Elevated CO2 and the magnitude and seasonal dynamics of root production and loss in t Betula papyrifera

The impact of elevated atmospheric CO₂ on belowground plant growth is poorly understood relative to its effects on aboveground growth. We carried out a study of the seasonal dynamics of gross root production and death to determine how elevated CO₂ affected the dynamics of net and gross root production through a full growing season. We quantified gross root production and root loss from sequential, in situ images of fine roots of t Betula papyrifera in ambient (375 ppm.) and elevated (700 ppm) CO₂ atmospheres from 2 weeks following germination through leaf senescence. We found that elevated CO₂ led to increases in the magnitude of cumulative gross production (P) and cumulative gross loss (L) of roots. However, the effect of elevated CO₂ on these processes was seasonally dependent. Elevated CO₂ led to greater levels of enhancement in P early in the growing season, prior to maximum standing root length (NP). In contrast, elevated CO₂ led to greater levels of enhancement in L in the last half of the growing season, after maximum NP had been reached. This difference in the timing of when elevated CO₂ affects P and L led to a transitory, early enhancement in NP. By the end of the growing season, there was no significant effect of elevated CO₂ on NP, and P was 87% greater than NP for ambient CO₂ and 117% greater in elevated CO₂. We conclude that static assessments of belowground productivity may greatly underestimate gross fine root productivity and turnover and this bias can be exaggerated with elevated CO₂.     

The effect of nutrient stress on developmental instability in leaves of Acer platanoides (Aceraceae) and Betula pendula (Betulaceae)

Studies of developmental stability can provide insights into the amount of genetic or environmental stress experienced by individuals or populations. In the present study, we used young plants of Acer platanoides (Norway maple) and Betula pendula (silver birch), two distantly related tree species with widely different leaf morphologies, to compare the expression of developmental instability in two contrasting environments: one with free access to nutrients and the other with a severely limited supply of nutrients. Using the difference in size between the right and left side of each leaf as a measure of developmental instability, we found no effect of nutrient deficiency on leaf asymmetry, despite large sample sizes (370-380 plants per species and treatment) and evidence for stress-related changes in overall leaf size and plant biomass. Moreover, there was no consistent relationship between individual leaf asymmetry and plant biomass within each nutrient treatment. In view of these observations, leaf asymmetry appears to be a poor indicator of nutrient stress in young plants of Acer platanoides and Betula pendula.     

The influence of elevated atmospheric CO2 on fine root dynamics in an intact temperate forest

Root dynamics are important for plant, ecosystem and global carbon cycling. Changes in root dynamics caused by rising atmospheric CO₂ not only have the potential to moderate further CO₂ increases, but will likely affect forest function. We used FACE (Free‐Air CO₂ Enrichment) to expose three 30‐m diameter plots in a 13‐year‐old loblolly pine (Pinus taeda) forest to elevated (ambient + 200 µL L^?1) atmospheric CO₂. Three identical fully instrumented plots were implemented as controls (ambient air only). We quantified root dynamics from October 1998 to October 1999 using minirhizotrons. In spite of 16% greater root lengths and 24% more roots per minirhizotron tube, the effects of elevated atmospheric CO₂ on root lengths and numbers were not statistically significant. Similarly, production and mortality were also unaffected by the CO₂ treatment, even though annual root production and mortality were 26% and 46% greater in elevated compared to ambient CO₂ plots. Average diameters of live roots present at the shallowest soil depth were, however, significantly enhanced in CO₂‐enriched plots. Mortality decreased with increasing soil depth and the slopes of linear regression lines (mortality vs. depth) differed between elevated and ambient CO₂ treatments, reflecting the significant CO₂ by depth interaction. Relative root turnover (root flux/live root pool) was unchanged by exposure to elevated atmospheric CO₂. Results from this study suggest modest, if any, increases in ecosystem‐level root productivity in CO₂‐enriched environments.     

