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 pre- and 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.

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; 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 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 Δδ¹³C 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.

In future, groundwater dependent ecosystems are likely to face increasing stress from direct and indirect effects of climate change. European floodplain forests already experience lower groundwater tables caused by river regulations and water extraction. Yet, the effects of more frequent and prolonged dry and hot periods in combination with groundwater extraction on floodplain forests are not well-understood. Here, we conducted a dendroecological study in floodplain forests dominated by pedunculate oak (Quercus robur), a widespread forest type in Europe. Since drought-related precipitation deficits are increasingly less compensated by available groundwater, we hypothesized that groundwater extraction would increase climate sensitivity of growth. Moreover, we expected that younger trees that established under the condition of lowered groundwater tables would be less sensitive to droughts in contrast to older trees that might not be able to adapt to abrupt changes in groundwater levels. In three regions in the Upper Rhine Valley of Germany, we analyzed retrospectively tree-ring growth in Q. robur stands where (a) water extraction had dramatically lowered groundwater levels and trees presumably lost direct access to it, or (b) trees have not been affected by water extraction. In addition, (c) nearby sessile oak (Q. petraea) stands at upland sites were analyzed to isolate the climate signal for each study region from a groundwater signal. In total, we analyzed ring-width chronologies from 18 forest stands and investigated their relationship with climate. Radial growth of oak trees was strongly correlated with summer drought (SPEI, Standardized Precipitation Evapotranspiration Index). Their sensitivity to summer drought increased significantly following the onset of groundwater extraction (88–49 years ago). However, sensitivity of oaks to summer drought at sites with groundwater contact remained relatively stable over time. Oaks at sites without water extraction took advantage of favorable growing conditions. Our results show that access to groundwater can buffer negative effects of summer drought on tree growth. Therefore, in the face of the ongoing climate change and more frequent occurrence of extreme drought events, we recommend that groundwater extraction from floodplain forests should be reduced rather than increased to maintain tree vitality in the long term.

Climate change is expected to pose major direct and indirect threats to groundwater-dependent forest ecosystems. Forests that concurrently experience increased rates of water extraction may face unprecedented exposure to droughts. Here, we examined differences in stem growth and xylem hydraulic architecture of 216 oak trees from sites with contrasting groundwater availability, including sites where groundwater extraction has led to reduced water availability for trees over several decades. We expected reduced growth and xylem hydraulic capacity for trees at groundwater extraction sites both under normal and unfavourable growing conditions. Compared to sites without extraction, trees at sites with groundwater extraction showed reduced growth and hydraulic conductivity both during periods of moderate and extremely low soil water availability. Trees of low vigour, which were more frequent at sites with groundwater extraction, were not able to recover growth and hydraulic capacity following drought, pointing to prolonged drought effects. Long-term water deficit resulting in reduced CO₂assimilation and hydraulic capacity after drought are very likely responsible for observed reductions in tree vitality at extraction sites. Our results demonstrate that groundwater access maintains tree function and resilience to drought and is therefore important for tree health in the context of climate change.