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Forest evapotranspiration (ET) is one of the most important factors influencing the terrestrial hydrological cycle and is prone to being influenced by increasing drought events. This highlights the need to understand the interaction between global forest ET and drought. Consequently, we drew 710 sets of ecosystem-scale ET observations from 69 forest sites around the world and then systematically assessed the ET anomalies during droughts across the dominant forest and climate types. Overall, the response of forest ET to drought is non-monotonic. Under severe and extreme droughts with long durations, ET was reduced in most of forests in the world. We attributed the decreased forest ET to both restricted water supply and restricted water consumption by forests; however, lower magnitude and short-term droughts generally increased forest ET, and in some cases, ET even exceeded precipitation during droughts. We attributed this to the increased surface evaporation and the utilization of deep water by deep root systems. Specifically, we find that the positive anomaly of ET under droughts implied the drought paradox, which acts to accelerate the terrestrial hydrological cycle and further amplify the drought. This study as a global synthetic analysis of case studies on the site scale may have great potential for understanding the importance of the drought-modulated forest water cycle and the possibility of increasing drought stress with the effects of the drought paradox. Key words: climate change, water balance, forest hydrology, drought paradox, evapotranspiration. L'evapotranspiration (ET) de la foret est un des plus importants facteurs qui influencent le cycle hydrologique terrestre tout en etant sujette a l'influence des episodes de secheresse de plus en plus frequents. Cela souligne la necessite de comprendre l'interaction entre l'ET globale de la foret et la secheresse. Par consequent, nous avons extrait 710 ensembles d'observations d'ET a l'echelle de l'ecosysteme dans 69 stations forestieres partout dans le monde et nous avons ensuite systematiquement evalue les anomalies de l'ET lors de secheresses parmi les types dominants de forets et de climats. Globalement, la reaction de l'ET de la foret a la secheresse n'est pas monotone. Lors de secheresses severes et extremes qui persistent longtemps, l'ET a diminue dans la plupart des forets dans le monde. Nous avons attribue la diminution de l'ET de la foret a l'approvisionnement restreint en eau et a la consommation reduite d'eau par les forets; cependant, les secheresses moins severes et de courte duree ont generalement augmente l'ET de la foret et dans certains cas l'ET a meme excede la precipitation durant les periodes de secheresse. Nous avons attribue cela a l'augmentation de l'evaporation superficielle et a l'utilisation de l'eau en profondeur par le systeme de racines profondes. Plus precisement, nous trouvons que l'anomalie positive de l'ET en periodes de secheresse implique le paradoxe de la secheresse qui accelere le cycle hydrologique terrestre et amplifie encore davantage la secheresse. Cette etude, en tant qu'analyse synthetique globale d'etudes de cas a l'echelle de la station peut avoir un potentiel eleve pour comprendre l'importance du cycle de l'eau en foret module par la secheresse et la possibilite que le stress du a la secheresse augmente a cause des effets du paradoxe de la secheresse. [Traduit par la Redaction] Mots-cles: changement climatique, bilan hydrique, hydrologie forestiere, paradoxe de la secheresse, evapotranspiration., Introduction Evapotranspiration (ET), which mainly includes soil evaporation and vegetation transpiration, is a critical component of the water cycle in terrestrial ecosystems (Kool et al. 2014) and links water and [...]

