Abstract

In 2018, Central Europe experienced one of the most severe and long-lasting summer drought and heat wave ever recorded. Before 2018, the 2003 millennial drought was often invoked as the example of a “hotter drought”, and was classified as the most severe event in Europe for the last 500 years. First insights now confirm that the 2018 drought event was climatically more extreme and had a greater impact on forest ecosystems of Austria, Germany and Switzerland than the 2003 drought. Across this region, mean growing season air temperature from April to October was more than 3.3°C above the long-term average, and 1.2°C warmer than in 2003. Here, we present a first impact assessment of the severe 2018 summer drought and heatwave on Central European forests. In response to the 2018 event, most ecologically and economically important tree species in temperate forests of Austria, Germany and Switzerland showed severe signs of drought stress. These symptoms included exceptionally low foliar water potentials crossing the threshold for xylem hydraulic failure in many species and observations of widespread leaf discoloration and premature leaf shedding. As a result of the extreme drought stress, the 2018 event caused unprecedented drought-induced tree mortality in many species throughout the region. Moreover, unexpectedly strong drought-legacy effects were detected in 2019. This implies that the physiological recovery of trees was impaired after the 2018 drought event, leaving them highly vulnerable to secondary drought impacts such as insect or fungal pathogen attacks. As a consequence, mortality of trees triggered by the 2018 events is likely to continue for several years. Our assessment indicates that many common temperate European forest tree species are more vulnerable to extreme summer drought and heat waves than previously thought. As drought and heat events are likely to occur more frequently with the progression of climate change, temperate European forests might approach the point for a substantial ecological and economic transition. Our assessment also highlights the urgent need for a pan-European ground-based monitoring network suited to track individual tree mortality, supported by remote sensing products with high spatial and temporal resolution to track, analyse and forecast these transitions.

Highlights

  • Since the beginning of instrumental climate recording, Central Europe has experienced several extreme hot and dry summers (Sch€ar et al, 2004; Orth, Vogel, Luterbacher, Pfister, & Seneviratne, 2016; Hanel et al, 2018), but the climatic conditions during the 2018 growing season were exceptional

  • Across Austria, Germany and Switzerland, the 2018 mean growing season air temperature (MGT; April to October) was the highest temperature ever recorded for that period (Fig. 1)

  • The 2018 growing season had the highest mean vapour pressure deficit (MVPD; April to October) and second most negative climatic water balance (CWB) ever recorded in the DACH region, which was even more negative than in 2003 (-238 mm versus -200 mm, respectively)

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Summary

OPINION PAPER

A first assessment of the impact of the extreme 2018 summer drought on Central European forests. W€urzburg, Julius-von-Sachs-Platz 3, 97082 W€urzburg, Germany bLand Surface-Atmosphere Interactions, Technische Universit€at M€unchen, Hans-Carl-von-Carlowitz-Platz 2, 85354. 2, 8092 Z€urich, Switzerland hEcophysiology of Plants, Department of Ecology and Ecosystem Management, Technische Universit€at M€unchen, Hans-Carl-von-Carlowitz-Platz 2, 85354 Freising, Germany iChair of Applied Vegetation Ecology, Institute of Forest Sciences, Faculty of Environment and Natural Resources, University of Freiburg, Tennenbacher Str. 4, 79106 Freiburg, Germany jDepartment of Geobotany, Faculty of Biology, University of Freiburg, Sch€anzlestr. 1, 79104 Freiburg, Germany kMax-Planck Institute for Biogeochemistry, Hans Knoell Str. 10, 07745 Jena, Germany lChair of Forest Growth, Faculty of Environment and Natural Resources, Albert-Ludwigs-University Freiburg, Tennenbacherstraße 4, 79106 Freiburg imBreisgau, Germany mPlant Ecology, Albrecht von Haller Institute for Plant Sciences, University of Goettingen, Untere Karsp€ule 2, 37073 G€ottingen, Germany nKarlsruhe Institute of Technology (KIT), Institute of Meteorology and Climate Research - Atmospheric Environmental. Research (IMK-IFU), Kreuzeckbahnstr. 19, 82467 Garmisch-Partenkirchen, Germany oChair of Ecosystem Physiology, University of Freiburg, Georges-K€ohler-Allee 53/54, Freiburg 79110, Germany

Picea abies
Bark exudation
Findings
Conclusion and outlook
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