Relationships between stomatal regulation, water-use, and water-use efficiency of two coexisting key Mediterranean tree species

This study aims to test the hypothesis that as leaf water potential decreases, stomatal conductance (g_s) and total water use decrease faster in trees tending toward isohydric behavior than in coexisting anisohydric trees.We measured leaf gas exchange rates in two key Mediterranean species: Pinus halepensis (isohydric) and Quercus calliprinos (anisohydric) growing together in two different sites during seven field campaigns over 14months. Intrinsic water-use efficiency (WUE_i) was calculated from gas exchange ratios, and independently from carbon isotopic composition, δ¹³C, of annual tree-ring sub-sections in four representative growth years.As expected, g_s was greatly restricted already at VPD<3kPa in pine trees whereas in oak trees g_s was dynamically adjusted even at VPD>5kPa. Consequently, mean transpiration rates were 0.2-2.2 and 0.5-3.9mmolm²s^-1 in coexisting pines and oaks, respectively. Mean δ¹³C values were 1.5‰ higher in tree-rings of the pine compared to the oak trees, consistent with the differences in WUE_i between 75 and 64μmol CO₂mol^-1 H₂O in pines and oaks, respectively, based on the short-term gas exchange measurements.A preliminary attempt to upscale the results to typical forest stands of the two species, on annual time-scales, demonstrated that the differences in stomatal regulation and water-use could imply ~30% higher water-use (or ~70% lower water yield) in oak stand compared to pine stand, related to its tendency toward anisohydric behavior. This sets the limit for typical 300treesha^-1 oak and pine stands at the 460 and 360mm iso-precipitation lines, respectively, consistent with their current distribution along the precipitation gradient in our region. The results can help predict or manage changes in species composition in the face of increasing water limitations in Mediterranean regions.