Vulnerability of European ecosystems to two compound dry and hot summers in 2018 and 2019

Abstract

Abstract. In 2018 and 2019, central Europe was affected by two consecutive extreme dry and hot summers (DH18 and DH19). The DH18 event had severe impacts on ecosystems and likely affected vegetation activity in the subsequent year, for example through depletion of carbon reserves or damage from drought. Such legacies from drought and heat stress can further increase vegetation susceptibility to additional hazards. Temporally compound extremes such as DH18 and DH19 can, therefore, result in an amplification of impacts due to preconditioning effects of past disturbance legacies. Here, we evaluate how these two consecutive extreme summers impacted ecosystems in central Europe and how the vegetation responses to the first compound event (DH18) modulated the impacts of the second (DH19). To quantify changes in vegetation vulnerability to each compound event, we first train a set of statistical models for the period 2001–2017, which are then used to predict the impacts of DH18 and DH19 on enhanced vegetation index (EVI) anomalies from MODIS. These estimates correspond to expected EVI anomalies in DH18 and DH19 based on past sensitivity to climate. Large departures from the predicted values can indicate changes in vulnerability to dry and hot conditions and be used to identify modulating effects by vegetation activity and composition or other environmental factors on observed impacts. We find two regions in which the impacts of the two compound dry and hot (DH) events were significantly stronger than those expected based on previous climate–vegetation relationships. One region, largely dominated by grasslands and crops, showed much stronger impacts than expected in both DH events due to an amplification of their sensitivity to heat and drought, possibly linked to changing background CO2 and temperature conditions. A second region, dominated by forests and grasslands, showed browning from DH18 to DH19, even though dry and hot conditions were partly alleviated in 2019. This browning trajectory was mainly explained by the preconditioning role of DH18 on the impacts of DH19 due to interannual legacy effects and possibly by increased susceptibility to biotic disturbances, which are also promoted by warm conditions. Dry and hot summers are expected to become more frequent in the coming decades, posing a major threat to the stability of European forests. We show that state-of-the-art process-based models could not represent the decline in response to DH19 because they missed the interannual legacy effects from DH18 impacts. These gaps may result in an overestimation of the resilience and stability of temperate ecosystems in future model projections.

Towards species‐level forecasts of drought‐induced tree mortality risk

“…Ultimately, if droughts result in widespread species dieback (Mantgem et al ., 2009; Peng et al ., 2011; Mitchell et al ., 2014), they may lead to sustained changes in biodiversity and community composition (Slik, 2004; Nepstad et al ., 2007; Anderegg et al ., 2013), altering land–atmosphere feedbacks (Swann et al ., 2018) on seasonal to decadal timescales. To develop a predictive framework and better understand the impacts and implications of changes in the timing, severity and spatial extent of climatic extremes, we need to combine insights across scales (McDowell et al ., 2016), including field ecology (Rowland et al ., 2015), remote sensing (Bastos et al ., 2021), modelling and climate science (Williams et al ., 2020).…”

Precipitation–temperature relationships over Europe in CORDEX regional climate models

“…Bastos et al . (2021) concluded that amplified evapotranspiration during the hot spring of 2018 in Europe was related to dry and hot conditions in summer that year. These lagged relationships and their simulation in the RCMs are beyond the scope of the present study but deserve further investigation.…”

“…Della-Marta et al (2007) and Russo et al (2019) showed that hot summers in the Mediterranean tend to be preconditioned by dry springs and, in addition, dry springs over the Mediterranean also contribute to the development of extreme heat in continental Europe (Zampieri et al, 2009). Bastos et al (2021) concluded that amplified evapotranspiration during the hot spring of 2018 in Europe was related to dry and hot conditions in summer that year. These lagged relationships and their simulation in the RCMs are beyond the scope of the present study but deserve further investigation.…”

Analysis of multi-seasonal meteorological storylines leading to reduced forest greenness in Europe in 2000–2020

“…Drought legacy effects are known to reduce tree-level growth (Anderegg et al, 2015), and, more broadly, resilience (Bose et al, 2020). Thus, the previous drought-induced low N DV I event could be a sign of reduced tree vigor and higher susceptibility for a following drought, as detected, e.g., in 2019 (Schuldt et al, 2020;Bastos et al, 2021). Moreover, forests potentially acclimate to increased drought stress, which might also lead to reduced leaf area and productivity (Gessler et al, 2020).…”