Sapling harvest: a predominant factor affecting future composition of tropical dry forests

Abstract

Degradation of tropical forests is of global concern and studies of the ecosystem processes concerned with the regeneration of trees have a great value for reforestation management. We analysed the structure of a tropical dry forest (TDF) in India in terms of the composition of small-diameter trees (saplings) at five distinct sites located along a gradient of soil moisture, and investigated the effects of biotic and abiotic disturbances on the diversity, aboveground biomass (AGB), AGB-accumulation, annual recruitment index (ARI) and annual mortality index (AMI) of saplings. Aim of the study was to analyse the relative impact of important disturbing factors on the species composition of saplings in the TDF. Study questions were: (1) how is the TDF structured in terms of tree saplings and their AGB? (2) what are the levels of biotic and abiotic disturbances which affect sapling population, forest regeneration and future composition of the community? (3) what is the relationship between species mortality and recruitment in the sites having different disturbance intensities? and (4) what is the relative importance of individual disturbing factors for the recruitment, mortality and structure of sapling community? Step-wise multiple regressions were performed for predicting species richness, stem density, mean AGB, AGB-accumulation, ARI and AMI from mean values of soil moisture content (SMC), harvest index, browse index, drought index and fire index. Results showed 80, 15, 4.5 and 0.5%, mortality through harvest, browse, drought, and fire, respectively. Floristic composition of the sapling population was different compared to that of the mature trees. Selective harvesting of the saplings of large canopy trees such as Terminalia tomentosa and Shorea robusta indicated the possiblity of change in floristic composition of the TDF in future. Tree saplings in our TDF are mainly damaged by illegal harvesting. Compared to browsing and natural mortality on account of drought and fire, damage to saplings due to harvesting was substantially higher at most of the study sites. In this study, the site which had the lowest level of SMC throughout the year, contained the lowest number of species and sapling stems and also had the greatest level of disturbance in terms of damaged saplings. In order to protect and improve the diversity and recruitment of tree saplings, we suggest to restrict the illegal (unauthorized) harvesting and to control browsing in the forest. Particularly the saplings of the dominant canopy trees and those with high biomass accumulation potential need to be protected.