Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4 year experiment in the field

Forest trees are major components of the terrestrial biome and their response to rising atmospheric CO₂ plays a prominent role in the global carbon cycle. In this study, loblolly pine seedlings were planted in the field in recently disturbed soil of high fertility, and CO₂ partial pressures were maintained at ambient CO₂ (Amb) and elevated CO₂ (Amb + 30 Pa) for 4 years. The objective of the study was to measure seasonal and long-term responses in growth and photosynthesis of loblolly pine exposed to elevated CO₂ under ambient field conditions of precipitation, light, temperature and nutrient availability. Loblolly pine trees grown in elevated CO₂ produced 90% more biomass after four growing seasons than did trees grown in ambient CO₂. This large increase in final biomass was primarily due to a 217% increase in leaf area in the first growing season which resulted in much higher relative growth rates for trees grown in elevated CO₂. Although there was not a sustained effect of elevated CO₂ on relative growth rate after the first growing season, absolute production of biomass continued to increase each year in trees grown in elevated CO₂ as a consequence of the compound interest effect of increased leaf area on the production of more new leaf area and more biomass. Allometric analyses of biomass allocation patterns demonstrated size-dependent shifts in allocation, but no direct effects of elevated CO₂ on partitioning of biomass. Leaf photosynthetic rates were always higher in trees grown in elevated CO₂, but these differences were greater in the summer (60–130% increase) than in the winter (14–44% increase), reflecting strong seasonal effects of temperature on photosynthesis. Our results suggest that seasonal variation in the relative photosynthetic response to elevated CO₂ will occur in natural ecosystems, but total non-structural carbohydrate (TNC) levels in leaves indicate that this variation may not always be related to sink activity. Despite indications of canopy-level adjustments in carbon assimilation, enhanced levels of leaf photosynthesis coupled with increased total leaf area indicate that net carbon assimilation for the whole tree was greater for trees grown under elevated CO₂ compared with ambient CO₂. If the large growth enhancement observed in loblolly pine were maintained after canopy closure, then these trees could be a large sink for fossil carbon emitted to the atmosphere and produce a negative feedback on atmospheric CO₂.     

Elevated CO2 and tree fecundity: the role of tree size, interannual variability, and population heterogeneity

Long‐term population effects of changes in atmospheric CO₂ will be largely determined by reproductive effort. Our research objectives were to quantify variability in seed production and rate of maturation among individual Pinus taeda L. (Pinaceae) trees growing in elevated CO₂ (ambient plus 200 μL L^?1) since 1996. Estimating tree fecundity in nature is frustrated by the difficulty of counting seeds from individual trees and the need for long‐term data. We have used a hierarchical Bayes approach to model individual tree fecundity, accounting for the complexity of experimentation in a natural setting over multiple years. The study presented here demonstrates large variability in natural fecundity rates and contributes to our understanding of how both interannual variation and population heterogeneity influence elevated CO₂ effects. We found that trees growing under elevated CO₂ matured earlier and produced more seeds and cones per unit basal area than ambient grown trees. By 2004, trees grown in high CO₂ had produced an average 300 more seeds per tree than ambient grown trees. Although there was a trend toward decreasing mean CO₂ effect (difference in fecundity between elevated and ambient treatments) over time, the hierarchical analysis indicates that this decrease comes from the emergence of a few highly fecund ambient grown trees by 2002, rather than acclimation or downregulation among the fumigated trees. The most important effect of increased CO₂ in forest ecosystems may be the increase in fecundity reported here. Although biomass responses can sometimes be large, the increase in fecundity can have long‐term impacts on forest dynamics that transcend the current generation.     

