PS 13-125 - Anthropogenic and environmental drivers of fine-scale patterns of tree species diversity in a remnant tropical forest of Nepal’s Himalaya

Background/Question/Methods

Spatial variability and anthropogenic disturbances influence the fine-scale patterns of tree diversity in forests, and the impact of human disturbances is even more pronounced when the dependency of local peoples on forest resources is high. Understanding how these environmental and anthropogenic drivers influence tree diversity is crucial to managing forest biodiversity and the maintenance of ecosystem services. Jalthal forest is one of the biodiverse and multifunctional forests surrounded by human settlements in the eastern lowland of Nepal. It is now confined to the area of 6300 ha from once continuous stands extending to the plains south of the Siwalik mountain. We divided the whole forest into 500 × 500m grids and randomly laid one 314 m² circular plot in each grid. Altogether 228 plots were sampled to record tree species, environmental and disturbance factors such as elevation, forest types, protection status, trail, lopping, substrate types in each plot. Slope was derived from the digital elevation model and distance from the boundary– a proxy of anthropogenic disturbance to the forest– was measured as the shortest distance from the forest edge. We used boosted regression tree analysis to assess which predictor drives tree richness and rank them based on their relative influence.

Results/Conclusions

A total of 116 tree species, probably the highest record in a single forest in Nepal, were recorded in the Jalthal forest with an average tree richness – calculated as the number of tree species per plot – of 4.21±0.22. Our best boosted regression tree model revealed that distance to the boundary (relative influence= 51%), slope (relative influence= 21%), and elevation (relative influence= 18%) are the most important explanatory variables explaining altogether >90% of the variation in tree richness and always appeared as the most influential predictors regardless of tree complexity and learning rate. Additionally, other factors representing anthropogenic disturbances such as lopping and the presence of trail also influenced the tree richness. Tree diversity increased with increasing distance from the boundary towards a relatively less disturbed forest center (core) having more micro-topographic variations as reflected by the spatial variability in elevation and slope. Our results demonstrated that tree diversity in the Jalthal forest is driven by both anthropogenic disturbances and topographic variations, and these factors should be considered while designing management plans, and most importantly the core areas of the Jalthal forest should be kept intact to maintain the biodiversity and conserve rare and unique tree species.