Context Elevated atmospheric CO2 (e[CO2]) and nitrogen (N) fertilisation stimulate biomass and yield of crops. However, their interactions depend on crop growth stages and may affect water use dynamics.Aims and methods This study investigated the interactive effects of two N rates, 0 and 100kgNha−1, and two CO2 concentrations, ambient (a[CO2], ~400μmolmol−1) and e[CO2] (~550μmolmol−1), on biomass, yield and water use of two wheat cultivars, Wyalkatchem (N-use efficient) and Yitpi (local), using a free air CO2 enrichment facility.Key results Elevated [CO2] stimulated leaf area (10%, P=0.003) and aboveground biomass (11%, P=0.03). In addition, e[CO2] reduced stomatal conductance (25%, P<0.001) and increased net assimilation rates (12%, P<0.001), resulting in greater (40%, P<0.001) intrinsic water use efficiency. During early growth stages, e[CO2] resulted in higher water use than a[CO2]; however, this difference disappeared later in the season, resulting in similar cumulative water use under both CO2 concentrations. Supplemental N stimulated grain yield of Yitpi by 14% while decreasing that of Wyalkatchem by 7% (N×cultivar, P=0.063). With supplemental N, Yitpi maintained greater post-anthesis leaf N, chlorophyll content, canopy cover and net assimilation rate than Wyalkatchem.Conclusions During early growth stages, the e[CO2]-induced stimulation of leaf-level water use efficiency was offset by greater biomass, resulting in higher water use. By the end of the season, similar cumulative water use under both CO2 concentrations indicates the dominating effect of the prevailing seasonal conditions in the study area. Observed yield responses of the studied cultivars to supplemental N were associated with their ability to maintain post-anthesis photosynthetic capabilities.Implications Our findings suggest that N-use efficiency traits and responsiveness need to be considered independently to optimise benefits from the ‘CO2 fertilisation effect’ through breeding.

Atmospheric carbon dioxide concentrations [CO2] are increasing steadily. Some reports have shown that root growth in grain crops is mostly stimulated in the topsoil rather than evenly throughout the soil profile by e[CO2], which is not optimal for crops grown in semi-arid environments with strong reliance on stored water. An experiment was conducted during the 2014 and 2015 growing seasons with two lentil (Lens culinaris) genotypes grown under Free Air CO2 Enrichment (FACE) in which root growth was observed non-destructively with mini-rhizotrons approximately every 2–3 weeks. Root growth was not always statistically increased by e[CO2] and not consistently between depths and genotypes. In 2014, root growth in the top 15 cm of the soil profile (topsoil) was indeed increased by e[CO2], but increases at lower depths (30–45 cm) later in the season were greater than in the topsoil. In 2015, e[CO2] only increased root length in the topsoil for one genotype, potentially reflecting the lack of plant available soil water between 30–60 cm until recharged by irrigation during grain filling. Our limited data to compare responses to e[CO2] showed that root length increases in the topsoil were correlated with a lower yield response to e[CO2]. The increase in yield response was rather correlated with increases in root growth below 30 cm depth.

Forest growth is considered as an important global carbon sink but its responses to environmental changes remain uncertain. Tree stems are a predominant carbon pool in temperate eucalypt forests, representing a susbstantive component of their net productivity and carbon dynamics. Despite their importance, our understanding of factors controlling stem growth in these evergreen forests remains limited partly because the dominant eucalypts lack distinct growth rings. Unravelling eucalypt species' growth responses to climate from other factors, such as competition and disturbances like fire, is challenging due to the lack of long-term growth data. To address this gap, we present six years of monthly measurements of stem-diamter changes (as basal area increment, BAI) of two co-occurring dominant eucalypts from different sub-genera (Eucalyptus obliqua and E. rubida) across seven sites in a natural temperate forest of south-eastern Australia. We used linear mixed-effect models to examine the relative importance to monthly BAI of species, monthly climate indices and their potential lag effects (temperature and rainfall), inter-tree competition, and recent fire history (long-unburnt, prescribed fires, wildfire). Monthly BAI peaked in spring and autumn and was lowest in summer with signficant differences between species during spring and summer. Overall BAI variation was most clearly associated with maximum mean temperature, having a hyperbolic relationship with increases in BAI up to species-specific temperature optima and decreases thereafter. Rainfall, particularly autumn rainfall, influenced seasonal patterns in BAI, while inter-tree competition and recent fire history were of comparatively minor importance. BAI also varied strongly between years reflecting the opportunistic growth behaviour of eucalypts including higher annual growth rates during and after periods of high rainfall and transient decreases in BAI during extended drier periods. Our study provides field-based evidence of different growth niches for co-existing eucalypts in natural temperate mixed forests and highlights the importance of intra-annual climate variation to better understand overall productivity in temperate evergreen forests.

