Variation in wood density, wood water content, stem growth and mortality among twenty-seven tree species in a tropical rainforest on Borneo Island

Interspecific variation among wood density (WD), wood water content (WWC), tree mortality and diameter at breast height (d.b.h.) increment was examined for 27 tree species (from 13 families), based on a 9-year interval data obtained from a permanent 1-ha forest plot setup for long-term studies of tree dynamics in Kuala Belong rainforest, Brunei, on Borneo Island. The species were also categorized into three adult stature groups of understorey (maximum height <= 15-20 m tall, n = 14), midcanopy (maximum height, 20-30 m tall, n = 8) and canopy/emergent (> maximum height, > 30 m tall, n = 5) tree species. All measured traits varied appreciably among species. Tree WD varied between 0.3 and 0.8 g cm(-3), and exhibited the least coefficient of variation (14.7%). D.b.h. increment was low, averaging 1.05 (95% confidence limits: 0.57-2.13) mm year(-1) and was attributed to predominance of understorey species in the sampled plot. Overall, annual mortality was also low, averaging 2.73% per year. The three adult stature groups differed significantly in d.b.h. increment and WWC but not in tree mortality and WD. Across species and especially more so when phylogenetic effect is minimized, WD was negatively related to tree mortality and d.b.h. increment, while a positive trend was observed between d.b.h. increment and tree mortality. A negative trend was also detected between maximum plant height and WWC, which was interpreted as a consequence of increased evaporative demand and use of xylem stored water by taller trees in order to compensate for hydraulic limitations to water transport induced by frictional resistance. No doubt, the traits chosen may vary spatially, but the consistent interspecific patterns observed in this study among coexisting species of differing adult stature reflect 'vertical' niche differentiation and may help to explain population regulation in a multispecies ecosystem like tropical rainforest.