Invasion and persistence of bird-dispersed, subtropical thicket and forest species in fire-prone coastal fynbos

Seedling abundance at four microsites (open fynbos, beneath emergent fynbos shrubs, beneath thicket, and beneath forest) was determined at three coastal dune landscapes, located along a gradient of increasing summer rainfall and where fire-dependent fynbos was the predominant vegetation. At all sites thicket seedlings were most common beneath emergent fynbos shrubs and under thicker clumps: seedlings of forest species were most abundant at forest microsites although some individuals were recorded beneath thicket. Very few thicket seedlings were observed in open fynbos. Birds play a keystone role in facilitating establishment of the fleshy fruit-bearing thicket flora. Seedling abundance at microsites of different thicket and forest species was generally unrelated to fruit abundance. Germination success of most species was highest under shaded conditions; soil organic content had no effect on germination. Removal of pulp and bird-ingestion enhanced the germination, relative to untreated controls, of two out Of three species rested. A simple Markov model predicted a gradual increase in cover of the thicket and forest component and a gradual decline in fynbos under a 'normal' (20-yr interval) fire regime simulated over 10 cycles. Although inter-fire seedling establishment under emergent fynbos shrubs is important in the initial colonisation of fynbos by obligate resprouting thicket shrubs, these species persist and expand by vegetative recruitment after and between fires, respectively. In the prolonged absence of fire, the endemic-rich and fire-dependent fynbos flora would be replaced by species-poor forest and thicket.