Seedling performance in heterogeneous light environments: A multiscale approach

I examined light-dependent growth, biomass allocation and survivorship of Dipteryx panamensis, Virola koschynii, and Brosimum alicastrum across a wide range of light conditions in NE Costa Rica. Furthermore, I explored the mechanistic bases of seedling performance in heterogeneous light environments by scaling leaf-level photosynthetic physiology to whole-plant carbon gain in closed-canopy understory microsites. Seedlings of Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum , three species of shade-tolerant canopy trees, were transplanted across a forest-pasture edge, in second-growth forests, and in tree plantations at La Selva Biological Station. Survivorship of Dipteryx and Brosimum was positively related to light availability within low light environments (\u3c1–7% diffuse transmittance), whilst Virola showed no relationship between survivorship and light. Species differed in light-dependent growth, biomass allocation and photosynthetic physiology. Although all species increased biomass growth with increasing light availability, increase in total biomass was accomplished through different allocation patterns. Dipteryx and Virola allocated carbon evenly among leaf, stem and root biomass across light environments. In contrast, Brosimum showed little aboveground growth, allocating carbon almost exclusively to root biomass. These among-species differences in growth and biomass allocation illustrate that shade tolerance can occur through diverse functional pathways. ^ Examining photosynthetic physiology and growth across a wider gradient of light (\u3c1–80% diffuse transmittance) revealed strong relationships between measures of physiology at the whole-plant level and growth. Although, maximum assimilation rates increased asymptotically with light in all species, estimates of daily whole-plant assimilation rates increased linearly. The translation of leaf- to whole-plant assimilation was mediated by efficient light interception as well as the frequency distribution of light in seedling micosites. Specifically, seedlings located in higher light microsites spent more time operating at photosynthetic capacity and thereby fixed more carbon on a daily basis. Whereas leaf-level assimilation rates were positively related to seedling growth, estimates of whole-plant carbon gain provided the strongest predictors of seedling growth in all species. This thesis represents a pioneering effort to quantitatively examine the relationships between leaf-level physiology, whole-plant physiology and growth. My results draw attention to the importance of whole-plant characters for understanding photosynthetic physiology and its relationship to plant growth.