Search

In savannas, partitioning of below‐ground resources by depth could facilitate tree–grass coexistence and shape vegetation responses to changing rainfall patterns. However, most studies assessing tree versus grass root‐niche partitioning have focused on one or two sites, limiting generalization about how rainfall and soil conditions influence the degree of rooting overlap across environmental gradients.We used two complementary stable isotope techniques to quantify variation (a) in water uptake depths and (b) in fine‐root biomass distributions among dominant trees and grasses at eight semi‐arid savanna sites in Kruger National Park, South Africa. Sites were located on contrasting soil textures (clayey basaltic soils vs. sandy granitic soils) and paired along a gradient of mean annual rainfall.Soil texture predicted variation in mean water uptake depths and fine‐root allocation. While grasses maintained roots close to the surface and consistently used shallow water, trees on sandy soils distributed roots more evenly across soil depths and used deeper soil water, resulting in greater divergence between tree and grass rooting on sandy soils. Mean annual rainfall predicted some variation among sites in tree water uptake depth, but had a weaker influence on fine‐root allocation.Synthesis. Savanna trees overlapped more with shallow‐rooted grasses on clayey soils and were more distinct in their use of deeper soil layers on sandy soils, consistent with expected differences in infiltration and percolation. These differences, which could allow trees to escape grass competition more effectively on sandy soils, may explain observed differences in tree densities and rates of woody encroachment with soil texture. Differences in the degree of root‐niche separation could also drive heterogeneous responses of savanna vegetation to predicted shifts in the frequency and intensity of rainfall. [ABSTRACT FROM AUTHOR]

Savannas are highly heterogeneous tree-grass mixtures, and the structural variation imposed by a discontinuous canopy cover results in spatial variation in soil properties such as plant-available nutrients, temperature and soil moisture. Many savannas are also dominated by large vertebrate herbivores, which impose a different suite of effects on soil properties related to consumption, excretion and physical disturbance., In nutrient-poor, water-limited systems, variation in soil resource availability may play a fundamental role in structuring plant communities, but the relative and interactive effects of key drivers of heterogeneity have received little attention in savanna ecosystems., We investigated the independent and interactive effects of herbivores and tree canopy cover on multiple soil properties up to a depth of 10 cm, including bulk density, soil C pools, total and plant-available nutrients, moisture dynamics and temperature at two contrasting sites in Kruger National Park, South Africa. We analysed samples from locations under and between adult tree canopies and inside and outside long-term herbivore exclosures using a nested design., We found strong positive effects of tree canopies on total and labile pools of soil C and N and negative effects on plant-available P and soil temperature. Tree canopy effects on soil moisture availability were small but clearly detectable. During precipitation events, trees increased moisture inputs on granite soils but decreased moisture inputs on basalt soils compared to open sites., Conversely, we found mixed evidence for herbivore effects on soil chemistry and temperature, but a clearer effect on soil moisture, with areas outside exclosures experiencing higher soil moisture retention, particularly immediately following precipitation events. Soil bulk density, a driver of both nutrient and soil moisture dynamics, was lower under trees and when herbivores were excluded., Synthesis. Our results suggest that variation in tree cover is the dominant biotic driver of C, N and P dynamics in these savanna systems in the top 10 cm of soil, that herbivores primarily affect soil moisture content, and that canopy and herbivore effects tend to be additive rather than synergistic. [ABSTRACT FROM AUTHOR]

Areas of locally intense and frequent grazing, or 'hotspots', are pervasive features in tropical grasslands and savannas. In some ecosystems, hotspot presence is clearly associated with edaphic factors ( e.g., high clay content and elevated soil fertility), such as those that develop in abandoned cattle bomas. Studies in a range of other savanna ecosystems, however, have failed to find intrinsic soil differences between hotspots and the surrounding matrix. Also, it remains unclear to what extent hotspots are associated with specific assemblages of nutrient-rich plant species, as opposed to being a manifestation of intraspecific variation in nutritive quality. We conducted simultaneous studies in Kruger (South Africa) and Serengeti (Tanzania) National Parks to re-evaluate the role of edaphic correlates of hotspot occurrence and to test whether intraspecific variation in plant quality occurs across hotspot-matrix boundaries. We sampled soils and plants in paired hotspot and matrix plots at multiple sites within each ecosystem to test our a priori hypothesis that hotspots would be associated with distinct species assemblages and differences in soil fertility. We found clear hotspot-matrix differences in foliar N, particularly within species, despite finding no differences in any soil or plant-soil variables, including N mineralization potential and mycorrhizal inoculation levels. We found only weak differences in community composition across the boundary, suggesting that intraspecific variation in foliar N rather than species turnover is mainly responsible for the enhanced nutritive value of hotspot vegetation. We propose that grazer-plant interactions may be stronger drivers of hotspot maintenance in these systems than plant-soil interactions. [ABSTRACT FROM AUTHOR]

Abstract: Fire exerts clear direct effects on savannah vegetation dynamics, but the indirect effects of fire, for example via changes in soil fertility, are less clear. In fire-dominated nutrient-poor savannahs, there is little evidence that fire affects plant-available nutrients, but plant performance on soils with contrasting fire histories has not been investigated. A bioassay experiment was conducted in Kruger National Park, South Africa, to test for effects of long-term fire manipulation on the ability of soils to influence tree growth. Combretum hereroense, a common savannah species, was used as a phytometer species in an experiment that combined fire history with N and P fertilization treatments. The results suggested no effect of fire history on a number of seedling traits after seven months of growth. Root mass and shoot:root ratios responded to P addition, with seedlings exhibiting lower allocation to root tissue under elevated P, particularly in the presence of N, suggesting some N and P co-limitation. Overall, it is concluded that there is little evidence that fire degrades or alters soil chemistry in a way that is likely to influence vegetation structure and dynamics in nutrient-poor savannah ecosystems. [Copyright &y& Elsevier]

A significant fraction of the terrestrial biosphere comprises biomes containing tree-grass mixtures. Forecasting vegetation dynamics in these environments requires a thorough understanding of how trees and grasses use and compete for key belowground resources. There is disagreement about the extent to which tree-grass vertical root separation occurs in these ecosystems, how this overlap varies across large-scale environmental gradients, and what these rooting differences imply for water resource availability and tree-grass competition and coexistence. To assess the extent of tree-grass rooting overlap and how tree and grass rooting patterns vary across resource gradients, we examined landscape-level patterns of tree and grass functional rooting depth along a mean annual precipitation (MAP) gradient extending from ~ 450 to ~ 750 mm year -1 in Kruger National Park, South Africa. We used stable isotopes from soil and stem water to make inferences about relative differences in rooting depth between these two functional groups. We found clear differences in rooting depth between grasses and trees across the MAP gradient, with grasses generally exhibiting shallower rooting profiles than trees. We also found that trees tended to become more shallow-rooted as a function of MAP, to the point that trees and grasses largely overlapped in terms of rooting depth at the wettest sites. Our results reconcile previously conflicting evidence for rooting overlap in this system, and have important implications for understanding tree-grass dynamics under altered precipitation scenarios.