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

Abstract

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.

Fig. 1. Climatic conditions during the 2018 heatwave period and dendrometer network.

a, b Spatial distribution of dendrometer sites and their respective mean atmospheric temperature (T_a in °C) and relative extractable water (REW) during the 2018 heatwave timeframe (day of year 208 until 264) for 2016–2018. Sites with incomplete dendrometer time series data are indicated with grey dots. c Tree stem radius monitored at the Swiss Lötschental (site N13) for a Picea abies (L.) Karst. tree. The period corresponding to the 2018 heatwave is shown in all three years (defined as Heatwave₂₀₁₈), in addition to tree water deficit (TWD) and extracted growth (GRO). d Inset of three days of tree stem radius monitored for the P. abies tree, where the concept of daily minimum and maximum TWD is shown (Max. TWD_daily and Min. TWD_daily, respectively).

Fig. 2. Species-specific ratios of tree water deficit (TWD) during the 2018 heatwave and 2018 annual growth (GRO) relative to control years (2016–2017).

a, b Boxplots of broadleaf and conifer minimum tree water deficit (min. TWD_2018:control; a) and annual radial stem growth (GRO_2018:control; b) in response to the 2018 heatwave relative to control years. Log transformed ratios are shown to linearise and normalise the response metric. log₁₀(TWD_2018:control) above zero indicates a larger shrinkage was registered during the 2018 heatwave compared to the control years, whereas log₁₀(GRO_2018:control) near zero indicates that stem growth in 2018 was similar to that in control years. Centerlines, box limits, and whiskers represent the median, upper and lower quartiles, and extremes excluding outliers (those further than the 1.5x interquartile range). n = 175 tree stems over 37 sites.

Fig. 3. Response of tree water deficit (TWD) ratio to vapor pressure deficit (VPD, kPa) and relative extractable water (REW, unitless [-]) of broadleaf (a, c) and conifer (b, d) species in the hydrometeorological space.

a–d Linear-mixed effect model output of the ratio of the daily minimum (a, b) and maximum (c, d) TWD during the 2018 heatwave compared to the 95^th percentile of the control period (TWD_2018:control), while VPD₂₀₁₈ and REW₂₀₁₈ refer to the absolute values during the 2018 heatwave. TWD_2018:control above 1 indicates that a larger shrinkage was registered during the 2018 heatwave relative to the control period. From the hydrometeorological space range of the entire database (indicated in grey), models have been adjusted for the common climatic range of broadleaf and conifer species only (indicated with dotted lines). e Bars indicate the percentage of the hydrometeorological space covering different ranges of TWD_2018:control values (see legend).

Fig. 4. Species-specific response of tree water deficit (TWD) ratio to vapour pressure deficit (VPD) and relative extractable water (REW) in the hydrometeorological space for most abundant and economically valuable tree species in Europe.

a–d, Linear-mixed effect model output of the ratio of the daily minimum TWD during the 2018 heatwave compared to the 95^th percentile of the control period (TWD_2018:control), while VPD₂₀₁₈ and REW₂₀₁₈ refer to the absolute values during 2018 heatwave. All trees of Fagus sylvatica, Quercus spp. (Q. robur/petraea), Picea abies and Pinus sylvestris within the database are considered. From the hydrometeorological space range of the entire database (indicated in grey), models have been adjusted for the common climatic range of the selected species only (indicated with dotted lines; see Supplementary Fig. 4 for the climate range of each species). e Bars indicate the percentage of the hydrometeorological space covering different ranges of min. TWD_2018:control values (see legend).