Dynamics of plant and insect communities across shifting cultvation fallow in Arunachal Pradesh, Eastern Himalaya

Abstract

Shifting cultivation or jhum is a form of agriculture practice that is practiced by several communities around the world. In jhum cultivation, a patch of forest is slashed, burned, and cultivated for a limited period of time, and later on the patch is left for the regeneration vis a vis recovery of vegetation. Ecologically, such habitats provide a clear disturbance gradient to understand biodiversity recovery, including the process of establishment. Several studies have been carried out in the tropical forest around the world; however limited studies have looked at the recovery in the shifting cultivation landscape of North Eastern region in general and specifically in Arunachal Pradesh. The present study was conducted in Nampong, Changlang district of Arunachal Pradesh to investigate the dynamics of plant communities across shifting cultivation fallows of different ages. The vegetation was sampled across five different age classes vis-à-vis., 1, 5, 15, 25, 35 and forest as a control site. For vegetation sampling, 10m circular nested plots were used to sample for trees and 3.5m circular plots were used to sample for shrubs. To understand the community composition, diversity indices using the vegan package in R, following which a species accumulation was run to check sampling adequacy, were estimated. Following, which a Non-Metric Multidimensional Scaling (NMDS) was run to see how communities changed across fallows, followed by Indicator species Analysis and Similarity Percentages (SIMPER) analysis. Generalized Linear Model (GLM) with Poisson was run to check which factors influence the species richness. Based on the sampling and field observations, change in communities across fallows of different ages, with the forest having the highest species count, was observed. Simpson showed how diversity changed across fallow along with a decrease in 25-year-old which may be due to the presence of several species of Bamboo and Musa. NMDS showed a distinct pattern across age class, though there were overlaps in the mid-successional changes. Further, indicator species analysis was used to understand the association of species with respect to successional stages; such as Mesua ferrea, Terminalia chebula were associated with forest, Agalaia edulis and Albizia sp. with 35 and 25-year-old fallows, and species such as Zanthozyllum rhetsa in the early fallows. The SIMPER analysis showed the dissimilarity in the species across fallows of different ages, which showed that Trema orientalis, Macaranga denticulata, Croton persimilis, bamboo species, and Musa species contributed to differences among successional stages, and species such as Baccaurea ramiflora and Mesua ferrea were the species distinct in forest classes, suggesting the requirement of longer periods of recovery for the fallows. The understory communities of young fallows were dominated by invasive species such as Chromolaena odorata, Erigeron sumatrensis, and Mikania micrantha. Their abundance declined with increasing fallow age, indicating that canopy development and habitat recovery reduce the dominance of invasive species. Older fallows and forest sites increasingly consisted forest-associated species such as Gnetum montanum, Calamus spp., Phrynium pubinerve, and Entada sp., reflecting progressive recovery of understory vegetation. Model selection based on AICc identified fallow age as the strongest predictor of both tree and understory species richness, highlighting the importance of successional stage in shaping vegetation recovery. The influence of topographic and spatial variables was comparatively weaker. The study highlights the importance of protecting the remnant forested areas, and avoiding as well as reducing fallow cycles for the biodiversity to be resilient in the shifting cultivation landscapes of Arunachal Pradesh.