Canopy photosynthesis and carbon cycling in a deciduous forest: Implications of species composition and rising concentrations of carbon dioxide

Atmospheric {dollar}rm COsb2{dollar} concentrations are rising due to fossil fuel combustion and deforestation. Temperate forests are a large component of the global carbon cycle, contributing to seasonal oscillations in the {dollar}rm COsb2{dollar} record. {dollar}rm COsb2{dollar} is a primary plant resource. As it becomes more plentiful in the atmosphere, the photosynthetic rate and net carbon uptake of vegetation may increase, acting as a negative feedback on future {dollar}rm COsb2{dollar} concentrations. However, the extent to which the forest carbon sink may increase due to elevated {dollar}rm COsb2{dollar} atmospheres is not clear.; My thesis has two components: (1) canopy photosynthesis in a mixed deciduous forest; and (2) a novel method for inferring mature tree foliar response to rising {dollar}rm COsb2{dollar} concentrations based on seedlings' responses. I consider the species-specific photosynthetic properties of foliage in the canopy top (20 m) and sub-canopy (14-16 m) of four co-occurring tree species: red oak, red maple, white birch, and yellow birch. Diurnal patterns of light levels, leaf and air temperature, and vapor pressure deficit, contribute to diurnally varying photosynthetic rates. Large differences in these environmental factors occur through the season, partly leading to seasonal differences in species-specific photosynthesis. Slopes of maximal photosynthetic rate (Pmax) versus nitrogen concentration are lower in autumn than summer.; Neighboring canopy trees of a species do significantly differ; however, I found that differences between species in Pmax and nitrogen content are greater than differences between individuals within a species. More variation is accounted for by differences among leaves in a single tree, than by differences among individuals of a species. This finding is important for studies of canopy physiology, in which data collection is costly and logistically difficult.; To model possible effects of elevated {dollar}rm COsb2{dollar} atmospheres on mature tree foliage, I considered ontogenetic and environmental differences in Pmax. Scaling factors, {dollar}alpha ,{dollar} summarize ontogenetic difference in Pmax of foliage grown in canopy-top and sub-canopy conditions. Differences between Pmax of seedlings grown in the field (on platforms in the canopy) and of seedlings grown in controlled environments (in simulated canopy heights) is the "glasshouse effect", {dollar}Gamma .{dollar} {dollar}beta{dollar} is the {dollar}rm COsb2{dollar}-induced photosynthetic enhancement for seedlings grown in Glasshouse-simulated canopy height conditions. The leaf-level scaling model is valid only if {dollar}alpha{dollar} and {dollar}Gamma{dollar} are near one. For white birch and red maple this model is feasible. In contrast, for red oak both ontogenetic and environmental factors were problematic, and for yellow birch ontogenetic differences were problematic.