Lower altitudinal variation in habitat associations, tree diversity and co-dominant population structures along moist deciduous forests dominated by sal (Shorea robusta) in Northeast India

Abstract

We used line transects of 10 × 500 m size to analyze the patterns of sal associations, diversity and population structure at lower elevation (<100 meters above mean sea level) of moist deciduous forests dominated by sal (Shorea robusta) in Tripura, Northeast India. Overall 216 woody species with 898.56 ha−1 mean density and 33.21 m2 ha−1 mean basal area were recorded using 32 line transects of 0.5 ha size sampled by measuring sal forests at ≥10 cm girth at breast height. Five different sal associations were recognized by cluster analysis, where sal typically associated with Terminalia bellirica, Careya arborea and Schima wallichii in moist deciduous habitats. Rare associations of sal with Artocarpus chama and Dipterocarpus turbinatusa were also identified. Species diversity (p < 0.001) and dominance (p < 0.0001) was significantly different across the sal associations; species richness was recorded higher in S. robusta – S. wallichii association with 152 species than in S. robusta – T. bellirica association with 85 species. Existing habitat disturbances and intensity of sal effects on diversity and structure were significant (p ≤ 0.05), as observed by the changes in species richness and abundance of most abundant, least abundant and unique species along the associations. Population trends and regeneration potential of sal and its most associated trees were predicted using steepness and negative slope values in regression of size class distributions of tree density per hectare at 10 girth classes for each sal association. The present investigation helps in the understanding of broad ecological aspects in lowland sal habitats, particularly when this ecosystem is shifting to these regions in relatively wetter and plain areas due to climate change.