Assessment of interspecific interactions in plant communities: an illustration from the cold desert saltbush grasslands of North America D. Carl Freeman* & John M. Emlen† *Department of Biological Sciences, W ayne State University, Detroit, M I 48202, U.S.A. †U.S. Fish and Wildlife Service, Seattle, Building 204, NAVAL Station, Seattle, Washington 98115, U.S.A. ( Received 9 October 1993, accepted 15 May 1994) Interspecific interactions influence both the productivity and composition of plant communities. Here, we propose new field procedures and analytical approaches for assessing interspecific interactions in nature and apply these procedures to the salt desert shrub grasslands of western Utah. Data were collected from two grazing treatments over a period of 2 years. The proposed equations were fairly consistent across both treatments and years. In addition to illustrating how to assess interspecific interactions within a community, we also develop a new approach for projecting the community composition as a result of some alteration, i.e. increase or decrease in the abundance of one or more species. Results demonstrate competition both within and between plant life-form groups. While introduced annuals were found to depress profoundly the likelihood of perennial plants replacing themselves, perennials had little influence on annuals. Thus, as native perennials die, they are more likely to be replaced by perennials than for the reverse to occur. Our results suggest that unless conditions change, these communities will become increasingly dominated by introduced annuals. ©1995 Academic Press Limited Keywords: species interaction; competition; cold desert; saltbush; grasslands Introduction Interspecific interactions impact the local abundance and distribution of species. A quantitative understanding of these interactions would enhance our abilities to predict the consequences of anthropogenic activities and natural disturbances (e.g. drought) as well as predicting the pattern of succession. The influence of drought and/or grazing on the population dynamics of the cold desert shrub communities has been the subject of numerous investigations (see Harper, 1959; Holmgren & Hutchings, 1972; Norton & Bermant, 1977; West, 1979, 1982; Clary & Holmgren, 1982; Chambers & Norton, 1993). Typically, these studies Journal of Arid Environments (1995) 31: 179–198 0140–1963/95/020179+ 20 $12.00/0 © 1995 Academic Press Limited