Spatial self-organization of vegetation in water-limited systems: mechanistic causes, empirical tests, and ecosystem-level consequences

Self-organized spatial patterns of vegetation are frequent in water-limited regions and have been suggested as important ecosystem health indicators. However, the mechanisms underlying their formation remain unclear. It has been hypothesized that patterns could emerge from a water-mediated scale-dependent feedback (SDF), whereby interactions favoring plant growth dominate at short distances while growth-inhibitory interactions dominate in the long-range. As precipitation declines, this framework predicts a sequential change from gapped to labyrinthine to spotted spatial patterns. However, we know little about how net plant-to-plant interactions may shift from positive to negative as a function of inter-individual distance, and in the absence of strong empirical support, the relevance of SDF for vegetation pattern formation remains disputed. Alternative theories show that the same sequence of patterns could emerge when interactions between plants are always inhibitory if their intensity decays sharply enough with inter-individual distance. Although these alternative hypotheses lead to visually indistinguishable spatial distributions of plants, they predict different ecosystem-level consequences for the patterns, thus limiting their potential use as ecosystem-state indicators. Therefore, to make reliable ecological predictions, models need to accurately capture the mechanisms at play in the systems of interest. Here, we review existing theories for vegetation self-organization and their conflicting ecosystem-level predictions. We discuss ways to reconcile these predictions. We focus on the mechanistic differences among models, which can provide valuable information to help researchers decide which model to use for a particular system and/or whether it requires modification.