Deriving ozone dose-response of photosynthesis in adult forest trees from branch-level cuvette gas exchange assessment.

Branch-level gas exchange provided the basis for assessing ozone flux in order to derive the dose-response relationship between cumulative O3 uptake (COU) and carbon gain in the upper sun crown of adult Fagus sylvatica. Fluxes of ozone, CO2 and water vapour were monitored simultaneously by climatized branch cuvettes. The cuvettes allowed branch exposure to an ambient or twice-ambient O3 regime, while tree crowns were exposed to the same O3 regimes (twice-ambient generated by a free-air canopy O3 exposure system). COU levels higher than 20mmolm(-2) led to a pronounced decline in carbon gain under elevated O3. The limiting COU range is consistent with findings on neighbouring branches exposed to twice-ambient O3 through free-air fumigation. The cuvette approach allows to estimate O3 flux at peripheral crown positions, where boundary layers are low, yielding a meso-scale within-crown resolution of photosynthetic foliage sensitivity under whole-tree free-air O3 fumigation.

Flux-based response of sucrose and starch in leaves of adult beech trees (Fagus sylvatica L.) under chronic free-air O3 fumigation.

Investigations on sucrose and starch contents in leaves of 60-year-old beech trees ( FAGUS SYLVATICA L.) are the focus of the present study. Five trees were exposed to a twice ambient ozone regime (2 x O(3)) with a free-air canopy exposure system throughout the seasons and five trees under the prevailing ambient ozone regime served as controls (1 x O(3)). In order to examine chronic ozone (O(3)) effects, leaf samples from the sun and shade crowns of the trees were analyzed five times throughout the growing seasons in 2003 and 2004. Sucrose concentrations of leaves collected in 2004 were consistently lower than those taken in 2003, regardless of the O(3) treatment and crown position. However, the opposite was found for starch. O(3) caused a reduction of sucrose and starch contents of sun leaves in both years. Due to the fact that O(3)-responsiveness depends on the O(3) uptake through stomata during the season, all carbohydrate data were related to the cumulative O(3) uptake (COU). Little differences were found comparing sucrose and starch contents in leaves of trees grown under ambient or elevated O (3) regimes, possibly indicating the high capacity of leaves of adult beech to cope with rising O(3) exposure. Even under 2 x O(3), leaves were still able to regulate the O(3) intake by narrowing their stomata at the cost of CO(2)-uptake and sugar synthesis. In order to clarify whole-tree response patterns carbohydrate data were compared with photosynthesis, stomatal conductance and electron transport rates. In 2004 all parameters revealed a significant common response pattern to COU that indicated a reduction for all parameters under 2 x O(3).

O3 flux-related responsiveness of photosynthesis, respiration, and stomatal conductance of adult Fagus sylvatica to experimentally enhanced free-air O3 exposure.

Knowledge of responses of photosynthesis, respiration, and stomatal conductance to cumulative ozone uptake (COU) is still scarce, and this is particularly the case for adult trees. The effect of ozone (O(3)) exposure on trees was examined with 60-year-old beech trees (FAGUS SYLVATICA) at a forest site of southern Germany. Trees were exposed to the ambient O(3) regime (1 x O(3)) or an experimentally elevated twice-ambient O(3) regime (2 x O(3)). The elevated 2 x O (3) regime was provided by means of a free-air O(3) canopy exposure system. The hypotheses were tested that (1) gas exchange is negatively affected by O(3) and (2) the effects of O(3) are dose-dependent and thus the sizes of differences between treatments are positively related to COU. Gas exchange (light-saturated CO(2) uptake rate A(max), stomatal conductance g (s), maximum rate of carboxylation Vc (max), ribulose-1,5-bisphosphate turnover limited rate of photosynthesis J (max), CO(2) compensation point CP, apparent quantum yield of net CO(2) uptake AQ, carboxylation efficiency CE, day- and nighttime respiration) and chlorophyll fluorescence (electron transfer rate, ETR) were measured IN SITU on attached sun and shade leaves. Measurements were made periodically throughout the growing seasons of 2003 (an exceptionally dry year) and 2004 (a year with average rainfall). In 2004 Vc(max), J(max), and CE were lower in trees receiving 2 x O(3) compared with the ambient O(3) regime (1 x O(3)). Treatment differences in Vc (max), J (max), CE were rather small in 2004 (i.e., parameter levels were lower by 10 - 30 % in 2 x O(3) than 1 x O(3)) and not significant in 2003. In 2004 COU was positively correlated with the difference between treatments in A (max), g (s), and ETR (i.e., consistent with the dose-dependence of O(3)'s deleterious effects). However, in 2003, differences in A(max), g (s), and ETR between the two O(3) regimes were smaller at the end of the dry summer 2003 (i.e., when COU was greatest). The relationship of COU with effects on gas exchange can apparently be complex and, in fact, varied between years and within the growing season. In addition, high doses of O(3) did not always have significant effects on leaf gas exchange. In view of the key findings, both hypotheses were to be rejected.

