Recent theory supports the long-held proposition that coexistence is promoted by species-specific responses to a spatially varying environment. The underlying coexistence mechanism, the spatial storage effect, can be quantified by the covariance between response to the environment and competition. Here, "competition" is generalized to encompass similar processes such as facilitation and apparent competition. In the present study, we use a model field system of desert annual plants to demonstrate this method and to provide insight into the dynamics of the field system. Specifically, we use neighborhood competition experiments to quantify the spatial storage effect and compare it to the separate (but not mutually exclusive) process of neighborhood-scale resource partitioning. As our basic experimental design has been used frequently in community ecology, these methods can be applied to many existing data sets, as well as future field studies.