The population dynamics of Anisantha sterilis in winter wheat: comparative demography and the role of management.
A 3-year field study designed to monitor the detailed population dynamics of Anisantha sterilis in winter wheat, and to explore the consequences of changing broad-scale patterns of management in the form of reduced fertilizer inputs, was conducted at Higham, Suffolk, UK. In the absence of herbicides, population dynamics were dominated by density-dependent population growth. Unusually, this occurred mainly through density-dependent recruitment. This was estimated to reduce population growth rates by 50-fold, compared with the effects of density-dependent competition between plants for resources. Density-dependent recruitment also tended to buffer populations against year-to-year variations in emergence levels. Little evidence for temporal variations in allometric seed production, the strength of competition between plants for resources or maximal mean plant performance was found in this study, or in comparison with previously published data. No aspect of the life cycle was significantly affected by variation in the level of applied nitrogen fertilizer. In the case of competitive interactions it is postulated that this lack of effect results from reduced intraspecific competitive effects as a consequence of decreased maximal plant size under low nitrogen conditions. The highly competitive nature of the environment in which A. sterilis occurs means that such changes tend to mask the effects of changing nitrogen levels. Estimates of the effects of cultivation, on the other hand, indicated that seed germination, and hence population growth, was reduced by up to 90% when ploughing was employed rather than minimum tillage. While to some extent the effects of variation in the form of cultivation on population numbers may be buffered by density-dependent recruitment, this effect outweighs any effects of nutrients or spatial or temporal variability in population dynamics. Using previously published models for the dynamics of Alopecurus myosuroides, Avena sterilis and Avena fatua, it is shown that the response of populations of Anisantha sterilis to cultivation is very different from that of other grass weeds. In addition it is shown how single-species models for population dynamics may be used to predict the responses of weed assemblages to changes in forms of management.