Contrasting effects of competition on plant fitness and distributions along environmental gradients

Species distributions depend on the interplay between demographic performance and biotic interactions along environmental gradients, such that competition can decouple species’ realized distributions from intrinsic responses to environmental heterogeneity. However, logistical challenges often necessitate simplifying assumptions about competition across environments, limiting our ability to explain or predict species distributions. Here, we experimentally quantify the consequences of spatially variable competition for the demography and distributions of eight annual plant species across an edaphically heterogeneous grassland. Competition had contrasting effects on demographic responses to the environment depending on the species and environmental gradient. Specifically, competition dampened species’ intrinsic responses to a soil texture gradient, but had relatively little effect on responses to a nutrient (Ca:Mg) gradient. We find congruence between demography and occurrence, as well as absences from suitable sites consistent with competitive exclusion and dispersal limitation. Our work highlights the complex relationship between demography, competition, and spatial distributions in real-world landscapes.