Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.

The biology and population dynamics of pine sawfly Diprion pini L. are extremely complex and variable. Among other factors, climatic conditions determine the potential for mass outbreaks of the species. In this paper, we investigate this influence and describe a statistical approach to identify responsible climatic variables in floating time windows, thus identifying the factors responsible for the transition from latency to outbreak events. Analyses were built upon a data base comprising outbreak events and fine-scaled climatic data for the period 2002–2016 for a model region in the state of Brandenburg, Germany. By applying Random Forest statistic classification analyses, we isolated a set of four variables. They cover precipitation, temperature, and potential evapotranspiration in distinct periods during the current and the previous year. These periods are not fixed in their position but attached to the floating phenological date of bud burst of the host species Pinus sylvestris L. The complete set of variables was able to distinguish forests likely to be defoliated from those not threatened at high probabilities (95% true-positive rate, 98% true-negative rate). The identified climatic windows offer insights into population dynamics in the study region, support adjustments in current monitoring algorithms, and indicate starting points for further investigations covering other regions or different years.

To develop measures to reduce the vulnerability of forests to drought, it is necessary to estimate specific water balances in sites and to estimate their development with climate change scenarios. We quantified the water balance of seven forest monitoring sites in northeast Germany for the historical time period 1961–2019, and for climate change projections for the time period 2010–2100. We used the LWF-BROOK90 hydrological model forced with historical data, and bias-adjusted data from two models of the fifth phase of the Coupled Model Intercomparison Project (CMIP5) downscaled with regional climate models under the representative concentration pathways (RCPs) 2.6 and 8.5. Site-specific monitoring data were used to give a realistic model input and to calibrate and validate the model. The results revealed significant trends (evapotranspiration, dry days (actual/potential transpiration < 0.7)) toward drier conditions within the historical time period and demonstrate the extreme conditions of 2018 and 2019. Under RCP8.5, both models simulate an increase in evapotranspiration and dry days. The response of precipitation to climate change is ambiguous, with increasing precipitation with one model. Under RCP2.6, both models do not reveal an increase in drought in 2071–2100 compared to 1990–2019. The current temperature increase fits RCP8.5 simulations, suggesting that this scenario is more realistic than RCP2.6.

Increasingly frequent summer droughts are intensifying stem dehydration across European forests, constraining tree growth and resilience. Even mild water-related stress can inhibit tree growth, although its impact is species-specific. Tree water deficit (TWD), a measure deduced from radial stem contraction during drought, reflects stem water status and can be used as a proxy for the physiological drought stress experienced by trees. Using high-resolution dendrometer measurements representing 29 tree species from 130 forest sites across Europe from 2003 to 2018, we examined how the length of TWD periods during summer is related to their recurrence within the same year, to TWD intensity, to climate factors, and to wood type (i.e., tracheid-based coniferous, ring-porous or diffuse-porous anatomy). A greater degree of incomplete rehydration, indicated by highest minimum TWD during TWD periods (70% increase of expected response for conifers and ring-porous species, 42% increase of expected response for diffuse-porous), together with warmer temperature (44% increase of expected response for all wood types) and stronger drought, prolonged the length of TWD periods. However, the recurrence of TWD periods only increased the length of the subsequent TWD period within the same year for diffuse-porous tree species (12% increase of expected response). Our results show that wood type and its associated functional traits influence tree responses to recurring stem dehydration periods. The greater sensitivity of diffuse-porous species to recurring TWD periods suggests that wood type anatomy may play an important role in determining how tree species cope with increasingly frequent summer droughts.

Climate change is expected to alter hydro-climatic conditions and water availability across Central Europe, with important implications for vegetation and land-use systems. This study assesses projected changes in climatic water balance (CWB), hydro-climatic stress indicators, and their relationship with root-zone soil moisture in Brandenburg, Germany. Four regional climate model realizations under RCP4.5 and RCP8.5 were analyzed for 2006–2100. Air temperature (tas), precipitation (pr), reference evapotranspiration (ET₀), and CWB were evaluated together with indicators describing drought frequency, water-deficit magnitude, aridity, and standardized CWB anomalies. All model realizations project substantial warming, exceeding +2 °C under RCP8.5 by the late century. While annual precipitation remains relatively stable, vegetation-period precipitation decreases and evaporative demand increases, resulting in increasingly negative growing-season CWB. Projected vegetation-period CWB changes range from −22 to −139 mm between near-future and far-future periods. Hydro-climatic stress indicators consistently show increasing seasonal water limitation. Comparisons between CWB and root-zone soil moisture reveal significant positive relationships (p < 0.001), although explanatory power remains low (R² < 0.14). Despite differences in the magnitude of projected changes, all model realizations agree on the direction of change, indicating a robust signal of increasing growing-season drying. These findings highlight the importance of seasonal hydro-climatic indicators for assessing future drought risk and ecosystem water availability.