Ellsworth, D.S., Thomas, R., Crous, K.Y., Palmroth, S., Ward, E., Maier, C., DeLucia, E. and Oren, R. 2012. Elevated CO2 affects photosynthetic responses in canopy pine and subcanopy deciduous trees over 10 years: a synthesis from Duke FACE. Global Change Biology 18: 223-242.
What was done
Ellsworth et al., as they describe it, "compiled a comprehensive dataset measured over ten years for a temperate pine forest of Pinus taeda, but also including deciduous species, primarily Liquidambar styraciflua," which they derived from "over one thousand controlled-response curves of photosynthesis as a function of environmental drivers (light, atmospheric CO2 concentration [Ca] and temperature) measured at canopy heights up to 20 meters over eleven years (1996-2006)," from which they generated "parameterizations for leaf-scale models for the Duke free-air CO2 enrichment (FACE) experiment."
What was learned
The eight researchers report that the enhancement of light-saturated leaf net photosynthesis (Anet) in P. taeda trees by elevated Ca of +200 ppm was 67% for current-year needles in the upper crown of the trees in summer conditions over the ten-year period, while previous-year foliage Anet was enhanced by 30%, with the result that "the mean stimulation in light-saturated Anet averaged over the growing season of all years and across canopy positions and needle age classes was 53 ± 7%." And they add that "the photosynthetic enhancement responses to elevated Ca are mirrored in part by the pine biomass accumulation responses to elevated Ca across different years."
The researchers also report that "co-dominant and subcanopy L. styraciflua trees showed Anet enhancement of 62%," while "various understory deciduous tree species showed an average Anet enhancement of 42%." In addition, they note that "the photosynthetic responses of shaded, understory leaves suggest a capacity to increase photosynthetic carbon capture in elevated Ca in shade-grown plants when measured in sunflecks," citing the work of DeLucia and Thomas (2000). And they note that this response suggests "a competitive advantage to shade-tolerant species adapted for carbon capture in high sunlight or sunflecks in the understory over less shade-tolerant species."
What it means
The comprehensive set of photosynthesis measurements compiled over the course of the Duke Forest FACE study clearly rebuts the progressive nitrogen limitation hypothesis, which posits that the initial growth stimulation of atmospheric CO2 enrichment will dwindle away as time progresses, especially in the case of the pine-hardwood forests of the southeastern United States, which often remove so much nitrogen from the soils in which they grow that they induce what Finzi and Schlesinger (2003) have described as "a state of acute nutrient deficiency that can only be reversed with fertilization." But as has been demonstrated by Ellsworth et al.'s study, such is simply not the case. And in another satisfying implication of their findings, the eight researchers conclude that the observed "differences in photosynthetic responses between the overstory pines and deciduous tree subcanopy suggest that increased Ca may have the potential to enhance the mixed-species composition of planted pine stands," and, by extension, "naturally regenerating pine-dominated stands."
DeLucia, E.H. and Thomas, R.B. 2000. Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory. Oecologia 122: 11-19.
Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.Reviewed 20 June 2012