Summary and Future Research

The ongoing increase in the concentration of carbon dioxide in the atmosphere, as well as potential changes in temperature and precipitation, may have important consequences for crop losses due to weeds. The physiological plasticity of weeds and their greater intraspecific genetic variation compared with most crops could provide weeds with a competitive advantage in a changing environment. However, because so little experimental work on crop/weed interactions under global change conditions has been carried out under field conditions, it is premature to conclude the magnitude or direction of changes in the interactions. Despite the lack of direct experimental evidence, several effects seem probable. One is that C3 species will be favoured relative to C4 species as [CO2] increases, although exceptions should be anticipated. The fact that many weeds are C4 and most crops are C3 may seem advantageous, but will be of little comfort to those trying to grow C4 crops in competition with C3 weeds. A second likely outcome, driven by increased [CO2] itself and potentially exacerbated by warming, is that some tropical and subtropical weeds will extend their ranges poleward and become troublesome in areas where they are not currently a problem. Thirdly, and most speculative, because of the stimulation of seedling emergence and root and rhizome growth by increased [CO2], and because of decreased stomatal conductance with increasing [CO2], weed control could become more difficult both for mechanical and chemical control measures.

Although much has been learned about responses to global change factors from studies of crops and weeds in controlled environment chambers and glasshouses, field facilities to simulate future environments are now sufficiently available that we urge the development of long-term field studies of crop/weed interactions and weed control under global change conditions. Designing meaningful experiments will be challenging. Perhaps it is worth keeping in mind that crop/weed interactions are local events, and generalizations about how interactions may change with global climate are best built from specific examples. Information obtained from such studies could be of substantial benefit in the development of new weed control strategies in a changing climate.

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