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, quantification of these effects is lacking. Here we studied water-use dynamics for five tree species in a mature, mixed, evergreen, and Mediterranean forest. We use empirical measurements of key tree structural attributes including root distribution, through DNA barcoding and soil cores, tree height and biomass along with measurements of species-specific water use for two years. These measurements at the tree-scale were used to parameterize an ecosystem model of coupled water, carbon and energy fluxes (Regional Hydro Ecologic Simulation System, RHESSys). Site-scale empirical measurements showed contrasting diurnal and seasonal transpiration and sap flow curves across tree species, with year-round activity in angiosperms, and mostly wet season-activity in gymnosperms. Water-use patterns matched the rooting depth patterns, with the deep- and shallow-rooted Ceratonia and Cupressus, showing year-round and seasonal behaviors, respectively. RHESSys estimates of species-specific and stand-scale transpiration, biomass and productivity across 20 years of climate variation showed substantial differences between mixed and monoculture scenarios. Stand-scale annual net primary productivity and transpiration increased by 20–70 g C m−1 yr−1 and 40–80 mm yr−1, respectively, for mixed stands relative to average fluxes aggregated across monocultures. Model results, collaborated by field data provide evidence for niche partitioning of the soil water resource among co-habiting tree species, and demonstrate that this mechanism can facilitate higher productivity and an enhanced forest carbon sink especially in semi-arid regions.