Elevated CO ₂ affects photosynthetic responses in canopy pine and subcanopy deciduous trees over 10 years: A synthesis from Duke FACE

Leaf responses to elevated atmospheric CO ₂ concentration (C _a) are central to models of forest CO ₂ exchange with the atmosphere and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated C _a 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 C _a 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 C _a 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 CO ₂ enrichment (FACE) experiment. The enhancement of leaf net photosynthesis (A _net) in P. taeda by elevated C _a 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 C _a has the potential to enhance the mixed-species composition of planted pine stands and, by extension, naturally regenerating pine-dominated stands.