Temperature-respiration relationships differ in mycorrhizal and non-mycorrhizal root systems of Picea abies (L.) Karst.

Root respiration has been shown to increase with temperature, but less is known about how this relationship is affected by the fungal partner in mycorrhizal root systems. In order to test respiratory temperature dependence, in particular Q (10) of mycorrhizal and non-mycorrhizal root systems, seedlings of PICEA ABIES (L.) Karst. (Norway spruce) were inoculated with the ectomycorrhizal fungus PILODERMA CROCEUM (Eriksson and Hjortstam, SR430; synonym: PILODERMA FALLAX: [Libert] Stalpers) and planted in soil respiration cuvettes (mycocosms). Temperature dependence of hyphal respiration in sterile cultures was determined and compared with respiration of mycorrhizal roots. Respiration rates of mycorrhizal and non-mycorrhizal root systems as well as sterile cultures were sensitive to temperature. Q (10) of mycorrhizal root systems of 3.0 +/- 0.1 was significantly higher than that of non-mycorrhizal systems (2.5 +/- 0.2). Q (10) of P. CROCEUM in sterile cultures (older than 2 months) was similar to that of mycorrhizal root systems, suggesting that mycorrhizae may have a large influence on the temperature sensitivity of roots in spite of their small biomass. Our results stress the importance of considering mycorrhization when modeling the temperature sensitivity of spruce roots.

Synopsis of the CASIROZ case study: carbon sink strength of Fagus sylvatica L. in a changing environment--experimental risk assessment of mitigation by chronic ozone impact.

Databases are needed for the ozone (O(3)) risk assessment on adult forest trees under stand conditions, as mostly juvenile trees have been studied in chamber experiments. A synopsis is presented here from an integrated case study which was conducted on adult FAGUS SYLVATICA trees at a Central-European forest site. Employed was a novel free-air canopy O(3) fumigation methodology which ensured a whole-plant assessment of O(3) sensitivity of the about 30 m tall and 60 years old trees, comparing responses to an experimental 2 x ambient O(3) regime (2 x O(3), max. 150 nl O(3) l (-1)) with those to the unchanged 1 x ambient O(3) regime (1 x O(3)=control) prevailing at the site. Additional experimentation on individual branches and juvenile beech trees exposed within the forest canopy allowed for evaluating the representativeness of young-tree and branch-bag approaches relative to the O(3) sensitivity of the adult trees. The 2 x O(3) regime did not substantially weaken the carbon sink strength of the adult beech trees, given the absence of a statistically significant decline in annual stem growth; a 3 % reduction across five years was demonstrated, however, through modelling upon parameterization with the elaborated database. 2 x O(3) did induce a number of statistically significant tree responses at the cell and leaf level, although the O(3) responsiveness varied between years. Shade leaves displayed an O(3) sensitivity similar to that of sun leaves, while indirect belowground O(3) effects, apparently mediated through hormonal relationships, were reflected by stimulated fine-root and ectomycorrhizal development. Juvenile trees were not reliable surrogates of adult ones in view of O(3) risk assessment. Branch sections enclosed in (climatized) cuvettes, however, turned out to represent the O(3) sensitivity of entire tree crowns. Drought-induced stomatal closure decoupled O(3) intake from O(3) exposure, as in addition, also the "physiologically effective O(3) dose" was subject to change. No evidence emerged for a need to lower the "Critical Level for Ozone" in risk assessment of forest trees, although sensitive tree parameters did not necessarily reflect a linear relationship to O(3) stress. Exposure-based concepts tended to overestimate O(3) risk under drought, which is in support of current efforts to establish flux-related concepts of O(3) intake in risk assessment.

