Structure and function in two tropical gallery forest communities: implications for forest conservation in fragmented systems

1. Composition, growth and turnover of trees in two species-rich tropical gallery forests were examined to evaluate what community reorganization may be needed to transform recently created tropical forest fragments into stable refugia for regional forest biotas. 2. Rates of tree growth and turnover over a 5-year interval were comparable to those recorded in continuous forests and in both communities there had been some tree species turnover in the measured stem size classes during the 5-year interval. 3. The more abundant tree species in both communities formed three functional groups along gradients between streams and forest edges: edge-concentrators, core-concentrators and generalists. 4. Soil fertility showed no consistent increase close to streams and neither tree growth nor recruitment rates were increased in this zone. In contrast, forest edge zones exhibited increased rates of tree growth and recruitment indicating that growth processes in these forests are light-limited rather than soil-limited, and that forest edge zones are generally favourable habitats for tree populations. 5. Both communities showed signs of past fire incursions, and the tendency of a subset of tree species to concentrate in the more growth-limited core habitats is attributed to their fire sensitivity. 6. Rapid development of an edge zone of fire-insensitive tree species is considered to be essential to the survival of forest community fragments in the fire-prone landscapes of the tropics, and the edges of gallery forests are recommended as potential sources of species with which to fashion these protective ecotones. 7. Preservation of a diverse forest biota in the fire-protected interiors of fragments will require natural or artificially enhanced immigration rates that are sufficient to offset local extinctions.