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Almost half of Earth’s terrestrial surface is covered by drylands, where limitation of water restricts vascular plant growth. In these ecosystems, a substantial part of primary production instead takes place directly at the soil surface, within complex microbial communities forming biological soil crusts (biocrusts) that include intricately bound soil particles. These communities, composed mainly of cyanobacteria, algae, fungi, and bryophytes, are fundamental actors for dryland biogeochemical cycles, as they fix atmospheric CO2 and N2 and constitute one of the primary, if not the only, sources of soil C and N. Due to the vast spatial extent of biocrusts, accounting for up to 70% of the living land cover in drylands, their importance for C and N cycling extend to the global scale, i.e. accounting for 7% of global net primary production of total terrestrial vegetation. However, warming temperatures and increasing soil dryness following climate change are estimated to have critical implications on these systems; recent studies show i.e. warming-induced reduction of biocrust cover and, thus, reduced CO2 uptake.Our aim is to contribute to this relatively new research field by providing insights into biocrust-soil-microorganism interactions under elevated temperatures and drought at the process-scale. This was realized in a phytotron incubation experiment of soil-biocrust mesocosms with experimental warming and drought, during which CO2 uptake and heterotrophic respiration was monitored. Dual labelling pulses (13CO2 and 15N2) were applied to follow the fate of recently fixed 13C and 15N into both particulate and mineral-associated SOM pools via physical fractionation and into microbial biomass via PLFA. Further, hyperspectral VIS-NIR images of the surface were recorded to quantify and determine crust cover and composition.The results revealed clear drought effects—not only in a distinct reduction in CO2 fixation by the biocrusts, but also in the elemental distribution of soil C underneath; the effect extended down into underlying soil layers, where biocrust-derived C contents were reduced by half due to drought. The change in the translocation of biocrust-derived C into the underlying soil was reflected in the 13C-PLFA profiles, showing how mainly fungi transform recently fixed C from the biocrust into their biomass in the biocrust layer, extending down into the underlying soil via fungal hyphae expansion. While drought clearly restricted the microbial abundance, warming further induced a microbial community shift, where a greater relative fungal dominance was determined under experimental warming—a shift, however, that was not reflected under dry conditions. A further combined effect was determined in N fixation, where we confirm a decrease in biocrust-derived N under drought under warming.Our results showcase the implications of elevated temperature and drought on C and N fixation and cycling—the two most fundamental ecosystem functions in biocrust-soil systems. The results support the growing body of evidence of major implications for biogeochemical cycles in drylands in a warming world.

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.

Besides soils, tree stems are known to emit nitrous oxide (N2O) into the atmosphere. However, it seems, stems of some tree species might also take up this important greenhouse gas from the atmosphere under certain conditions. Even though tree leaves dominate the tree surface area, they are entirely excluded from field N2O flux measurements, and their role in forest N2O exchange is still unknown.We aimed to investigate the contribution of leaf fluxes to the forest N2O exchange. We determined N2O exchange of stems and leaves of mature European beech (Fagus sylvatica), and adjacent soil in a typical temperate upland mixed forest in Southern Germany, using non-steady-state chamber methods and a system of scaffold towers reaching the top of tree crowns in 35 m. The measurements were accompanied by a parallel determination of stem, leaf and soil CO2 exchange and numerous environmental characteristics (soil N2O and CO2 concentrations and water content in vertical soil profiles, soil and air temperature).We found out that the beech stems and especially the leaves were net sinks of N2O from the atmosphere (–1.07 ± 3.47 and –249.9 ± 84.3 mg N2O ha−1 ground area h−1, respectively), whereas the soil was a net N2O source into the atmosphere (24.0 ± 10.8 mg N2O ha−1 h−1). The never studied tree leaves were identified as a key player in ecosystem N2O exchange, taking up in fact 10 times more N2O than the soil emits at the same time. Therefore, native Central European and widely spread European beech trees seem to contribute to forest N2O uptake markedly.For the first time, we revealed tree leaves being substantial N2O sinks. Our results clearly show that the current and ongoing exclusion of tree leaves from forest N2O flux measurements can lead to a severe underestimation of the overall tree and forest N2O exchange and, therefore, global forest greenhouse gas flux inventories. AcknowledgementThis research was supported by the Czech Science Foundation (17-18112Y) and project SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797). We thank Jan Hrdlička and Thomas Feuerbach for their technical support.

Anthropogenic activities such as industrialization, land use change and intensification of agriculture strongly contribute to changes in the concentrations of atmospheric trace gases. Carbon dioxide (CO2), oxidized N compounds(NOx), sulfur dioxide (SO2) and ozone(O3) have particularly significant impacts on plant physiology. CO2, the substrate for plant photosynthesis, is in the focus of interest as the ambiguous effect of its increasing concentration is controversially discussed. Is its increase beneficial for plants or are plants non-responsive? NOx, a product of combustion and lightning, can have either fertilizing or toxic effects depending on the concentration and form. This is also the case for reduced forms of nitrogen(NHy), which are mostly emitted from agricultural and industrial activities. In combination CO2and N compounds can have a fertilizing effect. SO2 and ground-level O3 are mostly phytotoxic, depending on their concentrations, daily and seasonal exposure dynamics, and tree health condition. Elevated concentrations of both substances arise from industrial combustion processes and car emissions. All of the above-mentioned gaseous compounds affect plant metabolism in their specific ways and to different degrees. This impacts the isotope fractionation leaving specific fingerprints in the C, O, (H) and N isotope ratios of organic matter. In this chapter we will show how the impact of increasing CO2 and air pollutants are reflected in the isotopic ratios of tree rings. Increasing CO2 shows a considerable variation in responses of δ13C and to a minor degree in δ18O. Ozone and SO2 exposure cause an overall increase of the δ13C values in tree rings and a slight decrease in δ18O, mimicking an increase in net photosynthesis (AN) and to a minor degree in stomatal conductance (gs). However, directly measured AN and gs values show the opposite, which does not always correspond with the isotope derived gas exchange data. NO2 concentration as it is found near highly frequented freeways or industrial plants causes an increase of δ13C while δ18O decreases. This indicates an increase in both AN and gs, which corresponds well with directly measured gas exchange data. Thus the air quality situation must be taken in consideration for the interpretation of isotope values in tree rings.

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.

