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Photosynthetic Acclimation Within Canopies of Young Aspen Trees
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
The authors grew aspen (Populus tremuloides) clones for two years in 30-m diameter FACE plots located in Wisconsin, USA, where they were exposed to atmospheric CO2 concentrations of 360 and 560 ppm, to study the interactive effects of light intensity and elevated CO2 concentration on photosynthetic acclimation in this deciduous tree species.

What was learned
During the second complete growing season, leaves of seedlings exposed to an atmospheric CO2 concentration of 560 ppm exhibited rates of net photosynthesis that were 26 and 3% greater than those of leaves on ambiently-grown seedlings located within the upper and lower canopy, respectively.  In addition, atmospheric CO2 enrichment reduced the amount of rubisco present in upper- and lower-canopy leaves by 28 and 50%, respectively.  Thus, elevated CO2 induced photosynthetic acclimation in a light-dependent manner; for leaves in the lower canopy received much less irradiance than did leaves in the upper canopy.

What it means
As the atmospheric CO2 concentration continues to rise, aspen seedlings will likely respond by displaying increased rates of net photosynthesis, which should lead to greater biomass production.  It is important to note, however, that the degree of photosynthetic stimulation induced by elevated CO2 at the leaf level will likely be influenced by canopy position, with upper-canopy leaves exhibiting greater photosynthetic enhancements than lower-canopy leaves.  Nonetheless, at the whole-tree level, the current data demonstrate that aspen seedlings will indeed respond positively to future increases in the air's CO2 content.

Reviewed 18 September 2002