Research: Life History Trade-offs & Functional Traits
Life history trade-offs & functional traits
Individual organisms face trade-offs in terms of how energy and resources are allocated during a lifetime. For example, a juvenile tree growing in the understory of a closed-canopy forest allocates resources to produce leaves: Those leaves may be inexpensive to produce and therefore require frequent replacement, but have great photosynthetic capacity enabling fast growth in height or, alternatively, those leaves may be stronger and more resistant to damage, and thus longer-lived, but more costly to produce. Such trade-offs are hugely important for understanding species' life-histories and for defining dominant axes of variation between species in functional trait strategies.
A recent review identified integrating an understanding of mechanisms into life-history theory as one of the most exciting tasks facing evolutionary biologists in the 21st century. It is also an exciting task facing ecologists who seek to understand the functional bases for how trade-offs at the individual and population levels influence the dynamics of plant communities, and this is a main focus of research in my laboratory.
My laboratory has been addressing these questions in the hyper-diverse rain forests of Borneo. One explanation for the exceptionally high tree species richness in these forests is that species coexist by occupying slightly different ecological niches and hence trade-off performance in different habitats. But how are plant species sorted among habitats that they could conceivably occupy in the absence of interspecific competition?
To address this question, I have been working in a large forest dynamics plot in Borneo (Lambir Hills National Park in Malaysian Borneo) with collaborators from the Center for Tropical Forest Science, Harvard University, and Malaysia's Forest Research Corporation. We have been examining possible demographic and functional mechanisms that may generate the striking soil-correlated tree species distributions observed at Lambir (Russo et al. 2005).
Our research has shown that these soil-correlated distributions can be explained in part by performance-based ecological sorting of species among soils. Sorting may be partly mediated by an interspecific demographic trade-off between a species' ability to tolerate low-resource conditions by having conservative demographic responses (slow growth and low mortality) vs. its ability to respond quickly to increases in resource availability (fast growth). Mortality risk for species with intrinsically fast growth is greater on the poorest soil, which is consistent with the trade-off hypothesis (Russo et al. 2008).
This interspecific demographic trade-off can be seen as an emergent property resulting from functional constraints related to physiology, morphology, and resource allocation expressed at the individual level. I am developing models relating this demographic trade-off to the possible functional mechanisms underlying it, by measuring in situ variation in ecophysiological traits of several tree species and relating this variation to plant performance. This research provides a link between plant ecophysiology and its consequences for the structure and diversity of forest communities.