Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco Changes in tree structure, composition, and diversity of a mixed-dipterocarp rainforest over a 40-year period Sisira Ediriweera a, ⁎ , Champika Bandara a , David J. Woodbury b , Xiangcheng Mi c , I.A.U.N. Gunatilleke d , C.V.S. Gunatilleke d , Mark S. Ashton b a Department of Science and Technology, Uva Wellassa University, Badulla, Sri Lanka b School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA c State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Xiangshan, Beijing 100093, China d Botany Department, University of Peradeniya, Peradeniya, Sri Lanka ARTICLE INFO Keywords: Borneo Climate change Elevation Forest dynamics Forest productivity Mesua spp. Shorea spp. South Asia Sri Lanka Tree diversity Tree mortality Tree recruitment ABSTRACT Little work has examined the spatial and temporal changes of a tropical rain forest for long time periods. Here, we present an analysis of long-term plot data from a Sri Lankan Mixed-Dipterocarp forest (MDF). The plots were established in 1978 at three different elevations (low - 335m, medium - 560 m, and high - 915 m). At each site we measured all trees ≥ 10 cm DBH in 16 or 18 plots. The total area sampled amounted to 12.5 ha. We recorded 166 tree species, with each site comprising between 95 and 121 species. Stem densities and basal areas were different among sites, and declined over the period from 642.8 stems ha -1 and 41.8 m 2 ha -1 for the 1978 census, to 496.2 stems ha -1 and 35.2 m 2 ha -1 for the 2018 census. The lowest elevation exhibited the highest recruitment, mortality and turnover among sites but patterns changed and became less marked over time. Forest wide tree recruitment was about 1% y -1 , while mortality was twice as much averaging about 2% y -1 . Four families made up between 44% (1978 census) and 54% (2018 census) of the trees sampled. Two families – Clusiaceae, Dipterocarpaceae contributed up to 33.4% and 37.3% respectively, depending upon site and year of census. The five most common species represent between 31 and 54% of the basal area and between 25 and 66% of the stem density depending upon site. Percentage dominance by the most common species increased over the 40-year time period. Though many trees showed some degree of spatial differentiation, canopy trees showed greater site overlap in their distribution than understory and sub-canopy species. Our results provide an im- proved picture of variations in MDF structure and composition across the ever-wet realms of equatorial south and southeast Asia. We suggest that dominance in dipterocarps and the degree of closed canopy structure, as measured by basal area, is influenced by time, elevation, the degree of topographic variability within elevation, and topographic interactions with variabilities in climate (drought and windstorms). We propose higher standing basal areas and super dominance of dipterocarps in MDF are linked to site, succession, and landform stability. Dipterocarps increase in dominance with succession (time), with topography (ridges greater than valleys) and with elevation (lowland < hill < lower montane). 1. Introduction There is growing need for long term demographic studies of forests in order to evaluate the dynamics of tropical trees (Condit 1995; Swaine and Whitmore 1988; Zimmerman et al. 1994). Worldwide only a few rain forests have been monitored for extensive periods of time such as in Budongo, Uganda (Sheil et al. 2000), La Selva, Costa Rica (Lieberman and Lieberman, 1987) and in Australasia (Solomon Islands – Burslem et al. 1998, 2000; Queensland, Australia – Murphy et al. 2013); all of which have records that are > 40-years old. Mixed dipterocarp forest (MDF) is the major forest type that covers most of southeast Asia (Malaysia, Indonesia, Thailand, Philippines, Vietnam, Cambodia, Laos) and the wetter parts of south Asia (Western Ghats, India; southwest Sri Lanka) (Ashton and Seidler, 2014). It comprises the largest commercial timber resource on a per hectare basis of any other tropical forest (Sist et al. 2003), primarily because of ca- nopy dominant trees in a single plant family - the Dipterocarpaceae - for which the forest is named (Ashton and Seidler, 2014). The longest term studies for MDF in Asia exist in Peninsular Malaysia (Manokaran and Kochummen, 1987; 1990) and Sarawak, Borneo (Ashton and Hall 1992; https://doi.org/10.1016/j.foreco.2019.117764 Received 2 September 2019; Received in revised form 8 November 2019; Accepted 8 November 2019 ⁎ Corresponding author. E-mail address: sisira@uwu.ac.lk (S. Ediriweera). Forest Ecology and Management 458 (2020) 117764 0378-1127/ © 2019 Elsevier B.V. All rights reserved. T