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Abstract. Plant transpiration links physiological responses of vegetation to water supply and demand with hydrological, energy, and carbon budgets at the land–atmosphere interface. However, despite being the main land evaporative flux at the global scale, transpiration and its response to environmental drivers are currently not well constrained by observations. Here we introduce the first global compilation of whole-plant transpiration data from sap flow measurements (SAPFLUXNET, https://sapfluxnet.creaf.cat/, last access: 8 June 2021). We harmonized and quality-controlled individual datasets supplied by contributors worldwide in a semi-automatic data workflow implemented in the R programming language. Datasets include sub-daily time series of sap flow and hydrometeorological drivers for one or more growing seasons, as well as metadata on the stand characteristics, plant attributes, and technical details of the measurements. SAPFLUXNET contains 202 globally distributed datasets with sap flow time series for 2714 plants, mostly trees, of 174 species. SAPFLUXNET has a broad bioclimatic coverage, with woodland/shrubland and temperate forest biomes especially well represented (80 % of the datasets). The measurements cover a wide variety of stand structural characteristics and plant sizes. The datasets encompass the period between 1995 and 2018, with 50 % of the datasets being at least 3 years long. Accompanying radiation and vapour pressure deficit data are available for most of the datasets, while on-site soil water content is available for 56 % of the datasets. Many datasets contain data for species that make up 90 % or more of the total stand basal area, allowing the estimation of stand transpiration in diverse ecological settings. SAPFLUXNET adds to existing plant trait datasets, ecosystem flux networks, and remote sensing products to help increase our understanding of plant water use, plant responses to drought, and ecohydrological processes. SAPFLUXNET version 0.1.5 is freely available from the Zenodo repository (https://doi.org/10.5281/zenodo.3971689; Poyatos et al., 2020a). The “sapfluxnetr” R package – designed to access, visualize, and process SAPFLUXNET data – is available from CRAN.

Stand-level competition and local climate influence tree responses to increased drought at the regional scale. To evaluate stand density and elevation effects on tree carbon and water balances, we monitored seasonal changes in sap-flow density (SFD), gas exchange, xylem water potential, secondary growth, and non-structural carbohydrates (NSCs) in Abies pinsapo. Trees were subjected to experimental thinning within a low-elevation stand (1200 m), and carbon and water balances were compared to control plots at low and high elevation (1700 m). The hydraulic conductivity and the resistance to cavitation were also characterized, showing relatively high values and no significant differences among treatments. Trees growing at higher elevations presented the highest SFD, photosynthetic rates, and secondary growth, mainly because their growing season was extended until summer. Trees growing at low elevation reduced SFD during late spring and summer while SFD and secondary growth were significantly higher in the thinned stands. Declining NSC concentrations in needles, branches, and sapwood suggest drought-induced control of the carbon supply status. Our results might indicate potential altitudinal shifts, as better performance occurs at higher elevations, while thinning may be suitable as adaptive management to mitigate drought effects in endangered Mediterranean trees.Water uptake and carbon metabolism are limited by drought according to the specific sensitivities of physiological processes, like hydraulic failure, carbon starvation, and phloem transport failure, to declining water availability [10][11][12]. Briefly, hydraulic failure appears to usually be related to a high xylem susceptibility to embolism [13][14][15], whereas carbon starvation is related to isohydric stomatal control [16,17]. Phloem transport failure is relative to carbon starvation and seems to be present when the drought-induced limitation of carbohydrate transport to growth exceeds the drought sensitivity of photosynthesis [12,18]. Consequently, the balance between water supply and demand, on the one hand, and sources and sinks of carbohydrates, on the other hand, characterize the inherent species' drought sensitivity and determine how water and carbon relationships affect tree survival [11,14,19]. Indeed, the timing of this sequential reduction, in functions, such as gas exchange and growth, has been largely related to recent events of forest decline and mortality worldwide [11,[16][17][18][19]. Nonetheless, the functional and structural dynamics of these critical processes in response to competition are still not well understood.We focused on the hydraulic characteristics and seasonal patterns of sap flow density (SFD), gas exchange, secondary growth, and non-structural carbohydrate (NSC) of the drought-sensitive fir Abies pinsapo Boiss [20][21][22][23]. We assumed that xylem vulnerability to embolism places a physical limit for A. pinsapo tolerance to water shortage while the dynamics of NSC would be related to drought sensitivity, for the extent o...

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The frequency and intensity of drought events are increasing worldwide, challenging the adaptive capacity of several tree species. Here, we evaluate tree growth patterns and climate sensitivity to precipitation, temperature, and drought in the relict Moroccan fir Abies marocana. We selected two study sites, formerly stated as harboring contrasting A. marocana taxa (A. marocana and A. tazaotana, respectively). For each tree, dendrochronological methods were applied to quantify growth patterns and climate–growth sensitivity. Further, ddRAD-seq was performed on the same trees and close saplings to obtain single nucleotide polymorphisms (SNPs) and related genotype–phenotype associations. Genetic differentiation between the two studied remnant populations of A. marocana was weak. Growth patterns and climate–growth relationships were almost similar at the two sites studied, supporting a negative effect of warming. Growth trends and tree size showed associations with SNPs, although there were no relationships with phenotypes related to climatic sensitivity. We found significant differences in the SNPs subjected to selection in the saplings compared to the old trees, suggesting that relict tree populations might be subjected to genetic differentiation and local adaptation to climate dryness. Our results illustrate the potential of tree rings and genome-wide analysis to improve our understanding of the adaptive capacity of drought-sensitive forests to cope with ongoing climate change.