Using tree rings to detect a CO2 fertilization effect: a global review

Key message

Evidence for the CFE was mixed. New dendrochronological experimental designs are needed, especially in tropical and boreal biomes. A weakness of current methodologies could impede the ability to detect a CFE.

Abstract

The effects of elevated atmospheric carbon dioxide on plant growth and the carbon cycle have stimulated extensive research. Over the last three decades, CO₂ enrichment experiments have tested the CO₂ fertilization effect (CFE) hypothesis. Yet, CO₂ enrichment studies have been criticized for having unrealistic designs. Multiple studies have sought to use tree-ring analyses to verify the CFE under more natural conditions. To assess the breadth of scholarship about CO₂ fertilization on tree radial growth, this study conducted a tri-lingual (English, French, and Chinese) literature review. Finding no French, just one Chinese, and 73 English articles directly related to this topic, all 74 relevant tree-ring studies (1984–2020) were reviewed in-depth. We found an underrepresentation of studies from key regions important to the global carbon cycle, particularly tropical and boreal biomes. Evidence for the CFE was mixed: about 32.4% of dendrochronological studies found a CFE, another 23% found radial growth trends explained by both CO₂ fertilization and other climatic variables, 5.4% attributed tree growth only to climate change, and 39.2% (primarily in tropical areas) did not find any evidence of a CFE. Synthesized results of global scholarship highlight the need for new dendrochronological experimental designs to exclude climatic and environmental variables and test whether CO₂ fertilization occurs in tropical regions. Furthermore, weakness of current methodologies could lessen the ability to detect a CFE. More focused research is necessary from under-represented regions, and to test the effects of increasing anthropogenic activities (such as localized elevated CO₂ emissions) on tree growth in nature settings.