No xylem phenotypic plasticity in mature Picea abies and Fagus sylvatica trees after 5 years of throughfall precipitation exclusion.

Forest trees are experiencing increasing frequency and intensity of drought events with climate change. We investigated xylem and phloem traits from mature Fagus sylvatica and Picea abies trees after 5 years of complete exclusion of throughfall precipitation during the growing season. Xylem and phloem anatomy, leaf and branch biomass were analysed along top branches of ~1.5 m lenght in 5 throughfall precipitation excluded (TE) and 5 control (CO) trees of both beech and spruce. Xylem traits were analysed on wood cores extracted from the stem at breast height. In the top branches of both species, the lumen diameter (or area) of xylem and phloem conduits did not differ between TE and CO trees. At breast height, TE trees of both species produced narrower xylem rings and conduits. While allocation to branch (BM) and needle biomass (LM) did not change between TE and CO in P. abies, TE F. sylvatica trees allocated proportionally more biomass to leaves (LM) than BM compared with CO. Despite artificial drought increased the mortality in the TE plots, our results revealed no changes in both xylem and phloem anatomies, undermining the hypothesis that successful acclimation to drought would primarily involve increased resistance against air embolism.

Tree species from two contrasting habitats for use in harsh urban environments respond differently to extreme drought.

The role of trees in city cooling has warranted much attention based on concerns over climate change and urban expansion. Simultaneously, there is an interest in introducing species from dry habitats to cope with the increasing risks of drought under climate change. The general understanding is that the evolutionary adaptation to respective resource supplies in species' habitats affects their environmental tolerance. The physical performances of six frequently planted species, originating from two contrasting habitats, were tested in a drought experiment. We (1) investigated if species from drier habitats are more drought tolerant than species that have evolved in Central European woodlands under a temperate climate regime and (2) discussed the effect of tolerance on the cooling potential of these trees. Native species from mesic habitats maintained only 48% of their controls sap flux and of these species, Tilia cordata had the worst performance with premature leaf senescence. Species from drier habitats had less reduction in sap flux (60%) but lower stem growth, possibly favouring (fine) root development into deeper soil layers, as observed when comparing linden species. Higher stem water exploitation and stronger regulation of water use at high evaporative demand were further reaction patterns that likely helped species from dry habitats maintain good physiological functions. Therefore, even under sustained drought, we expect them to have a higher cooling capacity. As a conclusion, they should be favoured for planting in extreme urban environments. Systematic screening and testing of promising species from target habitats is recommended to diversify the choice of species.

Physiological recovery of tree water relations upon drought release-response of mature beech and spruce after five years of recurrent summer drought.

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.

Multivariate analysis of physiological parameters reveals a consistent O3 response pattern in leaves of adult European beech (Fagus sylvatica).

• Increasing atmospheric concentrations of phytotoxic ozone (O(3) ) can constrain growth and carbon sink strength of forest trees, potentially exacerbating global radiative forcing. Despite progress in the conceptual understanding of the impact of O(3) on plants, it is still difficult to detect response patterns at the leaf level. • Here, we employed principal component analysis (PCA) to analyse a database containing physiological leaf-level parameters of 60-yr-old Fagus sylvatica (European beech) trees. Data were collected over two climatically contrasting years under ambient and twice-ambient O(3) regimes in a free-air forest environment. • The first principal component (PC1) of the PCA was consistently responsive to O(3) and crown position within the trees over both years. Only a few of the original parameters showed an O(3) effect. PC1 was related to parameters indicative of oxidative stress signalling and changes in carbohydrate metabolism. PC1 correlated with cumulative O(3) uptake over preceding days. • PC1 represents an O(3) -responsive multivariate pattern detectable in the absence of consistently measurable O(3) effects on individual leaf-level parameters. An underlying effect of O(3) on physiological processes is indicated, providing experimental confirmation of theoretical O(3) response patterns suggested previously.

Fine-Root Traits Reveal Contrasting Ecological Strategies in European Beech and Norway Spruce During Extreme Drought.

