Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

Abstract

Spatial heterogeneity in woody cover affects biodiversity and ecosystem function, and may be particularly influential in savanna ecosystems. Browsing and interactions with herbaceous plants can create and maintain heterogeneity in woody cover, but the relative importance of these drivers remains unclear, especially when considered across multiple edaphic contexts. In African savannas, abandoned temporary livestock corrals (bomas) develop into long-term, nutrient-rich ecosystem hotspots with unique vegetation. In central Kenya, abandoned corral sites persist for decades as treeless 'glades' in a wooded matrix. Though glades are treeless, areas between adjacent glades have higher tree densities than the background savanna or areas near isolated glades. The mechanisms maintaining these distinctive woody cover patterns remain unclear. We asked whether browsing or interactions with herbaceous plants help to maintain landscape heterogeneity by differentially impacting young trees in different locations. We planted the mono-dominant tree species (Acacia drepanolobium) in four locations: inside glades, far from glades, at edges of isolated glades and at edges between adjacent glades. Within each location, we assessed the separate and combined effects of herbivore exclusion (caging) and herbaceous plant removal (clearing) on tree survival and growth. Both caging and clearing improved tree survival and growth inside glades. When herbaceous plants were removed, trees inside glades grew more than trees in other locations, suggesting that glade soils were favorable for tree growth. Different types of glade edges (isolated vs. non-isolated) did not have significantly different impacts on tree performance. This represents one of the first field-based experiments testing the separate and interactive effects of browsing, grass competition and edaphic context on savanna tree performance. Our findings suggest that, by excluding trees from otherwise favorable sites, both herbaceous plants and herbivores help to maintain functionally important landscape heterogeneity in African savannas.

Conservation lessons from large‐mammal manipulations in East African savannas: the KLEE, UHURU, and GLADE experiments

“…In the absence of all LMH, plant communities had higher floral diversity and abundance, and were visited by a more diverse and abundant group of pollinators. These more diverse communities, in turn, formed interaction networks that were more generalized, more nested, and were characterized by higher interaction diversity—all properties which tend to stabilize plant–pollinator communities, and make them more robust to the extinction of member species . By feeding on plants, wild LMH suppress plant reproduction (see above), leading to lower floral and pollinator abundance, and more specialized and less robust pollination networks.…”

“…Throughout much of sub‐Saharan Africa, cattle are contained each night in temporary corrals (bomas), and are accompanied by herders while foraging each day. After abandonment (at intervals of weeks to months), bomas develop into ecosystem hotspots characterized by high nutrient concentrations and unique plant community composition . On both the red soil and black cotton ecosystem, abandoned bomas develop into highly productive, treeless “glades” supporting nutrient‐enriched grasses.…”

Lessons on the relationship between livestock husbandry and biodiversity from the Kenya Long-term Exclosure Experiment (KLEE)

“…Through experimental removals of grass around trees, both inside the KLEE experiment and outside, we have shown that grass can have a strong, negative (competitive) effect on trees of all sizes-halving their rate of growth (Riginos and Young 2007;Riginos 2009). At the same time, the absence of browsers (e.g., where cattle have replaced wild herbivores) also enables trees to grow and reproduce more rapidly (Riginos and Young 2007;Maclean et al 2011;Porensky and Veblen 2012). These studies together provide new insights into how livestock grazing affects woody cover.…”

Effects of traditional pastoralism on grasshopper (Caelifera) assemblages in East Africa

“…Bomas eventually develop into treeless grass-dominated glades that have lower local vegetative species richness than the surrounding savannah, but higher vegetative biomass (Western & Dunne, 1979;Young, Patridge & Macrae, 1995;Muchiru, Western & Reid, 2009;Porensky & Veblen, 2012). The species found in the glades are rarely found outside of abandoned bomas, which leads to increased species diversity at the landscape scale (Young, Patridge & Macrae, 1995).…”

“…The dominant early successional species in the glades is the grass Cynodon plectostachyus, which is highly palatable to large herbivores (Veblen & Young, 2010;Veblen, 2012). While livestock initially create the glade, it is maintained by high densities of wildlife that preferentially feed on the grass inside the glades (Young, Patridge & Macrae, 1995;Porensky & Veblen, 2012). Heavy grazing from wildlife favours continued grass dominance, and the high concentration of wildlife dung maintains the glades as nutrient enriched areas with more nutritious grass (Reid & Ellis, 1995;Young, Patridge & Macrae, 1995;Augustine, 2003;van der Waal et al, 2011).…”