Abstract: We seek to link plants’ demography to physiology to better understand species differences at the community level. In particular, growth and mortality are two key demographic axes characterizing plant fitness and have been linked to species’ coexistence, and species’ physiological variation may help us characterize their successional niches. Here we ask how species’ photosynthesis traits influence their growth and mortality rates. Compared to morphological traits, physiological traits such as photosynthesis rates have received less attention. Additionally, while these relationships have been examined in temperate forests, less work has been done in tropical communities. To examine these relationships, we measured light response curves for saplings of 30 tree species growing in a moist tropical forest of Panama. From this, we calculated species’ maximum photosynthetic capacity (Amax) and their light compensation point (LCP). LCP is the minimum amount of light needed for a species to maintain zero or positive carbon assimilation rates, below which plants face a carbon deficit and potentially mortality. From a 50-ha Forest Dynamic Plot nearby on Barro Colorado Island, we also calculated maximum relative growth and mortality rates for saplings for these species, as previously defined in Wright et al. 2010 (RGRsap95and MRTsap25). We examined the relationships between species’ physiology and demography using regression models and principal component analyses.
We found a strong growth-mortality trade-off for sapling of our 30 species, which agrees with earlier studies of demography in this community. We used species’ scores on the first principal component axis (PC1) to quantify this trade-off, as more than 90 percent of the variation between growth and mortality was captured in PC1. Of our physiological measurements, we found no relationship between Amax and LCP among saplings of our species. We found that Amax by itself was a strong and significant predictor of growth rates (RGRsap95), mortality rates (MRTsap25), and of the growth-mortality trade-off (PC1). We found weaker relationships between LCP, growth, mortality, and the growth-mortality trade-off. Models containing both Amax and LCP were marginally better at predicting growth than Amax on its own, and significantly improved predictions of mortality compared to LCP on its own. These results suggest that, at the sapling stage, species’ growth and mortality are strongly driven by their maximum photosynthetic capacities, and that resource acquisition is important. Tolerance for carbon starvation, as represented by LCP, plays a role in characterizing species’ mortality, but other sources of mortality may be more important.
