Interspecific soil water partitioning as a driver of increased productivity in a diverse mixed Mediterranean forest

It has been assumed that mixing of species with high physiological diversity reduces competition over water and light resources, compared to single-species forests. Although several mechanisms to explain this observation have been proposed, empiric evidence is lacking. Here we studied water-use dynamics at a monthly resolution for two years in five key tree species in a mature, mixed, evergreen, Mediterranean forest. Root distribution was measured with DNA barcoding and soil cores. Measurements at the tree-scale were up-scaled using an ecosystem model of coupled water, carbon and energy fluxes (Regional Hydro Ecologic Simulation System, RHESSys). Tree species showed contrasting water-use patterns, with year-round activity in angiosperms, and mostly wet season-activity in gymnosperms. Water-use patterns matched the rooting patterns, with the deep- and shallow-rooted Ceratonia and Cupressus, showing year-round and seasonal behaviors, respectively. RHESSys simulations captured well the species-specific behaviors in the mixed forest, and were further applied to simulate monocultures of each of the species, which proved less productive than the mixed forest. Our results provide evidence for niche partitioning of the soil water resource among co-habiting tree species. This partitioning is driven by spatiotemporal species differences in rooting depth and eco-physiology, and facilitates the higher productivity of the mixed forest.