Carbon balance of young birch trees grown in ambient and elevated atmospheric CO2 concentrations

We constructed a carbon budget for young birch trees grown in ambient and elevated CO₂ concentrations over their fourth year of growth. The annual total of net leaf photosynthesis was 110% more in elevated CO₂ than in ambient CO₂. However, the trees in elevated CO₂ grew only 59% more biomass than the trees in ambient CO₂ over the year. Modelling studies showed that larger loss of carbon from fine-root production and growth of the root-associated mycorrhiza by the trees in elevated CO₂ probably accounted for all the remaining difference in net photosynthesis between the two treatments. Our modelling also showed that the fraction of net photosynthate consumed by respiration of nonleaf tissue was similar in the two CO₂ treatments, and was 26% and 24% for trees in ambient and elevated CO₂, respectively. Trees in elevated CO₂ had 43% more leaves, and produced 110% more net photosynthate than trees in ambient CO₂, even though the maximum rate of carboxylation per unit leaf nitrogen decreased by 21%. Sensitivity studies showed that down-regulation reduced the annual net photosynthetic production of the trees in elevated CO₂ by only 6%. Direct effects of higher CO₂ on photosynthesis and greater leaf area of the trees in elevated CO₂ increased the net photosynthesis of the trees by 68% and 60%, respectively; and together accounted for most of the difference in net photosynthesis between the two treatments.     

Effects of Atmospheric CO(2) Enrichment on Photosynthesis, Respiration, and Growth of Sour Orange Trees

Numerous net photosynthetic and dark respiratory measurements were made over a period of 4 years on leaves of 24 sour orange (Citrus aurantium) trees; 8 of them growing in ambient air at a mean CO₂ concentration of 400 microliters per liter, and 16 growing in air enriched with CO₂ to concentrations approaching 1000 microliters per liter. Over this CO₂ concentration range, net photosynthesis increased linearly with CO₂ by more than 200%, whereas dark respiration decreased linearly to only 20% of its initial value. These results, together with those of a comprehensive fine-root biomass determination and two independent aboveground trunk and branch volume inventories, suggest that a doubling of the air's current mean CO₂ concentration of 360 microliters per liter would enhance the growth of the trees by a factor of 3.8.     

A branch bag and CO2 control system for long-term CO2 enrichment of mature Sitka spruce [Picea sitchensis (Bong.) Carr.]

This paper describes the construction and performance of branch bags and a CO₂ control system used to fumigate branches of mature Sitka spruce trees with air enriched in CO₂ (700 μmolmol^-1). It contains some examples of results obtained using the system over the course of the first two growing seasons. The branch bags have run continuously for 2 years with very few problems. CO₂ concentrations were within 20 μmol mol^-1 of the target concentration for more than 90% of the time. Temperatures within the bags were slightly higher than ambient (1–2 °C) and this had some effect on phenology. Attenuation of quantum flux density (photosynthetically active radiation) was 10–15%. The branch bag system has enabled investigation into the effects of elevated CO₂ on mature tissue without the problems and expense of fumigating whole trees. Growth in elevated CO₂ resulted in an increase in starch and a decrease in soluble protein content of needles. Stomatal conductance was higher in elevated CO₂ grown needles, and there was some evidence of an increase in photosynthetic capacity.     

Interactive effects of elevated atmospheric CO2, mycorrhization and drought on long-distance transport of reduced sulphur in young pedunculate oak trees (Quercus robur L.)

Pedunculate oak (Quercus robur L.) was germinated and grown under nutrient non-limiting conditions for a total of 10–15 weeks at ambient CO₂ concentration and 1100 μmol mol^–1 CO₂ either in the presence or the absence of the mycorrhizal fungus Laccaria laccata. Half of the oak trees of these treatments were exposed to drought during final growth by suspending the water supply for 21 d. Mycorrhization and elevated atmospheric CO₂ each enhanced total plant biomass per tree. Whereas additional biomass accumulation of trees grown under elevated CO₂ was mainly attributed to increased growth of lateral roots, mycorrhization promoted shoot growth. Water deficiency reduced biomass accumulation without affecting relative water content, but this effect was more pronounced in mycorrhizal as compared to non-mycorrhizal trees. Elevated CO₂ partially prevented the development of drought stress, as indicated by leaf water potential, but did not counteract the negative effects of water deficiency on growth during the time studied. Enhanced biomass accumulation requires adaption in protein synthesis and, as a consequence, enhanced allocation of reduced sulphur produced in the leaves to growing tissues. Therefore, allocation of reduced sulphur from oak leaves was studied by flap-feeding radiolabelled GSH, the main long-distance transport form of reduced sulphur, to mature oak leaves. Export of radiolabel proceeded almost exclusively in basipetal direction to the roots. The rate of export of radioactivity out of the fed leaves was significantly enhanced under elevated CO₂, irrespective of mycorrhization. A higher proportion of the exported GSH was transported to the roots than to basipetal stem sections under elevated CO₂ as compared to ambient CO₂. Mycorrhization did not affect ³⁵S export out of the fed leaves, but the distribution of radiolabel between stem and roots was altered in preference of the stem. Trees exposed to drought did not show appreciable export of the ³⁵S radioactivity fed to the leaves when grown under ambient CO₂. Apparently, drought inhibited basipetal transport of reduced sulphur at the level of phloem loading and/or phloem transport. Elevated CO₂ seemed to counteract this effect of drought stress to some extent, since higher leaf water potentials and improved ³⁵S export out of the fed leaves was observed in oak trees exposed to drought and elevated CO₂ as compared to trees exposed to drought and ambient CO₂.     

Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water status

The long-term interaction between elevated CO₂ and soil water deficit was analysed in N₂-fixing alfalfa plants in order to assess the possible drought tolerance effect of CO₂. Elevated CO₂ could delay the onset of drought stress by decreasing transpiration rates, but this effect was avoided by subjecting plants to the same soil water content. Nodulated alfalfa plants subjected to ambient (400 μmol mol^?1) or elevated (700 μmol mol^?1) CO₂ were either well watered or partially watered by restricting water to obtain 30% of the water content at field capacity (ampproximately 0.55 g water cm^?3). The negative effects of soil water deficit on plant growth were counterbalanced by elevated CO₂. In droughted plants, elevated CO₂ stimulated carbon fixation and, as a result, biomass production was even greater than in well-watered plants grown in ambient CO₂. Below-ground production was preferentially stimulated by elevated CO₂ in droughted plants, increasing nodule biomass production and the availability of photosynthates to the nodules. As a result, total nitrogen content in droughted plants was higher than in well-watered plants grown in ambient CO₂. The beneficial effect of elevated CO₂ was not correlated with a better plant water status. It is concluded that elevated CO₂ enhances growth of droughted plants by stimulating carbon fixation, preferentially increasing the availability of photosynthates to below-ground production (roots and nodules) without improving water status. This means that elevated CO₂ enhances the ability to produce more biomass in N₂-fixing alfalfa under given soil water stress, improving drought tolerance.     

Carbon partitioning to mobile and structural fractions in poplar wood under elevated CO2 (EUROFACE) and N fertilization

To determine whether globally increasing atmospheric carbon dioxide (CO₂) concentrations can affect carbon partitioning between nonstructural and structural carbon pools in agroforestry plantations, Populus nigra was grown in ambient air (about 370 μmol mol^?1 CO₂) and in air with elevated CO₂ concentrations (about 550 μmol mol^?1 CO₂) using free‐air CO₂ enrichment (FACE) technology. FACE was maintained for 5 years. After three growing seasons, the plantation was coppiced and one half of each experimental plot was fertilized with nitrogen. Carbon concentrations and stocks were measured in secondary sprouts in seasons of active growth and dormancy during 2 years after coppicing. Although FACE, N fertilization and season had significant tissue‐specific effects on carbon partitioning to the fractions of structural carbon, soluble sugars and starch as well as to residual soluble carbon, the overall magnitude of these shifts was small. The major effect of FACE and N fertilization was on cell wall biomass production, resulting in about 30% increased above ground stocks of both mobile and immobile carbon pools compared with fertilized trees under ambient CO₂. Relative C partitioning between mobile and immobile C pools was not significantly affected by FACE or N fertilization. These data demonstrate high metabolic flexibility of P. nigra to maintain C‐homeostasis under changing environmental conditions and illustrate that nonstructural carbon compounds can be utilized more rapidly for structural growth under elevated atmospheric [CO₂] in fertilized agroforestry systems. Thus, structural biomass production on abandoned agricultural land may contribute to achieving the goals of the Kyoto protocol.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

CO2 and N-fertilization effects on fine-root length, production, and mortality: a 4-year ponderosa pine study

