Forest Sensitivity to Elevated Atmospheric CO2 and its Relevance to Carbon Management

Experimental research investigating the responses of forest trees to rising concentrations of atmospheric CO2 has for the past two decades been justified on the basis of the prominent role of forests in the global carbon cycle. The C cycle regulates the airborne fraction of CO2 from fossil fuel combustion, and, therefore, the rate of increase of CO2 in the atmosphere and the associated effects on global climate. As the atmospheric CO2 concentration increases, the rate of uptake of CO2 by the terrestrial biosphere—particularly forests—may increase via so-called “CO2 fertilization”. Our ability to balance the global C cycle depends in part on different assumptions about the degree of CO2 fertilization. Experiments with trees and forest stands exposed to future concentrations of CO2 have helped to define and quantify the physiological and ecological processes involved in the CO2 fertilization effect and their representation in ecosystem models. Recent interest in the use of forests for C sequestration has thrust elevated CO2 research into a new role, often a role that was not intended in experimental designs. Headlines describing the absence of an additional growth pulse in response to additional CO2 inaccurately blare “Mature Forests Not Necessarily CO2 Sinks,” and “Role of Trees in Curbing Greenhouse Gases Is Challenged”. Here we will consider what current research is saying about the sensitivity of forests to rising CO2 and whether that information should be an important factor in forest management plans for enhancing C sequestration.