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Canopy Effects on Photosynthesis in CO2-Enriched Aspen Seedlings
Reference
Takeuchi, Y., Kubiske, M.E., Isebrands, J.G., Pregitzer, K.S., Hendrey, G. and Karnosky, D.F.  2001.  Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment.  Plant, Cell and Environment 24: 1257-1268.

What was done
In 1997, the authors began growing aspen (Populus tremuloides) clones in 30-m diameter FACE plots in Wisconsin, USA, at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times the ambient concentration) to study the long-term interactive effects of these parameters on growth in this deciduous tree species.  In this paper, they report the photosynthetic and growth responses of seedlings only in the CO2-enriched and control-air plots.  Specifically, they document the influence of canopy depth on net rates of seasonal photosynthesis for the 1999 growing season.

What was learned
Elevated CO2 stimulated the growth and closure of aspen canopies, as indicated by photon flux density measurements near the seedlings' lowermost branches that were only 17 and 9% of the values observed near their uppermost branches in the ambient and CO2-enriched environments, respectively.  Consequently, light availability for photosynthesis decreased with canopy depth, but more so for CO2-enriched seedlings.

Seasonal photosynthetic rates were always greater in the CO2-enriched seedlings; and the CO2-induced stimulation was greater in the upper as opposed to the lower canopy (26 and 3%, respectively).  Similarly, photosynthetic acclimation occurred in CO2-enriched seedlings in a depth dependent manner within the canopy, with less acclimation occurring in the upper as opposed to the lower canopy, as indicated by decreases in foliar rubisco contents of 28 and 50%, respectively, in those two locations.  Nonetheless, on a per-seedling basis, elevated CO2 caused greater total net carbon uptake; and the CO2-enriched seedlings grew 18% taller than the ambiently-grown seedlings during the 1999 study year.

What it means
As the atmospheric CO2 concentration increases, aspen seedlings will likely display enhanced rates of photosynthesis and growth that should allow them to sequester greater amounts of carbon.


Reviewed 6 March 2002