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Photosynthetic Acclimation of Five Trees to Elevated CO2 and Temperature
Tjoelker, M.G., Oleksyn, J. and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiology 18: 715-726.

What was done
Seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine were grown in controlled-environment chambers for three months at 370 or 580 ppm atmospheric CO2 and day/night temperatures ranging from 18/12 to 30/24C to investigate the interactive effects of these variables on various leaf gas exchange parameters.

What was learned
The 57% increase in the CO2 content of the air significantly stimulated net photosynthesis in all species by an average of 28%, regardless of temperature, over the entire three-month study period. Jack pine was the only species that increased its CO2-induced photosynthetic enhancement with increasing temperature, as the other species exhibited either comparable enhancements or declining trends as air temperature rose. Elevated CO2 decreased leaf nitrogen levels in all species and caused differing degrees of photosynthetic down regulation, with aspen and birch exhibiting a 24% average reduction in rubisco. The mobilization of nitrogen from leaves, coupled with the sustained enhancement of photosynthetic rates, led to increased photosynthetic nitrogen-use efficiencies in all plants grown in elevated CO2. Moreover, elevated CO2 decreased stomatal conductance by 10 to 25% in all species, leading to 40 to 80% increases in their instantaneous water-use efficiencies.

What it means
As the atmospheric CO2 concentration rises, seedlings of quaking aspen, paper birch, tamarack, black spruce and jack pine will likely exhibit increases in photosynthesis that lead to enhanced biomass production. Their CO2-induced photosynthetic enhancements, however, may not be further stimulated by increases in air temperature, except for jack pine. With more CO2 in the air, stomatal conductance should decrease for all five species, thereby allowing greater biomass to be attained with more efficient water-use. Hence, seedlings of all five trees should grow much better in a future world of elevated CO2, where their photosynthetic rates and water-use efficiencies will both be significantly enhanced.

Reviewed 1 November 1998