Forests under changing CO2 levels: The expected increase and the unexpected tree responses

Studying plants in a changing world involves a high level of uncertainty. On this background, the ongoing increase in atmospheric CO2 level is very consistent. The response of trees and forests to this change will affect major global cycles and processes, but it is poorly understood. In this talk, I will show three recent examples from our research on tree response to the CO2 increase.

In a free-air CO2 enrichment experiment in a mature forest in Switzerland, we measured a 37% increase in carbon assimilation in spruce trees exposed to 550 ppm CO2, relative to control trees at the same site. Stomatal conductance was similar across the groups, and the increase in photosynthesis was proportional to the CO2 level increase. Yet CO2 had a near-zero effect on tree growth.

Some tree species close stomata under elevated CO2, a response that inspired a drought experiment at three CO2 levels. Young lemon trees exposed to elevated CO2 decreased stomatal conductance and, hence, water-use. In turn, soil water content increased, and trees performed better under drought than trees grown at ambient CO2.

To better understand the divergence in stomatal sensitivity to CO2 among tree species, we collected data from 144 experiments on 57 species, as part of a meta-analysis. Gymnosperm species (conifers) were significantly less sensitive to CO2 than angiosperms (broadleaves). The functional difference can be attributed to morphological, physiological, and evolutionary differences between the groups.

Our studies lead the way to informed predictions of forest function in future CO2 levels.