Summary: 1. 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.2. 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 1 year 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, WUE_wood).3. 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.4. 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.5. The WUE_wood of the five‐species mixtures (2·1 g DM kg⁻¹ H₂O) was about half that of either monocultures, two‐ or three‐species mixtures.6. 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.7. 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.

Summary: Deep‐water access is arguably the most effective, but under‐studied, mechanism that plants employ to survive during drought. Vulnerability to embolism and hydraulic safety margins can predict mortality risk at given levels of dehydration, but deep‐water access may delay plant dehydration. Here, we tested the role of deep‐water access in enabling survival within a diverse tropical forest community in Panama using a novel data‐model approach. We inversely estimated the effective rooting depth (ERD, as the average depth of water extraction), for 29 canopy species by linking diameter growth dynamics (1990–2015) to vapor pressure deficit, water potentials in the whole‐soil column, and leaf hydraulic vulnerability curves. We validated ERD estimates against existing isotopic data of potential water‐access depths. Across species, deeper ERD was associated with higher maximum stem hydraulic conductivity, greater vulnerability to xylem embolism, narrower safety margins, and lower mortality rates during extreme droughts over 35 years (1981–2015) among evergreen species. Species exposure to water stress declined with deeper ERD indicating that trees compensate for water stress‐related mortality risk through deep‐water access. The role of deep‐water access in mitigating mortality of hydraulically‐vulnerable trees has important implications for our predictive understanding of forest dynamics under current and future climates.

Climate change‐related environmental stress has been recognized as a driving force in accelerating forest mortality over the last decades in Central Europe. Here, we aim to elucidate the thermal sensitivity of three native conifer species, namely Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver fir (Abies alba), and three non‐native species, namely Austrian pine (Pinus nigra), Douglas fir (Pseudotsuga menziesii) and Atlas cedar (Cedrus atlantica). Thermal sensitivity, defined here as a decline of the maximum quantum yield of photosystem II (F_v/F_m) with increasing temperature, was measured under varying levels of heat stress and compared with the turgor loss point (π_tlp) as a drought resistance trait. We calculated three different leaf thermotolerance traits: the temperature at the onset (5%) of the F_v/F_m decline (T5), the temperature at which F_v/F_m was half the maximum value (T50) and the temperature at which only 5% F_v/F_m remained (T95). T5 ranged from 38.5 ± 0.8 °C to 43.1 ± 0.6 °C across all species, while T50 values were at least 9 to 11 degrees above the maximum air temperatures on record for all species. Only Austrian pine had a notably higher T5 value than recorded maximum air temperatures. Species with higher T5 values were characterized by a less negative π_tlp compared to species with lower T5. The six species could be divided into ‘drought‐tolerant heat‐sensitive’ and ‘drought‐sensitive heat‐tolerant’ groups. Exposure to short‐term high temperatures thus exhibits a considerable threat to conifer species in Central European forest production systems.