大气二氧化碳浓度与温度升高对红桦幼苗养分积累和分配的影响

利用封闭式生长室,研究了CO2浓度升高(环境CO2 350 μmol·mol-1,EC)、温度升高(环境温度 2 ℃,ET)以及二者同时升高(ECT)对川西亚高山红桦幼苗养分积累和分配的影响.结果表明:经过一个生长季, EC处理下红桦幼苗单株N、P、K积累比对照分别增加44%、45%和11%(P《0.05),ET处理下分别增加37%、76%和9%(P《0.05),ECT处理下分别增加24%、88%和20% (P《0.05).EC处理使N向红桦幼苗叶中分配的比例降低11.68%(P《0.05),向枝、茎、根中分配的比例分别增加2.95%、3.39%和5.34%(P》0.05);ET处理使N向叶中分配的比例增加11.09%(P《0.05),向枝、茎、根中分配的比例分别降低0.69%、10.35%和0.05%(P》0.05).ECT处理下N的分配格局与EC处理相似.3种处理下P和K在红桦幼苗中的分配变化差异较大,CO2浓度和温度升高可能促进植物养分的积累,改变养分在植物各器官间的分配.     

Wood properties of Populus and Betula in long‐term exposure to elevated CO2 and O3

We studied the interactive effects of elevated concentrations of CO₂ and O₃ on radial growth and wood properties of four trembling aspen (Populus tremuloides Michx.) clones and paper birch (Betula papyrifera Marsh.) saplings. The material for the study was collected from the Aspen FACE (free‐air CO₂ enrichment) experiment in Rhinelander (WI, USA). Trees had been exposed to four treatments [control, elevated CO₂ (560 ppm), elevated O₃ (1.5 times ambient) and combined CO₂ + O₃] during growing seasons 1998–2008. Most treatment responses were observed in the early phase of experiment. Our results show that the CO₂‐ and O₃‐exposed aspen trees displayed a differential balance between efficiency and safety of water transport. Under elevated CO₂, radial growth was enhanced and the trees had fewer but hydraulically more efficient larger diameter vessels. In contrast, elevated O₃ decreased radial growth and the diameters of vessels and fibres. Clone‐specific decrease in wood density and cell wall thickness was observed under elevated CO₂. In birch, the treatments had no major impacts on wood anatomy or wood density. Our study indicates that short‐term impact studies conducted with young seedlings may not give a realistic view of long‐term ecosystem responses.     

Stomatal conductance, photosynthesis and respiration of temperate deciduous tree seedlings grown outdoors at an elevated concentration of carbon dioxide

Seedlings of temperate deciduous tree species were grown outdoors at ambient and at an elevated concentration of carbon dioxide to examine how aspects of their gas exchange would be altered by growth at elevated carbon dioxide concentration. Leaf conductances to water vapour and net carbon dioxide exchange rates were determined periodically near midday. Whole-plant carbon dioxide efflux rates in darkness were also determined. The stomatal conductance of leaves of plants grown and measured at 700 cm³ m^?3 carbon dioxide did not differ from that of plants grown and measured at 350 cm³ m^?3 in Malus domestica, Quercus prinus and Quercus robur at any measurement time. In Acer saccharinum, lower conductances occurred for plants grown and measured at elevated carbon dioxide concentration only at measurement temperatures above 33°C. Photo-synthetic adjustment to elevated carbon dioxide concentration was evident only in Q. robur. All species examined had lower rates of dark respiration per unit of mass when grown and measured at elevated carbon dioxide concentration.     

The effects of nitrogen fertilisation and elevated CO2 on the lipid biosynthesis and carbon isotopic discrimination in birch seedlings (Betula pendula)

The effects of nitrogen (N) fertilisation and elevated [CO₂] on lipid biosynthesis and carbon isotope discrimination in birch (Betula pendula Roth.) transplants were evaluated using seedlings grown with and without N fertiliser, and under two concentrations of atmospheric CO₂ (ambient and ambient+250 μmol mol^-1) in solar dome systems. N fertilisation decreased n-fatty acid chain length (18:0/16:0) and the ratios of α-linolenate (18:2)/linoleate (18:1), whereas elevated [CO₂] showed little effect on n-fatty acid chain length, but decreased the unsaturation (18:2+18:1)/18:0. Both N fertilisation and elevated [CO₂] increased the quantity of leaf wax n-alkanes, whilst reducing that of n-alkanols by 20–50%, but had no simple response in fatty acid concentrations. ¹³C enrichment by 1–2.5‰ under N fertilisation was observed, and can be attributed to both reduced leaf conductance and increased photosynthetic consumption of CO₂. Individual n-alkyl lipids of different chain length show consistent pattern of δ¹³C values within each homologue, but are in general 5–8‰ more depleted in ¹³C than the bulk tissues. Niether nitrogen fertilisation and elevated CO₂ influenced the relationship between carbon isotope discrimination of the bulk tissue and the individual lipids. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impacts of elevated atmospheric CO2 and O3 on paper birch (Betula papyrifera): reproductive fitness

