Persistence of Riverine Vegetation in a Semi-Arid African Savanna Subjected to Clearing, Floods and Elephants

Published Article.,This study investigated responses of juvenile Acacia karroo, Acacia nilotica and Acacia rehmanniana to simulated herbivory in a semi-arid southern African savanna. For each species, we clipped twigs in the dry season and measured the removed twig biomass. Seven months after clipping, we counted the number of resprouts on clipped stems and measured their lengths, diameters and biomass. We also measured dieback length on clipped stems and dieback biomass (DB). Removed twigs of A. rehmanniana had the greatest biomass, followed by A. karroo and least was A. nilotica. However, A. nilotica had the greatest resprout biomass, followed by A. rehmanniana and least was A. karroo. Consideration of DB as lost biomass resulted in A. karroo fully compensating lost biomass, A. nilotica overcompensating and A. rehmanniana under-compensating. Acacia rehmanniana had the shortest resprouts while A. nilotica had the longest. The number and the diameter of resprouts did not vary among the species. We conclude by predicting that in semi-arid African savannas, undercompensation has the greatest probability of occurrence among woody species prone to severe shoot dieback following herbivory.

IntroductionThe presence of larger trees in semi-arid African savannas creates sub-habitats, which influences on herbaceous plant communities grown under their canopies differently from opened areas. Knowledge of seed banks accumulated in the soils over time beneath larger trees could facilitate the recovery of plant communities that might disappear due to sustained heavy grazing, prolonged fire, or other anthropogenic factors in semi-arid African savannas. However, the impact of larger trees on soil seed bank composition and its similarity with plant communities grown under their canopies are less understood in semi-arid African savannas. Therefore, we studied the effect of leguminous and non-leguminous tree species and their canopies on soil seed bank (SSB) composition and its similarity with understory vegetation (USV) in a semi-arid savanna of Ethiopia.MethodsWe selected 20 matured trees from 3 dominant tree species, representing one leguminous (Acacia robusta Burch) and 2 non-leguminous tree species (Ziziphus spina-Christi and Balanites aegyptiaca (L.) Del), found in isolation, a total of 60 trees for this study. Under each selected individual tree, the species composition of USV were recorded using 1-m2 quadrat in four directions (north, south, east, and west) under the inside and outside tree canopies during the flowering stage. Similarly, soil samples in a 1-m2 quadrat were also collected under the inside tree canopies and their corresponding outside canopies, in each individual tree, for the determination of SSB composition, using a seed emergence method. Then, the soil was thoroughly mixed after removal of all roots and plant fragments, and spread over sand in plastic pots to a depth of 20 mm. The pots were placed at random in a glasshouse, examined every 3 days, for the first 2 months, and thereafter weekly for 6 months. A total of 960 soil samples were used for the determination of SSB composition during this study.ResultsA total of 64 species were emerged from the SSB samples, of which 27 were grasses (19 annual and 8 perennial grasses), 35 annual forbs and 2 woody species. Acacia robusta had a higher seedling density in the SSB compared to other tree species, whereas Z. spina-Christi had higher species diversity in the SSB than other tree species. Moreover, seedling density and species diversity were higher under the inside canopies than outside tree canopies. The mean similarity in species composition between the SSB and USV was low. However, it was higher under the leguminous trees than non-leguminous trees, and under the inside tree canopies than outside canopies.ConclusionsWe found that mature tree species maintained a higher SSB species diversity and abundance under their canopies than the surrounding opened areas. Therefore, conservation of mature dominant tree species is of paramount importance for ecological stability and possible restoration of degraded semi-arid savannas under the changing climate and global warming.

Research Highlight: Journal of Animal Ecology, 00, 1-13. https://doi.org/10.1111/1365-2656.70182. Beyond rising temperatures, several parts of Africa are affected by aridification (more frequent and worsening droughts, lengthening dry seasons). Such drier conditions are likely to affect in several ways not only the many large herbivore species but also the rich carnivore guild that characterise African savannas, with consequences on the behavioural ecology of predator-prey interactions. Using data sets of exceptional quality on the feeding behaviour and the reproduction of leopards and lions covering 4 years of contrasting environmental conditions in a semi-arid African savanna, Balme etal. analysed the effect of drought conditions on the carnivores' diet composition, kill rates, prey biomass acquisition but also cub production and survival. They showed that droughts led to a higher prey biomass consumption for the two carnivore species although the underlying mechanisms differed (higher kill rate for leopards and larger consumed prey for lionesses). Additionally, they revealed that the probability of cub survival was driven by factors other than drought-driven food acquisition (such as intraguild predation by hyaenas for leopards and sarcoptic mange for lions). Balme etal. (2025) convincingly showed that droughts influence not only predator-prey interactions through several pathways, but also carnivore intraguild interactions. Altogether, their findings illustrate the difficulty to predict the impact of drier conditions on carnivore populations if we do not better unravel the mechanisms through which climate change affects both predator-prey and predator-predator interactions. Overall, this inspiring study invites us to conceptualise a larger framework to study interspecific interactions in African mammals in a context of a drier (and hotter) climate.

