Tropical Ecology 58(2): 397–408, 2017 ISSN 0564-3295 © International Society for Tropical Ecology www.tropecol.com Community structure and species diversity of Pinus merkusii Jungh. & de Vriese forest along an altitudinal gradient in Eastern Himalaya, Arunachal Pradesh, India ANUP KUMAR DAS 1* , LAL BIHARI SINGHA 1 & MOHAMMED LATIF KHAN 2 1 Department of Forestry, North Eastern Regional Institute of Science and Technology, (NERIST), Deemed University, Nirjuli–791109, Arunachal Pradesh, India 2 Department of Botany, Dr. Harisingh Gour Central University, Sagar 470003, Madhya Pradesh, India Abstract: Pinus merkusii, the only pine species naturally distributed to the south of equator with a wide altitudinal range of 30–1800 m a.s.l., is under threat due to habitat destruction and over exploitation for its timber and resin. In India, the species is restricted to Arunachal Pradesh. The community characteristics of P. merkusii dominated forests were studied in Anjaw District of Arunachal Pradesh at three elevations i.e. at Walong (900–1200 m a.s.l.), Namti (1200–1500 m a.s.l.) and Dong (1500–1800 m a.s.l.) during the years 2012–2014. Species richness was highest at low elevation site followed by medium elevation and lowest at high elevation site with 76, 50 and 35 species, respectively. Fagaceae, Poaceae, Lauraceae and Rosaceae were the most dominant families. Overall tree density decreased with the increase in elevation (1124 to 896 individuals ha –1 ), whereas density of P. merkusii increased with the elevation (286 to 504 individuals ha –1 ). Basal cover was largest at medium elevation site with 54.34 m 2 ha –1 and lowest at low elevation site with 32.8 m 2 ha –1 . P. merkusii contributed maximum to the total stand basal cover and IVI at all the three sites. The largest basal area of P. merkusii was recorded at medium elevation site. Shannon’s diversity index and Menhinick’s species richness index decreased with the increase in elevation. Conversely, Simpson’s dominance index increased with elevation. Species similarity index was highest between medium and high elevation sites (59%) and lowest between low and high elevation sites (39%). Most of the plant species were distributed contagiously in all the three elevation sites. Present study revealed high plant species diversity at low elevation which decreased with the increase in elevation. Significant contribution of P. merkusii to density, basal cover and IVI depicted sheer domination by this vulnerable gymnosperm in these forests. Key words: Community structure and dominance, Pinus merkusii forest, vulnerable. Handling Editor: S. K. Barik Introduction Pinus L. is the largest genera with more than 110 species of the family Pinaceae which forms dominant vegetation in northern hemisphere viz. North America, Europe and Asia. Only Pinus merkusii Jungh. & de Vriese extends towards the southern hemisphere (Critchfield & Little 1966; Mirov 1967; Price et al. 1998; Richardson 1994). Distribution of P. merkusii in natural forests is reported from Arunachal Pradesh, Indo–Myanmar region, Thailand, Malaysia, * Corresponding Author; e-mail: anuplily@gmail.com