Markus Löw

Extraordinary drought of 2003 overrules ozone impact on adult beech trees (Fagus sylvatica)

The extraordinary drought during the summer of 2003 in Central Europe allowed to examine responses of adult beech trees (Fagus sylvatica) to co-occurring stress by soil moisture deficit and elevated O3 levels under forest conditions in southern Germany. The study comprised tree exposure to the ambient O3 regime at the site and to a twice-ambient O3 regime as released into the canopy through a free-air O3 fumigation system. Annual courses of photosynthesis (Amax), stomatal conductance (gs), electron transport rate (ETR) and chlorophyll levels were compared between 2003 and 2004, the latter year representing the humid long-term climate at the site. ETR, Amax and gs were lowered during 2003 by drought rather than ozone, whereas chlorophyll levels did not differ between the years. Radial stem increment was reduced in 2003 by drought but fully recovered during the subsequent, humid year. Comparison of AOT40, an O3 exposure-based risk index of O3 stress, and cumulative ozone uptake (COU) yielded a linear relationship throughout humid growth conditions, but a changing slope during 2003. Our findings support the hypothesis that drought protects plants from O3 injury by stomatal closure, which restricts O3 influx into leaves and decouples COU from high external ozone levels. High AOT40 erroneously suggested high O3 risk under drought. Enhanced ozone levels did not aggravate drought effects in leaves and stem.

Enhanced ozone strongly reduces carbon sink strength of adult beech (Fagus sylvatica) – Resume from the free-air fumigation study at Kranzberg Forest

Ground-level ozone (O(3)) has gained awareness as an agent of climate change. In this respect, key results are comprehended from a unique 8-year free-air O(3)-fumigation experiment, conducted on adult beech (Fagus sylvatica) at Kranzberg Forest (Germany). A novel canopy O(3) exposure methodology was employed that allowed whole-tree assessment in situ under twice-ambient O(3) levels. Elevated O(3) significantly weakened the C sink strength of the tree-soil system as evidenced by lowered photosynthesis and 44% reduction in whole-stem growth, but increased soil respiration. Associated effects in leaves and roots at the gene, cell and organ level varied from year to year, with drought being a crucial determinant of O(3) responsiveness. Regarding adult individuals of a late-successional tree species, empirical proof is provided first time in relation to recent modelling predictions that enhanced ground-level O(3) can substantially mitigate the C sequestration of forests in view of climate change.

Effects of chronic elevated ozone exposure on gas exchange responses of adult beech trees (Fagus sylvatica) as related to the within-canopy light gradient.

The effects of elevated O3 on photosynthetic properties in adult beech trees (Fagus sylvatica) were investigated in relation to leaf mass per area as a measure of the gradually changing, within-canopy light availability. Leaves under elevated O3 showed decreased stomatal conductance at unchanged carboxylation capacity of Rubisco, which was consistent with enhanced delta 13C of leaf organic matter, regardless of the light environment during growth. In parallel, increased energy demand for O3 detoxification and repair was suggested under elevated O3 owing to enhanced dark respiration. Only in shade-grown leaves,light-limited photosynthesis was reduced under elevated O3, this effect being accompanied by lowered F(v)/F(m). These results suggest that chronic O3 exposure primarily caused stomatal closure to adult beech trees in the field regardless of the within-canopy light gradient. However, light limitation apparently raised the O3 sensitivity of photosynthesis and accelerated senescence in shade leaves.

Light inhibition of leaf respiration in field-grown Eucalyptus saligna in whole-tree chambers under elevated atmospheric CO2 and summer drought

We investigated whether the degree of light inhibition of leaf respiration (R) differs among large Eucalyptus saligna grown in whole-tree chambers and exposed to present and future atmospheric [CO(2) ] and summer drought. Associated with month-to-month changes in temperature were concomitant changes in R in the light (R(light) ) and darkness (R(dark) ), with both processes being more temperature dependent in well-watered trees than under drought. Overall rates of R(light) and R(dark) were not significantly affected by [CO(2) ]. By contrast, overall rates of R(dark) (averaged across both [CO(2) ]) were ca. 25% lower under drought than in well-watered trees. During summer, the degree of light inhibition of leaf R was greater in droughted (ca. 80% inhibition) than well-watered trees (ca. 50% inhibition). Notwithstanding these treatment differences, an overall positive relationship was observed between R(light) and R(dark) when data from all months/treatments were combined (R(2)  = 0.8). Variations in R(light) were also positively correlated with rates of Rubisco activity and nitrogen concentration. Light inhibition resulted in a marked decrease in the proportion of light-saturated photosynthesis respired (i.e. reduced R/A(sat) ). Collectively, these results highlight the need to account for light inhibition when assessing impacts of global change drivers on the carbon economy of tree canopies.

