Research: Tree Species Diversity and Distribution
Scientists and the lay public alike have long been fascinated by the incredible diversity of tree species in tropical forests. Environmental gradients in the availability of resources are important because they contribute disproportionately to this diversity, as different species are often observed to specialize on particular habitats. It is well-established that tree species trade-off the ability to grow quickly in high light versus the ability to survive in shade, and this trade-off has been shown to be an important determinant of the distributions of species along gradients in light. Less well-understood, however, are the mechanisms by which habitat-specific distributions arise along gradients in belowground (soil fertility and moisture) resources in relation to this fundamental life history trade-off (Russo et al. 2008).
Researchers in my group are investigating the determinants of the growth-survival trade-off along gradients in soil resources, in relation to specialization of tree species to different soil types in a hyper-diverse rain forest in Borneo (NSF DEB 0919136). The high tree species richness and high species:genus ratios of this forest, as well as the large, existing dataset on tree growth and survival, provide ample power and replication of phylogenetically controlled comparisons to fit models of survival and to experimentally test hypotheses of potential mechanisms underlying the growth-survival trade-off.
The overarching hypothesis is that differences among tree species in allocation to growth, storage, and defense, and their associated costs, explain variation in the strength of the growth-survival trade-off along gradients in soil resources. Differences in allocation are important because, for example, resources used for defense against tissue loss or to make more costly, longer-lived tissues cannot then be used to grow and accrue more resources. Allocation to storage carbohydrates can be viewed as a unifying currency by which nutrient- or water-stresses (in addition to light-stress) that limit carbon-assimilation are translated into survival differences among soil types.
Amy Kochsiek, a postdoctoral fellow in my lab group, is conducting field experiments in which tree species specializing on rich (more fertile and moist) or poor (less fertile and well-drained) soils are reciprocally transplanted, combined with treatments that manipulate defoliation, water-availability, and competitive neighborhood. Results from these experiments will be used to identify whether the cost that fast-growing tree species pay in terms of reduced survival can be assessed based on their allocation to storage carbohydrates and defenses against pests and pathogens. This work will improve our understanding of the determinants of fundamental life history trade-offs on resource gradients and the mechanisms whereby soil-specific tree species distributions arise in forests.