Atmospheric CO2 and tropospheric O3 are rising in many regions of the world. Little is known about how these two commonly co-occurring gases will affect reproductive fitness of important forest tree species. Here, we report on the long-term effects of CO2 and O3 for paper birch seedlings exposed for nearly their entire life history at the Aspen FACE (Free Air Carbon Dioxide Enrichment) site in Rhinelander, WI. Elevated CO2 increased both male and female flower production, while elevated O3 increased female flower production compared to trees in control rings. Interestingly, very little flowering has yet occurred in combined treatment. Elevated CO2 had significant positive effect on birch catkin size, weight, and germination success rate (elevated CO2 increased germination rate of birch by 110% compared to ambient CO2 concentrations, decreased seedling mortality by 73%, increased seed weight by 17%, increased root length by 59%, and root-to-shoot ratio was significantly decreased, all at 3 weeks after germination), while the opposite was true of elevated O3 (elevated O3 decreased the germination rate of birch by 62%, decreased seed weight by 25%, and increased root length by 15%). Under elevated CO2, plant dry mass increased by 9 and 78% at the end of 3 and 14 weeks, respectively. Also, the root and shoot lengths, as well as the biomass of the seedlings, were increased for seeds produced under elevated CO2, while the reverse was true for seedlings from seeds produced under the elevated O3. Similar trends in treatment differences were observed in seed characteristics, germination, and seedling development for seeds collected in both 2004 and 2005. Our results suggest that elevated CO2 and O3 can dramatically affect flowering, seed production, and seed quality of paper birch, affecting reproductive fitness of this species.     

The effects of elevated atmospheric CO2 and soil P placement on cotton root deployment

Root proliferation into nutrient rich zones is an important mechanism in the exploitation of soil nutrients by plants. No studies have examined atmospheric CO₂ effects on cotton (Gossypium hirsutum L.) root distribution as affected by localized phosphorus (P). Cotton plants were grown in a Troup sand (loamy, thermic Grossarenic Kandiudults) using 17.2-l containers placed in open top field chambers (OTC) under ambient (360 mol mol^–1) or enriched (720 mol mol^–1) atmospheric CO₂ concentrations for 40 days. Equivalent amounts of P were added (150 mg P per kg of soil) to 100, 50, 25, 12.5, and 6.25% of the total soil volume; control containers with no added P were also included. Under extremely low P (controls), cotton was unresponsive to CO₂ enrichment. In treatments with both fertilized and unfertilized soil volumes, root proliferation was greater in the unfertilized soil under elevated CO₂ conditions. Stimulation of root growth occurred in the P-fertilized soil fraction; the pattern of stimulation was similar under both CO₂ levels. Under ambient CO₂, cotton plant response was positive (shoot mass, and total root mass and length) when soil P was confined to relatively small proportions of the total soil volume (6.25 and 12.5%). However, elevated CO₂ grown plants tended to respond to P regardless of its distribution.     

