DOI: 10.5061/dryad.wstqjq2q6
title: Grazing lawns and overgrazing in frequently grazed grass communities

description: 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.
description: <strong>Sampling procedures</strong>
 Grass communities were sampled using 0.25
 m<sup>2</sup> quadrats distributed evenly through the
 frequently grazed habitat. Most sites had 30 quadrats but the minimum was
 15 at one site where sampling was restricted by time. Full details of
 sampling areas and plot layout are provided in the
 "SiteInformation" worksheet. Overall, the average distance
 between quadrats was ~12 m (range: 8 m to 15 m). All
 grass species occurring within a quadrat were identified in the field and
 verified at the National Herbarium in Pretoria, South Africa. For each
 grass species within a quadrat we recorded percentage aerial cover, median
 leaf table height (mm), culm orientation (lateral, decumbent, geniculate
 or upright), stolons (present or absent) and rhizomes (absent, short or
 long). Leaf table height was assessed visually as the approximate 80th
 quantile of leaf biomass, with the main bulk of the leaf canopy occurring
 below this height. We classified short rhizomes as those that
 incrementally allowed an individual to expand the size of its base,
 forming a tuft, and long rhizomes as those facilitating the establishment
 of new ramets with spatially separate aboveground biomass. Percentage bare
 ground in each quadrat was recorded, and whether grazer dung was present
 or not. All data were collected by the same observer throughout the study
 (Gareth Hempson). <strong>Environmental data
  </strong> Soil samples were collected at the
 four corners of each site, and analysed for texture (percent sand, silt
 and clay), cations (K, Ca, Mg and Na), exchangeable acidity and pH. Cation
 exchange capacity (CEC) was calculated for each soil sample. South African
 soil samples were analysed at the Agricultural Research Council Institute
 for Soil, Climate and Water, in Pretoria, South Africa, and Serengeti
 soils were analysed at the Sokoine University of Agriculture, Morogoro,
 Tanzania. Daily rainfall data were extracted from the Climate Prediction
 Center (CPC) Africa Rainfall Climatology Version 2.0 (ARC2) dataset and
 used to calculate mean annual rainfall for each site for the 30-year
 period prior to the sampling date. Access to these data was obtained via
 the Columbia University International Research Institute for Climate and
 Society website (iri.columbia.edu). 
 <strong>Trait indices </strong>
 Grass species at our sites
 persist under frequent grazing, and we sought to characterise the key life
 history attributes that enable this (i.e. avoidance-attractance,
 resistance or tolerance). Four trait indices were derived from field
 measurements: 1) culm orientation index, 2) lateral index, 3) tuft index,
 and 4) grazer use index. Culm orientation index was calculated as the
 species mean value after converting each species × quadrat culm
 orientation record to a numerical value as follows: lateral = 1,
 geniculate-lateral = 2, geniculate or decumbent = 3, geniculate-upright =
 4, and upright = 5. The lateral index was calculated as the proportion of
 quadrat-level records where a species had stolons or long rhizomes.
 Similarly, the tuft index was calculated as the proportion of
 quadrat-level records where a species had a tufted base. To indicate the
 relative site-level grazing preference of a species, we derived a grazer
 use index based on the assumption that maximum grazer use is experienced
 by species with heights close to the median site-level leaf table height,
 via: 1) taking the ratio of the leaf table height for each species ×
 quadrat to the overall site-level median leaf table height, 2) for values
 &gt; 1 (i.e. where a species × quadrat is taller than the overall site
 median), taking the reciprocal of this value, and 3) calculating the
 overall species mean value across all quadrats ×
 sites.
description: Data are provided as a .xlsx file created in Microsoft
 Excel.