COS 17-3
Size variation in functional traits of tropical trees: Partitioning RGR along a light gradient
One of the biggest questions in ecology is how so many species can coexist. This is particularly pertinent in tropical forests, where huge numbers of closely-related species inhabit similar environments. We grew 12 tropical tree species from a hyper-diverse family the Dipterocarpaceae in three different light treatments. While this family is classed as a shade-tolerant guild, there is nevertheless a huge within guild variation in growth rates. Relative Growth Rate (RGR) slows with size. We therefore used non-linear mixed effects models to estimate growth at a common size. RGR can be partitioned into three components; Net Assimilation Rate (NAR); Specific Leaf Area (SLA) and Leaf Mass Ratio (LMR). These components of RGR inherently include plant size variation in the same way that RGR does and thus analysis of these traits can end up showing size effects rather than effects as a direct result of allocation. We estimate leaf mass and leaf area at a common reference mass for each species treatment combination. We use these estimates of leaf mass and area to calculate the three components of size specific RGR at a common reference size.
Results/Conclusions
Not only do larger seeds produce larger seedlings, but they also grow faster even when compared at a common size. We found that investing more biomass into leaves has a cost in terms of growth rate in the dark and low light, yet translates to a growth advantage in the high light. In general plants invested in more biomass by making thicker leaves, and we suggest that thin leaves may have an extra cost in the high light, due to having to spread limited nitrogen more widely. Size effects can dominate analysis and mask important biological differences between species. Comparing growth rates at a common size can produce different relationships than seen with conventional RGR, and this analysis demonstrates the same may be true if traits are also estimated at a common size.