We conducted a 4-year study of juvenile Pinus ponderosa fine root (≤2 mm) responses to atmospheric CO₂ and N-fertilization. Seedlings were grown in open-top chambers at three CO₂ levels (ambient, ambient+175 μmol/mol, ambient+350 μmol/mol) and three N-fertilization levels (0, 10, 20 g m⁻² year⁻¹). Length and width of individual roots were measured from minirhizotron video images bimonthly over 4 years starting when the seedlings were 1.5 years old. Neither CO₂ nor N-fertilization treatments affected the seasonal patterns of root production or mortality. Yearly values of fine-root length standing crop (m m⁻²), production (m m⁻² year⁻¹), and mortality (m m⁻² year⁻¹) were consistently higher in elevated CO₂ treatments throughout the study, except for mortality in the first year; however, the only statistically significant CO₂ effects were in the fine-root length standing crop (m m⁻²) in the second and third years, and production and mortality (m m⁻² year⁻¹) in the third year. Higher mortality (m m⁻² year⁻¹) in elevated CO₂ was due to greater standing crop rather than shorter life span, as fine roots lived longer in elevated CO₂. No significant N effects were noted for annual cumulative production, cumulative mortality, or mean standing crop. N availability did not significantly affect responses of fine-root standing crop, production, or mortality to elevated CO₂. Multi-year studies at all life stages of trees are important to characterize belowground responses to factors such as atmospheric CO₂ and N-fertilization. This study showed the potential for juvenile ponderosa pine to increase fine-root C pools and C fluxes through root mortality in response to elevated CO₂.     

Changes in net photosynthesis and growth of Pinus eldarica seedlings in response to atmospheric CO2 enrichment

Pinus eldarica L. trees, rooted in the natural soil of an agricultural field at Phoenix, Arizona, were grown from the seedling stage in clear-plastic-wall open-top enclosures maintained at four different atmospheric CO₂ concentrations for 15 months. Light response functions were determined for one tree from each treatment by means of whole-tree net CO₂ exchange measurements at the end of this period, after which rates of carbon assimilation of an ambient-treatment tree were measured across a range of atmospheric CO₂ concentrations. The first of these data sets incorporates the consequences of both the CO₂-induced enhancement of net photosynthesis per unit needle area and the CO₂-induced enhancement of needle area itself (due primarily to the production of more needles), whereas the second data set reflects only the first of these effects. Hence the division of the normalized results of the first data set by the normalized results of the second set yields a representation of the increase in whole-tree net photosynthesis due to enhanced needle production caused by atmospheric CO₂ enrichment. In the solitary trees we studied, the relative contribution of this effect increased rapidly with the CO₂ concentration of the air to increase whole-tree net photosynthesis by nearly 50% at a CO₂ concentration approximately 300 μmol mol⁻¹ above ambient.     

Response of Norway spruce root system to elevated atmospheric CO2 concentration

Root structure parameters, root biomass and allometric relationships between above- and belowground biomass were investigated in young Norway spruce (Picea abies [L.] Karst.) trees cultivated inside the glass domes with ambient (AC, 375 μmol(CO₂) mol^?1) and elevated (EC, A + 375 μmol(CO₂) mol^?1) atmospheric CO₂ concentrations ([CO₂]). After 8 years of fumigation, a mean EC tree in comparison with AC one exhibited about 37 % higher belowground biomass. The growth of primary root structure was unaffected by elevated [CO₂]; however, the biomass of secondary roots growing on the primary root structure and the biomass of secondary roots growing in the zone between the soil surface and the first primary root ramification were significantly higher in EC comparing with AC treatment about 58 and 70 %, respectively. The finest root’s (diameter up to 1 mm) biomass as well as length and surface area of both primary and secondary root structures showed the highest difference between the treatments; advancing EC to AC by 43 % on average. Therefore, Norway spruce trees cultivated under well-watered and rather nitrogen-poor soil conditions responded to the air elevated [CO₂] environment by the enhancement of the secondary root structure increment, by enlargement of root length and root absorbing area, and also by alternation of root to aboveground organ biomass proportion. Higher root to leaf and root to stem basal area ratios could be beneficial for Norway spruce trees to survive periods with limited soil water availability.     

