Ecological impacts of atmospheric CO2 enrichment on terrestrial ecosystems.

Global change has many facets, of which land use and the change of atmospheric chemistry are unquestioned primary agents, which induce a suite of secondary effects, including climatic changes. The largest single contribution to the compositional change of the atmosphere, CO(2) enrichment, has (besides its influence on climate) immediate and direct effects on plants. Quantitatively, CO(2) is the plant 'food' number one, and the rate of photosynthetic CO(2) uptake by leaves is not yet CO(2)-saturated. This abrupt change of the biosphere's diet does and will affect all aspects of life, including our food. However, the plant and ecosystem responses are more subtle than had been assumed from the results of responses of isolated, well-fertilized and well-watered plants in greenhouses during the early days of CO(2)-enrichment research. In this article, I discuss potential responses of complex natural grassland and diverse forests, and address three key themes: CO(2) and nutrients; CO(2) and water; CO(2) and plant-animal interactions. Examples from a suite of climatic regions emphasize that the most important ecosystem level responses to elevated CO(2) will be introduced by differential responses of species. Atmospheric CO(2) enrichment is a biodiversity issue. Classical physiological baseline responses of leaves to elevated CO(2) can be overrun by biodiversity effects to such an extent that some of the traditional predictions may even become reversed. For instance, biodiversity effects may cause humid tropical forests (those which avoid destruction) to become more dynamic and store less, rather than more, carbon as CO(2) enrichment continues. The abundance of certain life forms and species and their lifespans exert major controls over the half-life of carbon stored in forest biomass, and there is evidence that elevated CO(2) can affect these controls and most likely does so already. Also, long-term hydrological consequences of atmospheric CO(2) enrichment will be driven by biodiversity effects, given that some, but not all, species reduce their rate of water loss when exposed to a CO(2)-rich atmosphere. Such insights call for much more realistic experimental conditions and larger-scale test units, which permit biotic interactions across taxa and trophic levels to occur while simulating our CO(2) future. The evidence currently available suggests that ecosystem processes reflect the composition of their biological inventory and this will be affected by a shift in carbon supply.