Vegetation dynamics of an old-growth lowland tropical rainforest in North-east India: Species composition and stand heterogeneity

Abstract

The North East India forms an important hotspot of biodiversity by virtue of its unique biogeographical situation and healthier growing conditions. The region has an area of 13302.80 km2 under the protected area network (PAN), which is not yet systematically inventorised. In this paper, we investigated Namdapha National Park that comprises 14% of the total PAN of the region and forms the largest remnants of the tropical dipterocarp forest. We examined land use pattern of Namdapha National Park and studied the vegetation dynamics of lowland tropical rainforest communities in the buffer zone area. The study revealed a total of eight cover classes and four vegetation categories with three forest types, namely, Stand I: Altingia-mixed species, Stand II: Shorea-Dipterocarp (both are tropical wet evergreen forests), and Stand III: Albizia (riverine tropical semi-evergreen forest) types. These three stands were systematically studied for general physiognomy, vegetation stratification, species area curve, plant structure and stand heterogeneity (species richness, diversity and dominance), tree girth class structure, and dominance-diversity relationship. These three stands collectively comprised  198 species (including 135 tree species), thus exhibited 34% of total floral and 50% of total tree diversity of the Namdapha National park, which is significant. The tropical wet evergreen forests showed five distinct vegetation strata. The species area curve depicted 123 and 19 tree species ha-1 in tropical wet evergreen and riverine semi-evergreen forests, respectively. Higher frequencies of lauraceae, diperocarpaceae, Euphorbiaces, Fagaceae, Leguminosae, Meliaceae and Magnoliaceae species clearly show that the forest had all attributes of tropical rainforests. The Stands I, II and III showed densities of 418, 390, and 245 trees ha-1, and total basal area of 45.47, 49.68 and 18.33 m2 ha-1, respectively, which is well comparable with other similar forests. Species dominance and rarity analysis revealed that 63, 35, and 45% of the species were rare at local level in Stands I, II and III respectively, however such species contributed highly to forest stand diversity. The species number and density declined with increasing tree-girth sizes that denotes an evolving population with old growth trees. Comparisons of the floristic data with other similar stands have been discussed.   Key words: Rainforest, tropical evergreen and semi-evergreen forest, protected area network, tree diversity, species area curve, dominance-diversity curve, forest management.