The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests

Publisher Copyright: © 2022, The Author(s).Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.Peer reviewe

The 2018 European heatwave led to stem dehydration but not to consistent growth reductions in forests

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes

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

Rooting characteristics and their drivers for important vegetation communities in European Alps

The data collection contains 171 root samples, where the root mass, the root length and the root depth are given. Furthermore, the database contains the main environmental parameters that influence rooting (climate, land use, soil, vegetation composition). The rooting data are provided separately for three root categories: (1) very fine roots (diameter between 0 - 1 mm), (2) fine roots (diameter between 1 - 5 mm), and (3) coarse roots (diameter between 5 - 20 mm). Roots of woody species with a diameter larger than 20 mm were not considered, as the distribution and diameter of coarse roots (especially trees) in the soil vary greatly spatially. The rooting samples were taken between 1994 and 2017 in the most widespread vegetation communities and land-use types in 13 Alpine study sites along a north-south transect from Tyrol (Austria) via South Tyrol to Trentino (both in Italy). 15 samples were taken from arable land, 56 samples from intensively used hay meadows, 15 samples from extensively managed hay meadows, 16 samples from lightly stocked pastures, 32 samples from agriculturally unused grasslands, and 37 samples from forests. The roots were collected with core samplers of 6.8–7.7 cm diameter and a maximum core depth of 70 cm. In the laboratory, the soil cores were split into the O-horizon (if present) and mineral soil layers of various thicknesses (0–3 cm, 3–8 cm, 8–13 cm, 13–23 cm, 23–38 cm, 38–53 cm, and >53 cm). Root extraction was performed manually with the roots cleared of soil in sieving cascades under running water. The root mass was weighed and based on the mass, the root length and root depth distribution were determined according to the method of Tasser et al. (2005). As environmental parameters 79 potential impact variables on rooting, including 19 site variables, six land-use variables and 53 vegetation variables, are present in the database. The vegetation variables are based on vegetation relevées after Braun-Blanquet, in whose center the root samples were taken. Meteorological parameters were measured in the most study sites at a distance of < 150 m from the rooting samples using different microclimate stations. For detailed soil characterization, soil profiles were investigated directly at the sample site or at a representative site with the same land-use type and identical plant communities in the immediate vicinity (<50 m distance). For all soil samples, we analyzed pore size distribution, soil bulk density, soil particle density, total soil porosity, soil texture, and soil organic C and pH. Furthermore, we calculated mean Ellenberg's indicator values (EIV) for temperature (T), moisture (F), soil reaction (R) and soil productivity or fertility (N) for all study sites. Past and present management practices at the study sites were recorded by interviewing landowners

Impact of droughts on water provision in managed alpine grasslands in two climatically different regions of the Alps

International audienceThis study analyzes the impact of droughts, compared with average climatic conditions, on the supporting ecosystem service water provision in sub-watersheds in managed alpine grasslands in two climatically different regions of the Alps, Lautaret (French Alps) and Stubai (Austrian Alps). Soil moisture was modelled in the range of 0–0.3 m. At both sites, current patterns showed that the mean seasonal soil moisture was (1) near field capacity for grasslands with low management intensity and (2) below field capacity for grasslands with higher land-use intensity. Soil moisture was significantly reduced by drought at both sites, with lower reductions at the drier Lautaret site. At the sub-watershed scale, soil moisture spatial heterogeneity was reduced by drought. Under drought conditions, the evapotranspiration to precipitation ratios at Stubai was slightly higher than those at Lautaret, indicating a dominant 'water spending' strategy of plant communities. Regarding catchment water balance, deep seepage was reduced by drought at Stubai more strongly than at Lautaret. Hence, the observed 'water spending' strategy at Stubai might have negative consequences for downstream water users. Assessing the water provision service for alpine grasslands provided evidence that, under drought conditions, evapotranspiration was influenced not only by abiotic factors but also by the water-use strategy of established vegetation. These results highlight the importance of 'water-use' strategies in existing plant communities as predictors of the impacts of drought on water provision services and related ecosystem services at both the field and catchment scale

Data_Sheet_1_Tree growth at the limits: the response of multiple conifers to opposing climatic constraints along an elevational gradient in the Alps.docx

IntroductionClimate change affects the vitality of mountain forests through increasing temperatures and decreasing water availability due to changing precipitation patterns, earlier snowmelt, and increasing evaporative demand. Depending on species characteristics, tree growth might therefore increase in cold habitats near the forest line but decrease in water-limited conditions at low elevation.MethodsWe analyzed the tree-ring widths of five conifers (Picea abies, Larix decidua, Pinus sylvestris, Pinus nigra, and Pinus cembra) along an elevational gradient from 1,000 m to 2,320 m above sea level (a.s.l.) in Vinschgau/Val Venosta Valley in Northern Italy, one of the driest regions of the Alps (mean annual precipitation of 682 mm at 1,310 m a.s.l.).ResultsOur aim was to estimate the species-specific growth response to changing climate conditions along an elevational gradient. At low elevations, we observed a significant response to water availability not only during the actual growing season but also throughout the previous autumn for all species present. At mid-elevation, the correlation coefficients to precipitation and drought indices (SPEI) were highest for Picea abies. At high elevations, the positive correlation of growth with temperature was smaller than expected for Pinus cembra. In contrast, Larix decidua responded positively to temperature and grew faster in recent decades.DiscussionConsidering that a further increase in temperatures will reduce plant water availability during the growing season, our space-for-time approach provides an outlook on future growth conditions of conifers in larger regions of the European Alps. Water limitation will affect tree growth and vitality not only at low elevation in the valleys but also at mid elevation on mountain slopes, potentially impacting timber production and protective and recreative functions of forests. Near the forest line, the different capabilities of tree species to benefit from higher temperatures might lead to changes in species composition.</p

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