Increasing atmospheric carbon dioxide concentration ([CO2]) stimulates the leaf-level (intrinsic) water use efficiency (iWUE), which may mitigate the adverse effects of drought by lowering water use in plants. This study investigated the interactive effect of [CO2] and soil type on growth, yield and water use of canola (Brassica napus L.) in a dryland environment.

Forests are an important global carbon sink but their responses to climate change are uncertain. Tree stems, as the predominant carbon pool, represent net productivity in temperate eucalypt forests but the drivers of growth in these evergreen forests remain poorly understood partly because the dominant tree species lack distinct growth rings. Disentangling eucalypt species' growth responses to climate from other factors, such as competition and disturbances like fire, remains challenging due to a lack of long-term growth data. We measured monthly stem-diameter changes (as basal area increment, BAI) of two co-occurring dominant eucalypts from different sub-genera (Eucalyptus obliqua and E. rubida) over nearly four years. Our study included seven sites in a natural temperate forest of south-eastern Australia, and we used linear mixed-effects models to examine the relative importance to monthly BAI of species, monthly climate variables (temperature and rainfall), inter-tree competition, and recent fire history (long-unbumt, prescribed fire, wildfire). Monthly BAI peaked in spring and autumn and was significantly different between species during spring and summer. BAI variation was most dearly associated with temperature, increasing in hyperbolic response curves up to maximum mean temperatures of similar to 15-17 degrees C and thereafter decreasing. Temperature optima for maximum monthly BAI were 1 to 2 degrees C warmer for E. rubida than E. obliqua. While less important than temperature, rainfall, particularly autumn rainfall, also helped explain patterns in monthly BAI, with inter-tree competition and recent fire history of comparatively minor importance. Our study provides the first comprehensive field-based evidence of different growth niches for eucalypts from different subgenera in natural temperate mixed forests. It highlights the importance of intra-annual climate to understanding productivity variation in temperate evergreen forests and provides insights into the mechanisms underpinning the successful co-existence of different tree species as well as their relative vulnerabilities to changing climates. (C) 2019 Elsevier B.V. All rights reserved.

The 'CO2 fertilisation effect' is often predicted to be greater under drier than wetter conditions, mainly due to hypothesised early season water savings under elevated [CO2] (e[CO2]). However, water savings largely depend on the balance between CO2-induced improvement of leaf-level water use efficiency and CO2-stimulation of transpiring leaf area. The dynamics of water use during the growing season can therefore vary depending on leaf area development. Two canola (Brassica napes L.) cultivars of contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper) were grown under ambient [CO2] (aCO(2)], similar to 400 mu mol mol(-1)) or e[CO2] (similar to 700 mu mol mol(-1)) with two water treatments (well-watered and mild drought) in a glasshouse to investigate the interdependence of leaf area development and water use. Dynamics of water use during the growing season varied depending on [CO2] and cultivars. Early stimulation of leaf growth under e[CO2], which also depended on cultivar, overcompensated for the effect of increased leaf-level water use efficiency, so that weekly water use was greater and water depletion from soil greater under e [CO2] than aCO(2)]. This result shows that the balance between leaf area and water use efficiency stimulation by e[CO2] can tip towards early depletion of available soil water, so that e[CO2] does not lead to water savings, and the 'CO2 fertilisation effect' is not greater under drier conditions.