Key message: The responses of tree leaf δ13C to climatic and geographic factors were different among major climate zones in China, suggesting that the environmental influences on tree ecophysiology are different among climates. Leaf stable carbon isotope composition (δ13C) integrates information about environmental effects on plant carbon and water balance. However, extensive inconsistent findings have been reported regarding the variability of leaf δ13C along environmental gradients, impeding the interpretations of plant physiological responses to environmental changes using δ13C values. In this study, data from 505 tree leaf δ13C measurements and climatic and geographic information at 186 sites across China were collected from peer-reviewed articles. The dataset was grouped according to the Köppen–Geiger climate classification for investigating the variations in tree leaf δ13C with climatic and geographic factors among different climate zones. Multiple regression analyses were conducted using this dataset to provide a predictive model for tree leaf δ13C variability in China. The results showed that the tree leaf δ13C was − 28.05‰ on average and was highest in the polar climate (− 26.82‰), lowest in the warm temperate climate (− 29.66‰), and almost the same in the temperate arid (− 27.60‰) and cold temperate (− 27.69‰) climate zones. We found that the magnitude of the leaf δ13C values of certain climates was partly determined by the functional type of local tree species. Moreover, the relationship between tree leaf δ13C and mean annual precipitation (MAP) was found to be significantly positive only for the temperate arid climate zone, while significant negative correlations were found for the other climate zones. Tree leaf δ13C was significantly negatively related to mean annual temperature (MAT) in the warm temperate and polar climate zones, and it was positively related to altitude in the warm temperate and cold temperate climate zones. These results suggest that the widely observed inconsistent responses of leaf δ13C to environmental factors, as well as the inconsistent δ13C values of a specific tree functional type, can be attributed to the climate conditions in which the tree inhabits. Finally, a simple regression model incorporating MAT, MAP, and altitude was proposed, which explained 45% of the variation in tree leaf δ13C. These findings could be useful in evaluating potential changes in tree physiological traits under the changing climate in China. [ABSTRACT FROM AUTHOR]

Water retained on tree leaves after rainfall, dew, or fog impacts transpiration. To determine the impact of leaf retained water (LRW) on the transpiration of trees, the difference between transpiration rates of different species with and without water sprayed on the leaves was measured. Results show that transpiration was inhibited by LRW. Both the inhibition extent and duration of LRW were higher on broadleaf species than on coniferous species. Under conditions of saturated LRW of the tree crown, mean transpiration inhibition rates of the test species were 0.32 (Quercus mongolica Fisch. ex Ledeb.), 0.25 (Acer mono), 0.37 (Tilia amurensis Rupr.), 0.31 (Fraxinus mandshurica Rupr.), 0.22 (Pinus koraiensis Siebold & Zucc.), 0.22 (Abies nephrolepis (Trautv. ex Maxim.) Maxim.), and 0.23 (Picea asperata Mast.). Mean inhibition rate and inhibition time for broadleaf species were 0.31 and 115 min, which were 40% higher and 27 min longer than those for coniferous species, respectively. The transpiration inhibition rate of LRW increased linearly with the LRW amount, and there was a higher transpiration inhibition rate in broadleaf species than in coniferous species for the same amount of LRW. Mean rising velocity of the inhibition with LRW amount for broadleaf species was 0.53%, whereas it was only 0.15% for the coniferous species. Key words: temperate tree species, leaf retained water, transpiration inhibition. L'eau retenue sur les feuilles des arbres apportee par la pluie, la rosee ou le brouillard a un impact sur la transpiration. Pour determiner l'impact sur la transpiration des arbres de l'eau retenue sur les feuilles (ERF), la difference entre le taux de transpiration apres avoir ou non pulverise de l'eau sur les feuilles de differentes especes a ete mesuree. Les resultats montrent que la transpiration est inhibee par l'ERF. Tant l'ampleur de l'inhibition que la persistance de l'ERF etaient plus elevees chez les feuillus que chez les coniferes. Lorsque que la cime des arbres etait saturee par l'ERF, le taux moyen d'inhibition de la transpiration des especes testees atteignait 0,32 (Quercus mongolica Fisch. ex Ledeb.), 0,25 (Acer mono), 0,37 (Tilia amurensis Rupr.), 0,31 (Fraxinus mandshurica Rupr.), 0,22 (Pinus koraiensis Siebold & Zucc.), 0,22 (Abies nephrolepis (Trautv. ex Maxim.) Maxim.) et 0,23 (Picea asperata Mast.). Chez les feuillus, le taux moyen et la duree moyenne de l'inhibition etaient de 0,31 et 115 min, soit respectivement 40 % plus eleve et 27 min plus longue que chez les coniferes. Le taux d'inhibition de la transpiration de l'ERF augmentait lineairement avec la quantite d'ERF et le taux d'inhibition de la transpiration etait plus eleve chez les feuillus que chez les coniferes pour la meme quantite d'ERF. L'augmentation moyenne de la vitesse d'inhibition en fonction de la quantite d'ERF atteignait 0,53 % chez les feuillus mais seulement 0,15 % chez les coniferes. [Traduit par la Redaction] Mots-cles: especes d'arbres des zones temperees, eau retenue sur les feuilles, inhibition de la transpiration., Introduction Tree transpiration is a critical component of land surface evapo-transpiration (Baldocchi et al. 1991; Yang et al. 1997; Amarakoon et al. 2000). To understand changes in forest water resources, [...]