Trees adjust multiple structural and functional organ-specific characteristics, "traits", to cope with diverse soil conditions. Studies on traits are widely used to uncover ecological species adaptability to varying environments. However, fine-root traits are rarely studied for methodological reasons. We analyzed the adaptability of the fine-root systems of European beech and Norway spruce to extreme drought within species-specific tree groups at Kranzberger Forst (Germany), focusing on the seasonality of morphological, physiological, and biochemical key traits in view of carbon (C) and nitrogen dynamics. We hypothesized that fine roots of both species adjust to seasonal drought: with beech representing a "fast" (i.e. with fast C turnover), and spruce a "slow" (i.e. with long-term C retention) ecological strategy. We identified three functional fine-root categories, based on root function (absorptive or transport fine roots), and mycorrhizal status of the absorptive fine-roots (mycorrhizal or non-mycorrhizal). Solely the non-mycorrhizal absorptive roots adjusted in a species-specific manner supporting fine-root ecological strategy hypothesis. During drought, beech produced thin ephemeral (absorptive non-mycorrhizal) fine roots with high specific fine-root area and high respiratory activity, representing fast C turnover and enabling effective resource exploitation. These adjustments reflect a "fast" ecological strategy. Conversely, spruce absorptive fine roots did not respond to the soil moisture deficit by growth but instead increased root suberization. Drastically lowered respiratory activity of this functional category facilitated C retention and structural persistence during drought, indicating a "slow" ecological strategy in spruce. Absorptive mycorrhizal fine roots maintained respiration throughout the drought event in both tree species, but in spruce this was the only fine-root category with high respiration. This suggests, that spruce relies heavily on mycorrhizal associations as a method of drought resistance. Accumulation of non-structural carbohydrates and high C concentrations were observed in the transport fine roots of both species, indicating drought-induced osmotic protection of these roots. Thus, functional classification enabled us to determine that fine-root branches of each species are not tied to one sole ecological strategy. The suggested approach helps to better understand the complex interplay between structure and function belowground.

Close to the edge: effects of repeated severe drought on stem hydraulics and non-structural carbohydrates in European beech saplings.

Severe drought events threaten tree water transport system, productivity and survival. Woody angiosperms generally die when embolism-induced loss of hydraulic conductance (PLC) surpasses 80-90% under intense water shortage. However, the recovery capability and possible long-term carry-over effects of repeated drought events could dictate the fate of species' population under climate change scenarios. Potted saplings of European beech (Fagus sylvatica L.) were subjected to two drought cycles in two consecutive growing seasons, aiming to induce minimum leaf water potentials (Ψmd) of about -4 MPa, corresponding to hydraulic thresholds for survival of this species. In the first cycle, a well-irrigated (C) and a drought-stressed group (S) were formed, and, in the following summer, each group was divided in a well-irrigated and a drought-stressed one (four groups in total). The impact of the multiple drought events was assessed by measuring wood anatomical traits, biomass production, water relations, stem hydraulics and non-structural carbohydrate (NSC) content. We also investigated possible connections between stem hydraulics and carbon dynamics during the second drought event and following re-irrigation. S plants had lower Ψmd and maximum specific hydraulic conductivity (Ks) than C plants in the following growing season. Additionally, aboveground biomass production and leaf number were lower compared to C trees, resulting in lower water consumption. However, PLC was similar between groups, probably due to the production of new functional xylem in spring. The second drought event induced 85% PLC and promoted conversion of starch-to soluble sugars. Nevertheless, 1 week after re-irrigation, no embolism repair was observed and soluble sugars were reconverted to starch. The previous drought cycle did not influence the hydraulic performance during the second drought, and after re-irrigation S plants had 40% higher wood NSC content. Our data suggest that beech cannot recover from high embolism levels but multiple droughts might enhance stem NSC availability.

Photosynthetic and leaf water potential responses of Alnus glutinosa saplings to stem-base inoculaton with Phytophthora alni subsp. alni.

