Interspecific Soil Water Partitioning as a Driver of Increased Productivity in a Diverse Mixed Mediterranean Forest

Abstract

AbstractIt 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.