Using constraint lines to characterize plant performance

We used the self-thinning law to model constraints on the performance of desert annual plants in competitive situations which were also influenced by many other biotic and abiotic factors. We then applied the model to field data for two desert annual species measured over 384 permanent quadrats. Based on the self-thinning law, competition for limiting resources from conspecifics and heterospecifics places an upper boundary on the survivorship of desert annuals. Simple trade-off models are also developed to define constraint lines for another measure of performance, density under interspecific competition. Similar constraint lines can be potentially fitted to field data where still other factors also influence plant performance. Compared with most conventional methods, such as linear regression and multivariate techniques, models of constraint lines are better able to characterize the effects of the major limiting processes in complex systems in which many other factors contribute to the observed variation. This approach offers a powerful tool in dealing with complex ecological patterns; it should be generally applicable to many other systems.