Print PageControls over woody plant cover are a central topic of savanna ecology. In many savannas, spatial heterogeneity in woody cover can enhance ecosystem function and increase biodiversity. Herbivores, herbaceous vegetation and soil conditions can all help maintain spatial heterogeneity in woody cover, but the relative importance of these drivers remains unclear.
In the rangelands of eastern and southern Africa, one major source of heterogeneity in woody cover is derived from former livestock corrals. For centuries, pastoralists have used temporary corrals to protect cattle at night against predators and stock raiders. Once abandoned, corral sites develop into nutrient-rich patches of specialized vegetation. In the savanna woodlands of central Kenya, abandoned corral sites persist for decades as treeless, grass-dominated ‘glades’ embedded in a wooded matrix. Though glade interiors are treeless, areas between adjacent glades (<150m apart) have higher densities of small trees than either the background savanna or areas near isolated glades. It is not clear how the distinctive patterns of woody cover associated with glade interiors and glade edges are maintained over the long-term.
We asked whether browsing by large mammals or interactions with herbaceous plants help to maintain long-term landscape heterogeneity by altering the survival or growth of trees inside glades or in glade edges. We planted 560 seedlings and saplings of the mono-dominant tree species (Acacia drepanolobium) in five blocks, each of which included four locations: inside glades, far from glades, edges of isolated glades, and edges between adjacent glades. Within each location, we used a factorial design to assess the separate and combined impacts of caging (excluding browsers) and clearing (removing herbaceous plants) on tree survival and growth.
Results/Conclusions
After 10 months, we found that both clearing and caging had improved the survival and growth of seedlings and saplings inside glades. Moreover, when grasses were cleared, trees inside glades actually had higher basal diameter growth and produced more branches than trees in other locations. In contrast, clearing and caging did not differentially impact tree growth and survival in different glade edge types. Glade density-related patterns may be propagated during a different tree life stage, or may reflect long-term legacies of active corral use. Overall, our findings highlight the importance of both herbivores and herbaceous plants as drivers of woody cover in savannas. Moreover, our findings suggest that both herbaceous plants and herbivores can maintain landscape heterogeneity – in the form of treeless, anthropogenic glades – in this savanna ecosystem.
