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 ov er possible impacts on agricultural pests, All pests would be affected by the global warming and consequent changes in precipitation, wind p atterns, 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 stimulat e photosynthesis and growth in C-3 weeds and reduce stomatal aperture and increase water use efficiency in both C-3 and C-4 weeds, Respirati on, and photosynthate composition, concentration, and translocation ma y be affected, Perennial weeds may become more difficult to control, i f increased photosynthesis stimulates greater production of rhizomes a nd other storage organs, Changes in leaf surface characteristics and e xcess starch accumulation in the leaves of C-3 weeds may interfere wit h herbicidal control, Global warming and other climatic changes will a ffect the growth, phenology, and geographical distribution of weeds, A ggressive species of tropical and subtropical origins, currently restr icted to the southern U.S., may expand northward. Any direct or indire ct consequences of the CO2 increase that differentially affect the gro wth or fitness of weeds and crops will alter weed-crop competitive int eractions, sometimes to the detriment of the crop and sometimes to its benefit.