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

Abstract

<p>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.</p>

Responses of two Acacia species to drought suggest different water-use strategies, reflecting their topographic distribution

“…Our results here show an anisohydric 'waterspender' strategy with a diurnal pattern of peak canopy conductance at noon (Figure 3) when temperatures are at their maximum, and we suggest, indirectly, that this 'waterspender' strategy might be a way to cope with high temperatures and survive where other tree species cannot. Other tree species, from semi-arid and Mediterranean forests, which cope with occasional high temperatures (maximum 40 ˚C) and a long dry season (with VPD of ~4 kPa), usually show a more water-conserving strategy, minimizing their activity during the dry season (and often also middays) to prevent water loss (Maseyk et al, 2008;Klein, 2014;Rog et al, 2021). For example, in a mixed dry Mediterranean forest, transpiration rates during summer were almost zero in Pinus halepensis, Quercus calliprinos, and Cupressus sempervirens (Rog et al, 2021).…”

“…Other tree species, from semi-arid and Mediterranean forests, which cope with occasional high temperatures (maximum 40 ˚C) and a long dry season (with VPD of ~4 kPa), usually show a more water-conserving strategy, minimizing their activity during the dry season (and often also middays) to prevent water loss (Maseyk et al, 2008;Klein, 2014;Rog et al, 2021). For example, in a mixed dry Mediterranean forest, transpiration rates during summer were almost zero in Pinus halepensis, Quercus calliprinos, and Cupressus sempervirens (Rog et al, 2021). In addition, in a P. halepensis forest on the edge of the semi-arid region, reduction in stomatal conductance was observed with the seasonal increase in VPD (from 1 kPa to 4 kPa), resulting in transpiration rates of < 1 mmol m -2 s -1 in the summer months (Maseyk et al, 2008).…”