Grazing lawns and overgrazing in frequently grazed grass communities

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
(5 sites), covering wide rainfall (336–987 mm.yr-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 or
resister abundance both reduce forage value, although resisters may
provide better soil protection.

频繁放牧可形成具有高饲用价值的放牧草地（grazing lawns），支撑较高的放牧动物种群密度，但也可能催生过度放牧的草本群落——此类群落草类基盖度低下且适口性不佳，仅能维持少量放牧动物。因此，若未明确环境条件如何影响两种结果的发生概率，通过集中放牧构建放牧草地以提升放牧动物种群数量的尝试便存在风险。
本研究从南非东部（28个样地）与坦桑尼亚塞伦盖蒂国家公园（5个样地）的33个高频放牧草本群落中采集了草本物种与功能性状数据，研究样地覆盖了跨度较大的降雨梯度（336–987 毫米·年⁻¹）与土壤梯度（例如沙粒含量44%–93%）。本研究通过对性状数据开展主成分分析后进行层次聚类，划分出4种草本生长型，并借助四角落法（fourth corner method）与主成分分析，评估了性状-环境以及生长型-环境之间的关联。
我们区分出两种适口性优良的草本生长型，二者均能吸引放牧动物同时耐受采食，共同构成放牧草地：1）‘匍匐型吸引种’，可通过匍匐茎与根状茎进行营养繁殖；2）‘丛生型吸引种’，形成独立的丛生株丛，且可呈现替代型的高大生长型。与之相对，3）坚韧直立的丛生‘耐受种’，以及4）株丛结构稀疏或贴地生长的‘避食种’，饲用价值极低，且会被放牧动物避开。
放牧草地可在多种环境条件下形成，通常在干旱多沙的生境中以匍匐型吸引种草类为优势类群，而在湿润低沙的生境中则以丛生型吸引种草类为优势类群。耐受种多见于中生环境的黏质土壤中，而避食种分布广泛但种群密度较低。尽管放牧草地可在多数环境条件下构建，但监测其群落组成与盖度仍至关重要，因为过度放牧的风险同样普遍存在。以丛生型吸引种为主的放牧草地，相较于以匍匐型吸引种为主的放牧草地，似乎更易发生退化。避食种或耐受种的丰度提升均会降低群落饲用价值，不过耐受种可更好地保护土壤。