Three-year-old Alnus glutinosa (L.) Gaertn. (alder) saplings were single or double inoculated at the stem base with Phytophthora alni subsp. alni Brasier & S.A. Kirk under natural climatic conditions. Lesion formation on the bark showed a biphasic pattern of development, with extension occurring at a moderate rate in spring, and more rapidly during late summer. However, large variability was encountered in pathogen development within the population of infected saplings, ranging from high susceptibility to almost complete resistance. Infection resulted in severe growth retardation, and death within two years of inoculation in 75% of the saplings. During disease development, rates of transpiration and CO(2) uptake were significantly reduced. Consequently, minimum leaf water potentials were less negative in infected saplings than in control saplings. Surviving saplings matched control trees in photosynthetic capacity, transpiration rate and water potential during the second year of infection. Leaf starch concentration of infected saplings was significantly higher than in control saplings, possibly indicating that the destruction of bark tissue by the pathogen impaired phloem transport from leaves to roots.

Acclimation of branch and leaf hydraulics in adult Fagus sylvatica and Picea abies in a forest through-fall exclusion experiment.

Decreasing water availability due to climate change poses the question of whether and to what extent tree species are able to hydraulically acclimate and how hydraulic traits of stems and leaves are coordinated under drought. In a through-fall exclusion experiment, hydraulic acclimation was analyzed in a mixed forest stand of Fagus sylvatica L. and Picea abies (L.) Karst. In drought-stressed (TE, through-fall exclusion over 2 years) and control (CO) trees, hydraulic vulnerability was studied in branches as well as in leaves (F. sylvatica) and end-twigs (P. abies, entirely formed during the drought period) sampled at the same height in sun-exposed portions of the tree crown. In addition, relevant xylem anatomical traits and leaf pressure-volume relations were analyzed. The TE trees reached pre-dawn water potentials down to -1.6 MPa. In both species, water potentials at 50% loss of xylem hydraulic conductivity were ~0.4 MPa more negative in TE than in CO branches. Foliage hydraulic vulnerability (expressed as water potential at 50% loss of leaf/end-twig hydraulic conductance) and water potential at turgor loss point were also, respectively, 0.4 and 0.5 MPa lower in TE trees. Minor differences were observed in conduit mean hydraulic diameter and cell wall reinforcement. Our findings indicate significant and fast hydraulic acclimation under relatively mild drought in both tree species. Acclimation was well coordinated between branches and foliage, which might be essential for survival and productivity of mature trees under future drought periods.

Exemplifying whole-plant ozone uptake in adult forest trees of contrasting species and site conditions.

Whole-tree O3 uptake was exemplified for Picea abies, Fagus sylvatica and Larix decidua in stands at high and low altitude and contrasting water availability through sap flow measurement in tree trunks, intrinsically accounting for drought and boundary layer effects on O3 flux. O3 uptake of evergreen spruce per unit foliage area was enhanced by 100% at high relative to low elevation, whereas deciduous beech and larch showed similar uptake regardless of altitude. The responsiveness of the canopy conductance to water vapor and, as a consequence, O3 uptake to soil moisture and air humidity did not differ between species. Unifying findings at the whole-tree level will promote cause-effect based O3 risk assessment and modeling.

Post-drought hydraulic recovery is accompanied by non-structural carbohydrate depletion in the stem wood of Norway spruce saplings.

Hydraulic failure and carbon starvation are recognized as main causes of drought-induced forest decline. As water transport and carbon dynamics are strictly interdependent, it is necessary to clarify how dehydration-rehydration cycles are affecting the relations between stem embolism and non-structural carbohydrates (NSC). This is particularly needed for conifers whose embolism repair capability is still controversial. Potted Norway spruce saplings underwent two drought-re-irrigation cycles of same intensity, but performed in two consecutive summers. During the second cycle, stem percent loss of hydraulic conductivity (PLC) and NSC content showed no carry-over effects from the previous drought, indicating complete long-term recovery. The second drought treatment induced moderate PLC (20%) and did not affect total NSCs content, while starch was converted to soluble sugars in the bark. After one week of re-irrigation, PLC recovered to pre-stress values (0%) and NSCs were depleted, only in the wood, by about 30%. Our data suggest that spruce can repair xylem embolism and that, when water is newly available, NSCs stored in xylem parenchyma can be mobilized over short term to sustain respiration and/or for processes involved in xylem transport restoration. This, however, might imply dependency on sapwood NSC reserves for survival, especially if frequent drought spells occur.