Free-air exposure systems to scale up ozone research to mature trees.

Because seedlings and mature trees do not necessarily respond similarly to O(3) stress, it is critically important that exposure systems be developed that allow exposure of seedlings through to mature trees. Here we describe three different O(3) Free-Air Exposure Systems that have been used successfully for exposure at all growth stages. These systems of spatially uniform O(3) release have been shown to provide reliable O(3) exposure with minimal, if any, impact on the microclimate. This methodology offers a welcome alternative to chamber studies which had severe space constraints precluding stand or community-level studies and substantial chamber effects on the microclimate and, hence physiological tree performance.

Gas exchange and antioxidative compounds in young beech trees under free-air ozone exposure and comparisons to adult trees.

Three-year-old beech (Fagus sylvatica) seedlings growing in containers were placed into the sun and shade crown of a mature beech stand exposed to ambient (1 x O(3)) and double ambient (2 x O(3)) ozone concentrations at a free-air exposure system ("Kranzberg Forst", Germany). Pigments, alpha-tocopherol, glutathione, ascorbate, and gas exchange were measured in leaves during 2003 (a drought year) and 2004 (an average year). Sun-exposed seedlings showed higher contents of antioxidants, xanthophylls, and beta-carotene and lower contents of chlorophyll, alpha-carotene, and neoxanthin than shade-exposed seedlings. In 2003 sun-exposed seedlings showed higher contents of carotenoids and total glutathione and lower net photosynthesis rates (A(max)) compared to 2004. O(3) exposure generally affected the content of chlorophyll, the xanthophyll cycle, and the intercellular CO(2) concentration (c(i)). Seedlings differed from the adjacent adult trees in most biochemical and physiological parameters investigated: Sun exposed seedlings showed higher contents of alpha-tocopherol and xanthophylls and lower contents of ascorbate, chlorophyll, neoxanthin, and alpha-carotene compared to adult trees. Shade exposed seedlings had lower contents of xanthophylls, alpha-carotene, and alpha-tocopherol than shade leaves of old-growth trees. In 2003, seedlings had higher A(max), stomatal conductance (g(s)), and c(i) under 2 x O(3) than adult trees. The results showed that shade acclimated beech seedlings are more sensitive to O(3), possibly due to a lower antioxidative capacity per O(3) uptake. We conclude that beech seedlings are uncertain surrogates for adult beech trees.

Promoting the O3 flux concept for European forest trees.

Tropospheric ozone (O3) levels are predicted to stay high, being a factor within "global change" with potential effects on the carbon sink strength of forest trees. Hence, new approaches to O3 risk assessment and their validation are required, although appropriate databases for adult trees are scant. Approaches based on external O3 exposure are presently being evaluated against the ones on O3 flux into leaves, as the cumulative uptake has the capacity for deriving O3 risk from cause-effect relationships. The effective dose, however, needs to account for the trees' O3 defence and tolerance in addition to O3 uptake. The current status of promoting the preferable mechanistic O3 flux concept is highlighted for major regions of Europe, addressing refinements and simplifications needed for routine use. At the pan-European scale, however, the flux-based concept is ready for use in O3 risk assessment and has the potential of meso-scale application at the forest ecosystem level.

Woody-plant ecosystems under climate change and air pollution-response consistencies across zonobiomes?

