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. © 2011 Blackwell Publishing Ltd.
Ellsworth, DS; Thomas, R; Crous, KY; Palmroth, S; Ward, E; Maier, C; Delucia, E; Oren, R
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