Hydraulic redistribution under moderate drought among English oak, European beech and Norway spruce determined by deuterium isotope labeling in a split-root experiment.

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.

Testing the unifying theory of ozone sensitivity with mature trees of Fagus sylvatica and Picea abies.

The broad range in plant responses to chronic O(3) exposure compels a search for integrative, underlying principles. One such approach is the unifying theory proposed by Reich (1987), which combines the O(3) response of contrasting physiognomic classes of plants on the basis of their intrinsic leaf diffusive conductance and, hence, capacity for O(3) uptake. Physiognomic classes differ in the proportional decline in photosynthesis and growth when compared on the basis of cumulative O(3) exposure per unit time, but converge when compared on the basis of O(3) uptake per unit time or cumulative O(3) uptake over the entire lifetime of the leaf. The theory is based on observations on a large number of contrasting plant species, relying primarily on studies of juvenile trees subjected to short-term O(3) exposure. To test the applicability of the unifying theory to mature trees, broadleaf deciduous European beech (Fagus sylvatica L.) and the evergreen conifer Norway spruce (Picea abies (L.) Karst.) in a mature mixed stand were exposed to either ambient air (control) or air with twice the ambient O(3) concentration delivered into the canopy by means of a free-air fumigation system. We accounted for differences in growing season length, leaf longevity and O(3)-related effects on leaf diffusive conductance in determining total O(3) uptake over the lifetime of the leaf. On this basis, Norway spruce needles required 5 years to take up as much O(3) as did beech leaves in one growing season. The core of the unifying theory on O(3) sensitivity was substantiated in relation to O(3) exposure and uptake. However, contrary to the unifying theory, which was formulated on the basis of results with juvenile trees, the O(3) response of mature trees in a natural stand was more complex. The increased complexity was attributed to additional environmental stressors, stress compensation at the whole-tree level, and differential O(3) sensitivities of leaves according to age class and position within the canopy. Contrary to the theory, photosynthesis was no less sensitive to O(3) in Norway spruce than that of beech, and was reduced in the twice-ambient O(3) regime in the first year of exposure.

Response patterns in adult forest trees to chronic ozone stress: identification of variations and consistencies.

The responsiveness of adult beech and spruce trees to chronic O(3) stress was studied at a free-air O(3) exposure experiment in Freising/Germany. Over three growing seasons, gas exchange characteristics, biochemical parameters, macroscopic O(3) injury and the phenology of leaf organs were investigated, along with assessments of branch and stem growth as indications of tree performance. To assess response pattern to chronic O(3) stress in adult forest trees, we introduce a new evaluation approach, which provides a comprehensive, readily accomplishable overview across several tree-internal scaling levels, different canopy regions and growing seasons. This new approach, based on a three-grade colour coding, combines statistical analysis and the proficient ability of the "human eye" in pattern recognition.

Ozone induces stomatal narrowing in European and Siebold's beeches: a comparison between two experiments of free-air ozone exposure.

Stomata tend to narrow under ozone (O(3)) impact, leading to limitation of stomatal O(3) influx. Here, we review stomatal response under recently conducted free-air O(3) exposure experiments on two species of the same tree genus: Fagus sylvatica at Kranzberg Forest (Germany) and F. crenata at Sapporo Experimental Forest (Japan). Both beeches exhibited reduction in stomatal conductance (gs) by 10-20% under experimentally enhanced O(3) regimes throughout the summer relative to ambient-air controls. Stomatal narrowing occurred, in early summer, in the absence of reduced carboxylation capacity of Rubisco, although photosynthetic net CO(2) uptake rate temporarily reflected restriction to some minor extent. Observed stomatal narrowing was, however, diminished in autumn, suggesting gradual loss of stomatal regulation by O(3). Monotonic decline in gs with cumulative O(3) exposure or flux in current modeling concepts appear to be unrealistic in beech.

