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.
-In a mature beech stand located in north-eastern Germany, xylem sap flux measurements were continuously performed during the 2002−2004 growing seasons. Ten representative trunks were studied using heated thermal dissipation probes. The measurements aimed at identifying principles governing radial profiles of xylem flux in beech trunks. The measurements were taken up to a trunk depth of 132 mm. The sap flow density in the pericambial xylem was found to vary among trees of different diameters, but was not considerably smaller in suppressed trees. A model for the radial distribution of sap flux density was formulated relating trunk radius and sap flow density. The model takes into account different trunk diameter. About 90% of the sap flux was found to occur in the outer two fifths of the trunk. Using this model, an adequate estimate of transpiration can be achieved at tree and stand level, even when the sap flux measurements are restricted to the outer trunk sectors. Les mesures ont été réalisées jusqu'à une profondeur de 132 mm. La densité de flux de sève du xylème de la zone cambiale variait d'arbre en arbre en fonction du diamètre, mais cette densité ne diminuait pas sensiblement dans les arbres dominés. Un modèle de distribution radiale de la densité de flux de sève a été mis au point dans lequel le diamètre du tronc et la densité du flux de sève sont mis en relation. Le modèle prend en considération les arbres ayant des troncs de diamètres différents. Environ 90 % de l'eau circule dans les deux cinquièmes extérieurs du tronc. De cette façon, il est possible de calculer de manière suffisamment exacte la transpiration de l'arbre ou du peuplement tout entier, même si les mesures du flux de sève se limitent à la zone externe du tronc. densité de flux de sève / modèle radial / hêtre
Periods of drought frequently affect the development of winter rape at the shooting stage in spring. A growth chamber study was conducted to determine the effects of 13 days of strong water deficit at the shooting stage followed by rewatering on oilseed rape (Brassica napus). The osmolality and the efficiency of photosystem II in leaves were measured by means of a vapour pressure osmometer and a chlorophyll fluorometer, respectively. The activities of sucrose cleaving enzymes in leaves were determined through invertase as well as sucrose synthase tests. A higher osmotic pressure was observed 2 days after water deprivation. Extracellular and vacuolar invertases as well as chlorophyll fluorescence were decreased with delay. Rewatering led to a gradual decrease in the osmotic pressure and a delayed increase in the enzyme activities as well as the photosynthetic efficiency. A linear regression model could predict osmolality in well‐watered plants based on the extracellular enzyme activity and growth stage. The ontogenetic and stress‐related pattern of extracellular invertase activity could indicate a role of this enzyme in balancing source‐sink relations. The onset of flowering was not influenced by drought, but the period of this stage was prolonged. Though, the ontogenetic stages of stressed and unstressed plants were the same at the time of harvest, the development of seed biomass was significantly depressed under stress.
-To evaluate the impact of herb layer structure on the transpiration of Scots pine ecosystems in north-eastern Germany, we measured tree canopy and herb layer transpiration in three stands. Parameters
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