Comparison of community composition and species diversity of understorey and overstorey tree species in a dry tropical forest of northern India

Abstract

The study focuses on understorey–overstorey plant community dynamics in a dry tropical forest to facilitate appropriate management decisions. We compare community composition and species diversity of the understorey vegetation among five dry tropical forest sites in northern India. A total of 1500 quadrats distributed over 15 one-ha permanent plots in five sites differing in the degree of disturbance, were used to enumerate the understorey tree species and the results were compared with overstorey tree layer. The non-metric multidimensional scaling (NMS) ordination revealed that human disturbance intensity, as well as the overall disturbance regimes, and soil water holding capacity controlled the organisation of dry tropical forest understorey composition through effects on soil organic matter. The α-diversity and its components decreased with increasing human disturbance intensity, reflecting utilisation pressure and decreased soil fertility, as also revealed by the analysis of overstorey tree layer. There was a significant positive relationship between overstorey and understorey diversity. Results suggest that in the future, the existing understorey tree communities may replace the current dry tropical forest communities under prevailing environmental conditions. The study also asserts that the rate of species accumulation will be greater in more disturbed sites as well as at small spatial scale within each disturbance level.

Introduction

The quantification of understorey tree species diversity is important for understanding the forest dynamics because it influences nutrient cycling and energy flow (Moore and Allen, 1999); provides habitat for animals, partly regulates microclimatic conditions (Ramovs and Roberts, 2003); and contributes a significant part of ecosystem biomass particularly in younger open-canopy stands (Alaback, 1982), which characterise the dry tropical forests (Singh and Singh, 1991). The spatial distribution of understorey vegetation may provide a clue to the nature, degree and duration of processes or resources that structure understorey communities and also assist in formulating hypotheses about the relevant processes (Dale, 1999; Scheller and Mladenoff, 2002). A study of understorey tree species diversity is also important because it represents the regeneration potential and affects the future course of succession and species composition of the communities.

Tropical forests occupy only 7% of the Earth's land surface but they sustain more than half of the planet life-forms (Wilson, 1988). Dry tropical, sub-tropical and woodlands once covered more than half of the world's tropics (Janzen, 1988), but have decreased considerably during the past few decades. The dry tropical forests account for 38.2% of the forest cover of India (MoEF, 1999). These forests in northern India have been under tremendous human pressure and in large areas have been reduced to a mosaic of patches of forest, savanna and cropland (Singh et al., 1991). According to Sheil (1999), the disturbance of a suitable intensity will increase species richness in old growth communities in consonance with the intermediate disturbance hypothesis of Connell (1978). However, other studies maintain that disturbance cannot increase diversity in genuine old growth forests (e.g. Phillips et al., 1997).

We have previously examined the diversity of the overstorey tree vegetation of the tropical dry deciduous forest with reference to disturbance and dispersion (Sagar et al., 2003a), and fitted asymptotic models of species–area curve for measuring overstorey tree diversity (Sagar et al., 2003b). In this study, we ask: (i) Does disturbance directly or indirectly affect community composition? (ii) Does disturbance intensity increase or decrease species diversity? and (iii) Do species accumulate over spatial scales at greater rates in less or more disturbed sites?