Purpose of Review Despite the rapidly increasing use of resilience indices to analyze responses of trees and forests to disturbance events, there is so far no common framework to apply and interpret these indices for different purposes. Therefore, this review aims to identify and discuss various shortcomings and pitfalls of commonly used resilience indices and to develop recommendations for a more robust and standardized procedure with a particular emphasis on drought events. Recent Findings Growth-based resilience indices for drought responses of trees are widely used but some important drawbacks and limitations related to their application may lead to spurious results or misinterpretation of observed patterns. The limitations include (a) the inconsistency regarding the selection and characterization of drought events and the climatic conditions in the preand post-drought period and (b) the calculation procedure of growth-based resilience indices. Summary We discuss alternative options for metrics, which, when used in concert, can provide a more comprehensive understanding of drought responses in cases where common growth-based resilience indices are likely to fail. In addition, we propose a new analytical framework, the "line of full resilience," that integrates the three most commonly used resilience indices and show how this framework can be used for comparative drought tolerance assessments such as rankings of different tree species or treatments. The suggested approach could be used to harmonize quantifications of tree growth resilience to drought and it may thus facilitate systematic reviews and development of the urgently needed evidence base to identify suitable management options or tree species and provenances to adapt forests for changing climatic conditions.
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
Hydroclimate, the interplay of moisture supply and evaporative demand, is essential for ecological and agricultural systems. The understanding of long-term hydroclimate changes is, however, limited because instrumental measurements are inadequate in length to capture the full range of precipitation and temperature variability and by the uneven distribution of high-resolution proxy records in space and time. Here, we present a tree-ring-based reconstruction of interannual to centennial-scale groundwater level (GWL) fluctuations for south-western Germany and north-eastern France. Continuously covering the period of 265–2017 CE, our new record from the Upper Rhine Valley shows that the warm periods during late Roman, medieval and recent times were characterized by higher GWLs. Lower GWLs were found during the cold periods of the Late Antique Little Ice Age (LALIA; 536 to ~ 660 CE) and the Little Ice Age (LIA; between medieval and recent warming). The reconstructed GWL fluctuations are in agreement with multidecadal North Atlantic climate variability derived from independent proxies. Warm and wet hydroclimate conditions are found during warm states of the Atlantic Ocean and positive phases of the North Atlantic Oscillation on decadal scales.
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