Effect of CO2 enrichment and elevated temperature on methane emissions from rice, Oryza sativa

Methane emissions from rice grown within Temperature Gradient Greenhouse Tunnels under doubled CO₂ concentrations were 10–45 times less than emissions from control plants grown under ambient CO₂. For two cultivars of rice (cvs. Lemont and IR-72), methane emissions increased with a temperature increase of 2°, from outdoor ambient temperatures to the first cell of the ambient CO₂ tunnel (ambient temperature + 2 °C). Within both tunnels and for both cultivars methane emissions decreased with further temperature increases (from 2° to 5 °C above ambient). Carbon dioxide enrichment stimulated both above- and below-ground production. Our original hypothesis was that increased CO₂ would stimulate plant productivity and therefore stimulate methane emission, since direct linkages between these parameters have been observed. We hypothesize that CO₂ enrichment led to the attenuation of methane production due to increased delivery of oxygen to the rhizosphere because of increased root biomass and porosity. The increased root biomass due to elevated CO₂ may have more effectively aerated the soil, suppressing methane production. However, this study may be unique because the low organic content (< 1%) of the sandy soils in which the rice was grown created very little oxygen demand.     

Growth,loss, and vertical distribution of Pinus radiata fine roots growing at ambient and elevated CO2 concentration

Increased below-ground carbon allocation in forest ecosystems is a likely consequence of rising atmospheric CO₂ concentration. If this results in changes to fine root growth, turnover and distribution long-term soil carbon cycling and storage could be altered. Bi-weekly measurements were made to determine the dynamics and distribution of fine roots (< 1 mm diameter) for Pinus radiata trees growing at ambient (350 μmol mol^–1) and elevated (650 μmol mol^–1) CO₂ concentration in large open-top chambers. Measurements were made using minirhizotrons installed horizontally at depths of 0.1, 0.3, 0.5 and 0.9 m. During the first year, at a depth of 0.3 m, the increase in relative growth rate of roots occurred 6 weeks earlier in the elevated CO₂ treatment and the maximum rate was reached 10 weeks earlier than for trees in the ambient treatment. After 2 years, cumulative fine root growth (P_t) was 36% greater for trees growing at elevated CO₂ than at ambient CO₂ concentration, although this difference was not significant. A model of root growth driven by daily soil temperature accounted for between 43 and 99% of root growth variability. Total root loss (L_t) was 9% in the ambient and 14% in the elevated CO₂ treatment, although this difference was not significant. Root loss was greatest at 0.3 m. In the first year, 62% of fine roots grown between mid-summer and late-autumn disappeared within a year in the elevated CO₂ treatment, but only 18% in the ambient CO₂ treatment (P < 0.01). An exponential model relating L_t to time accounted for between 74 and 99% of the variability. Root cohort half-lives were 951 d for the ambient and 333 d for the elevated treatment. Root length density decreased exponentially with depth in both treatments, but relatively more fine roots grown in the elevated CO₂ treatment tended to occur deeper in the soil profile.     

Tree species diversity interacts with elevated CO2 to induce a greater root system response

As a consequence of land‐use change and the burning of fossil fuels, atmospheric concentrations of CO₂ are increasing and altering the dynamics of the carbon cycle in forest ecosystems. In a number of studies using single tree species, fine root biomass has been shown to be strongly increased by elevated CO₂. However, natural forests are often intimate mixtures of a number of co‐occurring species. To investigate the interaction between tree mixture and elevated CO₂, Alnus glutinosa, Betula pendula and Fagus sylvatica were planted in areas of single species and a three species polyculture in a free‐air CO₂ enrichment study (BangorFACE). The trees were exposed to ambient or elevated CO₂ (580 μmol mol^?1) for 4 years. Fine and coarse root biomass, together with fine root turnover and fine root morphological characteristics were measured. Fine root biomass and morphology responded differentially to the elevated CO₂ at different soil depths in the three species when grown in monocultures. In polyculture, a greater response to elevated CO₂ was observed in coarse roots to a depth of 20 cm, and fine root area index to a depth of 30 cm. Total fine root biomass was positively affected by elevated CO₂ at the end of the experiment, but not by species diversity. Our data suggest that existing biogeochemical cycling models parameterized with data from species grown in monoculture may be underestimating the belowground response to global change.     

