Determinants of tree species distributions across a floristic and climatic transition in SE Asia
A major floristic and climatic transition from aseasonal to seasonal evergreen tropical forest (the Kangar–Pattani Line; KPL) exists in the Indo-Sundaic region of Southeast Asia. Mechanisms constraining species distribution here were poorly understood, but it was hypothesized that species differences in drought tolerance might be important. Under this hypothesis, we anticipated differences in performance and drought tolerance traits of species differing in distribution with respect to the KPL. This research demonstrated definitively that the predicted trade-off between tolerance of abiotic stress, in this case seasonal drought, and performance exists between species occupying seasonally dry forests and those restricted to everwet forests including the identification of a novel interaction between tolerance of drought and herbivory. Furthermore, systematic differences between the two distributional groups in physiological and anatomical traits form the basis of this trade-off. These differences provide evidence of the role of climatic transitions in determining tree species distributions in this region. Such performance differences have important implications for our understanding of biodiversity gradients and responses of this hyperdiverse region to climate change.
Habitat associations in tropical trees
The importance of soil characteristics in determining the habitat associations of tree species in many tropical rain forests is well documented, but the underlying mechanisms are unclear. We made use of phylogenetically independent comparisons of survey data and experimental reciprocal transplantation to test whether tree species from a Bornean forest specialized to dry, sandy ridges have greater resource-use efficiency in comparison with specialists from the alluvial valleys and generalist species. We demonstrate that divergent water-use strategies are an important mechanism underlying differences in edaphic associations and that this results in systematic differences in whole-plant light requirements among these groups. We also used this dataset to examine how edaphic differentiation contributes to tradeoffs among leaf functional traits. The primary axis of trait variation in light-limited, lowland tropical forests was identical to the leaf economics spectrum and corresponded with the shade tolerance continuum. There was no separation with respect to edaphic variation along this primary axis of trait variation. However, a second orthogonal axis determined largely by foliar phosphorus (P) concentrations resulted in a near-perfect separation of species occupying distinct soil types within the forest. These findings were substantiated using data from two other tropical forests. We suggest that this second axis of variation represents a “leaf edaphic habitat spectrum” related to foliar P and potentially other nutrients closely linked to geological substrate, and may generally occur in plant communities characterized by strong edaphic resource gradients.
Quantification of shade tolerance in trees
Shade tolerance is associated with low light growth and survivorship. However, the importance of growth in this definition is under debate; also, the contribution of soil resources to shade tolerance remains unclear and may contribute importantly to regional differences in minimum light requirements in trees. In this research, survey and experimental studies were carried out in both tropical and temperate forests to develop a metric for quantifying shade tolerance with a view to addressing these issues. Using survey data across a range of light environments, we compared whole-plant light compensation points (WPLCP) for growth and survivorship of 20 Bornean tree species, and evaluated the importance of various plant traits in predicting WPLCP. We also examined both phenotypic and evolved differences in WPLCP among trees growing on two soil types distinct in both water and nutrient availability. Growth- and survival- based estimates of WPLCP correspond closely and dark respiration rates were found to be the best predictor of WPLCP. Species associated with the resource-poor soils had greater WPLCP than those associated with richer soils in keeping with ideas associated with tradeoffs in low resource tolerance. Similarly, gas-exchange- and field- based estimates of WPLCP were closely linked with metabolic costs in our temperate studies and resource availability impacted WPLCP significantly. These findings highlight the fact that shade tolerance is not a fixed quantity but that it shifts with resource availability due in part to changes in physiological parameters and will take the study of shade tolerance in exciting new directions.