Elevated CO2 affects photosynthetic responses in canopy pine and subcanopy deciduous trees over 10 years : a synthesis from Duke FACE

Leaf responses to elevated atmospheric CO2 concentration (Ca) are central to models of forest CO2 exchange with the atmosphere and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated Ca requires an understanding of how photosynthesis is regulated by diffusional and biochemical components and up-scaled to entire canopies. To test the sensitivity of leaf photosynthesis and stomatal conductance to elevated Ca in time and space, we compiled a comprehensive dataset measured over 10 years for a temperate pine forest of Pinus taeda, but also including deciduous species, primarily Liquidambar styraciflua. We combined over one thousand controlled-response curves of photosynthesis as a function of environmental drivers (light, air Ca and temperature) measured at canopy heights up to 20 m over 11 years (1996-2006) to generate parameterizations for leaf-scale models for the Duke free-air CO2 enrichment (FACE) experiment. The enhancement of leaf net photosynthesis (A net) in P. taeda by elevated Ca of +200 µmol mol-1 was 67% for current-year needles in the upper crown in summer conditions over 10 years. Photosynthetic enhancement of P. taeda at the leaf-scale increased by two-fold from the driest to wettest growing seasons. Current-year pine foliage A net was sensitive to temporal variation, whereas previous-year foliage A net was less responsive and overall showed less enhancement (+30%). Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N (N area), but statistically significant. In contrast, co-dominant and subcanopy L. styraciflua trees showed A net enhancement of 62% and no A net-N area adjustments. Various understory deciduous tree species showed an average A net enhancement of 42%. Differences in photosynthetic responses between overwintering pine needles and subcanopy deciduous leaves suggest that increased Ca has the potential to enhance the mixed-species composition of planted pine stands and, by extension, naturally regenerating pine-dominated stands.