Agroecosystern responses to combinations of elevated CO2, ozone, and global climate change

Global climate change, caused by increased emissions of greenhouse gases, is likely to affect agroecosystems in many ways, but the outcome, for instance, as a shift in productivity, depends on the combined effects of climate (temperature, precipitation) and other global change components. The focus of this review is on temperature, soil moisture, atmospheric CO2 and tropospheric ozone (O-3). Changes in agricultural productivity can be the result of direct effects of these factors at the plant level, or indirect effects at the system level, for instance, through shifts in nutrient cycling, crop-weed interactions, insect pest occurrence, and plant diseases. Based on results of factorial experiments under a range of experimental conditions, it is difficult to draw generalized conclusions. With respect to C-3 crops, the data suggest that elevated CO2 may have many positive effects, including yield stimulation, improved resource-use efficiency, more successful competition with C-4 weeds, reduced O-3 toxicity, and in some cases better pest and disease resistance. However, many of these beneficial effects may be lost-at least to some extent-in a warmer climate. Warming, accelerates plant development and reduces grain-fill, reduces nutrient-use efficiency, increases crop water consumption, and favors C-4 weeds over C-3 crops. Also, the rate of development of insects may be increased. In grasslands, elevated CO2 stimulates dry matter production, in particular, in N-fixing legumes, but warming again reduces the positive CO2 effect. A major effect of climate warming in the temperate zone could be a change in winter survival of insect pests, whereas at more northern latitudes shifts in phenology in terms of growth and reproduction, may be of special importance. However, climate warming disturbs the synchrony between temperature and photoperiod; because insect and host plant species show individualistic responses to temperature, CO2 and photoperiod, it is expected that climate change will affect the temporal and spatial association between species interacting at different trophic levels. Although predictions are difficult, it seems reasonable to assume that agroecosystem responses will be dominated by those caused directly or indirectly by shifts in climate, associated with altered weather patterns, and not by elevated CO2 per se. Overall, intensive agriculture may have the potential to adapt to changing conditions, in contrast to extensive agricultural systems or low-input systems which may be affected more seriously. (C) 2003 Elsevier Science B.V. All rights reserved.