Rapid assessment of tropical rain forest successional status using aerial photographs

Abstract

Four forest structural types were recognized and mapped from 1 : 20,000 black-and-white aerial photographs of the 2000 ha Central Catchment Nature Reserve in Singapore. These types were an essentially treeless unit of grassy areas and fern thickets (Type 1), a low forest with uniform canopy of many small trees (Type 2), a taller forest with larger trees (Type 3), and a yet taller forest with a structurally more heterogeneous canopy with some very large-crowned trees present (Type 4). The three vegetation units containing trees were sampled using clusters of circular plots totalling 0·2 ha each, in which all trees ≥30 cm gbh were measured and identified. Increasing canopy height and tree size among the structural types were reflected in increasing total basal area of the sample clusters. Mean tree species richness also increased with structural complexity. Ordination of the clusters based on their floristic composition showed that Type 4 was very varied but generally distinct from Types 2 and 3, which were much less diverse and not distinct from one another floristically. TWINSPAN was used to analyze the important floristic differences between the sample clusters. It was found that Types 2 and 3 were dominated by early successional species including Adinandra dumosa, Macaranga conifera and species of Calophyllum and Garcinia. Type 4 was typified more by the presence of species of Burseraceae and Dipterocarpaceae — primary forest trees. This mosaic of different successional stages is supported by the known history of the area, with massive deforestation in the mid and late 19th century. We conclude that the interpretation of aerial photographs is a useful tool for the fine-scale rapid assessment of the successional status of lowland tropical forest and can be used to infer relative levels of forest diversity. As such it can be of value in the rational management of tropical forest conservation areas.