Climate Change and Agriculture: Promoting Practical and Profitable Responses III - 2 Climate Change Impacts on Weeds Lewis H. Ziska, PhD USDA-ARS Crop Systems and Global Change Laboratory 10300 Baltimore Avenue, Beltsville, MD 20705 (lziska@asrr.arsusda.gov ) Among the questions farmers need to answer regarding weeds, carbon dioxide (CO 2 ), and climate change, the most important are: • How will increased CO 2 effect weed growth? • What effect does increased temperature have on weeds? • How does a change in precipitation effect weed growth? • Why can’t we simply control CO 2 , climate and weed? • If climate change/CO 2 alters weed biology, will this impact human health? How will increased CO 2 effect weed growth? Weeds have a greater genetic diversity than crops. Consequently, if a resource (light, water, nutrients or carbon dioxide) changes within the environment, it is more likely that weeds will show a greater growth and reproductive response. It can be argued that many weed species have the C 4 photosynthetic pathway and therefore will show a smaller response to atmospheric CO 2 relative to C 3 crops. However, this argument does not consider the range of available C 3 and C 4 weeds present in any agronomic environment. That is, at present, the U.S. has a total of 46 major crops; but, over 410 “troublesome” weed species (both C 3 and C 4 ) associated with those crops (Bridges 1992). Hence, if a C 4 weed species does not respond, it is likely that a C 3 weed species will. In addition, many growers recognize that the worst weeds for a given crop are similar in growth habit or photosynthetic pathway; indeed, they are often the same uncultivated or “wild” species, e.g. oat and wild oat, sorghum and shattercane, rice and red rice. To date, for all weed/crop competition studies where the photosynthetic pathway is the same, weed growth is favored as CO 2 is increased (Table 1, Ziska and Runion, In Press). In addition to agronomic weeds, there is an additional category of plants that are considered “noxious” or “invasive” weeds. These are plants, usually non-native whose introduction results in wide-spread economic or environmental consequences (e.g. kudzu). Many of these weeds reproduce by vegetative means (roots, stolons, etc.) and recent evidence indicates that as a group, these weeds may show a strong response to recent increases in atmospheric CO 2 (Ziska and George 2004). How rising CO 2 would contribute to the success of these weeds in situ however, is still unclear. Overall, the data that are available on the response of weeds and changes in weed ecology are limited. Additional details, particularly with respect to interactions with other environmental variables (e.g. nutrient availability, precipitation and temperature) are also needed. University of Vermont Extension, New York State College of Agriculture and Life Sciences 2006