Japanese maple (Acer palmatum var. Matsumurae, Aceraceae) recruitment patterns: seeds, seedlings, and saplings in relation to conspecific adult neighbors

We analyzed the spatial patterns among seeds, seedlings, saplings, and conspecific adult trees of the cool-temperate tree species Acer palmatum var. Matsumurae in a conifer-hardwood mixed forest in northern Japan, using two models that consider the influence of each adult within the neighborhood of the offspring. The results showed that recruitment patterns of each stage could be characterized and that significant shifts occur between successive stages. Sound seeds were more widely dispersed than unsound seeds; the mean dispersal distance (MDD) was 41.5 m for sound seeds, but only 12.6 m for unsound seeds. Most seedlings were located near conspecific adult trees, with a MDD of 14.3 m. Saplings, however, were more dispersed away from conspecific adult trees, with an MDD of more than 35 m. Light and gap distributions did not strongly affect the spatial distribution of the offspring; most saplings were located under nonconspecific canopies. These results suggest that the recruitment pattern of Japanese maple offspring is strongly affected by conspecific adult neighbors, rather than by light and gap distributions, with close proximity to conspecific adult trees reducing the growth and survival of seedlings during the transition to saplings.     

Performance of the invasive weevil Polydrusus sericeus is influenced by atmospheric CO2 and host species

• 1 Natural forest systems constitute a major portion of the world's land area, and are subject to the potentially negative effects of both global climate change and invasion by exotic insects. A suite of invasive weevils has become established in the northern hardwood forests of North America. How these insects will respond to increasing CO₂ or O₃ is unknown.
• 2 The present study examined the effects of elevated atmospheric CO₂ and O₃ on the invasive weevil Polydrusus sericeus Schaller at the Aspen Free Air CO₂ Enrichment (FACE) site near Rhinelander, Wisconsin. A performance assay was conducted in the laboratory during the summer of 2007 using mated pairs of P. sericeus fed a combination of aspen, birch and maple foliage. We recorded leaf area consumption, oviposition and adult longevity. We also conducted visual abundance surveys in the field from 2004 to 2007 on aspen and birch at Aspen FACE.
• 3 Elevated CO₂, but not O₃, significantly affected P. sericeus performance. Female, but not male, longevity was reduced under elevated CO₂. Polydrusus sericeus also produced fewer eggs under elevated CO₂ conditions compared with ambient conditions. Adult P. sericeus strongly preferred birch over both aspen and maple, regardless of fumigation treatment.
• 4 The effects of elevated CO₂ on P. sericeus populations at Aspen FACE were minimal, and varied among years and host tree species. Polydrusus sericeus abundance was significantly greater on birch than aspen. Over the long term, elevated CO₂ may reduce adult female longevity and fecundity of P. sericeus. Further studies are needed to evaluate how this information may scale to ecosystem impacts.

     

Pollination dynamics of arctic dwarf birch (Betula glandulosa; Betulaceae) and its role in the loss of seed production

At the northern limit of its distribution, arctic dwarf birch, Betula glandulosa, shows nearly complete absence of sexual reproduction (i.e., <;0.5% viable samaras) and maintains its populations vegetatively. To investigate the possible role of pollination dynamics in the loss of sexual reproduction, pollination biology of arctic dwarf birch was compared at two locations: Tarr Inlet on Baffin Island, Northwest Territories near the northern limit of the species, and Kuujjuaq, Quebec in the center of its distribution where sexual reproduction is the primary mode of reproduction. Relative production of staminate and pistillate flowers, pollen rain, pollen viability, and stigmatic pollen loads throughout pollen dispersal were compared. Plants at Tarr Inlet produced 15%–30% of pollen produced at Kuujjuaq, both as a result of a lower density of staminate catkins and less pollen per catkin. Potential seed productivity is limited at the northern limit because pistillate catkins produce 50% fewer flowers in the north than in the south. While stigmatic pollen loads were similar at both sites, lower pollen viability (68% vs. 93%) and a higher probability of geitonogamous pollen due to clonal growth pattern reduced fertilization success at the northern site. These data suggest that lack of sexual reproduction in B. glandulosa at its northern limit is in part due to pollen limitation.     

