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As climate change progresses, the frequency and duration of drought stress events are increasing. While the mechanisms of drought acclimation of trees has received considerable attention in recent years, the recovery processes remain critically understudied. We used a unique throughfall exclusion experiment in a mature temperate mixed forest consisting of the more isohydric Norway spruce and more anisohydric European beech, to study the recovery and resilience after drought release. We hypothesized that pre-dawn water potential (ΨPD) of both species will increase within 1 day after watering, while the recovery of stomatal conductance (gs) and the reversal of osmoregulation will be significantly delayed in the more isohydric spruce. Furthermore, we hypothesized that the xylem sap flow density (udaily) will not fully recover within the growing season due to the strong drought impact. After 5 years of summer drought, trees showed significantly reduced ΨPD, udaily and increased osmoregulation in leaves, but only isohydric spruce displayed increased leaf abscisic acid concentrations. In line with our hypothesis, ΨPD and gs recovered within 1 day in beech. Conversely, isohydric spruce showed delayed increases in ΨPD and gs. The delay in recovery of spruce was partially related to the replenishment of the stem water reservoir, as indicated by the missing response of udaily at the crown base compared with DBH level upon watering. However, udaily fully recovered only in the next growing season for beech and was still reduced in spruce. Nevertheless, in both species, osmotic acclimations of leaves were reversed within several weeks. While both species displayed full resilience to drought stress in water-related physiology, the recovery time was in several cases, e.g., udaily, ΨPD and gs, shorter for beech than for spruce. With future increases in the frequency of drought events under ongoing climate change, tree species that recover more quickly will be favored.

The past decade with the drought years of 2015, 2018 and 2019 in Central Europe revealed strikingly the devastating consequences of severe and repeated drought/heat events on forest ecosystems. Nevertheless, responses of the water balance of trees and forest stands to such repeated drought events and subsequent recovery are poorly understood. The estimation of the water consumption of trees and forest ecosystems is a crucial part of many process-based models, especially under severe drought events. One major factor for the calculation of the water consumption per tree by xylem sap flow measurements is the radial profile of the xylem sap flow density decreasing towards the inner part of the sapwood. However such profiles are very scarce, especially under repeated and severe drought events.Here, we present the changes of the profile of xylem sap flow density of European beech and Norway spruce within a five-year throughfall-exclusion (TE) experiment and a subsequent recovery. Two different methods were used to measure the xylem sap flow density up to 8cm sapwood depth, i.e. the heat dissipation and heat field deformation method. In beech, there was no difference in the linear radial profile between the TE and the CO (control) trees. However, under drought, for spruce the xylem sap flow density was strongly reduced along the profile by about 48 ± 16 % and the profile changed to an exponential decrease compared to the linear decrease for CO trees. Even two years upon drought release, the profile has not recovered. The reduction in the xylem sap flow density profile of drought stressed spruce was accompanied by heavy loss of fine roots and a reduction of the leaf area.The use of standardized xylem sap flow profiles without the consideration of drought induced changes would lead to an overestimation of the water consumption of more than 30%. These results stress the importance of the radial profile measurements for the calculation of water balance of trees and thus forest ecosystems.

Acclimation processes in a changing climate require the ability to tolerate and survive abiotic stress events (e.g. drought), challenging especially immobile and long-living species and ecosystems, such as forests. The drought years 2018/2019 in Central Europe have laid bare the vulnerability of temperate forest ecosystems to drought and heat. The recovery phase after the stress event represents a crucial phase, especially after intense and repeated drought periods, and might be different for anisohydric and isohydric species.The second phase of the Kranzberg Forest Roof (KROOF) experiment in southeast Germany focused on the watering of mature more anisohydric European beech (Fagus sylvatica L.) and more isohydric Norway spruce (Picea abies (L.) H. Karst.) after five years of repeated experimental summer drought. Both treatments, the former throughfall-exclusion (TE, recovering trees) and control (CO) plots were labeled with 2H enriched water by controlled watering, to end the experimental drought. Pre-dawn leaf water potential, stomatal conductance, xylem sap flow density (at breast height and crown base) and leaf osmoregulation were recorded for two growing seasons after drought release and the resilience and recovery times were calculated. All measured parameters were strongly reduced by on average 30% in both species due to the drought treatment. While the distribution of the labeled water upon irrigation across the soil profile occurred within a few days in both treatments, the water uptake and distribution within the trees was delayed by several days in recovering trees compared to control trees and in recovering spruce compared to recovering beech. Additionally, upon drought release recovering beech reached full resilience (i.e. same level as control trees) earlier than recovering spruce in water potential, stomatal conductance and xylem sap flow density and even showed signs of overcompensation by surpassing the control trees. No differences were found between the two species in the recovery of leaf osmoregulation.The “opposing” drought mitigation strategies seem to be responsible for the differences detected between more anisohydric beech and spruce during the recovery period. For example, the lack of recovery of xylem sap flow density at crown base in TE spruce indicates a re-filling of the stem water reservoirs upon watering. Additionally, we found fast responding parameters as water potential (hours to days) and stomatal conductance (days to weeks) compared to slow responding parameters such as osmoregulation (weeks to months) and full hydraulic recovery, i.e. xylem sap flow density, may even take years. Rapid physiological recovery after drought events, which are expected to increase in frequency and intensity with ongoing climate change, will be beneficial for overall recovery and might put faster-reacting trees in favor over slower responding species.

