Differential response of aspen and birch trees to heat stress under elevated carbon dioxide

The effect of high temperature on photosynthesis of isoprene-emitting (aspen) and non-isoprene-emitting (birch) trees were measured under elevated CO₂ and ambient conditions. Aspen trees tolerated heat better than birch trees and elevated CO₂ protected photosynthesis of both species against moderate heat stress. Elevated CO₂ increased carboxylation capacity, photosynthetic electron transport capacity, and triose phosphate use in both birch and aspen trees. High temperature (36-39 °C) decreased all of these parameters in birch regardless of CO₂ treatment, but only photosynthetic electron transport and triose phosphate use at ambient CO₂ were reduced in aspen. Among the two aspen clones tested, 271 showed higher thermotolerance than 42E possibly because of the higher isoprene-emission, especially under elevated CO₂. Our results indicate that isoprene-emitting trees may have a competitive advantage over non-isoprene emitting ones as temperatures rise, indicating that biological diversity may be affected in some ecosystems because of heat tolerance mechanisms.