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In recent years, important processes controlling ecosystem carbon dynamics have been connected to fine-root exudation of soluble carbon compounds. Root exudation patterns may change depending on plant interactions and plant susceptibility to and recovery from drought. Recent investigations suggest that root exudation tends to increase with stress events, but quantification of the amount of carbon released from roots across soil depths with differing water availability and species interactions are missing.We tested if root exudation rates were negatively correlated with soil water content across soil depths during and after drought. We further tested if species in mixture, often considered to be less stressed under drought, exuded less carbon than species in monospecific environments. Exudates were sampled in a mature Fagus sylvatica L. and Picea abies (L.) Karst. forest at the end of a five-year throughfall exclusion period and again one year after the drought ended. We quantified root exudates and their variation with soil depth for both tree species in monospecific and mixed species zones.Carbon exudation significantly increased in fine roots exposed to dry soils (

Forest soils are crucial for many ecosystem services that rely on soil organic matter (SOM) stability. Carbon allocated to roots and released as exudates to the rhizosphere plays a key role in SOM stabilization. Under periodic drought, elevated root exudation and SOM accumulation have been reported. Yet, whether root exudates control SOM formation and stability in mature forests once the drought ends is largely unknown. We examined whether root exudates from P. abies and F. sylvatica trees relate to SOM formation and stability in soil depth profiles one year following five years of experimental drought (Kroof experiment, Germany). We collected root exudates throughout the rooting zone and combined the data with thermogravimetric analysis of SOM in the rhizosphere and non-rooted soil. We found that the rhizosphere of both species was characterized by stable SOM fractions that did not decrease post-drought, suggesting potential protection of SOM due to rhizodeposition and root exudates. In contrast, stable SOM fractions decreased relative to controls in non-rooted topsoil below P. abies, indicating a loss of stabilized SOM from drought-affected and re-wetted soil. Our measurements provide valuable insights into post-drought SOM formation and mechanisms of SOM stabilization in forest ecosystems under climate change.

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.

Reduced carbon assimilation by plants and increased net ecosystem exchange are often considered to reduce the overall carbon sink function of drought-stressed ecosystems. However, plants and soil may respond differently under drought, leading to imprecise predictions of carbon sequestration in soil. We determined the net carbon assimilation and related it to soil organic carbon (SOC) stocks as well as to root exudate production to measure belowground carbon investment in mature trees (F. sylvatica and P. abies) exposed to experimental drought for five growing seasons. Despite more than 50 % reduction in net carbon assimilation under drought, SOC stocks increased on average by more than 30 %. The proportion of carbon allocated as root exudates increased two- to threefold under drought. Increasing amounts of carbon in organo-mineral associations suggest increased carbon stability under water-limited P. abies but not under F. sylvatica. Our data indicate that the belowground sink strength increased rapidly for the ecological and economic most relevant tree species in Europe. However, evaluating the ecosystem´s carbon sink strength by using the net ecosystem exchange alone neglects belowground SOC accumulation under drought. Although belowground-invested carbon could contribute to reducing the soil carbon-climate feedback temporarily and may support ecosystem resilience, SOC accumulated primarily in dry mineral topsoil may be vulnerable upon exposure to rewetting events.

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.

Under ongoing global climate change, drought periods are predicted to increase in frequency and intensity in the future. Under these circumstances, it is crucial for tree's survival to recover their restricted functionalities quickly after drought release. To elucidate the recovery of carbon (C) transport rates in c. 70-year-old Norway spruce (Picea abies [L.] KARST.) after 5 years of recurrent summer droughts, we conducted a continuous whole-tree $^{13}$C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the $^{13}$C label in stem and soil CO$_{2}$ efflux, and tips of living fine roots. In the first week after watering, aboveground C transport rates (CTR) from crown to trunk base were still 50% lower in previously drought-stressed trees (0.16 ± 0.01 m h$^{-1}$) compared to controls (0.30 ± 0.06 m h$^{-1}$). Conversely, CTR below ground, that is, from the trunk base to soil CO$_{2}$ efflux were already similar between treatments (c. 0.03 m h$^{-1}$). Two weeks after watering, aboveground C transport of previously drought-stressed trees recovered to the level of the controls. Furthermore, regrowth of water-absorbing fine roots upon watering was supported by faster incorporation of $^{13}$C label in previously drought-stressed (within 12 ± 10 h upon arrival at trunk base) compared to control trees (73 ± 10 h). Thus, the whole-tree C transport system from the crown to soil CO$_{2}$ efflux fully recovered within 2 weeks after drought release, and hence showed high resilience to recurrent summer droughts in mature Norway spruce forests. This high resilience of the C transport system is an important prerequisite for the recovery of other tree functionalities and productivity.

Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought-stressed tree, that is, how much of net-C assimilation is allocated to exudation at the tree level. - We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above- and belowground C fluxes with leaf, stem and fine-root surface area. - Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine-root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two- to three-fold. Water-limited P. abies released two-thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one-third. - Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought-imposed belowground C investment, which could be beneficial for ecosystem resilience.