Effects of root medium pH on root water transport and apoplastic pH in red‐osier dogwood (Cornus sericea) and paper birch (Betula papyrifera) seedlings

• Soil pH is a major factor affecting plant growth. Plant responses to pH conditions widely vary between different species of plants. However, the exact mechanisms of high pH tolerance of plants are largely unknown. In the present study, we compared the pH responses of paper birch (Betula papyrifera) seedlings, a relatively sensitive species to high soil pH, with red‐osier dogwood (Cornus sericea), reported to be relatively tolerant of high pH conditions. We examined the hypotheses that tolerance of plants to high root zone pH is linked to effective control of root apoplastic pH to facilitate nutrient and water transport processes
• In the study, we exposed paper birch and red‐osier dogwood seedlings for six weeks to pH 5, 7 and 9 under controlled‐environment conditions in hydroponic culture. Then, we measured biomass, gas exchange, root hydraulic conductivity, ferric chelate reductase (FCR) activity, xylem sap pH and the relative abundance of major elements in leaf protoplasts and apoplasts.
• The study sheds new light on the rarely studied high pH tolerance mechanisms in plants. We found that compared with paper birch, red‐osier dogwood showed greater growth, higher gas exchange, and maintained higher root hydraulic conductivity as well as lower xylem sap pH under high pH conditions.
• The results suggest that the relatively high pH tolerance of dogwood is associated with greater water uptake ability and maintenance of low apoplastic pH. These traits may have a significant impact on the uptake of Fe and Mn by leaf cells.

     

The effects of chamber pressurization on soil-surface CO2 flux and the implications for NEE measurements under elevated CO2

Soil and ecosystem trace gas fluxes are commonly measured using the dynamic chamber technique. Although the chamber pressure anomalies associated with this method are known to be a source of error, their effects have not been fully characterized. In this study, we use results from soil gas-exchange experiments and a soil CO₂ transport model to characterize the effects of chamber pressure on soil CO₂ efflux in an annual California grassland. For greater than ambient chamber pressures, experimental data show that soil-surface CO₂ flux decreases as a nonlinear function of increasing chamber pressure; this decrease is larger for drier soils. In dry soil, a gauge pressure of 0.5 Pa reduced the measured soil CO₂ efflux by roughly 70% relative to the control measurement at ambient pressure. Results from the soil CO₂ transport model show that pressurizing the flux chamber above ambient pressure effectively flushes CO₂ from the soil by generating a downward flow of air through the soil air-filled pore space. This advective flow of air reduces the CO₂ concentration gradient across the soil–atmosphere interface, resulting in a smaller diffusive flux into the chamber head space. Simulations also show that the reduction in diffusive flux is a function of chamber pressure, soil moisture, soil texture, the depth distribution of soil CO₂ generation, and chamber diameter. These results highlight the need for caution in the interpretation of dynamic chamber trace gas flux measurements. A portion of the frequently observed increase in net ecosystem carbon uptake under elevated CO₂ may be an artifact resulting from the impact of chamber pressurization on soil CO₂ efflux.     

Elevated CO(2) and elevated temperature have no effect on Douglas-fir fine-root dynamics in nitrogen-poor soil

Here, we investigate fine-root production, mortality and standing crop of Douglas-fir (Pseudotsuga menziesii) seedlings exposed to elevated atmospheric CO(2) and elevated air temperature. We hypothesized that these treatments would increase fine-root production, but that mortality would be greater under elevated temperature, leading to a smaller increase in standing crop. Seedlings were grown in outdoor, sun-lit controlled-environment chambers containing native soil. They were exposed in a factorial design to two levels of atmospheric CO(2) and two levels of air temperature. Minirhizotron methods were used to measure fine-root length production, mortality and standing crop every 4 wk for 36 months. Neither elevated atmospheric CO(2) nor elevated air temperature affected fine-root production, mortality, or standing crop. Fine roots appeared to root deeper in the soil profile under elevated CO(2) and elevated temperature. Low soil nitrogen (N) levels apparently limited root responses to the treatments. This suggests that forests on nutrient-poor soils may exhibit limited fine-root responses to elevated atmospheric CO(2) and elevated air temperature.     