Forests store the largest terrestrial pools of carbon (C), helping to stabilize the global climate system, yet are threatened by climate change (CC) and associated air pollution (AP, highlighting ozone (O3) and nitrogen oxides (NOx)). We adopt the perspective that CC-AP drivers and physiological impacts are universal, resulting in consistent stress responses of forest ecosystems across zonobiomes. Evidence supporting this viewpoint is presented from the literature on ecosystem gross/net primary productivity and water cycling. Responses to CC-AP are compared across evergreen/deciduous foliage types, discussing implications of nutrition and resource turnover at tree and ecosystem scales. The availability of data is extremely uneven across zonobiomes, yet unifying patterns of ecosystem response are discernable. Ecosystem warming results in trade-offs between respiration and biomass production, affecting high elevation forests more than in the lowland tropics and low-elevation temperate zone. Resilience to drought is modulated by tree size and species richness. Elevated O3 tends to counteract stimulation by elevated carbon dioxide (CO2). Biotic stress and genomic structure ultimately determine ecosystem responsiveness. Aggrading early- rather than mature late-successional communities respond to CO2 enhancement, whereas O3 affects North American and Eurasian tree species consistently under free-air fumigation. Insect herbivory is exacerbated by CC-AP in biome-specific ways. Rhizosphere responses reflect similar stand-level nutritional dynamics across zonobiomes, but are modulated by differences in tree-soil nutrient cycling between deciduous and evergreen systems, and natural versus anthropogenic nitrogen (N) oversupply. The hypothesis of consistency of forest responses to interacting CC-AP is supported by currently available data, establishing the precedent for a global network of long-term coordinated research sites across zonobiomes to simultaneously advance both bottom-up (e.g., mechanistic) and top-down (systems-level) understanding. This global, synthetic approach is needed because high biological plasticity and physiographic variation across individual ecosystems currently limit development of predictive models of forest responses to CC-AP. Integrated research on C and nutrient cycling, O3-vegetation interactions and water relations must target mechanisms' ecosystem responsiveness. Worldwide case studies must be subject to biostatistical exploration to elucidate overarching response patterns and synthesize the resulting empirical data through advanced modelling, in order to provide regionally coherent, yet globally integrated information in support of internationally coordinated decision-making and policy development.

Interactions between drought and O3 stress in forest trees.

Temperature increase and altered precipitation are facets of "Global Change", along with enhanced tropospheric ozone (O3) and CO2 levels. Both O3 and drought may curtail the probably limited capacity of "extra" carbon fixation in forest trees under a CO2-enriched atmosphere. In view of the exceptionally dry year of 2003 in Central Europe, this mini-review highlights O3/drought interactions in biochemical and ecophysiological responses of trees. Such interactions appear to vary, depending on the genotype and factorial scenarios. If O3 perturbs stomatal regulation, tolerance to both drought and persisting O3 exposure may be weakened, although drought preceding O3 stress may "harden" against O3 impact. Stomatal closure under drought may shield trees against O3 uptake and injury, which indeed was the case in 2003. However, the trees' "tuning" between O3 uptake and defence capacity is crucial in stress tolerance. Defence may be constrained due to limited carbon fixation, which results from the trade-off with O3 exclusion upon stomatal closure. Drought may cause a stronger reduction in stem growth than does ozone on an annual basis.

Acclimation to ozone affects host/pathogen interaction and competitiveness for nitrogen in juvenile Fagus sylvatica and Picea abies trees infected with Phytophthora citricola.

In a two-year phytotron study, juvenile trees of European beech (Fagus sylvatica) and Norway spruce (Picea abies) were grown in mixture under ambient and twice ambient ozone (O3) and infected with the root pathogen Phytophthora citricola. We investigated the influence of O3 on the trees' susceptibility to the root pathogen and assessed, through a 15N-labelling experiment, the impact of both treatments (O3 exposure and infection) on belowground competitiveness. The hypotheses tested were that: (1) both P. citricola and O3 reduce the belowground competitiveness (in view of N acquisition), and (2) that susceptibility to P. citricola infection is reduced through acclimation to enhanced O3 exposure. Belowground competitiveness was quantified via cost/benefit relationships, i.e., the ratio of structural investment in roots relative to their uptake of 15N. Beech had a lower biomass acquisition and captured less 15N under enhanced O3 and P. citricola infection alone than spruce, whereas the latter species appeared to profit from the lower resource acquisition of beech in these treatments. Nevertheless, in the combined treatment, susceptibility to P. citricola of spruce was increased, while beech growth and 15N uptake were not further reduced below the levels found under the single treatments. Potential trade-offs between stress defence, growth performance, and associated nitrogen status are discussed for trees affected through O3 and/or pathogen infection. With respect to growth performance, it is concluded that O3 enhances susceptibility to the pathogen in spruce, but apparently raises the defence capacity in beech..

