Global transpiration data from sap flow measurements : the SAPFLUXNET database

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.Peer reviewe

Effect of contrasting water supply on the diameter growth of Norway spruce and aspen in mixed stands: a case study from the southern Russian taiga

Stem radial growth of Norway spruce (Picea abies (L.) Karst.), aspen (Populus tremula L.), birch (Betula alba L.), alder (Alnus incana (L.) Moench) and mountain ash (Sorbus aucuparia L.) was measured in an unmanaged mixed uneven-aged tree stand in the southern taiga of European Russia by band dendrometers during the growing seasons of 2000–2001. In addition woody cores were taken from sample spruce and aspen trees using increment borers for dendrochronological analysis. Analysis of the tree rings was made for period from 1999 to 2002, the period while daily meteorological data were available. Spruce and aspen represented 40 and 44% of the entire stand basal area, respectively. However, basal area has grown over 60% of the total in spruce and less than 15% in aspen for each year during the period of 1999–2002. These results indicate the transformation processes of secondary aspen-spruce stands of the Southern European Taiga into a mixed spruce-aspen-birch stands. The most intensive radial growth of the spruce trees was observed in 2000 with high over-watering conditions in summer, whereas the highest radial growth in aspen was observed during the extremely dry growing season of 2002. The basal area growth for entire forest stand ranged from 5.3 m2 ha–1 in 1999 to 11.4 m2 ha–1 in 2000. Annual increments of radial growth measured by dendrometers and by microscopic evaluation of woody cores were quite similar in spruce. In contrast, this comparison was poor for aspen trees because zero growth in some of aspen trees (measured by dendrometers) was occurred.Effets d’une alimentation en eau variée sur la croissance en diamètre de l’épicéa et du tremble dans des peuplements mélangés étudiés dans le sud de la taïga russe. La croissance radiale saisonnière de l’épicéa (Picea abies (L.) Karst.), du tremble (Populus tremula L.), du bouleau (Betula alba L.), de l’aulne (Alnus incana (L.) Moench) et du sorbier (Sorbus aucuparia L.) a été mesurée, dans une forêt mixte non gérée dominée par l’épicéa et le tremble, et située dans la taïga du sud en Russie d’Europe. Les mesures ont été effectuées à l’aide de dendromètres à ruban pendant les saisons 2000–2001 en parallèle avec les données météorologiques. En plus des carottes de bois ont été prélevées sur tous les épicéas et trembles mesurés pour faire une analyse dendrochronologique. L’épicéa et le tremble représentaient 40 et 44 % de la surface terrière du peuplement. Néanmoins, annuellement dans la période 1999–2002, la croissance de la surface terrière de l’épicéa représentait plus de 60 % de la croissance totale de la surface terrière contre moins de 15 % pour le tremble. Ces résultats révèlent la transformation de la forêt secondaire de trembles en forêt mixte à dominance d’épicéa. La croissance la plus forte de l’épicéa été observée pendant la saison 2000, caractérisée par une surabondance de l’eau dans le sol pendant l’été, tandis que la croissance maximale du tremble a été observée pendant la saison extrêmement sèche du 2002. La croissance totale de la surface terrière du peuplement variait de 5.3 m2 ha–1 en 1999 jusqu’à 11.4 m2 ha–1 en 2000. La croissance annuelle radiale mesurée par les dendromètres et par la mesure des accroissements annuels sur les carottes de sondage était similaire chez l’épicéa. En contraste chez le tremble, cette concordance était faible à cause de l’absence de croissance radiale d’une partie des trembles mesurés par les dendromètres

Large variations in afforestation-related climate cooling and warming effects across short distances

The time for the climatic benefits of afforestation via carbon-sequestration to offset the warming effects of reduced albedo and longwave radiation emission varies greatly across short distances, according to a study of paired forested and non-forested ecosystems along an aridity gradient

Disentangling Soil, Shade, and Tree Canopy Contributions to Mixed Satellite Vegetation Indices in a Sparse Dry Forest