Herbivores and mycorrhizal fungi interactively influence growth, resource utilization, and plant defense responses. We studied these interactions in a tritrophic system comprising Quercus robur, the herbivore Lymantria dispar, and the ectomycorrhizal fungus Piloderma croceum under controlled laboratory conditions at the levels of gene expression and carbon and nitrogen (C/N) allocation. Taking advantage of the endogenous rhythmic growth displayed by oak, we thereby compared gene transcript abundances and resource shifts during shoot growth with those during the alternating root growth flushes. During root flush, herbivore feeding on oak leaves led to an increased expression of genes related to plant growth and enriched gene ontology terms related to cell wall, DNA replication, and defense. C/N-allocation analyses indicated an increased export of resources from aboveground plant parts to belowground. Accordingly, the expression of genes related to the transport of carbohydrates increased upon herbivore attack in leaves during the root flush stage. Inoculation with an ectomycorrhizal fungus attenuated these effects but, instead, caused an increased expression of genes related to the production of volatile organic compounds. We conclude that oak defense response against herbivory is strong in root flush at the transcriptomic level but this response is strongly inhibited by inoculation with ectomycorrhizal fungi and it is extremely weak at shoot flush.

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.

The Amazon rainforest has been hit by extreme drought events in recent decades. Thereby, plant hydraulics are essential to better understand the impacts of droughts on single plants and whole forest ecosystems. Plant hydraulic mechanisms such as stomatal closure and leaf water potential are very complex, still posing challenges for current vegetation model development and parameterization. Here, we present the new hydraulic architecture of the Dynamic Global Vegetation Model LPJ-GUESS, accounting for leaf stomatal responses to plant water status and subsequent drought-induced mortality. We show that when applying the model to the Amazon rainforest we can reproduce the observed increasing trend in carbon losses and the decreasing trend in net carbon sink from plot observations over the past two decades. Our model simulations suggest that the increasing historical trend in carbon losses from mortality can be explained by hydraulic failure and associated mortality.The high biodiversity of the Amazon tropical rainforest poses further challenges for process-based models. Here we present an approach to include the diversity of plant responses to drought by simulating 37 individual Plant Functional Types (PFTs) differing in their leaf water potential regulation- and resistance to soil water stress, and provide a simple solution how to cover a wide range of species and species-specific parameters. Future modelling studies should also take species interaction and competition of different hydraulic strategies into account.

The rapid rise in atmospheric CO2 concentration over the past century is unprecedented. It has unambiguously influenced Earth’s climate system and terrestrial ecosystems. Elevated atmospheric CO2 concentrations (eCO2) have induced an increase in biomass and thus, a carbon sink in forests worldwide. It is assumed that eCO2 stimulates photosynthesis and plant productivity and enhances water-use efficiency – the so-called CO2-fertilization effect, which may provide an important buffering effect for plants during adverse climate conditions. For these reasons, current global climate simulations consistently predict that tropical forests will continue to sequester more carbon in aboveground biomass, partially compensating human emissions and decelerating climate change by acting as a carbon sink. In contrast to model simulations, several lines of evidence point towards a decreasing carbon sink strength of the Amazon rainforest. Reliable predictions of eCO2 effects in the Amazon rainforest are hindered by a lack of process-based information gained from ecosystem scale eCO2 experiments. Here we report on baseline measurements from the Amazon Free Air CO2 Enrichment (AmazonFACE) experiment and preliminary results from open-top chamber (OTC) experiments with eCO2. After three months of eCO2, we find that understory saplings increased carbon assimilation by 17% (under light saturated conditions) and water use efficiency by 39% in the OTC experiment. We present our main hypotheses for the FACE experiment, and discuss our expectations on the potential driving processes for limiting or stimulating the Amazon rainforest carbon sink under eCO2. We focus on possible effects of eCO2 on carbon uptake and allocation, nutrient cycling, water-use and plant-herbivore interactions, which need to be implemented in dynamic vegetation models to estimate future changes of the Amazon carbon sink.

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.

This contribution summarizes the outcome of a five-year experiment on mature (60-80 years old) trees in a Central European forest. We studied roughly 100 trees of European beech and Norway spruce, two tree species of contrasting foliage (i.e. deciduous vs. evergreen) and stomatal sensitivity to drought (i.e. anisohydric vs. isohydric behavior). Trees were exposed to experimentally induced summer droughts from 2014 to 2018 with precipitation throughfall being completely excluded during the growing seasons. The throughfall-exclusion study was established on 12 plots with trees readily accessible by canopy crane (Kranzberg forest roof experiment, southern Germany). We aimed at bringing trees to the edge of survival to studying trees’ capability for acclimation under repeated, severe summer droughts as expected more frequently in future climate scenarios. Results come from a multidisciplinary approach focusing on mechanisms of acclimation, eventually reducing trees’ vulnerability to drought during the five-year study period. Presented data integrate responses from the level of soil/microbial interactions over tree organs and whole-tree morphology to responses at the stand level.During the first two years, restrictions caused by drought were most prominent, exemplified by pre-dawn leaf water potentials of down to -2.5 MPa and reductions in photosynthesis and growth by up to 50 and 80 % in European beech and Norway spruce, respectively. Nevertheless, percentage loss of conductivity in branch xylem was hardly affected. Likewise, concentrations of non-structural carbohydrates (sum of soluble sugars and starch) in tree organs remained largely unaffected, but translated to significantly lower carbohydrate pool sizes in view of strongly reduced tree growth. Nevertheless, two spruce trees died from drought, in the absence of bark beetle or pathogen interactions. During the fourth and fifth year of summer drought, trees showed clear signs of drought acclimation with e.g. some recovery of stomatal conductance, reductions of whole-tree leaf area, changes in rooting depth and acclimation of associated soil microbial communities. Accordingly, stem diameter growth recovered during the last years of the stress treatment, indicating reduced vulnerability of trees towards the end of the five-year drought treatment.

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.

Plant hydraulics are crucial to understand impacts of droughts on single plants and whole forest ecosystems. The complex interplay of hydraulic mechanisms still poses challenges for vegetation modellers, regarding development and parameterization. Here, we apply a new hydraulic module developed for the dynamic global vegetation model LPJ-GUESS to the Amazon Basin. Special focus is given to the newly developed mortality process based on hydraulic-failure and to differences in hydraulic behaviour of plants. The implemented hydraulic-failure process can explain observed mortality patterns at rainfall exclusion experiments in the Amazon. Modelled vegetation carbon is most sensitive to two of the hydraulic processes: The xylem vulnerability to water stress and the plant specific hydraulic behaviour, i.e. how plants regulate their water potential under drought stress. Applied to the whole Amazon Basin, our model shows a strong impact of the 2005 drought event across a wide margin of modelled species and parameters, which is in good agreement with empirical studies. We highlight the hydraulic behaviour of plants, for which little is known in the Amazon rainforest, and its relevance for ecosystem model development. Considering only one single plant functional type does not sufficiently capture the complex response of the Amazon rainforest to drought, hence future modelling studies should take the interaction and competition of different hydraulic strategies into account.

