Response of forest trees to increased atmospheric CO2

The CO2 fertilization hypothesis stipulates that rising atmospheric CO2 has a positive effect on tree growth due to increasing availability of carbon. The objective of this paper is to compare the recent literature related to both field CO2-enriched experiments with trees and empirical dendrochronological studies detecting CO2 fertilization effects in tree-rings. This will allow evaluation of tree growth responses to atmospheric CO2 enrichment by combining evidence from both ecophysiology and tree-ring research. Based on considerable experimental evidence of direct CO2 fertilization effect (increased photosynthesis, water use efficiency, and above- and belowground biomass), and predications from the interactions of enriched CO2 with temperature, nitrogen and drought, we propose that warm, moderately drought-stressed ecosystems with an ample nitrogen supply might be the most CO2 responsive ecosystems. Empirical tree-ring studies took the following three viewpoints on detecting CO2 fertilization effect in tree-rings: 1) finding evidence of CO2 fertilization effect in tree-rings, 2) attributing growth enhancement to favorable climate rather than atmospheric CO2 enrichment, and 3) considering that tree growth enhancement might be caused by synergistic effects of several factors such as favorable climate change, CO2 fertilization, and anthropogenic atmospheric deposition (e.g., nitrogen). At temperature-limiting sites such as high elevations, nonfindings of CO2 fertilization evidence could be ascribed to the following possibilities: 1) cold temperatures, a short season of cambial division, and nitrogen deficiency that preclude a direct CO2 response, 2) old trees past half of their maximum life expectancy and consequently only a small increase in biomass increment due to CO2 fertilization effect might be diminished, 3) the elimination of age/size-related trends by statistical detrending of tree-ring series that might remove some long-term CO2-related trends in tree-rings, and 4) carbon partitioning and growth within a plant that is species-specific. Our review supports the atmospheric CO2 fertilization effect hypothesis, at least in trees growing in semi-arid or arid conditions because the drought-stressed trees could benefit from increased water use efficiency to enhance growth. Copyright © Taylor & Francis Group, LLC.