Abstract:
Frequent grazing can establish high forage value grazing lawns supporting high grazer
densities, but can also produce overgrazed grass communities with unpalatable or
low grass basal cover, supporting few grazers. Attempts to create grazing lawns via
concentrated grazing, with a goal to increase grazer numbers, are thus risky without
knowing how environmental conditions influence the likelihood of each outcome. We
collected grass species and trait data from 33 frequently grazed grass communities
across eastern South Africa (28 sites) and the Serengeti National Park, Tanzania (five
sites), covering wide rainfall (336–987
mm year−1) and soil (e.g., 44%–93%
sand) gradients.
We identified four grass growth forms using hierarchical clustering on principal
components analyses of trait data and assessed trait–environment
and growth form–environment
relationships using fourth corner and principal components analyses.
We distinguished two palatable grass growth forms that both attract yet resist grazers
and comprise grazing lawns: (1) “lateral attractors” that spread vegetatively via stolons
and rhizomes, and (2) “tufted attractors” that form isolated tufts and may have alternate
tall growth forms. By contrast, (3) tough, upright, tufted “resisters,” and (4) “avoiders”
with sparse architectures or that grow appressed to the soil surface, are of little
forage value and avoided by grazers. Grazing lawns occurred across a wide range of
conditions, typically comprising lateral attractor grasses in drier, sandy environments,
and tufted attractor grasses in wetter, low-sand
environments. Resisters occurred on clay-rich
soils in mesic areas, while avoiders were widespread but scarce. While grazing
lawns can be established under most conditions, monitoring their composition and
cover is important, as the potential for overgrazing seems as widely relevant. Tufted
attractor-dominated
lawns appear somewhat more vulnerable to degradation than
lateral attractor-dominated
lawns. Increased avoider and resister abundance both reduce
forage value, although resisters may provide better soil protection.