Phloem sustains maintenance and growth processes through transport of sugars from source to sink organs. Under low water availability, tree functioning is impaired, i.e., growth/photosynthesis decline and phloem transport may be hindered. In a 3-year throughfall exclusion (TE) experiment on mature European beech (Fagus sylvatica L.) we conducted 13CO2 branch labeling to investigate translocation of recently fixed photoassimilates under experimental drought over 2 years (2015 and 2016). We hypothesized (H1) that mean residence time of photoassimilates in leaves (MRT) increases, whereas (H2) phloem transport velocity (Vphloem) decreases under drought. Transport of carbohydrates in the phloem was assessed via δ13C of CO2 efflux measured at two branch positions following 13CO2 labeling. Pre-dawn water potential (ΨPD) and time-integrated soil water deficit (iSWD) were used to quantify drought stress. The MRT increased by 46% from 32.1 ± 5.4 h in control (CO) to 46.9 ± 12.3 h in TE trees, supporting H1, and positively correlated (P < 0.001) with iSWD. Confirming H2, Vphloem in 2016 decreased by 47% from 20.7 ± 5.8 cm h-1 in CO to 11.0 ± 2.9 cm h-1 in TE trees and positively correlated with ΨPD (P = 0.001). We suggest that the positive correlation between MRT and iSWD is a result of the accumulation of osmolytes maintaining cell turgor in the leaves under longer drought periods. Furthermore, we propose that the positive correlation between Vphloem and ΨPD is due to a lower water uptake of phloem conduits from surrounding tissues under increasing drought leading to a higher phloem sap viscosity and lower Vphloem. The two mechanisms increasing MRT and reducing Vphloem respond differently to low water availability and impair trees' carbon translocation under drought.

Under drought, potential plant death from depletion of carbon (C) stores, referred to as carbon starvation, is thought to result from negative carbon balance during (partial) stomatal closure. As evidence for C starvation is rare for mature trees, we investigated the C dynamics in mature beech and spruce under drought, focusing on non-structural carbohydrates (NSC) as an indicator of their C balance. Trees were exposed to complete exclusion of precipitation throughfall over two consecutive growing seasons. We assessed NSC concentrations during the early dormant season in leaves, twigs, stem phloem/xylem, coarse and fine roots. Up-scaling to whole-tree NSC pools was achieved using a process-based single-tree model (“BALANCE”), estimating tree biomass increment. While there were distinct differences in NSC concentration among different tissues in both beech and spruce (root

The ability of treeline conifers in the Central European Alps to cope with recent climate warming and increasing CO2 concentration is still poorly understood. We determined basal area increment (BAI) and tree ring stable carbon isotope ratios (δ13C) of Pinus cembra trees from 1925 through 2013. Stable isotope ratios and BAI were compared with leaf level gas exchange measurements carried out in situ between 1934 and 2012, and thus, provided new insights into long-term trends of tree-ring derived intrinsic water-use efficiency (iWUE). Mature P. cembra trees at treeline responded to increasing Ca and air temperature with a parallel increase in maximum net CO2 uptake rate at ambient CO2 (Amax) and tree-ring-derived intercellular CO2 partial pressure (Ci). Amax tripled and was positively correlated to BAI and Ci. The latter increased in parallel with ambient CO2 concentration and stomatal conductance. In contrast to the instantaneous gas exchange parameters, δ13C derived iWUE informs about the long-term changes in the carbon water relations. These data showed three changes in the iWUE chronosequences, which could be identified with different long term gas exchange patterns: (1) from stomatal controlled functioning from 1925 to 1981, to a situation where (2) both net CO2 fixation (A) and leaf conductance for water vapour (gw), responded to the environment from 1982 to 1997, and (3) back to a stomata controlled pattern over iWUE from 1998 onwards. This temporal pattern was also mirrored in leaf level gas exchange assessments, suggesting a parallel increase of A and gw of P. cembra at treeline during the last nine decades.

Tree roots and mycorrhizal fungi form an important resource for soil microinvertebrates; however, there is little knowledge on the role of root feeders and microbial grazers on belowground carbon flux of forest trees. Using oak (Quercus robur) microcuttings (Clone DF159) inoculated with an ectomycorrhizal symbiont partner (Piloderma croceum), the present study investigated the impact of multitrophic interactions in the rhizosphere on the carbon flow to the soil food web in a pulse-chase labelling experiment. Changes in plant biomass, microbial communities (soil phospholipid fatty acids - PLFAs) and Collembola lipid pattern (total lipid fatty acids - TLFAs) as well as the 13C incorporation into fatty acids were assessed at 2, 5 and 20 days post labelling of oak microcuttings. Microcosms were inoculated with Pratylenchus penetrans (root-feeding nematode), Protaphorura armata (fungal-feeding Collembola) or their combination. The interaction of P. penetrans and P. armata synergistically reduced oak biomass. Both soil animals independently impacted growth of microorganisms in the rhizosphere, P. penetrans inhibited whilst P. armata promoted bacterial biomass. Irrespective of treatment and time, Gram-positive bacteria derived most of the recent photoassimilated oak carbon, with amounts ranging between 80 and 92% of the total 13C allocated in microbial PLFAs. Presence of either P. armata or P. penetrans enhanced the incorporation of plant carbon in bacteria, whereas collective effects of these two functional groups was scarce. Predominantly the impact of the root-feeding nematode on belowground carbon flow diminished with time. In sum, root feeders and detritivores played independent roles in carbon allocation patterns and community structure of microorganisms in the rhizosphere of oaks.

Hydraulic redistribution (HR) of soil water through plant roots is a crucial phenomenon improving the water balance of plants and ecosystems. It is mostly described under severe drought, and not yet studied under moderate drought. We tested the potential of HR under moderate drought, hypothesizing that (H1) tree species redistribute soil water in their roots even under moderate drought and that (H2) neighboring plants are supported with water provided by redistributing plants. Trees were planted in split-root systems with one individual (i.e., split-root plant, SRP) having its roots divided between two pots with one additional tree each. Species were 2- to 4-year-old English oak (Quercus robur L.), European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst). A gradient in soil water potential (ψsoil) was established between the two pots (-0.55 ± 0.02 MPa and -0.29 ± 0.03 MPa), and HR was observed by labeling with deuterium-enriched water. Irrespective of species identity, 93% of the SRPs redistributed deuterium enriched water from the moist to the drier side, supporting H1. Eighty-eight percent of the plants in the drier pots were deuterium enriched in their roots, with 61 ± 6% of the root water originating from SRP roots. Differences in HR among species were related to their root anatomy with diffuse-porous xylem structure in both beech and-opposing the stem structure-oak roots. In spruce, we found exclusively tracheids. We conclude that water can be redistributed within roots of different tree species along a moderate ψsoil gradient, accentuating HR as an important water source for drought-stressed plants, with potential implications for ecohydrological and plant physiological sciences. It remains to be shown to what extent HR occurs under field conditions in Central Europe.