Mistletoe-infected trees facilitate invasion of the alien shrub Lantana camara in a semi-arid African savanna

Increasing livestock densities and more severe drought events challenge sustainable management in South Africa’s semi-arid savannas. Effective mitigation strategies require accurate assessments of livestock foraging behaviour. By utilising high-throughput sequencing technology, this study evaluated the use of a dual-locus metabarcoding approach (trnL and rbcL) together with study-area-specific reference libraries, to analyse cattle diets in two bioregions of the eastern semi-arid South African savanna. Both markers demonstrated the ability to identify various plant families, but trnL exhibited a higher diversity in terms of family and genus identification at both sampling sites. Forbs, although comprising a diverse component of savanna plant communities, have relatively small above-ground biomass, but can still serve as crucial forage items, especially during dry periods. Our study underscores the significant role of forbs in cattle diets, demonstrating a shift in cattle foraging preferences from grass-based diets to higher inclusions of forbs and woody taxa during the drier season. Although grasses, such as Setaria, were still prevalent, forbs, belonging to the genera Malvastrum, Asparagus, Pollichia and Ipomoea were also important food items for cattle as well as woody taxa belonging to Fabaceae, Combretaceae, Ebenaceae, and Malvaceae with a selection of food items from trees and shrubs from genera Albizia, Combretum, Euclea and Vachellia. Furthermore, our study highlights the value of a dual-locus metabarcoding approach for understanding herbivorous diets. Using trnL and rbcL markers, with study-area-specific reference libraries, improves taxonomic resolution for accurately reconstructing cattle diets in semi-arid savannas. This study may improve biodiversity estimates and inform sustainable rangeland management strategies in semi-arid African savanna ecosystems.

Fire regimes are expected to change with climate change, resulting in a crucial need to understand the specific ways in which variable fire regimes impact important contributors to ecosystem functioning, such as mound-building termites. Termite mounds and fire are both important agents of savanna ecosystem heterogeneity and functioning, but there is little understanding of how they interact across savanna types. We used very high-resolution LiDAR remote sensing to measure the size and distribution of termite mounds across approximately 1300 ha of experimental burn plots in four South African savanna landscapes representing a wide range of fire treatments differing in seasonality and frequency of burning. In nutrient-poor granitic savannas, fire had no impact on termite mound size, densities and spatial distributions. In nutrient-rich basaltic savannas with high mammalian herbivore abundance and intermediate rainfall, very frequent fires caused a decrease in termite mound size, whereas in arid nutrient-rich basaltic savannas, fires that occurred at intermediate frequencies and in transitional seasons (i.e. late dry season and late wet season) decreased the degree of spatial overdispersal exhibited by mounds. Overall, our results suggest that termite mounds are resistant to variation in fire seasonality and frequency, likely indicating that ecosystem services provided by mound-building termites will be unaffected by changing fire regimes. However, consideration of changes to termite mound size and distribution could be necessary for land managers in specific savanna types, such as nutrient-rich soils with high mammalian herbivore abundance.

Fire pattern is predicted to vary across an African savanna in accordance with spatial variation in rainfall through its effects on fuel production, vegetation type (on account of differences in fuel load and in flammability), and distribution of herbivores (because of their effects on fuel load). These predictions were examined for the 23 651-ha Mkuzi Game Reserve, KwaZulu-Natal, based on a 37-year data set. Fire return period varied from no occurrence to a fire every 1.76 years. Approximately 75% of the reserve experienced a fire approximately every 5 years, 25% every 4.1–2.2 years and less than 1% every 2 years on average. Fire return period decreased in relation to an increase in mean annual rainfall. For terrestrial vegetation types, median fire return periods decreased with increasing herbaceous biomass, from forest that did not burn to grasslands that burnt every 2.64 years. Fire was absent from some permanent wetlands but seasonal wetlands burnt every 5.29 years. Grazer biomass above 0.5 animal units ha–1 had a limiting influence on the maximum fire frequency of fire-prone vegetation types. The primary determinant of long-term spatial fire patterns is thus fuel load as determined by mean rainfall, vegetation type, and the effects of grazing herbivores.

Important drivers of woody cover in African savannas are rainfall, soil and nutrients, and disturbance factors, such as fire and herbivory. However, very little is known about how these drivers influence woody cover at specific height classes. The main aim of this study was to identify which of these drivers are the best predictors of woody cover at three height classes: shrub (0.75–2.5 m), brush (2.5–5.5 m) and tree (>5.5 m). Percent woody cover estimates were collected using a rapid monitoring technique over 1 700 sites in Kruger National Park, South Africa. Geology (basalt and granite), mean annual rainfall, fire frequency and elephant density were analysed as potential drivers of woody cover. Results indicate that mean annual rainfall was negatively associated with shrub cover, fire frequency was negatively associated with brush cover, and elephant density was negatively associated with tree cover. Patterns of woody cover show that while geology influences the spatial distribution of horizontal woody cover, variation in vertical vegetation structure is created and maintained by top-down disturbance. This provides the first documentation of drivers associated with structurally differentiated woody cover at regional scales. Future studies on woody cover in disturbance-driven environments should not ignore vegetation structure.

