Land tenure and human disturbance influence the current distribution of aboveground biomass in Sri Lankan rainforest fragments

Abstract

This study examines the landscape distribution of aboveground biomass (AGB) in fragmented tropical rainforests in Sri Lanka. While most attribute AGB losses from fragmentation to abiotic edge effects (e.g., desiccation and wind), land tenure and human disturbance also play a significant role. In South Asia, forest fragments are essential food, medicine, and wood sources for large rural populations. Despite this, few studies have addressed AGB dynamics in smallholder landscapes with a long land clearance and cultivation history. This study explored three objectives: (a) assessing the accuracy of a common allometric height-diameter model, (b) comparing AGB stocks in forest fragments to primary forests, and (c) examining the influence of land tenure, abiotic edge effects, and human disturbance on AGB distribution. We estimated AGB using field-based measurements in 18 forest fragments and two primary forest sites and employed mixed effects models to explore the landscape distribution of AGB. The allometric height-diameter model overestimated fragment tree heights compared to measured heights, leading to an overestimation of AGB by 25 %. AGB stocks were significantly lower in fragmented forests (259.9 Mg ha⁻¹) than in primary forests (434.5 Mg ha⁻¹), and there was no significant correlation between plot AGB and variables associated with abiotic edge effects (i.e., distance-to-edge, fragment size, isolation). However, AGB positively correlated with factors related to decreased accessibility and local land ownership. These findings suggest that land tenure and human disturbance strongly influence the current AGB distribution in Sri Lankan rainforest fragments, emphasizing the need to understand human impacts for effective conservation and management.