Drought is a severe natural risk that increases drying-rewetting frequencies of soils. Yet, it remains largely unknown how forest ecosystems respond to dry-wet cycles, hampering our ability to evaluate the overall sink and source functionality for this large carbon pool. Recent investigations suggest that the release of soluble carbon via root exudation increases under drought, influencing soil carbon stabilization and mineralization. However, an integration of root exudation into the carbon allocation dynamics of drought stressed trees is missing. We hypothesized that roots in dry soil layers have a higher exudation rate than roots in more moist layers across different soil depths. Further, we tested if higher exudation rates under drought are attenuated by reduced root abundance in dry soils and if the fraction of root exudation from total carbon allocation increases with decreasing photosynthesis rates under drought. At the KROOF experimental site in southern Germany, where mature beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.) trees were exposed to artificial drought stress for five consecutive growing seasons, we show that at the root level root exudation rate increases in drier soils. Especially roots in the upper soil profile and roots of spruce trees increased root exudation under drought. When scaled to whole tree level, we did not find differences in total exudation between drought stressed and control trees, indicating sustained root exudation at the tree level under drought. As photosynthesis rates and therefore total carbon assimilation was substantially reduced under drought (by 50 % in beech and almost 70 % in spruce), the fraction of root exudation from total assimilation slightly increased for drought stressed trees. Our results demonstrate that stimulation of root exudation rates with drought exists in natural temperate forest ecosystems but might be mitigated by reduced fine root abundance under drought. Nevertheless, increased exudation per root surface area will have localized impacts on rhizosphere microbial composition and activity especially in the topsoil exposed to more extreme dry-wet cycles. Finally, also the exudate composition can help to determine how priming of soil organic matter relates to belowground carbon allocation dynamics and to disclose processes of complementary species interaction and should be emphasised in future studies.

This contribution presents the result of a free-air 13C labeling experiment on mature Norway spruce (P. abies [L.] KARST.) upon watering after five years of recurrent summer drought in southern Germany, focusing on whole tree allocation processes. Mature spruce trees had been exposed to recurrent summer drought from 2014 to 2018 through complete exclusion of precipitation throughfall from spring to late fall (i.e., March to November). In early summer 2019, the drought stressed spruce trees were watered to investigate their recovery processes. In parallel with the watering, we conducted a whole-tree 13C labeling in canopies and traced the signal in various C sinks, i.e. stem phloem and CO2 efflux, tree rings at different heights, coarse roots, fine root tips, mycorrhiza, root exudates, and soil CO2 efflux.We hypothesize that drought stressed spruce preferentially allocates newly assimilated C to belowground sinks upon drought release. Conversely to our expectations, allocation to belowground C sinks was not stimulated in drought stressed compared to control spruce. Likewise, the relative amount of recently fixed C allocated to aboveground sinks did not differ between treatments. Our findings suggest that the belowground C sinks are not of higher priority for the allocation of newly assimilated C upon watering after long-term drought. The observed allocation pattern is discussed taking total above- and belowground biomass as well as C source/sink relations into account.

Ground-level ozone (O3) is a widespread air pollutant causing extensive injuries in plants. However, its effects on perennial energy crops remain poorly understood due to technical difficulties in cultivating fast-growing shrubs for biomass production under O3 treatment on the field. Here we present the results of a two-year evaluation in the framework of which willow (Salix sachalinensis F. Schmid) shrubs were exposed to ambient (AOZ) or elevated (EOZ) O3 in two successive growing seasons (2014, 2015) and treated with 0 (EDU0) or 400 mg L−1 (EDU400) ethylenediurea spray in the second growing season. In 2014, EOZ altered the chemical composition of both top young and fallen leaves, and a novel mechanism of decreasing Mg in fallen leaves while highly enriching it in young top leaves was revealed in shrubs exposed to EOZ. In 2015, EDU400 alleviated EOZ-induced decreases in leaf fresh mass to dry mass ratio (FM/DM) and leaf mass per area (LMA). While EDU400 protected against EOZ-induced suppression of the maximum rate at which leaves can fix carbon (Amax) in O3-asymptomatic leaves, it did not alleviate EOZ-induced suppression of the maximum rates of carboxylation (VCmax) and electron transport (Jmax) and chlorophylls a, b, and a + b in the same type of leaves. In O3-symptomatic leaves, however, EDU400 alleviated EOZ-induced suppression of chlorophylls a and a + b, indicating different mode of action of EDU between O3-asymptomatic and O3-symptomatic leaves. Extensive herbivory occurred only in AOZ-exposed plants, leading to suppressed biomass production, while EOZ also led to a similar suppression of biomass production (EDU0 × EOZ vs. EDU400 × EOZ). In 2016, carry-over effects were also evaluated following cropping and transplantation into new ambient plots. Effects of EOZ in the preceding growing seasons extended to the third growing season in the form of suppressed ratoon biomass production, indicating carry-over effect of EOZ. Although EDU400 protected against EOZ-induced suppression of biomass production when applied in 2015, there was no carry-over effect of EDU in the absence of EDU treatment in 2016. The results of this study provide novel mechanistic understandings of O3 and EDU modes of action and can enlighten cultivation of willow as energy crop.