To elucidate how trees adjust their C dynamics under future climate, we conducted a unique growth chamber experiment with tree saplings grown for three years under the present climate conditions (PC), a mitigation scenario (RCP2.6) and a worst-case scenario (RCP8.5). The RCPs were simulated by increased air temperature, vapor pressure deficit, irradiance, CO2 and O3 concentrations at hourly resolution. Both species, European beech and Norway spruce, grew more biomass under RCP8.5 than PC and RCP2.6, accompanied by increased partition to belowground. Towards the end of the three-year experiment, the mean residence time (MRT) and mean transit time (MTT) of C along the tree-soil system were assessed in a 13CO2 tracer experiment. MRT in leaves was unaffected in both species but decreased significantly in beech stem CO2 efflux under both RCP scenarios. MTT for beech from leaves to stem CO2 efflux decreased under RCP2.6. For both species, we found a negative correlation between MTT and maximum assimilation, indicating faster C flux through trees at higher assimilation rates. Conversely, no positive correlation was found between biomass growth and maximum assimilation rates. After three years of growth under the simulated RCP scenarios, we conclude that C turnover would accelerate under future climate conditions. The additional C fixed by photosynthesis will be released back to the atmosphere more rapidly, as a smaller fraction could remain in the biomass. These results support the implementation of faster C turnover in process-based dynamic vegetation models, although confirmation for mature trees is still needed.

Hydraulic redistribution (HR) by deep-rooting trees can provide water from deeper soil layers to shallow-rooted understory plants, yet species-specific and temporal patterns of HR water uptake remain poorly understood. We investigated whether HR water from mature oaks (Quercus robur L./Quercus petraea (Matt.) Liebl.) is preferentially taken up by oak seedlings compared to co-occurring understory species, and how HR water use varies over the course of the day. During drought periods in mature oak stands in Germany, two field experiments were conducted. In a deep-soil 2H-labeling experiment of mature oak trees, HR water was detected in the roots of three different understory plants. Six days after labeling, HR water amounted to 16 ± 8% (oak), 13 ± 7% (black cherry, Prunus serotina Ehrh.), and 8 ± 4% (small balsam, Impatiens parviflora DC.) of total root water content. Although intraspecific oak–oak HR was initially the highest, after 60 days all species contained approximately 20% HR water, indicating no persistent species-specific advantage. A second experiment analyzing natural δ18O abundance revealed pronounced diurnal dynamics of HR water use in oak seedlings. Contrary to expectations, the HR water fraction peaked at midday, when transpiration was highest. Overall, 29 ± 6% of the daily transpired water in oak seedlings originated from HR. These results demonstrate that HR can substantially contribute to seedling water use during dry periods, highlighting the ecological importance of HR for understory plant water supply in temperate forests. Future studies should analyze specific pathways through which HR water is transported from redistributing plants to the roots of receiving plants to assess the impact of plant species, degree and type of mycorrhization or soil physical properties on HR.

Rejuvenation of forests is one of the most important ecological and economic challenges in central Europe. In an existing large-scale experiment in southern German forests, c. 500 thousand c. 2 years old tree seedlings of Beech, Douglas fir, Silver, Oak have been planted in rows. However, our knowledge of how potential small-scale factors like tree stumps and tree mixture mechanistically enhance tree establishment after planting is limited.We focused on 6 plots (c. 2500 seedlings) of Douglas fir, planted on a comparably dry site without mature trees. We observed that those trees differed in height, 3 years after planting. Moreover, trees of comparably greater height cluster together on a small spatial scale of 3 m radius. However, conventional tree planting methods (in rows) neglect those beneficial small-scale sites for tree establishment.Using an app we programmed (Shiny package in R) for quickly finding trees clustered by e.g. height, clusters of 3 m radius of well- and poorly-established trees were identified. A combination of multispectral drone-derived optical parameters, morphological analyses of twigs and 13C analyses of tree needles was then used to provide insight into the factors driving the trees’ height differences.First results showing the positive effects on the establishment of young trees are presented, i.e. incorporating spatial proximity to tree stumps and using a tree mixture in the planting method. Further steps to gain a better understanding of the mechanisms driving tree seedling establishment are discussed.