In 2018, Central Europe experienced one of the most severe and long-lasting summer drought and heat wave ever recorded. Before 2018, the 2003 millennial drought was often invoked as the example of a “hotter drought”, and was classified as the most severe event in Europe for the last 500 years. First insights now confirm that the 2018 drought event was climatically more extreme and had a greater impact on forest ecosystems of Austria, Germany and Switzerland than the 2003 drought. Across this region, mean growing season air temperature from April to October was more than 3.3°C above the long-term average, and 1.2 °C warmer than in 2003. Here, we present a first impact assessment of the severe 2018 summer drought and heatwave on Central European forests. In response to the 2018 event, most ecologically and economically important tree species in temperate forests of Austria, Germany and Switzerland showed severe signs of drought stress. These symptoms included exceptionally low foliar water potentials crossing the threshold for xylem hydraulic failure in many species and observations of widespread leaf discoloration and premature leaf shedding. As a result of the extreme drought stress, the 2018 event caused unprecedented drought-induced tree mortality in many species throughout the region. Moreover, unexpectedly strong drought-legacy effects were detected in 2019. This implies that the physiological recovery of trees was impaired after the 2018 drought event, leaving them highly vulnerable to secondary drought impacts such as insect or fungal pathogen attacks. As a consequence, mortality of trees triggered by the 2018 events is likely to continue for several years. Our assessment indicates that many common temperate European forest tree species are more vulnerable to extreme summer drought and heat waves than previously thought. As drought and heat events are likely to occur more frequently with the progression of climate change, temperate European forests might approach the point for a substantial ecological and economic transition. Our assessment also highlights the urgent need for a pan-European ground-based monitoring network suited to track individual tree mortality, supported by remote sensing products with high spatial and temporal resolution to track, analyse and forecast these transitions. ISSN:1439-1791 ISSN:1618-0089

Key message Under severe drought, growth of Norway spruce suffered much more than European beech. Norway spruce benefited from growing in the environment of beech, and both species acclimated slightly to 5 years of experimentally extended drought. Abstract Recent studies show that the detrimental effects of drought on stand growth are mitigated when the stand contains mixed tree species. We analysed the growth responses of Norway spruce and European beech to episodic and experimentally extended drought in intra- and inter-specific mature stands. We used annual diameter growth records dating back to 1998 to determine the impact of the natural episodic drought in 2003 and 2015. To analyse extended drought, spruce and beech trees were exposed to extreme drought under automatic throughfall exclusion roofs from 2014 to 2018. The growth of spruce in an inter-specific environment with beech was 20–50% less affected by natural episodic drought compared with an intra-specific constellation. When beech grew in an inter-specific environment, it was by 23% more affected by drought compared to intra-specific conditions, but seemed to recover faster. The induced drought from 2014 to 2018 resulted in a strong growth reduction in the first year particularly for spruce, followed by a slight acclimation to the dry conditions. Beech acclimated and recovered faster than spruce across all growing conditions, while spruce only acclimatized faster in the environment of beech. Both species showed a higher mortality under induced drought compared with the controls; for spruce, the mortality rate was fivefold higher compared to the long-term mortality. The long-term moderate-growth stabilization and the growth increase after the 5-year exposure to drought suggest a gradual acclimation to drought by beech. The resistance and acclimation to drought of spruce when growing in mixture should be considered when designing resource efficient and productive mixed conifer-broadleaved stands for future climates.

Hydraulic redistribution (HR) of soil water through plant roots is widely described; however its extent, especially in temperate trees, remains unclear. Here, we quantified HR of five temperate tree species. We hypothesized that both, HR within a plant and into the soil increase with higher water‐potential gradients, larger root conduit diameters and root‐xylem hydraulic conductivities as HR driving factors. Saplings of conifer (Picea abies, Pseudotsuga menziesii), diffuse‐porous (Acer pseudoplatanus) and ring‐porous species (Castanea sativa, Quercus robur) were planted in split‐root systems, where one plant had its roots split between two pots with different water‐potential gradients (0.23–4.20 MPa). We quantified HR via deuterium labelling. Species redistributed 0.39 ± 0.14 ml of water overnight (0.08 ± 0.01 ml/g root mass). Higher pre‐dawn water‐potential gradients, hydraulic conductivities and larger conduits significantly increased HR quantity. Hydraulic conductivity was the most important driving factor on HR amounts, within the plants (0.03 ± 0.01 ml/g) and into the soil (0.06 ± 0.01 ml/g). Additional factors as soil‐root contact should be considered, especially when calculating water transfer into the soil. Nevertheless, trees maintaining high‐xylem hydraulic conductivity showed higher HR amounts, potentially making them valuable ‘silvicultural tools’ to improve plant water status. A free Plain Language Summary can be found within the Supporting Information of this article.

Hydraulic redistribution (HR) can buffer drought events of tree individuals, however, its relevance for neighbouring trees remains unclear. Here, we quantified HR to neighbouring trees in single- and mixed-species combinations. We hypothesized that uptake of HR water positively correlates with root length, number of root tips and root xylem hydraulic conductivity and that neighbours in single-species combinations receive more HR water than in phylogenetic distant mixed-species combinations. In a split-root experiment, a sapling with its roots split between two pots redistributed deuterium labelled water from a moist to a dry pot with an additional tree each. We quantified HR water received by the sapling in the dry pot for six temperate tree species. After 7 days, one quarter of the water in roots (2.1 ± 0.4 ml), stems (0.8 ± 0.2 ml) and transpiration (1.0 ± 0.3 ml) of the drought stressed sapling originated from HR. The amount of HR water transpired by the receiving plant stayed constant throughout the experiment. While the uptake of HR water increased with root length, species identity did not affect HR as saplings of Picea abies ((L.) Karst) and Fagus sylvatica (L.) in single- and mixed-species combinations received the same amount of HR water.