Mycorrhizosphere responsiveness to atmospheric ozone and inoculation with Phytophthora citricola in a phytotron experiment with spruce/beech mixed cultures.

The aim was to analyze functional changes in the mycorrhizosphere (MR) of juvenile spruce and beech grown in a mixture under ambient and twice ambient ozone and inoculated with the root pathogen Phytophthora citricola. The phytotron experiment was performed over two vegetation periods, adding the pathogen at the end of the first growing season. Root biomass data suggest that the combined treatment affected spruce more than beech and that the negative influence of ozone on stress tolerance against the root pathogen P. citricola was greater for spruce than for beech. In contrast, beech was more affected when the pathogen was the sole stressor. The functional soil parameter chosen for studies of MR soil samples was activity of extracellular enzymes. After the first year of ozone exposure, MR soil samples of both species showed increased activity of almost all measured enzymes (acid phosphatase, chitinase, beta-glucosidase, cellobiohydrolase) in the O3 treatment. Species-specific differences were observed, with a stronger effect of P. citricola on beech MR and a stronger ozone effect on spruce MR. In the second year, the effects of the combined treatment (ozone and P. citricola) were a significant increase in the activity of most enzymes (except cellobiohydrolase) for both tree species. The results indicated that responsiveness of MR soils towards ozone and P. citricola was related to the severity of infection of the plants and the reduction of belowground biomass, suggesting a strong, direct influence of plant stress on MR soil enzyme activity. Additional research is needed using different species and combined stresses to determine the broader ecological relevance of shifts in rhizosphere enzymes.

The plant's capacity in regulating resource demand.

Regulation of resource allocation in plants is the key to integrate understanding of metabolism and resource flux across the whole plant. The challenge is to understand trade-offs as plants balance allocation between different and conflicting demands, e.g., for staying competitive with neighbours and ensuring defence against parasites. Related hypothesis evaluation can, however, produce equivocal results. Overcoming deficits in understanding underlying mechanisms is achieved through integrated experimentation and modelling the various spatio-temporal scaling levels, from genetic control and cell metabolism towards resource flux at the stand level. An integrated, interdisciplinary research concept on herbaceous and woody plants and its outcome to date are used, while drawing attention to currently available knowledge. This assessment is based on resource allocation as driven through plant-pathogen and plant-mycorrhizosphere interaction, as well as competition with neighbouring plants in stands, conceiving such biotic interactions as a "unity" in the control of allocation. Biotic interaction may diminish or foster effects of abiotic stress on allocation, as changes in allocation do not necessarily result from metabolic re-adjustment but may obey allometric rules during ontogeny. Focus is required on host-pathogen interaction under variable resource supply and disturbance, including effects of competition and mycorrhization. Cost/benefit relationships in balancing resource investments versus gains turned out to be fundamental in quantifying competitiveness when related to the space, which is subject to competitive resource exploitation. A space-related view of defence as a form of prevention of decline in competitiveness may promote conversion of resource turnover across the different kinds of biotic interaction, given their capacity in jointly controlling whole plant resource allocation.

Short term effects of ozone on the plant-rhizosphere-bulk soil system of young beech trees.

Plant growth largely depends on microbial community structure and function in the rhizosphere. In turn, microbial communities in the rhizosphere rely on carbohydrates provided by the host plant. This paper presents the first study on ozone effects in the plant-rhizosphere-bulk soil system of 4-year-old beech trees using outdoor lysimeters as a research platform. The lysimeters were filled with homogenized soil from the corresponding horizons of a forest site, thus minimizing field heterogeneity. Four lysimeters were treated with ambient ozone (1 x O3) and four with double ambient ozone concentrations (2 x O3; restricted to 150 ppb). In contrast to senescence, which was almost unaffected by ozone treatment, both the photochemical quantum yield of photosystem II (PSII) and leaf gas exchange were reduced (11 - 45 %) under the elevated O3 regime. However, due to large variation between the plants, no statistically significant O3 effect was found. Even though the amount of primary metabolites, such as sugar and starch, was not influenced by elevated O3 concentrations, the reduced photosynthetic performance was reflected in leaf biochemistry in the form of a reduction in soluble phenolic metabolites. The rhizosphere microbial community also responded to the O3 treatment. Both community structure and function were affected, with a tendency towards a lower diversity and a significant reduction in the potential nutrient turnover. In contrast, litter degradation was unaffected by the fumigation, indicating that in situ microbial functionality of the bulk soil did not change.

