Size-related variation in physiology, carbon gain, and growth of trees in deciduous and evergreen forests

Abstract

A better understanding of species’ light requirements throughout developmental stages is required to improve models of forest dynamics. Although it has recently been shown that light requirements of many species may change as trees increase in size, the traits that underlie these ontogenetic patterns are not well understood. This dissertation characterizes the relationships of tree size with physiological and growth responses of eight tree species varying in leaf habit, biome, and shade tolerance. I examined morphological traits and physiological mechanisms operating at the leaf and whole-plant scale, thus allowing for identification of mechanisms that underlie observed ontogenetic variations in growth rates and shade tolerance. Size-related variation in leaf nutrients and gas exchange rates of each of the eight species were measured. While the general light conditions under which each of the eight species evolved were similar (i.e. all are trees common in forest gaps and understories and vary in shade tolerance within those environments), four of the eight species were temperate deciduous and four were subtropical evergreens. Therefore, my data are most relevant at the species or local community level and in qualitative comparisons across leaf habit. Generally speaking, shade-tolerant species and species with long-lived leaves expressed low rates of gas exchange and nitrogen content, while light-demanding species and species with shorter leaf lifespans showed higher rates of gas exchange and leaf nitrogen. However, size-related patterns of leaf-level traits were inconsistent in both deciduous and evergreen species, making it difficult to disentangle the factors responsible for these changes. To further investigate the effect of tree size on shade tolerance, we examined a combination of leaf and whole-plant traits of juvenile trees. Increased sapling size caused significant declines in shade tolerance and relative growth rates (RGR). Among the four evergreens, the slope of the relationship between light availability and RGR was steepest in light-demanding species, leading to crossovers in RGR between shade-tolerant and light-demanding species at low light, independent of sapling size. We found that no single factor can explain reductions in growth and shade tolerance with increased tree size, but that variations in self-shading and net daily carbon gain rates per gram of aboveground tissue were strong predictors of the declines. My results indicate that work on saplings and mature trees may detect differences in shade tolerance and associated traits that are not evident in studies utilizing seedlings alone. These differences could have important implications for modeling the dynamics of forest types.