This study investigated the interactive effects of atmospheric CO2 concentration ([CO2]) and water availability on yield, root growth and water use of two canola cultivars with contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper). Plants were grown under ambient [CO2) [a[CO2], similar to 400 mu mol mol(-1)) or elevated [CO2] (e[CO2], similar to 700 mu mol mol(-1)) in a glasshouse. Two water treatments (well watered and drought) were established in each [CO2] treatment. During the growing season leaf gas exchange parameters were measured. Leaf area was measured at 80 days after sowing. Aboveground biomass, seed yield, yield components and root biomass in four different soil layers (Layer 1: 0-20 cm, Layer 2: 21-40 cm, Layer 3: 41-60 cm and Layer 4: 61-80 cm depth) were measured at maturity. Weekly water use was determined grayimetrically. Elevated [CO2] stimulated seed yield (38%), aboveground biomass (34%), root biomass (42%), leaf area (42%) and leaf biomass (41%). Whilst e[CO2] stimulated root biomass in all soil layers, this stimulation was greater in the deeper than upper soil layers, and was associated with greater extraction of deeper soil water under e[CO2]. The cultivar with greater stimulation of deeper root biomass under e[CO2] showed greater yield benefit from the 'CO2 fertilisation effect'. Under well-watered conditions, e[CO2]-induced reductions of stomatal conductance (g(s)) balanced the effect of increased leaf area on water use, resulting in similar water use compared to a[CO2]. In contrast, under drought conditions, water use was greater under e[CO2] than a[CO2]. The 'CO2 fertilisation effect' depended on cultivar and water treatment. Under well-watered conditions, aboveground biomass of the hybrid cultivar benefitted more from the 'CO2 fertilisation effect'. However, under drought both aboveground biomass and seed yield of the non-hybrid cultivar benefitted more from the 'CO2 fertilisation effect'. These findings show that interactions between environmental conditions (here experimental water treatments) and expression of genotypic traits (here differences between cultivars) play a decisive role in determining potential yield and growth benefits from rising [CO2].

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.

This study investigated crop water use of wheat grown in a dryland Mediterranean-type environment under elevated atmospheric CO2 concentrations ([CO2]). Two related cultivars, contrasting in agronomic features (cvs. Scout and Yitpi; Scout has good early vigour and high transpiration efficiency), were grown under ambient [CO2] (a [CO2], similar to 400 mu mol mol(-1)) and elevated [CO2] (e[CO2), similar to 550 mu mol mol(-1)) in the Australian Grains Free Air CO2 Enrichment (AGFACE) facility for two growing seasons. Each year, an irrigation treatment (rainfed versus irrigated) was imposed within the CO2-treatments. Normalised difference vegetation index (as surrogate for canopy cover) and root length in the upper (0 cm-32 cm) and deeper (33 cm-64 cm) soil layers were measured at stem-elongation and anthesis. Elevated [CO2] stimulated root length of wheat in both upper and deeper soil layers, and this stimulation was modified by cultivars and irrigation regimes. Across cultivars and all treatments, water use, biomass and grain yield were positively associated with root length in the deeper soil layer but not with root length in the upper soil layer. The 'CO2 fertilisation effect' on biomass and grain yield was of similar magnitude under both irrigated and rainfed conditions. Although e[CO2] did not increase canopy cover in these experiments, the CO2 effect on water use depended on cultivars and irrigation regimes. Despite greater e[CO2]-induced stimulation of tillers and spikes, the cv. Scout did not receive more biomass or grain yield benefit from the 'CO2 fertilisation effect' compared to cv. Yitpi.

Elevated CO2 stimulates crop yields but leads to lower tissue and grain nitrogen concentrations [N], raising concerns about grain quality in cereals. To test whether N fertiliser application above optimum growth requirements can alleviate the decline in tissue [N], wheat was grown in a Free Air CO2 Enrichment facility in a low-rainfall cropping system on high soil N. Crops were grown with and without addition of 50-60 kg N/ha in 12 growing environments created by supplemental irrigation and two sowing dates over 3 years. Elevated CO2 increased yield and biomass (on average by 25%) and decreased biomass [N] (3%-9%) and grain [N] (5%). Nitrogen uptake was greater (20%) in crops grown under elevated CO2. Additional N supply had no effect on yield and biomass, confirming high soil N. Small increases in [N] with N addition were insufficient to offset declines in grain [N] under elevated CO2. Instead, N application increased the [N] in straw and decreased N harvest index. The results suggest that conventional addition of N does not mitigate grain [N] depression under elevated CO2, and lend support to hypotheses that link decreases in crop [N] with biochemical limitations rather than N supply.