Both facilitative and competitive interactions between trees affect water relations and fluxes in temperate forests. During drought, hydraulic redistribution (HR) by deep-rooting species such as oak (Quercus robur L.) can provide water from deeper soil layers to shallow-rooted understorey plants. Assuming that shallow-rooted plants take up HR water from mature oaks during drought, we tested two hypotheses: (1) mature oaks share more HR water with oak seedlings than with other understorey plants, and (2) seedlings accumulate most HR water in their roots at sunrise because HR occurs over night. We also quantified how much HR water seedlings used in daily transpiration.These hypotheses were tested in two experiments. 1) Over a period of six days in July 2023, a total of 7.2 L of ²H-labelled water (5 atom%) was added to a depth of 50-70 cm around mature oak trees in a forest in Brandenburg, Germany. We sampled soils, stem xylem of mature oaks, and roots of two tree species seedlings (oak and black cherry, Prunus serotina EHRH.) and a herbaceous plant (small balsam, Impatiens parviflora DC.) near the labelled trees. From all samples in the water was extracted via cryogenic water extraction and the isotopic composition of the water was analysed. After six days, recovery of δ²H in 0-10 cm soil depths indicated HR via oak roots. Also, seedling roots were enriched in δ²H, confirming HR water uptake with 16 ± 8 % (oak), 13 ± 7 % (black cherry) and 8 ± 4 % (small balsam) of root water originating from HR. Oak seedlings initially had more HR water in root tissues than other species, suggesting faster transport of HR water to oak seedlings, possibly due to shared mycorrhizae or root contact. However, after 60 days, the HR water content of all shallow-rooted understorey plants equalised (~20 %), rejecting our hypothesis 1 that HR water is preferentially found in seedlings of the same species (here: oak).2) In August 2024, in a Bavarian forest (Germany), soil and stem xylem samples of mature oaks were collected together with root samples from oak seedlings at five daily intervals. Water was again extracted from all samples as in experiment 1. Following the natural gradient of stable water isotope composition in the soil profile, we considered HR water uptake by seedlings, if δ18O values in seedlings did not match δ18O in the surrounding soil, but reflected deeper soil values. At each time interval, seedling transpiration was measured before root excavation. The highest HR water content was found at midday, not at sunrise, in seedlings’ root water, rejecting hypothesis 2. Nevertheless, 29 ± 6 % of the oak seedlings’ daily transpired water originated from HR, emphasising the importance of HR for shallow-rooted understorey plants during drought. Future research should focus on the transport pathways of HR water from mature trees to shallow-rooted understorey plants to improve mechanistic understanding.

Climate extremes like drought are threatening forests worldwide. Record breaking forest mortality has been observed in central Europe in the past five years. Meanwhile, more and more experiments are being set up that enable measurements of the hydraulic states of dying trees under extreme drought stress. These experimental data can be exploited by mechanistic vegetation models, offering the possibility to disentangle environmental drought stressors, e.g. atmospheric and soil moisture dryness, and their effects on a plant’s hydraulic system, such as stomatal closure and loss of hydraulic conductivity. Here, we show how a next generation plant hydraulic modelling is able to accurately reproduce the water potential dynamics of dying trees. We apply this plant hydraulic model to European drought experimental sites, including the canopy crane experiment II in Basel, Switzerland, and the KROOF experiment in Freising, Germany. We find that soil heterogeneity, rooting depth and stem hydraulic capacity are critical in determining whether a tree survives or succumbs to drought. Furthermore, good knowledge of four parameters is crucial to accurately capture the magnitude and temporal development of observed leaf and stem water potential: (1) stem hydraulic capacitance, (2) P50 (the water potential at which 50% of a plant’s hydraulic conductivity is lost), (3) saturated xylem hydraulic conductivity, and (4) the reference leaf water potential associated with full stomatal closure. Finally, when implemented into the terrestrial biosphere model QUINCY, our hydraulic scheme produces a clear mortality signal associated with recent drought events, giving confidence in our capacity to project the impact of future droughts on European forests