Beech leaf colonization by the endophyte Apiognomonia errabunda dramatically depends on light exposure and climatic conditions.

Ozone and light effects on endophytic colonization by Apiognomonia errabunda of adult beech trees (Fagus sylvatica) and their putative mediation by internal defence compounds were studied at the Kranzberg Forest free-air ozone fumigation site. A. errabunda colonization was quantified by "real-time PCR" (QPCR). A. errabunda-specific primers allowed detection without interference by DNA from European beech and several species of common genera of plant pathogenic fungi, such as Mycosphaerella, Alternaria, Botrytis, and Fusarium. Colonization levels of sun and shade leaves of European beech trees exposed either to ambient or twice ambient ozone regimes were determined. Colonization was significantly higher in shade compared to sun leaves. Ozone exhibited a marginally inhibitory effect on fungal colonization only in young leaves in 2002. The hot and dry summer of 2003 reduced fungal colonization dramatically, being more pronounced than ozone treatment or sun exposure. Levels of soluble and cell wall-bound phenolic compounds were approximately twice as high in sun than in shade leaves. Acylated flavonol 3- O-glycosides with putatively high UV-B shielding effect were very low in shade canopy leaves. Ozone had only a minor influence on secondary metabolites in sun leaves. It slightly increased kaempferol 3- O-glucoside levels exclusively in shade leaves. The frequently prominent hydroxycinnamic acid derivative, chlorogenic acid, was tested for its growth inhibiting activity against Apiognomonia and showed an IC50 of approximately 8 mM. Appearance of Apiognomonia-related necroses strongly correlated with the occurrence of the stress metabolite, 3,3',4,4'-tetramethoxybiphenyl. Infection success of Apiognomonia was highly dependent on light exposure, presumably affected by the endogenous levels of constitutive phenolic compounds. Ozone exerted only minor modulating effects, whereas climatic factors, such as pronounced heat periods and drought, were dramatically overriding.

Tree age dependence and within-canopy variation of leaf gas exchange and antioxidative defence in Fagus sylvatica under experimental free-air ozone exposure.

We characterized leaf gas exchange and antioxidative defence of two-year-old seedlings and 60-year-old trees of Fagus sylvatica exposed to ambient (1 x O3) or two-fold ambient (2 x O3) O3 concentrations (maximum of 150 ppb) in a free-air canopy exposure system throughout the growing season. Decline in photosynthesis from sun-exposed to shaded conditions was more pronounced in adult than juvenile trees. Seedling leaves and leaves in the sun-exposed canopy had higher stomatal conductance and higher internal CO2 concentrations relative to leaves of adult trees and leaves in shaded conditions. There was a weak overall depression of photosynthesis in the 2 x O3 variants across age classes and canopy positions. Pigment and tocopherol concentrations of leaves were significantly affected by canopy position and tree age, whereas differences between 1 x O3 and 2 x O3 regimes were not observed. Glutathione concentrations were significantly increased under 2 x O3 across both age classes and canopy levels. Seedlings differed from adult trees in relevant physiological and biochemical traits in ozone response. The water-soluble antioxidative systems responded most sensitively to 2 x O3 without regard of tree age or canopy position.

Comparison of ozone uptake and sensitivity between a phytotron study with young beech and a field experiment with adult beech (Fagus sylvatica).

Chamber experiments on juvenile trees have resulted in severe injury and accelerated loss of leaves along with reduced biomass production under chronically enhanced O3 levels. In contrast, the few studies conducted on adult forest trees in the field have reported low O3 sensitivity. In the present study, young beech in phytotrons was more sensitive to O3 than adult beech in the field, although employed O3 regimes were similar. The hypotheses tested were that: (1) differences in O3 uptake were caused by the ontogenetically higher stomatal conductance of young compared to adult trees, (2) the experimental settings in the phytotrons enhanced O3 uptake compared to field conditions, and (3) a low detoxification capacity contributes to the higher O3 sensitivity of the young trees. The higher O3 sensitivity of juvenile beech in the phytotrons is demonstrated to relate to both the experimental conditions and the physiological responsiveness inherent to tree age.