Increasing atmospheric CO2 concentration ([CO2]) has profound effects on plant growth, yield and water use. Elevated atmospheric [CO2] (e[CO2]) increases aboveand below ground biomass production of wheat. The increased root growth under e[CO2] may allow the extraction of subsoil water, which is especially important in hot and dry Mediterranean conditions when the top soil dries off quickly. A glasshouse experiment was conducted with hydraulically separated top and bottom layered soil column to investigate the effect of soil drying at different depths on growth and gas exchange of wheat under e[CO2]. Drought stress was induced either in the top or the bottom soil layer or both by withholding 33% of the irrigation. Drought treatments started at stem elongation. At the flowering growth stage, net photosynthetic assimilation rate (Anet) stomatal conductance (gs), as well as aboveand below ground biomass were measured. In our study, Anet was higher and gs was lower under e[CO2] than a[CO2], which resulted in greater intrinsic water use efficiency (iWUE). Aboveand below ground biomass production of wheat under e[CO2] were higher compared to a[CO2]. Under e[CO2], wheat produced more biomass than under a[CO2] well-watered conditions even if either the top or bottom layer were subjected to drought treatment. In the DW treatment (D = drought, W = well-watered, where the first letter denotes the top layer and second letter denotes the bottom layer) vigorous root growth under e[CO2] increased the accessibility to the moisture in the bottom layer and resulted in gs similar to that of the WD. In addition, the reduction of above ground biomass in DW treatment was lower compared to DD conditions under e[CO2]. Therefore higher below ground production of wheat under e[CO2] could be able to ameliorate the potential effect of drought by improving accessibility of subsoil water.

Wheat production will be impacted by increasing concentration of atmospheric CO2 [CO2 ], which is expected to rise from about 400 μmol mol(-1) in 2015 to 550 μmol mol(-1) by 2050. Changes to plant physiology and crop responses from elevated [CO2 ] (e[CO2 ]) are well documented for some environments, but field-level responses in dryland Mediterranean environments with terminal drought and heat waves are scarce. The Australian Grains Free Air CO2 Enrichment facility was established to compare wheat (Triticum aestivum) growth and yield under ambient (~370 μmol(-1) in 2007) and e[CO2 ] (550 μmol(-1) ) in semi-arid environments. Experiments were undertaken at two dryland sites (Horsham and Walpeup) across three years with two cultivars, two sowing times and two irrigation treatments. Mean yield stimulation due to e[CO2 ] was 24% at Horsham and 53% at Walpeup, with some treatment responses greater than 70%, depending on environment. Under supplemental irrigation, e[CO2 ] stimulated yields at Horsham by 37% compared to 13% under rainfed conditions, showing that water limited growth and yield response to e[CO2 ]. Heat wave effects were ameliorated under e[CO2 ] as shown by reductions of 31% and 54% in screenings and 10% and 12% larger kernels (Horsham and Walpeup). Greatest yield stimulations occurred in the e[CO2 ] late sowing and heat stressed treatments, when supplied with more water. There were no clear differences in cultivar response due to e[CO2 ]. Multiple regression showed that yield response to e[CO2 ] depended on temperatures and water availability before and after anthesis. Thus, timing of temperature and water and the crop's ability to translocate carbohydrates to the grain postanthesis were all important in determining the e[CO2 ] response. The large responses to e[CO2 ] under dryland conditions have not been previously reported and underscore the need for field level research to provide mechanistic understanding for adapting crops to a changing climate.