International audience; Hydraulic redistribution (HR), the passive reallocation of water along plant structures following a water potential gradient, is an important mechanism for plant survival under drought. For example, trees with deeper roots reallocate water from deeper moist to shallower, drier soil layers sustaining their upper fine root system. The relevance of HR for temperate forest ecosystems is hardly investigated. Both environmental and tree internal factors limiting the capacity for HR, such as low water potential gradients or root anatomy, respectively, are not well understood. Here we investigate fine root anatomy and related capacity for reverse flow of water of six temperate tree species, i.e. Acer pseudoplatanus, Castanea sativa, Fagus sylvatica, Picea abies, Pseudotsuga menziesii and Quercus robur both in forward and reverse flow direction. Additionally, anatomy of primary and secondary roots was analyzed, to test the hypotheses that root anatomy is similar in primary and secondary roots (H1) and conductivity for forward and reverse flow of water in fine roots is identical (H2). In contrast to the two conifer species, most anatomical parameters, e.g. hydraulic conduit diameter and conduit density, were distinctly different between primary and secondary roots in the angiosperms. Therefore, H1 was not supported for angiosperm trees. The reverse flow of water in fine roots was reduced by approx. 40 % compared to the forward flow in angiosperms, while there was no difference in the conifers. Thus, H2 was confirmed for conifers while there was a significant difference for angiosperms. This reduction may be caused by vessel structure (e.g. tapering or secondary thickening elements), or perforation plate and pit architecture (e.g. width of aperture opening). Because of the reduced conductivity of reverse water flow, the ability of angiosperm trees to redistribute water along their root system might be lower than expected.

Mixed stands of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) frequently over-yield, when compared to respective monospecific stands. Over-yielding is attributed to enhanced resource uptake efficiency through niche complementarity alleviating species competition, for example through enhanced root stratification in mixture. Under severe and frequent summer drought, however, water limitation may become crucial in modifying the prevailing competitive interaction in mixed beech-spruce forests. We hypothesize, therefore, that under drought (H I) inter-specific interaction with beech reduces water accessibility for spruce more than intra-specific conditions, thus (H II) exacerbating drought susceptibility of spruce in terms of reduced photosynthesis and stem growth. Reactions at the organ (leaf, fine root), tree and stand scale were analysed in a mature forest with beech-spruce group mixture. Under inter-specific conditions spruce’s fine-root production and depth of water uptake (assessed via δ18O of xylem water) shifted to shallow, drought-prone soil horizons, in agreement with H I. Overall, lowered fine root production and ramification along with a reduction in long-distance explorative ectomycorrhizal types resulted in decreased soil exploitation in spruce when growing together with beech. Spruce’s drought sensitivity was exemplified by a distinct decrease in stomatal conductance, net CO2 uptake rate and stem growth during periods of water limitation. Notwithstanding, species interaction effects were absent in leaf gas exchange and stem diameter growth, during a six-week summer drought period in 2013 as well as in the extremely dry year of 2003, hence rejecting H II. Based on results from soil moisture measurements and water uptake depth, we interpret the conflicting findings for H I and H II to result from: (i) seasonal shifts between positive (during spring drought) and negative (during summer drought) effects of beech neighbourhood on soil water availability for spruce, possibly overriding each other in their effect on annual stem diameter growth and (ii) the group-wise mixture pattern, where spruce is exposed to competition with beech only along group edges, i.e. laterally only, so that the putatively adverse beech effect on water accessibility stays limited. Our results suggest, compared to single tree mixture, group-wise mixture of beech and spruce to be a favourable silvicultural option in the face of climate change.

The (13/12) C ratio in plant roots is likely dynamic depending on root function (storage versus uptake), but to date, little is known about the effect of season and root order (an indicator of root function) on the isotopic composition of C-rich fractions in roots. To address this, we monitored the stable isotopic composition of one evergreen (Picea abies) and one deciduous (Fagus sylvatica), tree species' roots by measuring δ(13) C of bulk, respired and labile C, and starch from first/second and third/fourth order roots during spring and fall root production periods. In both species, root order differences in δ(13) C were observed in bulk organic matter, labile, and respired C fractions. Beech exhibited distinct seasonal trends in δ(13) C of respired C, while spruce did not. In fall, first/second order beech roots were significantly depleted in (13) C, whereas spruce roots were enriched compared to higher order roots. Species variation in δ (13) C of respired C may be partially explained by seasonal shifts from enriched to depleted C substrates in deciduous beech roots. Regardless of species identity, differences in stable C isotopic composition of at least two root order groupings (first/second, third/fourth) were apparent, and should hereafter be separated in belowground C-supply-chain inquiry.

As long-lived organisms, trees use resources to support both growth and below- and aboveground trophic interactions. Resources fluctuate in relation to periods of growth cease that are regulated by internal and external factors, and these fluctuations feed backs to trophic partners. Some major forest trees display an endogenous growth rhythm, and related pulses of variation in allocation of resources have been detected. As this trait makes it possible to separate growth into defined phases, it offers an opportunity to disentangle the intermingled complex regulation of growth and multitrophic interactions in trees. We present “TrophinOak”, a platform using microcuttings of pedunculated oak, a tree that displays endogenous rhythmic growth characterized by alternating shoot and root growth flushes. We select seven beneficial or detrimental above- and belowground partners including animals (Lymantria dispar, Pratylenchus penetrans, Protaphorura armata), fungi (Piloderma croceum, Microsphaera alphitoides, Phytophthora quercina) and bacteria (Streptomyces sp.), to synthesize bi- and tripartite trophic interactions, including ectomycorrhizal symbioses, and monitor fluctuations of carbon and nitrogen allocation as well as plant gene expression at distinct phases of oak growth. We use this model to identify and resolve the experimental challenges inherent in synthesizing diverse types of associations in a common microcosm system, in labeling plants with stable N and C isotopes and in analyzing transcripts in a non-model plant, a process which requires generating a specific contig library. We develop hypotheses and experimental design to test them in order to identify core mechanisms that help trees to modulate their own development and their multitrophic interactions for optimizing their long term performance in their environment. First results constitute a proof of concept that the platform works and enables us to test the hypotheses.

Absorptive fine roots are an important driver of soil biogeochemical cycles. Yet, the spatio-temporal dynamics of those roots in the presence of neighboring species remain poorly understood. The aim of this study was to analyze shifts in absorptive fine-root traits in monoculture or mixtures of Fagus sylvatica [L.] and Picea abies [L.] Karst. We hypothesized that root competition would be higher under single-species than mixed-species interactions, leading to changes in (i) root survivorship, diameter and respiration and (ii) spatio-temporal patterns of root growth and death. Using minirhizotron methods, we monitored the timing and location of absorptive fine-root growth and death at an experimental forest in southern Germany from 2011 to 2013. We also measured root respiration in the spring and fall seasons of 2012 and 2013. Our findings show that the absorptive fine roots of F. sylvatica had a 50% higher risk of root mortality and higher respiration rates in the single-species compared to mixed-species zones. These results support our hypothesis that root competition is less intense for F. sylvatica in mixture versus monoculture. We were unable to find confirmation for the same hypothesis for P. abies. To analyze spatio-temporal patterns of absorptive fine-root production and mortality, we used a mixed-effects model considering root depth (space) and seasons (time) simultaneously. This analysis showed that F. sylvatica shifts root production towards shallower soil layers in mixed-species stands, besides significant seasonal fluctuations in root production depths for both species. Ultimately, the impact of neighbor species identity on root traits observed in this study has important implications for where, when and how fast root-facilitated carbon cycling takes place in single-species versus mixed-species forests. In addition, our study highlights the need for inclusion of absorptive fine-root spatio-temporal dynamics when examining belowground plant interactions and biogeochemical cycles.

