DISTURBANCE AND TREE SPECIES COEXISTENCE IN TROPICAL RIPARIAN FOREST FRAGMENTS

Patterns of past fire incursions and tree falls were documented in a tropical riparian forest. Fire-scarred trees were concentrated on gentler slopes near the savanna:forest boundary, while tree fall sites were of highest frequency on convex upper valley slopes. Most tree falls were in a down-slope direction while the remainder were concentrated in directions that suggested an origin in extreme winds of atypical orientation. Tree species response to these two forms of disturbance was evaluated by assuming that the zones identified were inherently more disturbance-prone, and comparing the tree populations within and beyond the areas identified. Fire incursions had a larger measurable effect in augmenting species richness: 19% of species evaluated were significantly more abundant in burned zones, while only 8% of those evaluated were more abundant in tree fall zones. This difference is tentatively attributed to fire having a larger proximal effect on tree species recruitment as well as a spatially more persistent pattern of occurrence. Observations made at sites of recent fire incursion indicated that this caused little tree death or canopy opening but eliminated seedlings, saplings, litter and root mats: it thus created new seedbed conditions without the high light levels that would promote herbaceous establishment. Both disturbances play an augmentative, rather than exclusive, role in promoting species coexistence in these forests: they provide a varied micro-environment for seedling establishment but their patchy distribution ensures that disturbance-sensitive species can persist elsewhere in the forest patch.