Grass competition and the savanna-grassland ‘treeline’: A question of root gaps?

Abstract

Upland grasslands in South Africa occur in climates warm enough and wet enough for forests yet, unlike savannas, trees are absent. We hypothesize that the lack of trees in grasslands is due to the scarcity of gaps suitable for tree seedling establishment in the grass root layer. We assessed which demographic bottlenecks, seedling recruitment or sapling release, were limiting large tree populations of Acacia species in 229 sites on a gradient from lowland savannas to upland grasslands in eastern South Africa. We explored the frequency of ‘root gaps’ by quantifying grass root biomass in grids of 18 points at six sites distributed from lowland savannas to upland grasslands.Across the altitudinal gradient, sapling release was the most common bottleneck at low elevation sites switching to seedling recruitment in upland savannas and adjacent grasslands. Root studies showed grass root biomass at upland sites to be double than that of lowland sites. When grass root biomass of all 108 sample points was ranked, lowland savanna sites had the most frequent ‘gaps’ defined as the lowest percentiles of values (5, 15, 25, 35, 45). The number of ‘gaps’ decreased along the elevation gradient with none in the grassland site. Dense grass roots may prevent seedling establishment and ‘root gaps’ may be safe sites with reduced competition where seedlings can establish. We suggest that a lack of root gaps could explain the treeless nature of many upland grasslands.