Tree Functional Traits and Environmental Gradients explain Forest Dynamics in a Southeast Asian Forest in Singapore

M. S. KOENIGER, S. K. Y. LUM, K. M. NGO, K. M. ANDERSEN

Asian School of the Environment, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

Southeast Asian tropical forests are a global biodiversity hotspot, while simultaneously being disproportionally affected by human activities, which makes them highly vulnerable to further impacts caused by global change. Forest dynamics are determined by the performance of tree species or individuals in their environment, which depends on adaptations in their tissues. These distinct characteristics or “functional traits”, that help trees survive in one environment usually lead to a disadvantage in another habitat, which results in these functional traits mediating the filtering effect of environmental variables on tree community composition. This can provide an explanation for changes in species distribution over time and space, altering taxonomic and functional compositions of forests. To determine the relationship of forest demography to tree functional traits and environmental variables in two types of tropical forest (old growth and secondary), this study uses trait imputation techniques to assess the functional trait spaces in a highly diverse forest community. We used these trait-dimensions together with soil variables to predict tree demography from a time series spanning 26 years in a highly urbanized tropical forest in Bukit Timah Nature Reserve (BNTR) in Singapore.  The 414 tree species in our analysis exhibited a growth-mortality functional trait axis, of wood density contrasting with leaf nitrogen. Additionally, we observed a trait axis contrasting leaf phosphorus content and specific leaf area, which might be associated with P conservation strategies. Mortality, recruitment, and growth were elevated in secondary forest plots, which suggests the community there is more dynamic than in the primary forest. Lower soil P increased mortality and reduced recruitment and growth, emphasizing the importance of this nutrient for tropical trees. Our findings indicate that the secondary forest in BNTR could be limited in succession to old-growth forest, and further suggests that this type of forest might be less resilient to future challenges by global change.