The frequency and intensity of droughts and corresponding surges of forest dieback events around the globe are projected to increase in the 21st century (Allen et al., 2010;IPCC, 2014). This critically endangers the world's forests and the variety of ecosystem services they sustain, such as their potential to act as carbon sink (Anderegg et al., 2020) and as a nature-based solution for climate change mitigation (Griscom et al., 2017). Recent drought events, moreover, belong to a new category, so called 'hotter droughts', where low precipitation coincides with heat waves, which creates a positive feedback loop between soil water depletion through evapotranspiration and increased surface temperatures through reduced cooling by latent heat production (Allen et al., 2015;Buras et al., 2020). In 2018-2019, Central Europe was hit by two consecutive and hotter drought events, a phenomenon unprecedented at least in the last 250 years but likely to occur more frequently with intensifying climate change (Hari et al., 2020). The 2018 hotter drought alone had

Droughts increasingly threaten the worlds 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 trees growth resistance and physiological stress responses (increase in carbon isotope composition; Δδ13C) 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 in 2018 was comparable to former single drought years, but the cumulative drought stress in 2019 induced drastic decreases in growth resistance and increases in Δδ13C across all species. Consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with high levels of water supply.

<p>Intensifying climate change is successively increasing the frequency and intensity of extreme climate events such as droughts. In 2018–2019, Central European forests were hit by two consecutive hotter drought years that were unprecedented in their severity at least in the last 250 years. Such hotter droughts, where drought coincides with a heat wave, may have severe detrimental impacts on forest ecosystems as highlighted by reports of widespread tree defoliation and mortality across Central Europe in 2018–2019. Here, we examine the effect of this unprecedented event on tree growth and physiological stress responses (measured as increase in wood carbon isotope composition, Δδ<sup>13</sup>C) in a Central European floodplain forest ecosystem. We used tree rings of the dominant tree species <em>Quercus robur</em>, <em>Acer pseudoplatanus </em>and <em>Fraxinus excelsior </em>to compare growth responses, Δδ<sup>13</sup>C and drought legacy effects during the consecutive drought years 2018–2019 with effects observed in former single drought years (2003, 2006, 2015). We found that tree growth was, except for <em>F. excelsior</em>, not reduced in 2018 and that drought responses in 2018 were comparable to responses in former single drought years. This indicates that water availability in floodplain forests can partly buffer drought effects and meteorological water deficits. Nonetheless, the 2018 drought – which was the hottest and driest year since the start of records – induced drought legacies in tree growth while former drought years did not. Consistent with this observation, all tree species showed strong decreases in growth and increases in Δδ<sup>13</sup>C in the second hotter drought year 2019. The observed stress responses in 2019 were stronger than in any other examined drought year. We posit that the cumulative effect of two consecutive hotter drought years likely caused this unprecedented stress response across all species. Drought responses were consistent for both drought-stress indicators (growth response and Δδ13C), but the timing and magnitude of responses were species-specific: <em>Q. robur</em> exhibited the overall smallest response, followed by <em>A. pseudoplatanus</em> with the strongest response in <em>F. excelsior</em>. We discuss these species-specific differences in light of the species’ stomatal control (inferred from high-resolution sap flow measurements during drought at our site) and species’ resistance to xylem cavitation. Overall, our findings highlight that consecutive hotter droughts constitute a novel threat to forests, even in floodplain forests with comparably high levels of water supply. These results and similar research may contribute towards understanding and forecasting tree species responses to more frequent hotter droughts under intensifying climate change.</p>