Forest ecosystems face considerable long-term risks in the context of escalating drought and rising temperatures. Consequently, studies examining the water balance of individual trees and entire forest stands are imperative to assess potential impacts and explore potential silvicultural strategies to mitigate the effects of climate change.A range of methods have been employed to measure tree sapflow density (SFD), including TD (thermal dissipation), TTD (transient thermal dissipation), HRM (heat ratio method), and HFD (heat field deformation). Each method comes with advantages and disadvantages. However, when calculating the total water use of trees, two additional variables to SFD must be considered. The first is the conducting sapwood depth, and the second is the xylem sapflow profile, which represents the decrease in SFD from the outer to the inner part of the conductive sapwood. In the case of oak, the literature suggests that the conducting sapwood area/sapwood depth has primarily been determined using the light transmission method in combination with coloration due to the ring-porous properties and the formation of different colored heartwood.In this study, the HFD method was used to measure the SFD at 1 cm intervals up to a total depth of 7.5 cm in Quercus petraea (Matt.) Liebl. (sessile oak). In addition, the sapwood depth of each tree was assessed via the light transmittance method in combination with heartwood coloring and yielded an average sapwood depth of 2.7 ± 0.6 cm for the measurement trees within a diameter range from 31.5 cm to 42.0 cm. These results are consistent with the results on sapwood depth reported in the literature for various oak species, which indicate and were interpreted with more or less zero sap flow with the beginning of the heartwood. However, the HFD data showed that xylem sapflow extended to an average depth of 5.5 cm with a steep logarithmic decline, but resulting in relative sapflow rates still around 30% at the visual axis between sapwood and heartwood.Furthermore, both the actual measured sap flow profile and a sapwood depth-based model were used to calculate the whole tree water use. Within a range of daily water use per tree of 6 to 60 L d-1, the calculation based on the measured sap flow profile was on average 19.3 ± 0.6 % higher than with the sapwood-based profile. This daily offset of about 20 % is particularly important when calculating the water use of trees and stands under good weather conditions with high sap flow rates.

Increasingly frequent and intense drought events can jeopardize the current and future productivity and health of forests. Consequently, the ability of dynamic vegetation models (DVMs) to simulate drought impacts is paramount to improving their representation of the carbon cycle. To capture the physiological damage inflicted by drought, many state-of-the-art DVMs have implemented representations of plant hydraulic architecture in recent years. Although the understanding of the underlying processes governing hydrodynamic behavior in plants has steadily increased, the parameterization of hydraulic traits for different plant functional types (PFTs) remains a source of uncertainty in model output – in part due to limited data availability. Here, we use LPJ-GUESS-HYD, an extension of LPJ-GUESS with new parameters and processes to simulate plant hydraulic architecture, isohydrodynamic water-potential regulation, and hydraulic failure mortality. Using latin hypercube sampling we create 6000 sets of hydraulic parameter combinations based on values found in the literature. Based on these parameter sets, we conduct a comprehensive variance-based sensitivity analysis for a set of 12 common European tree species across 37 sites from the FLUXNET 2020 warm winter dataset, encompassing a wide range of European ecosystems. Subsequently, we determine which parameters and parameter interactions contribute the most to variations in model outputs. Our results indicate that of the seven parameters used in the hydraulic architecture model of LPJ-GUESS-HYD, only a few have a significant effect on the model outcomes. More specifically, Ѱ50, the water potential at which 50 percent of conductance is lost, and maximum specific leaf conductance had the largest impact on simulated processes. Parameters related with the isohydric strategy of plants, had a lesser but still substantial role in shaping the model output. These results suggest that certain hydraulic parameters – and combinations thereof – play a disproportionate role in modulating simulated forest fluxes and states in LPJ-GUESS-HYD. Specific parameterization choices can drastically alter model performance, including whether PFTs can survive in a given climate or not. Aside from encouraging careful consideration of the available trait data when parameterizing new PFTs, our results may guide future experiments in choosing which hydraulic traits to focus on.