International audience; nitori Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land-atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global mong network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.

RATIONALE We investigated the applicability of tree-ring whole-wood material for δ(18)O and δ(13)C analysis in comparison with the more time- and resource-intensive use of cellulose, by considering possible variability between (i) five different tree species (Fagus sylvatica, Quercus robur, Picea abies, Abies alba, Pseudotsuga menziesii), (ii) two sites that differ in soil moisture, and (iii) climate conditions within a 10-year period. METHODS Stem cores of 30 individual trees (n = 3 trees per each species and site) were sampled from two sites in south Germany (Bavaria), and tree rings within sapwood of the years 2001-2010 were separated. The δ(18)O and δ(13)C values from homogenized tree-ring whole wood and from extracted cellulose were measured by mass spectrometry. Species-specific offsets in isotope values were analyzed and the responses in isotopic signature to climate variability including a single drought event were compared between whole-wood and cellulose. RESULTS A constant offset in δ(18)O values of ca 5‰ between wood and cellulose was observed for most species independent of site conditions, with a significant difference between beech and Douglas-fir, while inter-annual variability was only observed in oak. The offset in δ(13)C values ranged between 1.45 and 1.84‰ across species, sites and years. Both materials generally showed similar strength in responses to temperature, precipitation and soil water availability, particularly for conifers. Resistance to severe drought stress--partly more strongly reflected in the δ(13)C values of cellulose--was lower for conifers than for the deciduous species. CONCLUSIONS Wood material from the sapwood of the studied tree species is as useful as cellulose for studying environmental effects on tree-ring δ(18)O and δ(13)C values at a short-term scale as considered in most ecophysiological studies. The more variable response of oak may require further investigations.

Highlight Increased resource availability has no impact on endogenous rhythmic growth and related allocation shifts in oak trees; an internal clock regulates core genes during shoot and root growth cessation phases., Common oak trees display endogenous rhythmic growth with alternating shoot and root flushes. To explore the mechanisms involved, microcuttings of the Quercus robur L. clone DF159 were used for 13C/15N labelling in combination with RNA sequencing (RNASeq) transcript profiling of shoots and roots. The effect of plant internal resource availability on the rhythmic growth of the cuttings was tested through inoculation with the ectomycorrhizal fungus Piloderma croceum. Shoot and root flushes were related to parallel shifts in above- and below-ground C and, to a lesser extent, N allocation. Increased plant internal resource availability by P. croceum inoculation with enhanced plant growth affected neither the rhythmic growth nor the associated resource allocation patterns. Two shifts in transcript abundance were identified during root and shoot growth cessation, and most concerned genes were down-regulated. Inoculation with P. croceum suppressed these transcript shifts in roots, but not in shoots. To identify core processes governing the rhythmic growth, functions [Gene Ontology (GO) terms] of the genes differentially expressed during the growth cessation in both leaves and roots of non-inoculated plants and leaves of P. croceum-inoculated plants were examined. Besides genes related to resource acquisition and cell development, which might reflect rather than trigger rhythmic growth, genes involved in signalling and/or regulated by the circadian clock were identified. The results indicate that rhythmic growth involves dramatic oscillations in plant metabolism and gene regulation between below- and above-ground parts. Ectomycorrhizal symbiosis may play a previously unsuspected role in smoothing these oscillations without modifying the rhythmic growth pattern.

Tree rings include retrospective information about the relationship between climate and growth, making it possible to predict growth reaction under changing climate. Previous studies examined species-specific reactions under different environmental conditions from the perspective of tree ring growth and 13C discrimination (Δ13C). This approach is extended to monospecific versus mixed stands in the present paper. We investigated the resistance and resilience of Norway spruce (Picea abies [L.] Karst) and European beech (Fagus sylvatica [L.]) in response to the drought event in 2003. The study was carried out along a precipitation gradient in southern Germany. Responses of basal area increment (BAI) and Δ13C were correlated with a Climate-Vegetation-Productivity-Index (CVPI). The species showed different strategies for coping with drought stress. During the summer drought of 2003, the BAI of spruces reveal a lower resistance to drought on dry sites than those of beech. For beech, we found an increasing resistance in BAI and Δ13C from dry to moist sites. In mixture with spruce, beech had higher resistance and resilience for Δ13C with increasing site moisture. The combination of Δ13C and tree ring growth proxies improves our knowledge of species-specific and mixture-specific reactions to drought for sites with different moisture conditions.

In the present study, we performed a large-scale protein analysis based on 2-DE DIGE to examine the effects of ozone on the leaves of juvenile European beech ( Fagus sylvatica L.), one of the most important deciduous tree species in Central Europe. To this end, beech trees were grown under field conditions and subjected to ambient and twice ambient ozone concentrations during the vegetation periods of four consecutive years. The twice ambient ozone concentration altered the abundance of 237 protein spots, which showed relative ratios higher than 30% compared to the ambient control trees. A total of 74 protein spots were subjected to mass spectrometry identification (LC–MS/MS), followed by homology-driven searches. The differentially expressed proteins participate in key biological processes including the Calvin cycle and photosynthesis, carbon metabolism, defense- and stress-related responses, detoxification mechanisms, protein folding and degradation, and mechanisms involved in senescence. The ozone-induced responses provide evidence of a changing carbon metabolism and counteraction against increased levels of reactive oxygen species. Biological significance This study provides useful information on how European beech, an economically and ecologically important tree species, reacts on the molecular level to increased ozone concentrations expected in the near future. The main emphasis in the present study was placed on identifying differentially abundant proteins after long-term ozone exposure under climatically realistic settings, rather than short-term responses or reactions under laboratory conditions. Additionally, using nursery-grown beech trees, we took into account the natural genotypic variation of this species. As such, the results presented here provide information on molecular responses to ozone in an experimental plant system at very close to natural conditions. Furthermore, this proteomic approach was supported by previous studies on the present experiment. Ultimately, the combination of this proteomic approach with several approaches including transcriptomics, analysis of non-structural carbohydrates, and morphological effects contributes to a more global picture of how beech trees react under increased ozone concentrations.

