The population dynamics of Anisantha sterilis in winter wheat: comparativedemography and the role of management

1. We report on a 3-year field study designed to monitor the detailed popul ation dynamics of Anisantha sterilis in winter wheat, as well as to explore the consequences of changing broad-scale patterns of management, in the fo rm of reduced fertilizer inputs. 2. In the absence of herbicides, population dynamics were dominated by dens ity-dependent population growth. Unusually, this occurred mainly through de nsity-dependent recruitment. This was estimated to reduce population growth rates by 50-fold, compared with the effects of density-dependent competiti on between plants for resources. Density-dependent recruitment also tended to buffer populations against year-to-year variations in emergence levels. 3. Little evidence for temporal variations in allometric seed production, t he strength of competition between plants for resources or maximal mean pla nt performance was found in this study, or in comparison with previously pu blished data. 4. No aspect of the life cycle was significantly affected by variation in t he level of applied nitrogen fertilizer. In the case of competitive interac tions we postulate that this lack of effect results from reduced intraspeci fic competitive effects as a consequence of decreased maximal plant size un der low nitrogen conditions. The highly competitive nature of the environme nt in which A. sterilis occurs means that such changes tend to mask the eff ects of changing nitrogen levels. 5. Estimates of the effects of cultivation, on the other hand, indicated th at seed germination, and hence population growth, was reduced by up to 90% when ploughing was employed rather than minimum tillage. While to some exte nt the effects of variation in the form of cultivation on population number s may be buffered by density-dependent recruitment, this effect outweighs a ny effects of nutrients or spatial or temporal variability in population dy namics. 6. Using previously published models for the dynamics of Alopecurus myosuro ides, Avena sterilis and Avena fatua, we show that the response of populati ons of Anisantha sterilis to cultivation is very different from that of oth er grass weeds. In addition we show how single species models for populatio n dynamics may be used to predict the responses of weed assemblages to chan ges in forms of management.