Large African herbivores decrease herbaceous plant biomass while increasing plant species richness in a semi-arid savanna toposequence

Habitat suitability of Vachellia stuhlmannii and Vachellia tortilis was evaluated using MaxEnt. Location data was collected, and nineteen bioclimatic variables data downloaded from the WorldClim database. Soils were analysed to investigate their influence on habitat suitability of the two species. MaxEnt effectively predicted current habitat suitability with an average test Area Under Curve value of 0.936 for V. stuhlmannii and 0.689 for V. tortilis. Key influential variables were BIO-1 (Annual mean temperature) with 71.7% highest gain for V. stuhlmannii and BIO-17 (Precipitation of driest quarter) with 65.3% highest gain for V. tortilis. However, BIO-14 (Precipitation of driest month) exhibited limited influence in predicting habitat suitability. V. stuhlmannii is estimated to cover an area of ~3 255.8 hectares, while V. tortilis is estimated to cover ~11 180.7 hectares. Both species predominantly occur on Prismacutanic/pedocutanic B-horizons. V. tortilis also thrive on Glenrosa and Mispah soils, which are crucial for diverse plant and animal life. These findings have practical implications for conservation efforts aimed at protecting these species. Identifying suitable habitats and preserving soil types is crucial as soil affects microclimate conditions and influences moisture retention. Overall, this study provides insights for the conservation of V. stuhlmannii and V. tortilis in the semi-arid African savanna.

Question: Can satellite time series be used to identify tree and grass green-up dates in a semi-arid savanna system, and are there predictable environmental cues for green-up for each life form? Location: Acacia nigrescens/Combretum apiculatum savanna, Kruger National Park, South Africa (25° S, 31° E). Methods: Remotely-sensed data from the MODIS sensor were used to provide a five year record of greenness (NDVI) between 2000 and 2005. The seasonal and inter-annual patterns of leaf display of trees and grasses were described, using additional ecological information to separate the greening signal of each life form from the satellite time series. Linking this data to daily meteorological and soil moisture data allowed the cues responsible for leaf flush in trees and grasses to be identified and a predictive model of savanna leaf-out was developed. This was tested on a 22-year NDVI dataset from the Advanced Very High Resolution Radiometer. A day length cue for tree green-up predicted 86% of the gree...

Loss of biodiversity caused by impact of elephants (Loxodonta africana) on African woodlands may require a management response, but any action should be based on an understanding of why elephants choose to utilise trees destructively. Comprehension of elephant feeding behaviour requires consideration of the relative value of the plant groups they may potentially consume. Profitability of available food is partly determined by the time to locate a food patch and, therefore, as a foundation for understanding the influence of food availability on diet selection, key controls on the density of grass, forb, and browse patches were investigated across space and time in a semi-arid African savanna. Density of food patches changed seasonally because plant life-forms required different volumes of soil water to produce green forage; and woody plants and forbs responded to long-term changes in soil moisture, while grasses responded to short-term moisture pulses. Soil texture, structure of woody vegetation and fire added further complexity by altering the soil water thresholds required for production of green forage. Interpolating between regularly-timed, ground-based measurements of food density by using modelled soil water as the predictor in regression equations may be a feasible method of quantifying food available to elephants in complex savanna environments.

Effect of tree species on understory vegetation, herbaceous biomass and soil nutrients in a semi-arid savanna of Ethiopia

Changes in grass plant populations and temporal soil seed bank dynamics in a semi-arid African savanna: Implications for restoration

Although studies have shown that mammalian herbivores often limit aboveground carbon storage in savannas, their effects on belowground soil carbon storage remain unclear. Using three sets of long-term, large herbivore exclosures with paired controls, we asked how almost two decades of herbivore removal from a semiarid savanna in Laikipia, Kenya affected aboveground (woody and grass) and belowground soil carbon sequestration, and determined the major source (C3 vs. C4 ) of belowground carbon sequestered in soils with and without herbivores present. Large herbivore exclusion, which included a diverse community of grazers, browsers, and mixed-feeding ungulates, resulted in significant increases in grass cover (~22%), woody basal area (~8m2 /ha), and woody canopy cover (31%), translating to a ~8.5t/ha increase in aboveground carbon over two decades. Herbivore exclusion also led to a 54% increase (20.5t/ha) in total soil carbon to 30-cm depth, with ~71% of this derived from C4 grasses (vs. ~76% with herbivores present) despite substantial increases in woody cover. We attribute this continued high contribution of C4 grasses to soil C sequestration to the reduced offtake of grass biomass with herbivore exclusion together with the facilitative influence of open sparse woody canopies (e.g., Acacia spp.) on grass cover and productivity in this semiarid system.