Summary The root-rot pathogen Phytophthora quercina is a key determinant of oak decline in Europe. The susceptibility of pedunculate oak (Quercus robur) to this pathogen has been hypothesized to depend on the carbon availability in roots as an essential resource for defense. Microcuttings of Q. robur undergo an alternating rhythm of root and shoot growth. Inoculation of mycorrhizal (Piloderma croceum) and nonmycorrhizal oak roots with P. quercina was performed during both growth phases, that is, root flush (RF) and shoot flush (SF). Photosynthetic and morphological responses as well as concentrations of nonstructural carbohydrates (NSC) were analyzed. Infection success was quantified by the presence of pathogen DNA in roots. Concentrations of NSC in roots depended on the alternating root/shoot growth rhythm, being high and low during RF and SF, respectively. Infection success was high during RF and low during SF, resulting in a significantly positive correlation between pathogen DNA and NSC concentration in roots, contrary to the hypothesis. The alternating growth of roots and shoots plays a crucial role for the susceptibility of lateral roots to the pathogen. NSC availability in oak roots has to be considered as a benchmark for susceptibility rather than resistance against P. quercina.

We review causes of synergies in mixed-species stands, develop guiding hypotheses for revealing their mechanisms and present a rainfall exclusion experiment along with a transect (KROOF) for exploring drought effects. While monocultures have dominated forest research and practice in the past, in face of growing resource scarcity and climate change, mixed-species stands are on the advance. Long-term observations show that mixed-species stands frequently over-yield monocultures, and they further suggest that the over-yielding is often higher on poor than on fertile sites and in low-growth than in high-growth years. However, the underlying causes have not yet been clarified. We start with a review of, among others, hydraulic redistribution, complementary eco-physiological traits, and ectomycorrhizal networks as possible causes behind the observed productivity gains in mixed-species stands. Then, we develop guiding hypotheses for further exploration of synergies in mixed-species stands. Finally, we introduce into the concept and model approach of the rainfall exclusion experiment for examining the role of water supply in mixed versus pure forest stands of spruce and beech. At the Kranzberg site, six plots are equipped with water retention roofs, which only close during rain events. The remaining six plots serve as non-roofed control. Together with the rainfall exclusion experiment, an ecological gradient with five sites extending through SE-Germany forms the “Kranzberg Roof Experiment” (in short KROOF). Kranzberg Forest is a part of this gradient from moist to dry conditions, with each site providing mixed and pure associations each of spruce and beech. The ecophysiological approach will be complemented by tree ring analysis and modelling of productivity of the tree associations under intense drought.

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.

Our study provides evidence that neither elevated CO 2 nor elevated O 3 alters the positive asymmetric competition for light and the symmetric competition for water among beech and spruce individuals grown in monoculture. We conclude that the mechanism of competition (i.e. symmetric/asymmetric) above (e.g shading or overtopping effect) and belowground (e.g. non-preemption or foraging) rather than abiotic treatments such as elevated CO 2 , O 3 and CO 2 /O 3 regimes, plays a dominant role for ensuring competitive success among tree saplings. Despite numerous studies conducted on plant responses to increasing CO2 and O3 concentrations, there is still a gap in understanding on how these gasses would affect the mode of competition (e.g., the ability by which larger and smaller plants capture resources) at the individual level of intra-specific beech and spruce saplings. Using empirical data and simulations from the plant-growth model PLATHO, we analyzed underlying mechanisms of competition and extrapolated effects beyond the time span of the experiment. We hypothesized that among juvenile beech and spruce trees planted in monoculture, +CO2 would diminish the positive asymmetric competition for light. Conversely, +O3 would enhance this outcome. In addition, we hypothesized that the symmetric mode of competition belowground for water would remain unchanged, irrespective of +CO2 and/or +O3 treatments. Our results showed that +CO2 and/or +O3 treatments did not alter the mode of competition aboveground for light. Conversely, we accepted our hypothesis that the mode of competition for water would remain unchanged under both treatments. Overall, we conclude that neither +CO2 nor +O3 alters the positive asymmetric competition for light and the symmetric competition for water among beech and spruce individuals grown in monoculture. We further conclude that competitive mechanism above (e.g., shading or overtopping effect) and belowground (e.g., non-preemption or foraging) rather than abiotic treatments, such as elevated CO2, O3 and CO2/O3 regimes, plays a dominant role for ensuring competitive success among tree saplings.

Leaf photosynthesis of the sensitive plant Mimosa pudica displays a transient knockout in response to electrical signals induced by heat stimulation. This study aims at clarifying the underlying mechanisms, in particular, the involvement of respiration. To this end, leaf gas exchange and light reactions of photosynthesis were assessed under atmospheric conditions largely eliminating photorespiration by either elevated atmospheric CO2 or lowered O2 concentration (i.e. 2000 μmol mol(-1) or 1%, respectively). In addition, leaf gas exchange was studied in the absence of light. Under darkness, heat stimulation caused a transient increase of respiratory CO2 release simultaneously with stomatal opening, hence reflecting direct involvement of respiratory stimulation in the drop of the net CO2 uptake rate. However, persistence of the transient decline in net CO2 uptake rate under illumination and elevated CO2 or 1% O2 makes it unlikely that photorespiration is the metabolic origin of the respiratory CO2 release. In conclusion, the transient knockout of net CO2 uptake is at least partially attributed to an increased CO2 release through mitochondrial respiration as stimulated by electrical signals. Putative CO2 limitation of Rubisco due to decreased activity of carbonic anhydrase was ruled out as the photosynthesis effect was not prevented by elevated CO2 .

Summary Oaks (Quercus spp.), which are major forest trees in the northern hemisphere, host many biotic interactions, but molecular investigation of these interactions is limited by fragmentary genome data. To date, only 75 oak expressed sequence tags (ESTs) have been characterized in ectomycorrhizal (EM) symbioses. We synthesized seven beneficial and detrimental biotic interactions between microorganisms and animals and a clone (DF159) of Quercus robur. Sixteen 454 and eight Illumina cDNA libraries from leaves and roots were prepared and merged to establish a reference for RNASeq transcriptomic analysis of oak EMs with Piloderma croceum. Using the Mimicking Intelligent Read Assembly (MIRA) and Trinity assembler, the OakContigDF159.1 hybrid assembly, containing 65 712 contigs with a mean length of 1003 bp, was constructed, giving broad coverage of metabolic pathways. This allowed us to identify 3018 oak contigs that were differentially expressed in EMs, with genes encoding proline-rich cell wall proteins and ethylene signalling-related transcription factors showing up-regulation while auxin and defence-related genes were down-regulated. In addition to the first report of remorin expression in EMs, the extensive coverage provided by the study permitted detection of differential regulation within large gene families (nitrogen, phosphorus and sugar transporters, aquaporins). This might indicate specific mechanisms of genome regulation in oak EMs compared with other trees.