After drought events, tree recovery depends on sufficient carbon (C) allocation to the sink organs. The present study aimed to elucidate dynamics of tree-level C sink activity and allocation of recent photoassimilates (C$_{new}$) and stored C in c. 70-year-old Norway spruce (Picea abies) trees during a 4-week period after drought release. We conducted a continuous, whole-tree $^{13}$C labeling in parallel with controlled watering after 5 years of experimental summer drought. The fate of C$_{new}$ to growth and CO$_{2}$ efflux was tracked along branches, stems, coarse- and fine roots, ectomycorrhizae and root exudates to soil CO$_{2}$ efflux after drought release. Compared with control trees, drought recovering trees showed an overall 6% lower C sink activity and 19% less allocation of C$_{new}$ to aboveground sinks, indicating a low priority for aboveground sinks during recovery. In contrast, fine-root growth in recovering trees was seven times greater than that of controls. However, only half of the C used for new fine-root growth was comprised of C$_{new}$ while the other half was supplied by stored C. For drought recovery of mature spruce trees, in addition to C$_{new}$, stored C appears to be critical for the regeneration of the fine-root system and the associated water uptake capacity.

Due to climate change suitable forest management measures are required to mitigate the proposed exacerbating drought events as already observed in central Europe in summer 2003, 2015, 2018, and 2019. This contribution summarizes the findings of two long-term field experiments studying different forest management measures aiming at mitigating drought in forest stands.The first study investigated the potential of mixing tree composition for mitigating drought stress in Norway spruce (Picea abies [L.] Karst.). To this end, a five-year study with repeated summer droughts, experimentally induced via throughfall-exclusion (TE), was performed. The study objects were mature (60-80 years) old stands of Norway spruce in monoculture and mixture with European beech (Fagus sylvatica (L.). The impacts of repeated summer droughts were assessed on about 100 trees distributed on 12 plots and accessible via canopy crane (Kranzberg forest ROOF experiment in southern Germany). Predawn leaf water potentials of Norway spruce reached minima of -1.8 MPa, but were not affected by species mixture. Nevertheless, daily xylem sapflow density was increased up to 40% in mixture compared to pure strands. Likewise, stem growth, i.e. relative basal area increment, showed significantly higher drought resistance in mixture compared to monocultures.While altering forest stand composition seems to be promising on the long-term, the conversion from monocultures is often economically not suitable for young stands in the short- to medium-term. Therefore, reducing intraspecific competition via thinning is a frequently discussed option investigated in the second, eight-year-long study. In a 26-year old Norway spruce monoculture, three thinning intensities, i.e. unchanged (control), moderate thinning (MT with reduced basal area by 43%) and heavy thinning (HT, reduced basal area by 67%) were applied, potentially mitigating drought by reducing intraspecific competition and increasing soil water availability. Indeed, in both thinning intensities the duration of drought stress for the trees (soil water content below critical value) was reduced compared to controls for up to 5-7 years following the event. However, increased radiation and higher growth rates of the individual trees accompanied by the fast establishment of a vital ground vegetation diminished the difference in stand transpiration between MT and HT within two years. Moreover, belowground competition with the understorey vegetation suppressed fine root recovery under HT compared to MT in contradiction to increased leaf area on HT on tree-level and therefore increased transpirational demand.The presented studies suggest admixing of broadleaved beech into monocultures of Norway spruce to be a promising management measure in the long-term. In juvenile monocultures of Norway spruce frequent and intense thinning interventions while preventing the establishment of a vital understorey vegetation appears to be a promising forest measure, mitigating drought without losing sight of economic needs.

