WEEDS IN A CHANGING CLIMATE

Current and projected increases in the concentrations of CO2 and other radiatively-active gases in the Earth's atmosphere lead to concern over possible impacts on agricultural pests, All pests would be affected by the global warming and consequent changes in precipitation, wind patterns, and frequencies of extreme weather events which may accompany the ''greenhouse effect.'' However, only weeds are likely to respond directly to the increasing CO2 concentration, Higher CO2 will stimulate photosynthesis and growth in C-3 weeds and reduce stomatal aperture and increase water use efficiency in both C-3 and C-4 weeds, Respiration, and photosynthate composition, concentration, and translocation may be affected, Perennial weeds may become more difficult to control, if increased photosynthesis stimulates greater production of rhizomes and other storage organs, Changes in leaf surface characteristics and excess starch accumulation in the leaves of C-3 weeds may interfere with herbicidal control, Global warming and other climatic changes will affect the growth, phenology, and geographical distribution of weeds, Aggressive species of tropical and subtropical origins, currently restricted to the southern U.S., may expand northward. Any direct or indirect consequences of the CO2 increase that differentially affect the growth or fitness of weeds and crops will alter weed-crop competitive interactions, sometimes to the detriment of the crop and sometimes to its benefit.