Climate simulations anticipate an increase in mean summer temperature with synchronous decrease in summer precipitation during the course of the current century in Central Europe. As a consequence, transpiration of forest trees and stands may be altered along with soil water availability. In this study, the effect of reduced plant water availability to conifers was investigated into an open Pinus sylvestris forest (Erico-Pinetum typicum; P. sylvestris 60 %, Picea abies 20 %; and Larix decidua 20 %) within the inner alpine dry valley of the Inn River in Tyrol, Austria. For reducing plant water availability, we installed a transparent roof construction above the forest floor to prevent precipitation to reach the soil. The roofed area covered 240 m² and included 10 trees. A respective number of 11 trees served as controls in the absence of any manipulation. Roofing significantly reduced plant water availability as indicated in lower predawn needle water potentials. Sap flow density (Q s) was 63, 47, and 24 % lower in roofed P. sylvestris, P. abies, and L. decidua trees, respectively, as compared to control trees. Our findings suggest that P. sylvestris and P. abies behaves “isohydric” as they close their stomata relatively early under conditions of reduced plant water availability and thus stabilize their water relations, whereas L. decidua behaves “anisohydric” and maintains high transpiration rates.

• The CO(2) efflux of adult trees is supplied by recent photosynthates and carbon (C) stores. The extent to which these C pools contribute to growth and maintenance respiration (R(G) and R(M), respectively) remains obscure. • Recent photosynthates of adult beech (Fagus sylvatica) and spruce (Picea abies) trees were labeled by exposing whole-tree canopies to (13) C-depleted CO(2). Label was applied three times during the year (in spring, early summer and late summer) and changes in the stable C isotope composition (δ(13) C) of trunk and coarse-root CO(2) efflux were quantified. • Seasonal patterns in C translocation rate (CTR) and fractional contribution of label to CO(2) efflux (F(Label-Max)) were found. CTR was fastest during early summer. In beech, F(Label-Max) was lowest in spring and peaked in trunks during late summer (0.6 ± 0.1, mean ± SE), whereas no trend was observed in coarse roots. No seasonal dynamics in F(Label-Max) were found in spruce. • During spring, the R(G) of beech trunks was largely supplied by C stores. Recent photosynthates supplied growth in early summer and refilled C stores in late summer. In spruce, CO(2) efflux was constantly supplied by a mixture of stored (c. 75%) and recent (c. 25%) C. The hypothesis that R(G) is exclusively supplied by recent photosynthates was rejected for both species.

Respiration is a substantial driver of carbon (C) flux in forest ecosystems and stable C isotopes provide an excellent tool for its investigation. We studied seasonal dynamics in δ¹³C of CO₂ efflux (δ¹³C(E)) from non-leafy branches, upper and lower trunks and coarse roots of adult trees, comparing deciduous Fagus sylvatica (European beech) with evergreen Picea abies (Norway spruce). In both species, we observed strong and similar seasonal dynamics in the δ¹³C(E) of above-ground plant components, whereas δ¹³C(E) of coarse roots was rather stable. During summer, δ¹³C(E) of trunks was about -28.2‰ (Beech) and -26.8‰ (Spruce). During winter dormancy, δ¹³C(E) increased by 5.6-9.1‰. The observed dynamics are likely related to a switch from growth to starch accumulation during fall and remobilization of starch, low TCA cycle activity and accumulation of malate by PEPc during winter. The seasonal δ¹³C(E) pattern of branches of Beech and upper trunks of Spruce was less variable, probably because these organs were additionally supplied by winter photosynthesis. In view of our results and pervious studies, we conclude that the pronounced increases in δ¹³C(E) of trunks during the winter results from interrupted access to recent photosynthates.

Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO2 with a δ13C of −46.9‰. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO2 gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO2 concentration ([CO2]) increase was ambient +100 μmol mol−1. Canopy [CO2] stayed within 10% of the target during more than 57% of the time and resulted in a drop of δ13C in canopy air by 7.8‰. After 19 labeling days about 50% of C in phloem sugars and stem CO2 efflux were turned over and 20–30% in coarse root CO2 efflux and soil CO2. The isoFACE system successfully altered δ13C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.

In a lysimeter study with young beech trees, the effects of elevated ozone concentration on the decomposition and fate of nitrogen in 15N‐labeled leaf litter were analyzed after one growing season. Nitrogen in the litter was dominated by a relatively inert, residual fraction, but easily decomposable nitrogen was present in substantial amounts. Nitrogen loss was significantly higher at twice‐ambient ozone which was largely attributed to an enhanced mobilization of residual nitrogen. Enhanced mobilization of nitrogen from litter at twice‐ambient ozone exposure resulted in additional 15N incorporation into the soil down to 30 cm depth. Only 0.41–0.62% of the nitrogen in the litter was incorporated into plant material at both ozone concentrations. Twice‐ambient ozone exposure changed the distribution of the nitrogen taken up from litter inside the beech trees in favor of the shoot, where it may have been used in biosynthetic processes required for defense reactions.

Ecuadorian mountain rainforests are declining dramatically due to deforestation. Exploitation of remaining forests has led to low abundances of native, valuable timber species. Enrichment planting of selected native tree species into forest gaps is a strategy that may increase their abundance and maintain biodiversity. However, the development of successful planting strategies requires knowledge of environmental demands on, and ecological requirements of, native species during their establishment. This knowledge is currently lacking for mid- and late-successional species in Central American forests. Two deciduous, mid-successional (Cedrela montana, Tabebuia chrysantha) and two evergreen, late-successional native tree species (Nectandra membranacea, Podocarpus sprucei) were planted into felling gaps. Photosynthetic performance and growth in height of these species were assessed along light gradients during seedling establishment to test whether species-specific light responses were related to plant successional traits. Both mid-successional species benefited from higher light levels in gaps up to 30% canopy openness60°. In larger gaps, C. montana exhibited a significant decline in growth. As expected, growth of the late-successional species was only marginally increased at higher light levels. Nevertheless, the photosynthetic apparatus of N. membranacea displayed rapid acclimation to higher light conditions in gaps. Plant response to felling gaps may not always be predicted based on successional status. Our results suggest that the four investigated species may coexist in the same gap by occupying different niches along light gradients. This arrangement may offer an ecological basis to increase the abundance of valuable timber species through enrichment planting in Ecuador mountain rainforests.