Every drought period will eventually end and plants will have access to water again. This phase of “re-watering” is a critical point that will either ensure survival or collapse of ecosystems. The drought years 2018 and 2019 have laid bare how vulnerable Central European forest-systems are, even under short-term water scarcity. To understand the effects of repeated summer drought and its release on mature forest stands we investigated the recovery of a mixed forest stand. After 5 years of repeated experimental summer drought on roughly 100 trees (with n = 6 plots) the second phase of the Kranzberg Forest Roof (k.roof) experiment was started, which focuses on the re-watering with Deuterium labeled water (2H2O) of the mature stand composed of European beech (Fagus sylvatica (L.)) and Norway spruce (Picea abies (L.)H.Karst.). According to our hypotheses the water household of the more anisohydric beech will recover faster and “stronger” (higher resilience) than the more isohydric spruce, due to the differences in stomatal control (hypothetical hydraulic regulation in beech vs. hormonal (ABA) control in spruce). We simulated a rainfall event to end our experimental drought and labeled the throughfall-exclusion (TE) and control (CO) plots of the k.roof experiment with roughly 13000 L for TE and 2000 L for CO of 2H2O enriched water, i.e. δ2H 1500 and 400 ‰ respectively. We traced the 2H2O signal along the soil-plant-atmosphere continuum (SPAC) from the soil through the stems and branches up to the leaves with conventional and real-time techniques (xylem sensors connected to CRDS system). Additionally, we measured leaf water potential and pressure-volume (PV) curves to assess the release of the drought stress. The distribution of the “new” water within the soil happened within a few days and we could not find any differences between the beech, mix or spruce dominated sites. However, the water uptake of the trees was significantly delayed in spruce compared to beech, evident from both the deuterium tracer signal (in stems and leaves) and leaf water potential. However, release of osmotic adjust was not different in the two species. The data allow for estimating the drought resilience of the water household of a mature forest stand after five-years of repeated summer drought and subsequent re-watering. While both species recovered their water household after several months to the same level as the control trees, we found beech to react faster and stronger than spruce.

Predicted intense and prolonged drought events challenge forest management. Thinning is debated as a silvicultural measure for reducing drought risk in densely established forest stands. We report on a thinning experiment in a 26-year-old Norway spruce stand (Picea abies), comprising of two thinning intensities and one unthinnned control. The removal of 43% (moderate thinning, MT) and 67% (heavy thinning, HT) of the initial basal area led to increased water availability during the entire three year observation period. Stand-level transpiration (Es) was decreased by about 25% upon moderate, and by about 50% upon heavy thinning during the first year after the interventions had been carried out. However, differences in Es across the treatments decreased within three years after thinning mainly due to increased single-tree transpiration and additional understory evapotranspiration at HT. Nevertheless, due to lower interception and transpiration on the thinned plots three years after treatment MT and HT still showed a substantial surplus in extractable soil water. The results showed that the main determinants concerning the extent of the mitigation effect with increasing thinning intensity were the available soil water storage capacity and the emerging understory vegetation. We conclude that repeated moderate thinning, through enhancing the water availability to the remaining trees, can mitigate drought risk in young spruce stands and thus, represent a viable silvicultural measure in anticipating possible water limitations due to climate change.

We hypothesize that reductions in stand density through thinning improve the recovery of radial stem growth in Norway spruce trees ( Picea abies ) from severe drought. However, thinning may not lead to higher relative radial growth during drought. Annual stem growth and stable carbon and oxygen isotopes in early- and latewood were assessed in trees from heavily thinned (HT), moderately thinned (MT) and un-thinned control stands at two sites in southern Germany. Physiological performance of trees as inferred from stable isotope analysis was used to interpret annual stem growth in response to the drought events in 1976 and 2003. Only in recently thinned stands, trees maintained growth probably through higher soil water availability during the drought year when compared to controls. In contrast, thinning improved the growth recovery in the years following the drought irrespective of the time span between thinning and drought. We conclude that thinning improves drought recovery response in the short and long term and should be considered as an effective management strategy to increase drought tolerance of Norway spruce stands.