The response of plants to increasing atmospheric CO2 concentration depends on several factors such as life history of specific species, availability of water, nutrients and light, and the ecological context that the plants are found. Although several experiments with elevated CO2 (eCO2) have been done worldwide, none was performed in the Amazon forest understory focusing in a community growing naturally. The understory of the central Amazon is limited by both light and phosphorus. Understanding how such ecosystem responds to eCO2 is important to foresee how the forest will function in the future. Also, quantifying the response of this forest compartment helps to constrain Ecosystem Models that compute carbon and water fluxes. For this study, we used the open-top chamber (OTC) approach, with a CO2 enrichment of +250 ppm above the ambient concentration. Eight OTC were installed (4 with ambient CO2 and another 4 with eCO2) in the understory of a natural forest in the Central Amazon, approximately 70 km from Manaus city. The eCO2 experiment started in November 2019 and, after 120 days, we quantified the average community response of the following photosynthetic parameters: light saturated carbon assimilation rate (Asat), stomatal conductance (gs), transpiration rate (E), intrinsic water use efficiency (iWUE), apparent quantum yield (Φ), light compensation point (LCP), maximum carboxylation capacity (Vcmax), maximum electron transport rate (Jmax). After 240 days of treatment, we quantified mean individual leaf production and accumulated leaf production, leaf area (Lfarea). After 300 days, we quantified the increment in base diameter (BD), height (Ht) and relative growth rate (RGR). Under eCO2, we observed increases in Asat (67%), Jmax (19%), Φ (56%), and iWUE (78%), in agreement with the hypothesis that plants near the light compensation point respond strongly to eCO2. We also detected an increase in Lfarea (51%) and BD (65%), indicating that the extra primary productivity was not allocated to growth in height, but to supporting more light intercepting organs (leaf and conducting tissues). No detectable changes were observed for the other variables. Apart from the expected increase in assimilation rates, understory plants in Central Amazon responded positively to eCO2 by increasing their ability to capture and use light (leaf size, Φ, and Jmax). The increment in leaf area while maintaining E rates signifies that this forest compartment will increase its contribution to the whole forest water fluxes to the atmosphere. That might be related to the prevailing acquisitive strategy necessary for competing for phosphorus brought by water flow through plants. As a possible consequence, this forest might be less resistant to extreme drought associated with El Niño years. Funding: Coordination for the Improvement of Higher Education Personnel - CAPES (grants 312589/2022-0) and São Paulo Research Foundation-FAPESP processes numbers (2022/07735-5) and (2015/02537-7).

Plant carbon (C) allocation describes the distribution of carbon among different organs and processes and is sensitive to environmental conditions. As climate change proceeds, it will introduce additional uncertainties to the forest’s function as a crucial terrestrial carbon sink. Previous studies have explored the effect of single environmental variables on plant carbon allocation. Still, they cannot provide insights into the combined effects of changing environment in the future. To understand how European tree species will alter their C allocation after acclimating to the future environment, beech and spruce seedlings were grown in controlled environment facilities (CEFs) of different scenarios for three years. The scenarios represent the present condition of 1987 to 2016 (PC) and, in accordance with the IPCC scenarios, a mitigation scenario (RCP2.6) and a worst-case scenario (RCP8.5) of 2017 to 2100. This implies an increase in air temperature by approximately 1°C (RCP2.6) and 3°C (RCP8.5), an increase in CO2 concentration by approximately 30 ppm (RCP2.6) and 500 ppm (RCP8.5), and changes in other variables over the three years, including the irradiance, the relative humidity, and the O3 concentration. After three years of treatment, the plants were labeled with 13C-enriched CO2 for three days to understand the allocation and turnover of new photoassimilates. Both beech and spruce had greater biomass under RCP8.5 compared to RCP2.6 and PC, accompanied by enhanced allocation to belowground biomass. The concentration of non-structural carbohydrates (NSC) showed no significant difference across the scenarios, neither in leaves nor fine roots. Yet, the mean residence time of carbon (MRT) of the soil respiratory pool had shortened in the RCP scenarios in both species. Specifically for beech, a compartmental model showed an increased pool size of mobile carbon and confirmed the shortened MRT of the mobile carbon pool under RCP8.5. The unchanged NSC concentration in the sink organ with the shortened MRT has indicated a more rapid carbon turnover under both RCP scenarios accompanied by more substantial C allocation to the immediate respiration. Additionally, the fixed C was substantially invested in biomass growth, i.e., structural carbon, only under RCP8.5, which indicates that the doubled CO2 concentration has alleviated the environmental stress. In contrast, the minor increase in CO2 concentration under RCP2.6 had no such effect. We thus recommend that the relationship between C fixation and biomass growth should be interpreted more cautiously under changing environmental conditions in the future.

