EFFECT OF IRRADIANCE AND VAPOR-PRESSURE DEFICIT ON STOMATAL RESPONSE TO CO2 ENRICHMENT OF 4 TREE SPECIES

The stomatal response of seedlings grown in 360 or 720 mu mol mol(-1) to irradiance and leaf-to-air vapour pressure deficit (VPD) at both 36 0 and 720 mu mol mol(-1) CO2, was measured to determine how environmen tal factors interact with CO2 enrichment to affect stomatal conductanc e, Seedlings of four species with different conductances and life hist ories, Cercis canadensis (L.), Quercus rubra (L.), Populus deltoides ( Bartr. ex Marsh.) x P. nigra (L.), and Pinus taeda (L.), were measured in hopes of identifying general responses, Conductance of seedlings g rown at 360 and 720 mu mol mol-l CO2 were similar and responded in the same manner to measurement CO2 concentration, irradiance and VPD, Con ductance was lower for all species where measured at 720 than when mea sured at 360 mu mol mol(-1) CO2 at both VPDs (similar to 1.5 and simil ar to 2.5 kPa) and all measured irradiances greater than zero (100, 30 0, 600, > 1600 mu mol m(-2) s(-1)). The average decrease in conductanc e due to measurement in elevated CO2 concentration was 32% for Cercis, 29% for Quercus, 26% for Populus, and 11% for Pinus, For all species, the absolute decrease in conductance due to measurement in CO2 enrich ment decreased as irradiance decreased or VPD increased, The proportio nal decrease due to measurement in CO2 enrichment decreased in three o f eight cases: from 0.46 to 0.10 in Populus and from 0.18 to 0.07 in P inus as irradiance decreased from > 1600 to 100 mu mol m(-2) s(-1) and from 0.35 to 0.24 in Cercis as VPD increased from 1.3 to 2.6 kPa.