Remote sensing (RS) for vegetation monitoring can involve mixed pixels with contributions from vegetation and background surfaces, causing biases in signals and their interpretations, especially in low-density forests. In a case study in the semi-arid Yatir forest in Israel, we observed a mismatch between satellite (Landsat 8 surface product) and tower-based (Skye sensor) multispectral data and contrasting seasonal cycles in near-infrared (NIR) reflectance. We tested the hypothesis that this mismatch was due to the different fractional contributions of the various surface components and their unique reflectance. Employing an unmanned aerial vehicle (UAV), we obtained high-resolution multispectral images over selected forest plots and estimated the fraction, reflectance, and seasonal cycle of the three main surface components (canopy, shade, and sunlit soil). We determined that the Landsat 8 data were dominated by soil signals (70%), while the tower-based data were dominated by canopy signals (95%). We then developed a procedure to resolve the canopy (i.e., tree foliage) normalized difference vegetation index (NDVI) from the mixed satellite data. The retrieved and corrected canopy-only data resolved the original mismatch and indicated that the spatial variations in Landsat 8 NDVI were due to differences in stand density, while the canopy-only NDVI was spatially uniform, providing confidence in the local flux tower measurements

Quantifying transpirable soil water and its relations to tree water use dynamics in a water-limited pine forest

Knowledge of the relationship between soil water dynamics and tree water use is critical to understanding forest response to environmental change in water-limited ecosystems. However, the dynamics in soil water availability for tree transpiration (Tt) cannot be easily deduced from conventional measurements of soil water content (SWC), notably because Tt is influenced by soil water potential (Ψs) that, in turn, depends on soil characteristics. Using tree sap flow and water potential and deriving depth-dependent soil water retention curves, we quantified the ‘transpirable soil water content’ (tSWC) and its seasonal and inter-annual variations in a semi-arid Pinus halepensis forest. The results indicated that tSWC varied in time and with soil depth. Over one growing season Tt was 57% of rain and 72% of the infiltrated SWC. In early winter, Tt was exclusively supported by soil moisture at the top 10 cm (tSWC = 11 mm), whereas in spring (tSWC > 18 mm) and throughout the dry season, source water for Tt shifted to 20–40 cm, where the maximum fine root density occurs. Simulation with the soil–plant–atmosphere water and energy transport model MuSICA supported the idea that consistent tSWC at the 20–40 cm soil layer critically depended on limited water infiltration below 40 cm, because of high water retention below this depth. Quantifying tSWC is critical to the precise estimation of the onset and termination of the growing season (when tSWC > 0) in this semi-arid ecosystem

Data from: Mortality versus survival in drought‐affected Aleppo pine forest depends on the extent of rock cover and soil stoniness

Drought-related tree mortality had become a widespread phenomenon in forests around the globe. Recent drought years led to 5-10% mortality in the semi-arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low predawn leaf water potential at the dry season (-2.8 MPa, vs. -2.3 MPa in non-stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 mg g-1 dw). Trends in tree ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15-20 years prior to mortality. At the plot scale, no differences in topography, soil type, tree age, or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the site’s long-term hydrological data supported the idea that these differences could result in higher soil water concentration (m3/m3) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity is therefore critical to assessing stand level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends

Data from: Mortality versus survival in drought‐affected Aleppo pine forest depends on the extent of rock cover and soil stoniness

Drought-related tree mortality had become a widespread phenomenon in forests around the globe. Recent drought years led to 5-10% mortality in the semi-arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low predawn leaf water potential at the dry season (-2.8 MPa, vs. -2.3 MPa in non-stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 mg g-1 dw). Trends in tree ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15-20 years prior to mortality. At the plot scale, no differences in topography, soil type, tree age, or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the site’s long-term hydrological data supported the idea that these differences could result in higher soil water concentration (m3/m3) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity is therefore critical to assessing stand level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends

Preisler et al. 2019_ Climatic panel_Figure 1

Climatic data and evapotranspiration recorded measured by the Yatir forest Eddy covariance flux towe

Preisler et al. 2019_ BAI-tree rings_Figure 3

Tree ring data recorded from all tree in the study period for the period of 1972 to 2012. Annual rainfall for the same period is also note

Preisler et al. 2019_ WP_figure 2

Pre-dawn water potential measurements data from 5 seasons that were held at the trees in different stress stages. rainfall amount of the corresponding period is also note