PARENT SESSION
Poster Session #28: Gas Exchange.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Vertical gradients in canopy CO₂-exchange and leaf N in Quercus and Populus at a flux site.

Francis, Janelle^*,1, Curtis, Peter², Ellsworth, David¹, ¹ School of Natural Resources and Environment, Ann Arbor, MI² Dept. of Evolution, Ecology, and Organismal Biology, Columbus, OH

ABSTRACT- While tree canopy foliage accounts for net carbon uptake within a forest, the relative importance of individual tree species and their canopy-level gas exchange characteristics is not well documented. I examined leaf-level canopy light response within a vertical gradient in bigtooth aspen and northern red oak trees in order to determine: (1) Do bigtooth aspen and red oak differ in their photosynthetic light response as predicted by their differing successional niches? (2) Do light-saturated photosynthesis rates (P_max), light compensation points, and dark respiration rates (R_d) increase with height in the canopy, paralleling the expectation that possible carbon uptake is higher for leaves in the upper canopy in order to maximize canopy-level carbon uptake? (3) Do leaf mass per area (LMA), nitrogen per unit mass (N/mass), and nitrogen per area (N/area) vary throughout the canopy of bigtooth aspen and red oak, and if so, are these variables useful predictors of the shape and magnitude of the photosynthetic light response? We found height-related differences in P_max, R_d, light compensation point, LMA, N/mass, and N/area. LMA and N/area were correlated with P_max, R_d, and light compensation point. The species did not differ significantly in these correlations. Overall, LMA was the best predictor of light response variables. Canopy level light response measurements are hard to attain due to logistics and expense, and thus a surrogate measure of leaf-level light response that does not require canopy access will allow easier study of canopy photosynthesis. Our results suggest that LMA and N/area may serve as these proxy measures.

KEY WORDS: photosynthesis, leaf mass per area, nitrogen, gas exchange