Plant data
Vegetation was sampled in the plots during the peak growing season (see also Pyšek et al. 2020, Hejda et al. 2022). Surveys were done during two rainy seasons, from 16 January to 4 February 2019 (33 plots) and 17 January to 3 February 2020 (27 plots); the plots sampled in each year were equally stratified among habitats and bedrocks. All plant species (for simplicity, intraspecific taxa such as subspecies are further also referred to as ‘species’) were recorded in each 2,500 m2 plot, separately for the shrub and herb layer. Their abundance was visually estimated using the Braun-Blanquet cover-abundance seven-grade scale (Mueller-Dombois and Ellenberg 1974). To quantify the occurrence of species in plots, the Braun-Blanquet scores were transformed to percentage values as follows: 5 = 87.5%, 4 = 62.5%, 3 = 37.5%, 2 = 15%, 1 = 2.5%, + = 1.0%, r = 0.02% (van der Maarel 1979).
For each plot, we calculated (i) grass species richness , i.e., the total number of grass species recorded in the plot, and (ii)grass cover as the sum of the ground covers of all grass species present in the plot; the grass cover ranged between 1% and 103% (because leaf layers may overlap across species, summary values over 100% are possible).
Grass traits were extracted from van Oudtshoorn (2018), namely: maximum culm length; maximum leaf length; maximum leaf width; perenniality (as a continuous variable: 1 = annual, 2 = weak perennial, 3 = perennial); grazing value (as a continuous variable: 1 = low, 2 = medium, 3 = high); grazing status (decreaser, increaser I, increaser II, increaser III). Species were classified as annuals if they grow for one season, weak perennials if they grow for two to five seasons, and perennials if persisting longer than five seasons. Decreasers are palatable grasses that decrease under over- or undergrazing, increasers I are unpalatable grasses growing in underutilized veld, increasers II are common in dry and overgrazed grasslands, and increasers III are unpalatable climax grasses of overgrazed veld (van Oudtshoorn 2018). Climax status that describes the species’ position on the succession continuum was also converted to a continuous variable (1 = pioneer, 2 = pioneer/subclimax, 3 = subclimax, 4 = subclimax/climax, 5 = climax).
The nomenclature of plant species follows van Oudtshoorn. (2018).