Age effects on Norway spruce (Picea abies) susceptibility to ozone uptake: a novel approach relating stress avoidance to defense.

Cumulative ozone (O3) uptake and O3 flux were related to physiological, morphological and biochemical characteristics of Norway spruce (Picea abies (L.) Karst.) trees of different ages. Under ambient CO2 conditions, photosynthetic capacity (Amax) declined in mature trees when cumulative O3 uptake into needles, which provides a measure of effective O3 dose, exceeded 21 mmol m-2 of total needle surface area. A comparable decline in Amax of seedlings occurred when cumulative O(3) uptake was only 4.5 mmol m-2. The threshold O3 flux causing a significant decline in Amax ranged between 2.14 and 2.45 nmol m-2 s-1 in mature trees and seedlings subjected to exposure periods of > or = 70 and > or = 23 days, respectively. The greater O3 sensitivity of young trees compared with mature trees was associated with needle morphology. Biomass of a 100-needle sample increased significantly with tree age, whereas a negative correlation was found for specific leaf area, these changes parallel those observed during differentiation from shade-type to sun-type needles with tree ontogeny. Age-dependent changes in leaf morphology were related to changes in detoxification capacity, with area-based concentrations of ascorbate increasing during tree ontogeny. These findings indicate that the extent of O3-induced injury is related to the ratio of potentially available antioxidants to O3 influx. Because this ratio, when calculated for ascorbate, increased with tree age, we conclude that the ratio may serve as an empirical basis for characterizing age-related differences in tree responses to O3.

Fate of recently fixed carbon in European beech (Fagus sylvatica) saplings during drought and subsequent recovery.

Drought reduces the carbon (C) assimilation of trees and decouples aboveground from belowground carbon fluxes, but little is known about the response of drought-stressed trees to rewetting. This study aims to assess dynamics and patterns of C allocation in beech saplings under dry and rewetted soil conditions. In October 2010, 5-year-old beech saplings from a forest site were transplanted into 20 l pots. In 2011, the saplings were subjected to different levels of soil drought ranging from non-limiting water supply (control) to severe water limitation with soil water potentials of less than -1.5 MPa. As a physiologically relevant measure of drought, the cumulated soil water potential (i.e., drought stress dose (DSD)) was calculated for the growing season. In late August, the saplings were transferred into a climate chamber and pulse-labeled with (13)C-depleted CO2 (δ(13)C of -47‰). Isotopic signatures in leaf and soil respiration were repeatedly measured. Five days after soil rewetting, a second label was applied using 99 atom% (13)CO2. After another 12 days, the fate of assimilated C in each sapling was assessed by calculating the (13)C mass balance. Photosynthesis decreased by 60% in saplings under severe drought. The mean residence time (MRT) of recent assimilates in leaf respiration was more than three times longer than under non-limited conditions and was positively correlated to DSD. Also, the appearance of the label in soil respiration was delayed. Within 5 days after rewetting, photosynthesis, MRT of recent assimilates in leaf respiration and appearance of the label in soil respiration recovered fully. Despite the fast recovery, less label was recovered in the biomass of the previously drought-stressed plants, which also allocated less C to the root compartment (45 vs 64% in the control). We conclude that beech saplings quickly recover from extreme soil drought, although transitional after-effects prevail in C allocation, possibly due to repair-driven respiratory processes.

How closely does stem growth of adult beech (Fagus sylvatica) relate to net carbon gain under experimentally enhanced ozone stress?

