Leaves are key organs that assimilate light and carbon into carbohydrates available for plant growth. Yet measures of leaf photosynthetic capacity, such as leaf mass per area (LMA) and nitrogen concentration (Nmass), are often poor predictors of species’ differences in sapling growth rates. Here we considered whether leaf traits might correlate with components of growth more attributable to light response. Specifically, we considered whether species’ growth rates in favorable environments (optimal growth) were higher or lower than expected based on their average growth rates – all species grew faster in favorable environments, but some grew even faster than expected. We considered this extra increase (or lack thereof) a metric of species’ acquisitiveness. We examined two aspects of traits against this metric of optimal growth for saplings growing in tropical understories. First, we considered how species’ mean LMA, Nmass, and wood density (WD) correlated with this optimal growth once average growth was controlled for. Second, we correlated this metric of growth with species’ ability to make more acquisitive leaves when growing in high light environments, quantified as the percent change in LMA and Nmass between individuals growing in sun versus shade.
Results/Conclusions
Previous research has shown that across species, lower values of LMA are associated with higher photosynthetic capacity, while within species, plants increase LMA to better harvest light when growing in high light conditions. Here, we found both trends also correlated with species’ differences in optimal growth after controlling for average growth. Species with lower mean LMA grew faster than expected in favorable environments, and species with a greater percent increase in LMA when exposed to higher light conditions also had higher optimal growth than expected. WD was not correlated with this measure of acquisitiveness. Nmass results were more ambiguous. Within species, there was a significant decrease in Nmass when exposed to sun for species that grew faster than expected. This counter-intuitive decrease in leaf nitrogen concentration may be due to a concurrent, faster increase in LMA when leaves are more exposed to light. When we did not control for average growth rates, species’ growth in favorable environments were primarily predicted by WD. These results suggest that if we controlled for average growth rates first, LMA becomes an important predictor of species’ acquisitiveness, and that the intraspecific plasticity of Nmass may be more important to consider than average values.