Response of rock-outcrop and fringing vegetation to disturbance by fire and drought

Abstract

A comparative study of vegetation responses to fire and drought investigated whether species regeneration mode, seedling density response (SDR) and seedling shoot growth rate varied significantly in relation to disturbance agent (fire and drought), habitat type (rock-outcrop and fringing vegetation) and plant growth form. A three-way ANOVA showed that SDR varied significantly in relation to all three categorical variables and most strongly in relation to disturbance agent. Seeders comprised 87% of the post-fire flora and 99.3% of the peak seedling population in rock-outcrop habitat, while resprouters were much more prominent in fringing woodland. Species SDRs and seedling growth rates were generally much higher after fire. Fire produced a high SDR with high shrub, grass and ephemeral therophyte components, and drought produced a low SDR dominated by grasses and herbs and inhibited shrub and ephemeral recruitment. Post-fire obligate-seeder shrubs behaved as facultative resprouters after drought. Some species exhibited SDRs equivalent to fire and drought, others appeared to require fire for regeneration, while others recruited more successfully after drought. This spectrum of responses indicated a range of optimal disturbance environments, depending on species, which was only partly consistent with the hypothesis that species exhibit essentially the same life-history syndromes in response to fire and drought. The dominance of seeders in outcrop vegetation appeared to be related to skeletal soil, higher disturbance frequency and soil trophic conditions, rather than low fire frequency.