Due to climate change, severe-drought events have become increasingly commonplace across Europe in recent decades, with future projections indicating that this trend will likely continue, posing questions about the continued viability of European forests. Observations from the most recent pan-European droughts suggest that these types of “hotter droughts” may acutely alter the carbon balance of European forest ecosystems. However, substantial uncertainty remains regarding the possible future impacts of severe drought on the European forest carbon sink. Dynamic vegetation models can help to shed light on such uncertainties; however, the inclusion of dedicated plant hydraulic architecture modules in these has only recently become more widespread. Such developments intended to improve model performance also tend to add substantial complexity, yet the sensitivity of the models to newly introduced processes is often left undetermined. Here, we describe and evaluate the recently developed mechanistic plant hydraulic architecture version of LPJ-GUESS and provide a parameterization for 12 common European forest tree species. We quantify the uncertainty introduced by the new processes using a variance-based global sensitivity analysis. Additionally, we evaluate the model against water and carbon fluxes from a network of eddy covariance flux sites across Europe. Our results indicate that the new model is able to capture drought-induced patterns of evapotranspiration along an isohydric gradient and manages to reproduce flux observations during drought better than standard LPJ-GUESS does. Further, the sensitivity analysis suggests that hydraulic process related to hydraulic failure and stomatal regulation play the largest roles in shaping the model response to drought.

Increasingly frequent and intense drought events can jeopardize the current and future productivity and health of forests. Consequently, the ability of dynamic vegetation models (DVMs) to simulate drought impacts is paramount to improving their representation of the carbon cycle. To capture the physiological damage inflicted by drought, many state-of-the-art DVMs have implemented representations of plant hydraulic architecture in recent years. Although the understanding of the underlying processes governing hydrodynamic behavior in plants has steadily increased, the parameterization of hydraulic traits for different plant functional types (PFTs) remains a source of uncertainty in model output – in part due to limited data availability.    Here, we use LPJ-GUESS-HYD, an extension of LPJ-GUESS with new parameters and processes to simulate plant hydraulic architecture, isohydrodynamic water-potential regulation, and hydraulic failure mortality. Using latin hypercube sampling we create 6000 sets of hydraulic parameter combinations based on values found in the literature. Based on these parameter sets, we conduct a comprehensive variance-based sensitivity analysis for a set of 12 common European tree species across 37 sites from the FLUXNET 2020 warm winter dataset, encompassing a wide range of European ecosystems. Subsequently, we determine which parameters and parameter interactions contribute the most to variations in model outputs.    Our results indicate that of the seven parameters used in the hydraulic architecture model of LPJ-GUESS-HYD, only a few have a significant effect on the model outcomes. More specifically, Ѱ50, the water potential at which 50 percent of conductance is lost, and maximum specific leaf conductance had the largest impact on simulated processes. Parameters related with the isohydric strategy of plants, had a lesser but still substantial role in shaping the model output.    These results suggest that certain hydraulic parameters – and combinations thereof –  play a disproportionate role in modulating simulated forest fluxes and states in LPJ-GUESS-HYD. Specific parameterization choices can drastically alter model performance, including whether PFTs can survive in a given climate or not. Aside from encouraging careful consideration of the available trait data when parameterizing new PFTs, our results may guide future experiments in choosing which hydraulic traits to focus on.