The hypothesis was tested that O(3)-induced changes in leaf-level photosynthetic parameters have the capacity of limiting the seasonal photosynthetic carbon gain of adult beech trees. To this end, canopy-level photosynthetic carbon gain and respiratory carbon loss were assessed in European beech (Fagus sylvatica) by using a physiologically based model, integrating environmental and photosynthetic parameters. The latter were derived from leaves at various canopy positions under the ambient O(3) regime, as prevailing at the forest site (control), or under an experimental twice-ambient O(3) regime (elevated O(3)), as released through a free-air canopy O(3) fumigation system. Gross carbon gain at the canopy-level declined by 1.7%, while respiratory carbon loss increased by 4.6% under elevated O(3). As this outcome only partly accounts for the decline in stem growth, O(3)-induced changes in allocation are referred to and discussed as crucial in quantitatively linking carbon gain with stem growth.

Transcriptional signatures in leaves of adult European beech trees (Fagus sylvatica L.) in an experimentally enhanced free air ozone setting.

Tropospheric ozone causes severe oxidative stress in plants. To investigate the transcriptional responsiveness of adult trees to ozone, fully-expanded sun and shade leaves of mature beech trees were harvested at four time points over the entire vegetation period in 2005 and 2006. Microarray analyses were conducted on leaves from trees grown in the field under ambient and twice-ambient ozone concentrations at Kranzberger Forst (Bavaria). Beech trees changed their transcript levels in response to ozone. In the years 2005 and 2006 different transcription patterns were observed; this may have been a result of different weather conditions and ozone uptake. Furthermore, we obtained differences in mRNA expression patterns between shade and sun leaves. In the ozone-treated sun leaves of 2005, slightly up- and down-regulated transcript levels were detected, particularly in the spring and autumn, whereas shade leaves clearly exhibited reduced mRNA levels, particularly at the end of the vegetation period. In 2006, this pattern could not be confirmed, and in the autumn, four other transcripts were slightly up-regulated in ozone-treated shade leaves. In addition, two other transcripts were found to be influenced in sun leaves in the spring/summer. While we detected changes in the levels of only a few transcripts, the observed effects were not identical in both years. In conclusion, elevated ozone exhibited very small influence on the transcription levels of genes of mature beech trees.

Belowground effects of enhanced tropospheric ozone and drought in a beech/spruce forest (Fagus sylvatica L./Picea abies [L.] Karst).

The effects of experimentally elevated O(3) on soil respiration rates, standing fine-root biomass, fine-root production and delta(13)C signature of newly produced fine roots were investigated in an adult European beech/Norway spruce forest in Germany during two subsequent years with contrasting rainfall patterns. During humid 2002, soil respiration rate was enhanced under elevated O(3) under beech and spruce, and was related to O(3)-stimulated fine-root production only in beech. During dry 2003, the stimulating effect of O(3) on soil respiration rate vanished under spruce, which was correlated with decreased fine-root production in spruce under drought, irrespective of the O(3) regime. delta(13)C signature of newly formed fine-roots was consistent with the differing g(s) of beech and spruce, and indicated stomatal limitation by O(3) in beech and by drought in spruce. Our study showed that drought can override the stimulating O(3) effects on fine-root dynamics and soil respiration in mature beech and spruce forests.

Ozone fumigation (twice ambient) reduces leaf infestation following natural and artificial inoculation by the endophytic fungus Apiognomonia errabunda of adult European beech trees.

In 2006, a controlled infection study was performed in the 'Kranzberger Forst' to address the following questions: (1) Will massive artificial inoculation with Apiognomonia errabunda override the previously observed inhibitory effect of chronic ozone? (2) Can biochemical or molecular markers be detected to account for the action of ozone? To this end six adult beech trees were chosen, three ozone fumigated (2x ozone) and three control trees (ambient = 1x ozone). Spore-sprayed branches of sun and shade crown positions of each of the trees, and uninoculated control branches, were enclosed in 100-L plastic bags for one night to facilitate infection initiation. Samples were taken within a five-week period after inoculation. A. errabunda infestation levels quantified by real-time PCR increased in leaves that were not fumigated with additional ozone. Cell wall components and ACC (ethylene precursor 1-amino cyclopropane-1-carboxylic acid) increased upon ozone fumigation and may in part lead to the repression of fungal infection.