Tree community responses to disturbance: Testing the intermediate disturbance hypothesis in different forest management regimes of Nepal

Abstract

The Intermediate Disturbance Hypothesis (IDH) posits that local species diversity peaks at moderate levels of disturbance. Despite its broad theoretical appeal, empirical validation across ecosystems and disturbance regimes remains limited. This study assesses the IDH in the Terai Sal (Shorea robusta) forests of Nepal by examining responses of tree communities to anthropogenic disturbances across three management regimes: buffer zone forests, user group-managed community forests and collaborative forests. The use of the importance value index allowed comparison of stand structure, species composition and diversity indices (alpha diversity measures: Shannon, Simpson, and Margalef indices) and beta diversity (Jaccard similarity). We used redundancy analysis to identify key environmental and disturbance variables that correlated with plant communities. The findings demonstrated significant structural and compositional shifts across the disturbance regimes. While low-disturbance buffer zones are characterised by mature, dominant Sal stands, high-disturbance collaborative forests had an overabundance of seedlings and an absence of mature trees. As predicted by IDH, the most moderately disturbed community forest recorded the highest values of alpha diversity. Beta diversity was pronounced, with each regime having a unique species composition. Community structure was significantly shaped by disturbance intensity, canopy cover and anthropogenic pressures. These results indicate that intermediate levels of disturbance at periodic intervals can sustain tree diversity in the high tropics of the Terai by maintaining a balance in the competition–colonisation trade-offs in a long-term process. Furthermore, managed silviculture practices can enhance diversity by producing heterogeneous conditions in gap settings. The quantitative insights derived from this study could help inform sustainable forest management policies in Nepal, balancing biodiversity conservation with community resource needs. More broadly, the findings underscore the role of disturbance regimes in shaping diversity patterns in tropical production forests and emphasise the necessity of context-specific empirical validation of ecological theories to guide management interventions.

Keywords: intermediate disturbance hypothesis; Shorea robusta; community forestry; alpha diversity; beta diversity; Nepal