Response of Forest Trees to Increased Atmospheric CO2

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Abstract
The CO 2 fertilization hypothesis stipulates that rising atmospheric CO 2 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 CO 2 -enriched experiments with trees and empirical dendrochronological studies detecting CO 2 fertilization effects in tree-rings. This will allow evaluation of tree growth responses to atmospheric CO 2 enrichment by combining evidence from both ecophysiology and tree-ring research. Based on considerable experimental evidence of direct CO 2 fertilization effect (increased photosynthesis, water use efficiency, and above- and belowground biomass), and predications from the interactions of enriched CO 2 with temperature, nitrogen and drought, we propose that warm, moderately drought-stressed ecosystems with an ample nitrogen supply might be the most CO 2 responsive ecosystems. Empirical tree-ring studies took the following three viewpoints on detecting CO 2 fertilization effect in tree-rings: 1) finding evidence of CO 2 fertilization effect in tree-rings, 2) attributing growth enhancement to favorable climate rather than atmospheric CO 2 enrichment, and 3) considering that tree growth enhancement might be caused by synergistic effects of several factors such as favorable climate change, CO 2 fertilization, and anthropogenic atmospheric deposition (e.g., nitrogen). At temperature-limiting sites such as high elevations, nonfindings of CO 2 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 CO 2 response, 2) old trees past half of their maximum life expectancy and consequently only a small increase in biomass increment due to CO 2 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 CO 2 -related trends in tree-rings, and 4) carbon partitioning and growth within a plant that is species-specific. Our review supports the atmospheric CO 2 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.