Abstract, Ecological Society of America Annual Meeting, Madison, WI, August, 2001.

Impacts of environmental variation on photosynthetic enhancement in a forest exposed to atmospheric CO2 enrichment

Carla A Gunderson1 and Johnna D Sholtis2

1Oak Ridge National Laboratory, Oak Ridge TN 37831-6422 USA; 2Texas Tech University, Lubbock TX 79409 USA

A mature broad-leaved forest canopy is subject to dynamic environmental variation in temperature, humidity, and precipitation which may modify photosynthetic and stomatal responses to increasing atmospheric [CO2]. Environmental influences on photosynthetic and stomatal responses were evaluated within the canopy of a sweetgum (Liquidambar styraciflua L.) plantation exposed to free-air CO2 enrichment (FACE) for three growing seasons. Photosynthetic CO2 assimilation averaged 46% higher in elevated CO2 (560 ppm) than in controls (360 ppm), in foliage throughout the canopy. The enhancement did not decline with length of exposure, and varied (in both directions) only when vapor pressure deficits were high and soil water potentials low, late in the first two growing seasons. The response of stomatal conductance to CO2 enrichment was less consistent (-24%, on average), and was inversely related to relative photosynthetic response. Drought and high vapor pressure deficits reduced absolute conductance and photosynthesis in both treatments, and increased variability in relative effects of [CO2] on gas exchange. Increasing leaf temperature had a small positive effect on photosynthesis and conductance, but there was no apparent relationship between prevailing temperature and the CO2 enhancement of photosynthesis. Because variation in photosynthetic response occurred chiefly at very low stomatal conductances (< 0.2 mol m-2 s-1), environmental or seasonal limitations to stomatal function may have transitory impacts on relative CO2 effects, but will not impact long-term photosynthetic stimulation.

Key-words: elevated CO2, photosynthesis, Liquidambar styraciflua (sweetgum), FACE, trees, stomatal conductance, vapor pressure deficit, drought, temperature