (Online Appendix 2) define this sub-community. These species are completely absent or occur with very low cover-abundance values in other communities and sub-community on the sodic site.
Diagnostic species:
Constant species:
Dominant species:
This sub-community is mostly represented by growing season 2 (December 2016) at the onset of the rainy season after the severe drought. Furthermore, this sub-community is defined by the presence of species from Species Group B, which include two geophytic species. There is also a complete absence of the species from Species Group A in this sub-community. Very notable in this sub-community is the almost complete absence of
Diagnostic species:
Constant species:
Dominant species: Bare soil 4
This community is defined by the presence of species from Species Group F. Although some of the species that occur in this Species Group were also present in community 1, they occur with much higher cover-abundance values in community 2:
Diagnostic species:
Constant species:
Dominant species:
Vegetation in this sub-community is mostly from growing season 5 (April 2018) with a single occurrence of vegetation from growing season 3 (April 2017). Although
Diagnostic species:
Constant species:
Dominant species: Bare soil 7
Sub-community 2.2 is defined by the presence of perennial grasses from Species Group E, which are absent from sub-community 2.1. Although having low cover abundances and not occurring in all relevés, this is the only season in which these grass species were found. All three of these grass species (
Species such as
The communities occur between the sodic site on the lower midslope of the hill and the drainage line. Soil forms found in this area include Dundee, Mispah, Bonheim and Sterkspruit. The depth of these soils varies from 100 mm to 600 mm with an average pHH20 of between 6.21–6.73. Soil texture also varies from sandy loam to loamy to sandy clay loam. In contrast to the other terrain units depicted along the catena, the riparian area’s classification did not result in communities that could depict the different seasons of sampling. The vegetation classification resulted in five communities (Online Appendix 3). The vegetation of the riparian communities can be compared to communities 1 (
Riparian area community descriptions:
Diagnostic species:
Constant species: Bare soil 67,
Dominant species:
Diagnostic species:
Constant species:
Dominant species:
Community 2 is defined by the presence of species from Species Group C, which are mostly restricted to this community although they occur with low cover-abundance values. Notable in this community is the strong presence of
Diagnostic species:
Constant species:
Dominant species:
Community 3 is the community with the lowest number of species in all the communities found in the riparian area, and there are no species that clearly distinguish this community from all the other communities in the riparian area. The cover abundance of species in this community is also low, and species do not occur in all the relevés found in this community. It is only the grass
Diagnostic species:
Constant species:
Dominant species:
Vegetation in this community is dominated by species from Species Group E, which are mostly absent from the other communities in the riparian area. Furthermore,
Diagnostic species:
Constant species: Bare soil 62,
Dominant species: Bare soil 8
This is the only community that is solely represented by vegetation sampled during sampling season 4. The vegetation is mostly dominated by the presence of species from Species Group F, which is mostly absent or occurs with low cover-abundance values in other communities of the riparian area. The grasses
Although there is no distinction to be made between the sampling seasons in the riparian area of the study site, there are differences in the vegetation composition over the study period. When comparing the vegetation of the riparian area with communities 1 and 2 (Theron et al.
From
Bar chart representing the species richness (Chao estimate) and diversity (Shannon index) for the different sampling seasons at the different topographical units. The height of each column represents the mean, and the error bar represents the upper portion of the 95% confidence interval.
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 plant functional groups. Indeed, in all three habitats, the proportional representation of plant functional groups differed between 2015 and 2016, with climax and subclimax species being replaced by pioneers, perennials, annuals and – in some cases, especially in the sodic habitat – bare soil (
Graphs showing the different percentage covers of different growth forms ([a] crest, [b], sodic site and [c] riparian) during the different sampling seasons.
With this study, we aimed to determine how savanna plant communities along a catenal gradient changed over time following a severe drought. The catenal gradient studied could be divided into three plant communities – crest and midslope with the highest diversity; sodic site, and riparian areas. The crest and sodic sites further showed a definite change in species composition among the different sampling seasons. There was also an association between April sampling seasons for the crest as well as associations between the December and April sampling sites for the sodic site. Vegetation in the riparian section of the study revealed no clear distinction between different sampling seasons or any correlation between April and December. In a study by Scholes (
Previous studies have indicated that the physical and chemical properties of soils would affect grass mortality rates during drought conditions (Khomo & Rogers
Definite changes in plant community composition were seen in the crest, midslope and sodic sites during the different sampling seasons. Shifts were also seen in terms of species composition at certain times of the year. This was not always clear in terms of richness and diversity of plant species. We would, however, be cautious to extrapolate these findings to all vegetation successions along a catena.
In the riparian area, no distinctions were clear between the different sampling seasons and no cyclic correspondence was observed between April and December. This phenomenon might be ascribed to water movement through the process of hydraulic lift from deeper soil layers which lessen the impact of drought on the vegetation.
We recommend that future studies following droughts should be done over more sampling seasons than reported here to better relate seasons to plant assemblages. Lastly, the recovery of the plant growth forms from 2015 to 2018 might be an indication of the resilience of the savanna ecosystem, in spite of the recovery not being complete.
The authors thank the South African National Parks for providing them with access to the research sites within Kruger National Park. A special thanks goes to the field rangers who accompanied them during the surveys. The authors also thank Louis Scott and Leslie Brown for suggestions on the writing of the manuscript.
The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.
E.J.T. and A.C.v.A. (partially) were responsible for the fieldwork and data collection during field surveys. A.C.v.A. and P.J.d.P. contributed towards the analysis and interpretation of the plant communities. D.C. contributed towards the analysis and interpretation of the statistical elements of the article. All authors contributed to the writing of the manuscript.
The authors are grateful to the University of the Free State (UFS) Strategic Research Fund for partially funding this multidisciplinary research.
Study data are available and may be provided, on request, by the corresponding author. Data from all research done within Kruger National Park is placed within the SANParks repository (not for free, open access).
The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of any affiliated agency of the authors.
, 1960-2019.