Coastal Forest in Eastern Southern Africa has Savanna Bush-clump Origins

Abstract

The extensive coastal forests in eastern southern Africa persist as a putative alternative stable state in an open ecosystem mosaic that includes grassland and savanna. We examine two current hypotheses of the state-transition origins of these forests: (1) facilitation—where light-demanding pioneer cohorts facilitate establishment by species that tolerate shade and whose functional traits align with forest; and (2) nucleation—where tree species of savanna origin establish on fire refugia in grassland, and where forest develops from the coalescence of these bush-clumps. We compared tree species diversity and composition on old-lands colonized by the savanna pioneer Vachellia kosiensis, with bush-clumps established on old-lands and on untransformed grassland, to determine their resemblance to intact coastal forest. The facilitation pathway in V. kosiensis woodland comprised impoverished tree assemblages differing markedly in richness, diversity, and composition from the nucleation pathway. By contrast, the bush-clump nucleation pathway comprised random assemblages of savanna species. Overlap in savanna tree composition between the bush-clump pathway and forest was notable. A suite of functional traits related to growth, reproduction and dispersal revealed that the life histories of most coastal forest tree species (~ 88%, n = 83 species) correspond with those expected from savanna. A combination of shade intolerance, multi-stemmed architecture, and growth form plasticity indicate species origins and persistence under frequent fire and variable light regimes typical of savanna environments. While these coastal woody formations satisfy the structural definition of forest, their constituents are clearly derived from savanna and questions the efficacy of their management as successional forest.