Within-canopy and ozone fumigation effects on δ13C and Δ18O in adult beech (Fagus sylvatica) trees: relation to meteorological and gas exchange parameters

In this study, the effects of different light intensities either in direct sunlight or in the shade crown of adult beech (Fagus sylvatica L.) trees on delta13C and Delta18O were determined under ambient (1 x O3) and twice-ambient (2 x O3) atmospheric ozone concentrations during two consecutive years (2003 and 2004). We analysed the isotopic composition in leaf bulk, leaf cellulose, phloem and xylem material and related the results to (a) meteorological data (air temperature, T and relative humidity, RH), (b) leaf gas exchange measurements (stomatal conductance, g(s); transpiration rate, E; and maximum photosynthetic activity, A(max)) and (c) the outcome of a steady-state evaporative enrichment model. Delta13C was significantly lower in the shade than in the sun crown in all plant materials, whilst Delta18O was increased significantly in the shade than in the sun crown in bulk material and cellulose. Elevated ozone had no effect on delta13C, although Delta18O was influenced by ozone to varied degrees during single months. We observed significant seasonal changes for both parameters, especially in 2004, and also significant differences between the study years. Relating the findings to meteorological data and gas exchange parameters, we conclude that the differences in Delta18O between the sun and the shade crown were predominantly caused by the Péclet effect. This assumption was supported by the modelled Delta18O values for leaf cellulose. It was demonstrated that independent of RH, light-dependent reduction of stomatal conductance (and thus transpiration) and of A(max) can drive the pattern of Delta18O increase with the concomitant decrease of delta13C in the shade crown. The effect of doubling ozone levels on time-integrated stomatal conductance and transpiration as indicated by the combined analysis of Delta18O and delta13C was much lower than the influence caused by the light exposure.

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.

How sensitive are forest trees to ozone? - New research on an old issue.

Studies on juvenile individuals under artificial environments dominate knowledge about the sensitivity of trees to O3. Field approaches based on free-air O3 fumigations of adult forest trees are a novel choice. Such a case study on beech and spruce (at the Kranzberg Forest near Munich, Germany) is used to address four long-standing issues in O3 research: (1) Can a “unifying theory” of O3 sensitivity be verified? (2) Are responses to O3 consistent at different scaling levels in trees? (3) Are branch-bag experiments relevant for O3 risk assessment of crowns? (4) Are saplings surrogates of adult trees when both are assessed under the same field conditions? Preliminary evidence from the ongoing long-term study confirms (1) and (3) but negates (2) and (4). In the absence of acute risks for adult trees, responsiveness of leaves cannot rule out long-term constraints by chronic O3 stress.

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.

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.

Elevated [CO2] mitigates the effect of surface drought by stimulating root growth to access sub-soil water

Through stimulation of root growth, increasing atmospheric CO2 concentration ([CO2]) may facilitate access of crops to sub-soil water, which could potentially prolong physiological activity in dryland environments, particularly because crops are more water use efficient under elevated [CO2] (e[CO2]). This study investigated the effect of drought in shallow soil versus sub-soil on agronomic and physiological responses of wheat to e[CO2] in a glasshouse experiment. Wheat (Triticum aestivum L. cv. Yitpi) was grown in split-columns with the top (0–30 cm) and bottom (31–60 cm; ‘sub-soil’) soil layer hydraulically separated by a wax-coated, root-penetrable layer under ambient [CO2] (a[CO2], ∼400 μmol mol-1) or e[CO2] (∼700 μmol mol-1) [CO2]. Drought was imposed from stem-elongation in either the top or bottom soil layer or both by withholding 33% of the irrigation, resulting in four water treatments (WW, WD, DW, DD; D = drought, W = well-watered, letters denote water treatment in top and bottom soil layer, respectively). Leaf gas exchange was measured weekly from stem-elongation until anthesis. Above-and belowground biomass, grain yield and yield components were evaluated at three developmental stages (stem-elongation, anthesis and maturity). Compared with a[CO2], net assimilation rate was higher and stomatal conductance was lower under e[CO2], resulting in greater intrinsic water use efficiency. Elevated [CO2] stimulated both above- and belowground biomass as well as grain yield, however, this stimulation was greater under well-watered (WW) than drought (DD) throughout the whole soil profile. Imposition of drought in either or both soil layers decreased aboveground biomass and grain yield under both [CO2] compared to the well-watered treatment. However, the greatest ‘CO2 fertilisation effect’ was observed when drought was imposed in the top soil layer only (DW), and this was associated with e[CO2]-stimulation of root growth especially in the well-watered bottom layer. We suggest that stimulation of belowground biomass under e[CO2] will allow better access to sub-soil water during grain filling period, when additional water is converted into additional yield with high efficiency in Mediterranean-type dryland agro-ecosystems. If sufficient water is available in the sub-soil, e[CO2] may help mitigating the effect of drying surface soil.