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ABSTRACT Mangroves are highly salt‐tolerant species, which live in saline intertidal environments, but rely on alternative, less saline water to maintain hydraulic integrity and plant productivity. Foliar water uptake (FWU) is thought to assist in hydration of mangroves, particularly during periods of acute water deficit. We investigated the dynamics of FWU in Avicennia marina and Aegiceras corniculatum by submerging and spraying excised branches and measuring leaf water potential (Ψ) at different time intervals. Daily changes in xylem sap composition (ionic concentrations, pH and surface tension) were monitored during 2 days characterised by the presence of morning dew and difference in tides. In both species, FWU occurred over relatively short times, with leaf Ψ recovering from −4.5 MPa to about −1.5 MPa in 120–150 min. At predawn, Ψ was higher (−1.5 MPa) than sea water Ψ, indicating that leaves had been partially rehydrated by absorbed dew. Tides did not affect Ψ, but high tides increased the overall ionic content of xylem sap. The results indicated mangroves are extremely efficient in absorbing non‐saline water via the leaves and restoring the water balance to Ψ higher than seawater. Changes in xylem sap composition, which were strongly influenced by tides, were not affected by observed FWU. [ABSTRACT FROM AUTHOR]

Heatwaves and soil droughts are increasing in frequency and intensity, leading many tree species to exceed their thermal thresholds, and driving wide‐scale forest mortality. Therefore, investigating heat tolerance and canopy temperature regulation mechanisms is essential to understanding and predicting tree vulnerability to hot droughts. We measured the diurnal and seasonal variation in leaf water potential (Ψ), gas exchange (photosynthesis Anet and stomatal conductance gs), canopy temperature (Tcan), and heat tolerance (leaf critical temperature Tcrit and thermal safety margins TSM, i.e., the difference between maximum Tcan and Tcrit) in three oak species in forests along a latitudinal gradient (Quercus petraea in Switzerland, Quercus ilex in France, and Quercus coccifera in Spain) throughout the growing season. Gas exchange and Ψ of all species were strongly reduced by increased air temperature (Tair) and soil drying, resulting in stomatal closure and inhibition of photosynthesis in Q. ilex and Q. coccifera when Tair surpassed 30°C and soil moisture dropped below 14%. Across all seasons, Tcan was mainly above Tair but increased strongly (up to 10°C > Tair) when Anet was null or negative. Although trees endured extreme Tair (up to 42°C), positive TSM were maintained during the growing season due to high Tcrit in all species (average Tcrit of 54.7°C) and possibly stomatal decoupling (i.e., Anet ≤0 while gs >0). Indeed, Q. ilex and Q. coccifera trees maintained low but positive gs (despite null Anet), decreasing Ψ passed embolism thresholds. This may have prevented Tcan from rising above Tcrit during extreme heat. Overall, our work highlighted that the mechanisms behind heat tolerance and leaf temperature regulation in oak trees include a combination of high evaporative cooling, large heat tolerance limits, and stomatal decoupling. These processes must be considered to accurately predict plant damages, survival, and mortality during extreme heatwaves. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Slot et al. (2024), 244: 1238–1249. [ABSTRACT FROM AUTHOR]

Summary: Global warming and droughts push forests closer to their thermal limits, altering tree carbon uptake and growth. To prevent critical overheating, trees can adjust their thermotolerance (Tcrit), temperature and photosynthetic optima (Topt and Aopt), and canopy temperature (Tcan) to stay below damaging thresholds. However, we lack an understanding of how soil droughts affect photosynthetic thermal plasticity and Tcan regulation.In this study, we measured the effect of soil moisture on the seasonal and diurnal dynamics of net photosynthesis (A), stomatal conductance (gs), and Tcan, as well as the thermal plasticity of photosynthesis (Tcrit, Topt, and Aopt), over the course of 1 yr using a long‐term irrigation experiment in a drought‐prone Pinus sylvestris forest in Switzerland.Irrigation resulted in higher needle‐level A, gs, Topt, and Aopt compared with naturally drought‐exposed trees. No daily or seasonal differences in Tcan were observed between treatments. Trees operated below their thermal thresholds (Tcrit), independently of soil moisture content.Despite strong Tcan and Tair coupling, we provide evidence that drought reduces trees' temperature optimum due to a substantial reduction of gs during warm and dry periods of the year. These findings provide important insights regarding the effects of soil drought on the thermal tolerance of P. sylvestris. [ABSTRACT FROM AUTHOR]