不同密度红桦幼苗苗冠结构与竞争对CO2浓度升高的响应

研究了两个种植密度下,红桦（Betula albosinensis）苗冠结构特征对CO2浓度的响应,在此基础上探讨了CO2浓度升高对植物竞争压力的影响。结果表明,冠幅、冠高、苗冠表面积和苗冠体积均受CO2浓度升高的影响而增加,但是受密度增加的影响而降低。CO2浓度升高对苗冠的促进效应在低密度条件下大于高密度处理,高密度条件下苗冠基本特征部分地受到CO2浓度升高的促进作用;升高种植密度的效应则在高CO2浓度条件下大于现行CO2浓度处理。高CO2浓度和高密度条件下,LDcpa（单位苗冠投影面积叶片数）、LDcv（单位苗冠体积叶片数）和苗冠底部枝条的枝角均低于相应的现行CO2浓度处理和低密度处理,这主要是由于冠幅和冠高的快速生长所造成的。升高CO2浓度对枝条长度的影响与枝条在主茎上所处位置有关。总之,升高CO2浓度有利于降低增加种植密度对苗冠所带来的负效应,而增加种植密度降低了升高CO2浓度的正效应。LDcpa和LDcv的降低表明,红桦在升高CO2浓度和种植密度的条件下,会作出积极的响应,从而缓解由于生长的增加所带来的竞争压力的增加。     

Quantification of soil carbon inputs under elevated CO2: C3 plants in a C4 soil

The objective of this investigation was to quantify the differences in soil carbon stores after exposure of birch seedlings (Betula pendula Roth.) over one growing season to ambient and elevated carbon dioxide concentrations. One-year-old seedling of birch were transplanted to pots containing C₄ soil derived from beneath a maize crop, and placed in ambient (350 L L^–1) and elevated (600 L L^–1) plots in a free-air carbon dioxide enrichment (FACE) experiment. After 186 days the plants and soils were destructively sampled, and analysed for differences in root and stem biomass, total plant tissue and soil C contents and ¹³C values. The trees showed a significant increase (+50%) in root biomass, but stem and leaf biomasses were not significantly affected by treatment. C isotope analyses of leaves and fine roots showed that the isotopic signal from the ambient and elevated CO₂ supply was sufficiently distinct from that of the C₄ soil to enable quantification of net root C input to the soil under both ambient and elevated CO₂. After 186 days, the pots under ambient conditions contained 3.5 g of C as intact root material, and had gained an additional 0.6 g C added to the soil through root exudation/turnover; comparable figures for the pots under elevated CO₂ were 5.9 g C and 1.5 g C, respectively. These data confirm the importance of soils as an enhanced sink for C under elevated atmospheric CO₂ concentrations. We propose the use of C₄ soils in elevated CO₂ experiments as an important technique for the quantification of root net C inputs under both ambient and elevated CO₂ treatments.     

Combined effects of CO2 concentration and nutrient status on the biomass production and nutrient uptake of birch seedlings (Betula pendula)

Birch seedlings (Betula pendula) were grown for four months in a greenhouse at three nutrient levels (fertilization of 0, 100 and 500 kg ha^-1 monthy) and at four CO₂ concentrations (350, 700, 1050 and 1400 ppm). The effect of CO₂ concentration on the biomass production depended on the nutrient status. When mineralization of the soil material was the only source of nutrients (0 kg ha^-1), CO₂ enhancement reduced the biomass production slightly, whereas the highest production increase occurred at a fertilization of 100 kg ha^-1, being over 100% between 350 and 700 ppm CO₂. At 500 kg ha^-1 the production increase was smaller, and the production decreased beyond a CO₂ concentration of 700 ppm. The CO₂ concentration had a slight effect on the biomass distribution, the leaves accounting for the highest proportion at the lowest CO₂ concentration (350 ppm). An increase in nutrient status led to a longer growth period and increased the nutrient concentrations in the plants, but the CO₂ concentration had no effect on the growth rhythm and higher CO₂ reduced the nutrient concentrations.     

Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3- enriched atmospheres

Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open‐top chambers over 3 years (age 7–9 years). The treatments were increased CO₂ concentration (+CO₂, 72 Pa), increased O₃ concentration (+O₃, 2 × ambient O₃ with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO₂ + O₃). Thirty‐seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer‐operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b₆f, Rubisco specificity and K_m (CO₂) were not influenced by treatments. Net photosynthetic rate responded to +CO₂ with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO₂ in clones K1659 and V5952, respectively. The elevated O₃ caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O₃ had less effect in the presence of +CO₂ than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O₃ treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.