Tree diversity enhances tree transpiration in a Panamanian forest plantation.
Tree plantations play an important role in meeting the growing demand for wood, but there is concern about their high rates of water use. Recent approaches to reforestation in the tropics involve the establishment of multispecies plantations, but few studies have compared water use in mixed vs. monospecific stands. We hypothesized that tree species diversity enhances stand transpiration. Tree water use rates were estimated in monocultures (n=5), two-species mixtures (n=3), three-species mixtures (n=3) and five-species mixtures (n=4). Sap flux densities were monitored with thermal dissipation probes in 60 trees for 1year in a 7-year-old native tree plantation in Panama. We also estimated changes in the amount of wood produced per unit water transpired (i.e. water use efficiency, WUEwood). Annual stand transpiration rates in two-/three-species mixtures (464±271 mm year-1) and five-species mixtures (900±76 mm year-1) were 14% and 56% higher than those of monocultures (398±293 mm year-1), respectively. Trees growing in mixtures had larger diameters, conductive sapwood and basal area than those in monocultures, which partly explained the enhanced stand transpiration in mixtures. The five-species mixtures maintained equally high stand transpiration rates during wet (2.64±0.30 mm day-1) and dry seasons (2.51±0.21 mm day-1), whereas monocultures and two-species mixtures had significantly lower transpiration rates during the dry season, because of the presence of dry season deciduous species. The WUEwood of the five-species mixtures (2.1 g DM kg-1 H2O) was about half that of either monocultures, two- or three-species mixtures. The comparably high stand transpiration rates in the five-species plots may arise from enhanced vegetation-atmosphere-energy exchange through higher canopy roughness and/or complementary use of soil water. Synthesis and applications. Stand transpiration increased linearly with tree species richness and basal area in monocultures, two- and three-species mixtures, but the ratio of stand transpiration to basal area was larger for five-species mixtures. In conclusion, species selection and consideration of species richness and composition is crucial in the design of plantations to maximize wood production while conserving water resources.