Accurate predictions of coexistence in natural systems require the inclusion of facilitative interactions and environmental dependency

1. Coexistence between plant species is well known to depend on the outcomes of species interactions within an environmental context. The incorporation of environmental variation into empirical studies of coexistence are rare, however, due to the complex experiments needed to do so and the lack of feasible modelling approaches for determining how environmental factors alter specific coexistence mechanisms. 2. In this paper, we present a simple modelling framework for assessing how variation in species interactions across environmental gradients impact on niche overlap and fitness differences, two core determinants of coexistence. We use a novel formulation of an annual plant population dynamics model that allows for competitive and facilitative species interactions, and for variation in the strength and direction of these interactions across environmental gradients. Using this framework, we examine outcomes of plant-plant interactions between four commonly co-occurring annual plant spe...