Role of nurse rocks on woody plant establishment in a South African grassland

Many empirical studies have supported the facilitative effect of nurse plants, and several recent studies have reported similar phenomena with non-plant features, such as rocks. Few studies have explored the effect of rock height on plant establishment, although variation in height can affect establishment. This study examined whether rocky outcrops have positive effects on plant establishment, as do classic nurse plants, and explored the mechanisms involved in the nurse effects in a South African grassland. To answer these questions, we compared the number of individual woody plants at the edges of rocky outcrops and in the adjacent grassland matrix, as well as beneath a putative nurse plant (Euclea crispa) and in the adjacent grassland matrix. We also measured the heights of rocky outcrops and E. crispa and the proportions of grass cover. The results showed that larger numbers of woody seedlings occurred at the edges of rocky outcrops and beneath E. crispa compared with in the adjacent grassland matrix. A generalised linear model (GLM) showed that rock and E. crispa height positively affected the number of seedlings; the diaspores of most recorded species showed characteristics associated with dispersal by vertebrates. The GLM analysis showed that the proportion of grass cover had a negative effect on the number of seedlings. These results indicate that in addition to shrubs, rocky outcrops appear to have positive effects on the establishment of woody plants in South African grasslands, likely serving as perching structures for birds and providing favourable microhabitats.