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

Abstract

Leaf responses to elevated atmospheric CO₂ concentration (C_a) 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 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 CO₂ 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.