This study investigated trade-offs between parameters determining water use efficiency of wheat under elevated CO2 in contrasting growing seasons and a semi-arid environment. We also evaluated whether previously reported negative relationships between nutrient content and transpiration efficiency among wheat genotypes will be maintained under elevated CO2 conditions. Two cultivars of wheat (Triticum aestivum L.), Scout and Yitpi, purportedly differing in water use efficiency related traits (e.g. transpiration efficiency) but with common genetic backgrounds were studied in a high yielding, high rainfall (2013), and in a low yielding, very dry growing season (2014) under Free-Air CO2 Enrichment (FACE, CO2 concentration of approximately 550 mu mol mol(-1)) and ambient (approximately 390 mu mol mol(-1)) CO2. Gas exchange measurements were collected diurnally between stem elongation and anthesis. Aboveground biomass and nutrient content (sum of Ca, K, S, P, Cu, Fe, Zn, Mn and Mg) were determined at anthesis. Yield, yield components and harvest index were measured at physiological maturity. Cultivar Scout showed transiently greater transpiration efficiency (measured by gas exchange) over cultivar Yitpi under both ambient and elevated CO2 conditions, mainly expressed in the high yielding but not in the low yielding season. Nutrient content was on average 13% greater for the lower transpiration efficiency cultivar Yitpi than the cultivar with higher transpiration efficiency (Scout) in the high yielding season across both CO2 concentrations. Elevated CO2 stimulated grain yield to a greater extent in the high yielding season than in the low yielding season where increased aboveground biomass earlier in the season did not translate into fertile tillers in cultivar Yitpi. Yield increased 27 and 33% in the high yielding and 0 and 19%in the low yielding season for cultivars Yitpi and Scout, respectively. Intraspecific variation in CO2 responsiveness related mechanisms of grain yield were observed. These results suggest CO2-driven trade-offs between traits governing water use efficiency are related to both growing season and intraspecific variations, and under very dry finishes, the trade-offs may even reverse. The negative relationship between nutrient content and transpiration efficiency among wheat genotypes will be maintained under elevated CO2 conditions. (C) 2016 Elsevier B.V. All rights reserved.

Stomatal conductance is a land-surface attribute that links the water and carbon cycles. Analysis of a global database covering a wide range of plant functional types and biomes now provides a framework for predicting the behaviour of stomatal conductance that can be applied to model ecosystem productivity, energy balance and ecohydrological processes in a changing climate. Stomatal conductance (gs) is a key land-surface attribute as it links transpiration, the dominant component of global land evapotranspiration, and photosynthesis, the driving force of the global carbon cycle. Despite the pivotal role of gs in predictions of global water and carbon cycle changes, a global-scale database and an associated globally applicable model of gs that allow predictions of stomatal behaviour are lacking. Here, we present a database of globally distributed gs obtained in the field for a wide range of plant functional types (PFTs) and biomes. We find that stomatal behaviour differs among PFTs according to their marginal carbon cost of water use, as predicted by the theory underpinning the optimal stomatal model1 and the leaf and wood economics spectrum2,3. We also demonstrate a global relationship with climate. These findings provide a robust theoretical framework for understanding and predicting the behaviour of gs across biomes and across PFTs that can be applied to regional, continental and global-scale modelling of ecosystem productivity, energy balance and ecohydrological processes in a future changing climate.

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 [CO2] and summer drought. Associated with month-to-month changes in temperature were concomitant changes in R in the light (Rlight) and darkness (Rdark), with both processes being more temperature dependent in well-watered trees than under drought. Overall rates of Rlight and Rdark were not significantly affected by [CO2]. By contrast, overall rates of Rdark (averaged across both [CO2]) 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 Rlight and Rdark when data from all months/treatments were combined (R2 = 0.8). Variations in Rlight 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/Asat). 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.

The effect of free-air ozone fumigation and crown position on antioxidants were determined in old-growth spruce (Picea abies) trees in the seasonal course of two consecutive years (2003 and 2004). Levels of total ascorbate and its redox state in the apoplastic washing fluid (AWF) were increased under double ambient ozone concentrations (2xO3), whilst ascorbate concentrations in needle extracts were unchanged. Concentrations of apoplastic and symplastic ascorbate were significantly higher in 2003 compared to 2004 indicating a combined effect of the drought conditions in 2003 with enhanced ozone exposure. Elevated ozone had only weak effects on total glutathione levels in needle extracts, phloem exudates and xylem saps. Total and oxidised glutathione concentrations were higher in 2004 compared to 2003 and seemed to be more affected by enhanced ozone influx in the more humid year 2004 compared to the combined effect of elevated ozone and drought in 2003 as observed for ascorbate.

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 (A max), stomatal conductance (g s), 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, A max and g s 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.

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.

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.