Biosphere responses to CO2 enrichment

Atmospheric changes such as elevated CO2 are of global extent, exert prime influences in the remaining wilderness areas, and are second in importance only to effects of land use on ecosystems in most parts of the world. This study is an attempt to summarize, from a biological viewpoint, knowledge of the influences of atmospheric CO2 enrichment on terrestrial ecosystems, as derived from empirical data. I first briefly recall key aspects of the global carbon cycle, mention important conceptual aspects and research tools, and then discuss in greater depth how elevated CO2 is likely to affect vegetation processes. Besides a stimulation of photosynthesis, the most robust findings on plant responses to elevated CO2 are changes in active tissue quality (wider C/N ratio) and effects on community dynamics. Results of experimental work offer a number of plausible projections with respect to future ecosystem processes and organismic interactions, but manipulative experiments appear unsuitable to prove or disprove C sequestration by terrestrial ecosystems. In certain regions, consequences of climatic changes and soluble-nitrogen deposition are likely to be greater than direct CO2 effects on the carbon balance of vegetation. The significance of the ecosystem approach, the use of fully coupled plant-soil systems, and the consideration of nonlinear responses are highlighted. The current understanding of the CO2 problem offers sufficient justification to urge measures for moderating human forcing of atmospheric change.