Photosynthetic responses of yellow poplar and white oak to long term atmospheric CO2 enrichment in the field

A critical consideration in evaluating forest response to rising atmospheric CO2 is whether the enhancement of net photosynthesis (PN) by elevated CO2 can be sustained over the long term. There are reports of declining enhancement of PN with duration of exposure to elevated CO2, associated with decreases in photosynthetic capacity and carboxylation efficiency. We investigated whether this photosynthetic acclimation occurs in two tree species under field conditions. Seedlings of yellow-poplar (Liriodendron tulipifera L.) and white oak (Quercus alba L.) were planted in the ground within six open-top field chambers in May 1989 and have been exposed continuously to CO2 enrichment during the last two growing seasons. The three CO2 treatment levels were: ambient, ambient +150, and ambient +300 μL/L. Throughout the second season, gas exchange of upper, light-saturated leaves was surveyed periodically, and leaves of different ages and canopy positions were measured occasionally. Net photosynthesis remained higher at higher CO2 levels (28-32% higher in +150 and 49-67% higher in +300 seedlings) in both species throughout the season, regardless of increasing leaf age and duration of exposure to CO2 enrichment. Stomatal conductance remained unchanged or decreased slightly with increasing CO2, but instantaneous water use efficiency (PN/transpiration) increased significantly with CO2. Analysis of PN versus internal CO2 concentration indicated no significant treatment differences in carboxylation efficiency, CO2-saturated PN, or CO2 compensation point. There was no evidence of a downward acclimation of photosynthesis to CO2 enrichment in this system