Cumulative growth and stress responses to the 2018-2019 drought in a European floodplain forest.

Florian Schnabel,Ronny Richter,Christian Wirth,Rolf A Engelmann,Lara Schmitt,Anja Kahl,Sarah Purrucker,Carolin Seele‐Dilbat,Georgios Skiadaresis

doi:10.1111/gcb.16028

Abstract

Droughts increasingly threaten the world's forests and their potential to mitigate climate change. In 2018-2019, Central European forests were hit by two consecutive hotter drought years, an unprecedented phenomenon that is likely to occur more frequently with climate change. Here, we examine tree growth and physiological stress responses (increase in carbon isotope composition; Δδ13 C) to this consecutive drought based on tree rings of dominant tree species in a Central European floodplain forest. Tree growth was not reduced for most species in 2018, indicating that water supply in floodplain forests can partly buffer meteorological water deficits. Drought stress responses in 2018 were comparable to former single drought years but the hotter drought in 2018 induced drought legacies in tree growth while former droughts did not. We observed strong decreases in tree growth and increases in Δδ13 C across all tree species in 2019, which are likely driven by the cumulative stress both consecutive hotter droughts exerted. Our results show that consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with comparably high levels of water supply.

Highlights

The frequency and intensity of droughts and corresponding surges of forest dieback events around the globe are projected to increase in the 21st century[1,2]
We found pronounced responses to drought stress in terms of tree growth and Δδ13C across the examined tree species, with strongest stress responses in the second of two consecutive drought years (2019)
Drought ranged around zero for oak and maple, while growth resistance in ash tended to be below zero (Fig. 2a-c). This indicates a similar tree growth in single drought years and in climatically ‘normal’ years for oak and maple but not for ash (drought year classification based on the Standardized Precipitation Evapotranspiration Index (SPEI)[39]; see methods)

Summary

Introduction

The frequency and intensity of droughts and corresponding surges of forest dieback events around the globe are projected to increase in the 21st century[1,2]. This critically endangers the world’s forests and the variety of ecosystem services they sustain, such as their potential to act as carbon sink[3] and as a nature-based solution for climate change mitigation[4]. An increasing number of studies has shown that droughts can affect tree growth and carbon cycling in forests for years after the actual drought event and that such ‘legacy effects’ are widespread in forest ecosystems[9,10,11,12,13]. The consecutive hotter drought in 2019 may have critically amplified drought stress as trees were hit that already had emptied carbon reserves, impaired hydraulic functioning due to embolism and weakened defense systems[8,13] and only access to emptied soil water reserves

Methods

Results

Conclusion