Increased atmospheric CO2 concentrations have already caused large-scale physiological responses of European forests. In particular, the efficiency of water-use of trees, which is coupled to the uptake of CO2 during photosynthesis of leaves and needles has changed significantly. According to the study of a large, interdisciplinary team of researchers, European broadleaf and coniferous trees have increased their water-use efficiency since the beginning of the 20th century by 14% and 22%, respectively.
During photosynthesis trees take up carbon dioxide (CO2) from the air. In return they loose water vapor (H2O) through tiny pores of their leaves or needles, so-called stomata. This gas exchange between trees and the atmosphere is regulated through the opening widths (aperture) of their stomata. Wider apertures of the stomata allow the uptake of higher numbers of CO2 molecules, but promote an increased loss of water vapor (transpiration) into the atmosphere. The opposite holds for narrowed stomatal apertures.
"Assuming that the trees demand for CO2 does not change, they can reduce the aperture of the stomates of their leaves and needles under increasing atmospheric CO2 concentrations. This should lower the rates of transpiration and minimize the tree's water loss", says Gerhard Helle at the GFZ German Research Centre for Geosciences, co-author of the study. "Nevertheless, a 5% increase in European forest transpiration was calculated over the twentieth century. This can likely be attributed to a lengthened growing season, increased transpiration due to a warmer environment, and an enhanced leaf area."