Interactive effects of water and CO2 on light response efficiency and gas exchange traits in pine (Pinus) and spruce (Picea) species

Photosynthetic light response curves were parameterized for eight species in two genera, Pinus and Picea, grown in a 2 × 2 factorial of atmospheric CO 2 and soil moisture treatments. Four of the pines and three of the spruces are native to eastern North America, and the fourth spruce, Norway spruce (Picea abies (L.) Karst.), is native to Europe. There was a significant genus × CO 2 interaction in apparent quantum efficiency (AQE): spruce AQE was greater under ambient CO 2 (CO 2 ) than elevated CO 2 (eCO 2 ), but pine AQE were equal. Under drought treatment (DR), AQE declined for both genera. Assimilation at light saturation (A lsat ) was greater for spruces than pines, and for both genera, A lsat decreased under eCO 2 and DR. Water-use efficiency was greater for pines than spruces and greater for pines and unchanged for spruces under DR. Examining AQE and A lsat change (%) from aCO 2 to eCO 2 , there was a significant positive relationship to biomass growth stimulation (%) across species. These relationships support the theory of sink (biomass growth) regulation of assimilation traits and also the importance of needle nitrogen. Our results in response to eCO 2 and DR suggest a shift toward increased use of pines in forest management for eastern North America.