Mechanisms behind facilitation-competition transition along rainfall gradients.

Woody cover is rapidly changing due to mortality, shrub encroachment, and afforestation, reshaping herbaceous communities and ecosystem functioning worldwide. Trees and shrubs often facilitate herb growth in dry sites but suppress it in wetter environments. Explanations for this facilitation-to-competition transition lack a clear link to resource competition theory, despite being a fundamental aspect of the broader Stress Gradient Hypothesis. While recent quantitative frameworks exist, explanations remain contested and have yet to reproduce the shift along a rainfall gradient from first principles. Here, we present a mechanistic framework consisting of two submodels: i) canopy shading, which reduces photosynthesis (competition) and evapotranspiration (facilitation), and ii) root effects, including water uptake (competition) and increased moisture via hydraulic redistribution (facilitation). We elucidate the conditions under which interactions shift from facilitation to competition. The models reproduce this reversal only when water is not the sole limiting factor at high rainfall or when woody density increases with precipitation. The two pathways leave distinct signatures: Canopy shading produces a hump-shaped pattern with maximum facilitation at intermediate stress, while the root pathway predicts a shift from positive to negative interactions as water availability increases. More generally, the reversal is robust across mechanisms and gradients: Enhanced infiltration can replace hydraulic redistribution, evaporative demand can replace rainfall, and combined canopy-root interactions preserve the transition. By translating a classic idea into a quantitative framework, this model enhances ecosystem management in a changing world.