The Amazon rainforest is highly biodiverse and has the largest extent of the remaining intacttropical forests in the world. To this day, undisturbed tropical forests act as a carbon sink by takingup about 15% of anthropogenic carbon emissions per year. However, in the past decades, adeclining trend in the carbon sink capacity in the Amazon rainforest has been observed due toincreased carbon losses and tree mortality. The causes are disputed, but increasing temperaturesand more frequent severe droughts are potentially major drivers. We employ a novel modelingframework and hypothesize that previously rare, extreme droughts in the Amazon, such as theones in 2005 and 2010, constitute the main cause behind the decline of the net carbon sink inaboveground biomass. Our dynamic vegetation model simulates process-based plant hydraulicsand drought-induced mortality, and accounts for the diversity of strategies in plant responses todrought based on observed hydraulic vulnerability curves. The simulated impact of the 2005drought event temporarily turned the annual Amazon net carbon sink to a carbon source of about0.25MgCha-1. In contrast to other dynamic vegetation models our model simulated anincreasing trend in carbon losses and a declining trend in the Amazon carbon sink over the past25 years (net sink rate of-0.015MgCha-1year-1or-0.18MgCha-1per decade) whichcorresponds well with long-term forest monitoring data (net sink rate of-0.016MgCha-1year-1). We show that this trend is entirely attributable to drought-induced forest mortalityduring extreme years. The simulations show a threshold-like behavior between drought intensityand biomass loss, which is due to xylem vulnerability, indicating the potentially high sensitivity ofAmazon forests to extreme drought. Further increases in the severity and frequency of droughtsmight thus lead to greater carbon release and tree mortality than previously assumed.

The composition of soil fungal communities is known to impact tree performance. However, fungal communities differ among soils with different precipitation histories and may change during drought. This study aimed to determine the influence of soil origin and associated climate adaptation of fungal communities on European beech seedlings’ drought responses. Seedlings were established from the same seed source and grown in three soils with different precipitation histories but similar water retention properties. One year after establishment, half of the seedlings were exposed to a two-month drought with predawn leaf water potentials of about –1.5 MPa, the other half remained well-watered (control). Before and during the drought, soil and root fungal community composition, root architecture, seedling growth, carbon allocation and leaf physiology were determined. The drought effect on the fungal community composition was the lowest in dry region soils, suggesting a natural adaptation of the fungal communities to dry environments. Nevertheless, contrary to our expectations, the seedlings grown in dry region soils with respective adapted fungal communities were most affected by drought. This was evidenced by a lower predawn water potential, probably due to shorter root systems with higher root branching compared to those grown in moist region soils where a greater taproot length was observed. Beech seedlings´ drought responses depend largely on their different rooting patterns and less on the soil fungal communities that are adapted to long-term precipitation conditions. Yet, microbial effects cannot be excluded. Future research should focus more on the role of specific microbial species on plant root growth and drought responses.

<p>This paper addresses the knowledge gaps on what determines tree responses to drought, their recovery, and survival following drought release, integrating physiological and morphological responses from the rhizosphere to the canopy of mature trees. I summarize 10 years of research on an experimentally induced drought and subsequent recovery in a maturing beech-spruce forest in southern Germany. Study objects 70- to 80-year-old trees that are readily accessible via canopy crane and grow in either mono-specific or mixed stands. We studied about 100 trees in 12 plots of roughly 150 m2 each. Tress were exposed to experimentally induced summer drought for five consecutive years by complete throughfall exclusion during the growing season (Kranzberg forest ROOF experiment, kroof.wzw.tum.de).</p> <p>During the first two drought summers, when both species were not yet acclimated, drought stress intensity peaked with pre-dawn leaf water potentials near -2.0 MPa and concomitant severe declines in physiological (e.g. leaf gas exchange, phloem transport) and morphological (e.g. growth) responses. After overcoming the critical first two years of drought, significant morphological acclimation in the following three years, e.g. by adjustment of leaf area or rooting depths, resulted in relaxation of physiological stress, as evidenced, for example, by increased stomatal conductance and pre-dawn leaf water potential. Reduced water consumption of spruce and thus higher water availability, also for neighboring beech trees, significantly alleviated drought stress in the trees.</p> <p>After a total of five years of experimentally induced summer drought, drought release was initiated in early summer 2019 by controlled watering. Physiological parameters such as stomatal conductance or xylem sap flow recovered with hours or days after drought release, including resilience of C allocation, e.g. sugar transport along the stems, as an important prerequisite for the recovery of tree functionality and productivity. Restored coupling between canopy and rhizosphere significantly supported spruce root growth, which recovered within a few days. In contrast, other morphological responses (e.g., leaf area recovery) took years to recover. With future increases in the frequency of drought events under ongoing climate change, tree species that recover more quickly will be favored.</p>

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.

