Abstract
Aim To assess the combined influences of nutrient enrichment, invasive species and climate on assembly processes in natural annual plant communities. Location S outh‐west W estern A ustralia. Methods A comprehensive survey of winter annual plant communities (more than a thousand communities sampled in total) was undertaken across a natural moisture availability gradient in phosphorus ( P )‐limited Y ork gum woodlands exposed to different levels of anthropogenic P enrichment. Three key functional traits (height, seed mass and specific leaf area) were measured incorporating intraspecific variation. Community richness, dominance of exotic species and trait distributions were investigated along local nutrient and regional climate gradients using hierarchical linear models. Results were assessed against expectations of moisture‐regulated release from nutrient limitation based on trade‐off theories and experimental findings of synergistic water and nutrient effects. Results Consistent with theoretical expectations, we identified significant interactions between local P levels and regional moisture availability for many of the response variables examined. Specifically, in communities exposed to both high P and high moisture we found: (1) reduced species richness, (2) high dominance of exotic species, (3) increases in community mean trait values and (4) strongly narrowing trait ranges. These results are consistent with competitive exclusion (via light competition). In naturally low‐ P situations we also identified shifting trait distributions and narrowing ranges as moisture stress increased, a finding consistent with environmental filtering. Main conclusion In this P‐ and water‐limited system, plant community responses to P enrichment are contingent on regional moisture availability in a non‐additive fashion. The most dramatic changes are seen under high‐ P and high‐moisture conditions, i.e. where productivity is high and light has become a major limiting resource. By empirically validating theory, this study enhances our ability to predict ecological responses to multifaceted drivers of global change.