Whole-tree seasonal nitrogen uptake and partitioning in adult Fagus sylvatica L. and Picea abies L. [Karst.] trees exposed to elevated ground-level ozone.

The effect of long-term exposure of twice-ambient O(3) (2 × O(3)) on whole-tree nitrogen (N) uptake and partitioning of adult beech and spruce was studied in a mixed forest stand, SE-Germany. N uptake as (15)N tracer and N pools were calculated using N concentrations and biomass of tree compartments. Whole-tree N uptake tended to be lower under 2 × O(3) in both species compared to trees under ambient O(3) (1 × O(3)). Internal partitioning in beech showed significantly higher allocation of new N to roots, with mycorrhizal root tips and fine roots together receiving about 17% of new N (2 × O(3)) versus 7% (1 × O(3)). Conversely, in spruce, N allocation to roots was decreased under 2 × O(3). These contrasting effects on belowground N partitioning and pool sizes, being largely consistent with the pattern of N concentrations, suggest enhanced N demand and consumption of stored N with higher relevance for tree-internal N cycling in beech than in spruce.

Canopy-level stomatal narrowing in adult Fagus sylvatica under O3 stress - means of preventing enhanced O3 uptake under high O3 exposure?

Spatio-temporally consistent O(3) doses are demonstrated in adult Fagus sylvatica from the Kranzberg Forest free-air fumigation experiment, covering cross-canopy and whole-seasonal scopes through sap flow measurement. Given O(3)-driven closure of stomata, we hypothesized enhanced whole-tree level O(3) influx to be prevented under enhanced O(3) exposure. Although foliage transpiration rate was lowered under twice-ambient O(3) around noon by 30% along with canopy conductance, the hypothesis was falsified, as O(3) influx was raised by 25%. Nevertheless, the twice-ambient/ambient ratio of O(3) uptake was smaller by about 20% than that of O(3) exposure, suggesting stomatal limitation of uptake. The O(3) response was traceable from leaves across branches to the canopy, where peak transpiration rates resembled those of shade rather than sun branches. Rainy/overcast-day and nightly O(3) uptake is quantified and discussed. Whole-seasonal canopy-level validation of modelled with sap flow-derived O(3) flux becomes available in assessing O(3) risk for forest trees.

Effect of fertilization on ozone-induced changes in the metabolism of birch (Betula pendula) leaves.

Cloned cuttings of Betula pendula Roth were grown in field fumigation chambers at Birmensdorf throughout one growing season in filtered air with either < 3 (control) or 90/40 nl l-1 O3 (day/night; ozone generated from pure oxygen). Each ozone regime was split into high and low soil nutrient regimes by watering plants with either a 0.05 % or a 0.005% solution of a fertilizer which contained macronutrients and micronutrients. Fertilization had a strong effect on plant growth, enzyme activities and the expression of ozone-induced effects at the biochemical level. The activities of PEPC and Rubisco were enhanced about threefold in the plants with high fertilization (HF). Significant effects of ozone were in most cases found only in the older leaves of the plants with low fertilization (LF), There, sucrose, glucose and fructose levels were enhanced. In both fertilization treatments, the number of starch granules along the minor veins was increased. These ozone effects point to a decreased or inhibited phloem loading. The increased PEPC activity and the enhanced malate levels in the ozone-exposed plants might be the result of a redirection of carbon flow from sucrose synthesis and translocation towards anapleurotic processes, which can feed detoxification and repair of ozone injury as indicated by enhanced respiration. These findings agree well with the observed effects of ozone in lowering the root: shoot biomass ratio. Although there was a marked reduction in the O3 /LF plants, O3 /HF plants showed no significant response. Inositol was decreased under ozone exposure in both fertilizer treatments, contrasting with the pattern for carbohydrates. These results demonstrate the role of fertilization as an important modifier of ozone-induced effects at the plant biochemical level. Well fertilized plants appear to cope better with the impact of ozone on metabolism.