Water consumption of trees is one of the most important processes connected to their survival under ongoing climate change and extreme events such as drought. Radial profiles of xylem sap flow density are an integral component to quantify the water transport for the level of an individual tree and that of ecosystems. However, knowledge of such radial profiles, in particular under stress, is very scarce. Here we show the radial profile of the xylem sap flow density in mature European beech (Fagus sylvatica L) and Norway spruce (Picea abies (L) Karst.) under repeated summer drought induced by throughfall exclusion (TE) and subsequent recovery compared to untreated control trees (CO). We measured xylem sap flow density (udaily in L dm-2 d-1) down to 8 cm sapwood depth at breast height using two different approaches, a thermal dissipation system and the heat field defor-mation method. In beech, repeated throughfall exclusion did not affect the radial xylem sap flow profile. However, in spruce, udaily was strongly reduced across the profile under repeated drought, changing the profile from a linear to a logarithmic regression. Even two years after drought release, the xylem sap flow profile did not fully recover in TE spruce. The reduction of udaily along the radial profile was accompanied by a reduction of the leaf area in TE spruce by c. 50%, while sapwood depth remained constant. The reduction of the xylem sap flow density along the profile reduced the calculated water consumption of TE spruce trees by more than 33% compared to CO trees, also after drought release. The impact of stressors such as repeated drought on the xylem sap flow density across the radial profile and its consequences for trees' and stands' water consumption needs to be addressed in more detail to minimize uncertainties in quantifying ecosystem water cycles.

Quantifying carbon dioxide (CO 2 ) fluxes between soil and atmosphere is key in understanding net ecosystem C exchange and biogeochemical C cycling in plant‐soil systems. In ecosystems with low primary production and sparse vegetation, for example, dry lands or subpolar regions where C fluxes are small, measurement sensitivity is key—even so when measurements are combined with isotopic labeling. Here, we present a simplified gas sampling system developed to facilitate sampling and measurement of low soil CO 2 fluxes as well as in situ 13 CO 2 labeling in the same setup. The capacity of the system was tested in a set of feature tests along with gas measurements of dryland soil‐biocrust systems. The system's sensitivity to capture minor changes in CO 2 concentration was confirmed in respiration and photosynthesis measurements of soil‐biocrust systems, where fluxes down to 0.1 μmol CO 2 m −2 s −1 were quantified. A balloon, implemented to counterbalance underpressure build‐up during gas withdrawal, mitigated 72% of pressure differences at sampling. The overall system volume was reduced to a minimum to limit contamination caused by residual air, and the design enabled one‐step flushing and evacuation of system compartments and gas sample bags, successfully ruling out cross‐contamination between samples. Ultimately, this system offers a flexible and accessible solution for CO 2 measurements that can be applied not only on arid soils with low biological activity and turnover rates, but also on plant‐soil systems. The modifications enabled larger, and thereby more representative, sample volumes to be collected while limiting incubation, contamination, and pressure effects on the intact soil system.

Summary Drought affects the fine-root systems of European beech ( Fagus sylvatica L.) and Norway spruce ( Picea abies [L.] KARST) in different ways, but little is known about how this impacts their fine-root-associated fungal communities. In a five-year throughfall exclusion experiment (KROOF) in a mature stand, we investigated whether recurrent drought periods progressively alter fine-root associated fungal communities, fine-root vitality, and ectomycorrhizal functionality in relation to the tree root zone (pure beech, pure spruce, or their mixture) and abiotic soil parameters. We found that the influence of recurrent droughts on root fungal communities peaked in the third year of the experiment and affected fungal functional groups in different ways. The root zone was the predominant factor in structuring all functional groups of root-associated fungi, while we did not find a prominent effect of root mixture. The importance of other factors (year of sampling, soil depth) varied among fungal functional groups. Our results indicate a robust biotrophic root-fungal system relying mainly on surviving root tips, complemented by a fluctuating saprotrophic fungal assembly.

Data set containing data from feature tests (1-3) as well as photosynthesis and respiration measurements.

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.