An overview of long-term effects of elevated atmospheric CO2 concentration on the growth and physiology of birch (Betula pendula Roth.)

Summary

An extensive number of biochemical and physiological measurements were made over the third and fourth years of the growth of birch (Betula pendula Roth.) in elevated CO₂. Trees in elevated CO₂ had 58% more biomass than trees grown in ambient CO₂ although relative growth rate was not affected in the last year of the study. No changes in biomass allocation were observed. Elevated CO₂ caused an increase in starch accumulation in the leaves that resulted in a series of feedback mechanisms to re-establish the source-sink balance of the trees. A decrease in Rubisco activity and to a lesser extent in chlorophyll and soluble proteins led to a decrease in the photosynthetic activity. Although the positive CO ₂effect on photosynthetic activity was maintained in the field over the whole experiment, the photosynthetic capacity of the trees was reduced by long-term exposure to elevated CO₂. Both maximum electron transport capacity (J _max) and maximum carboxylation capacity (V _emax) were reduced to a similar extent, so the ratio of J _max:V _cmax was not altered. Root biomass, fine root density and mycorrhizal infection were increased in elevated CO ₂. The mycorrhizal species of fungi associated with the trees grown in elevated CO₂ were late-successional species whereas the species associated with trees grown in ambient CO₂ were early successional species. This lends support to the hypothesis of a CO₂ effect on the ontogeny of the trees.     

Poa annua shows inter-generational differences in response to elevated CO2

Inter-generational effects on the growth of Poa annua (L.) in ambient and elevated atmospheric CO₂ conditions (350 and 550 μl l^–1, respectively) were studied in two different experiments. Both experiments showed similar results. In a greenhouse experiment growth, measured as the numbers of tillers produced per week, was compared for plants grown from first and second generation seeds. Second generation seeds were obtained from plants grown for one whole generation in either ambient or elevated atmospheric CO₂ (‘ambient’ and ‘elevated’ seeds, respectively). First generation plants and second generation ‘ambient’ plants did not respond to elevated CO₂. Second generation ‘elevated’ plants produced significantly more tillers in elevated CO₂. In the second experiment model terrestrial ecosystems growing in the Ecotron and which included Poa annua were used. Above-ground biomass after one and two generations of growth were compared. At the end of Generation 1 no difference was found in biomass production while at the end of Generation 2 biomass increased in elevated CO₂ by 50%. The implications for climate change research are discussed.     

Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees with lifetime exposure to naturally elevated [CO2]

An important question with respect to plant performance in future climatic scenarios is whether the offspring of mature trees that have experienced lifelong exposure to elevated [CO₂] show altered physiological responses to elevated [CO₂] compared with those originating from current ambient CO₂ concentrations. To investigate this question, acorns were collected from two seed sources, denoted as ‘control’ and ‘spring’, from Quercus ilex mother trees grown at ambient (36 Pa) and at about twice ambient CO₂ concentrations, respectively, close to a natural CO₂ spring, Laiatico, central Italy. The seedlings were raised for 8 months under controlled conditions at ambient and elevated [CO₂] in a reciprocal experimental design and were used for the determination of biomass, photosynthesis and foliar carbohydrate concentrations, as well as the accumulation of structural biomass and lignin during leaf maturation. Under ambient [CO₂], biomass and foliar carbon acquisition in control progeny were not significantly different from spring progeny. However, under elevated [CO₂], spring seedlings showed less CO₂ acclimation than control seedlings but no significant differences in non‐structural carbohydrate concentrations and structural biomass per unit leaf dry mass. Developmental lignin accumulation in leaves was delayed under elevated [CO₂] compared with ambient [CO₂], but only in control progeny. Under elevated [CO₂], whole‐plant biomass, leaf area and stem diameter were significantly increased in Quercus ilex seedlings from both seed sources but with a higher stimulation of above‐ground biomass in spring than in control seedlings and a higher stimulation of below‐ground biomass in control seedlings. These results indicate that life history and/or progeny may determine the species‐specific CO₂ response and suggest that positive CO₂ acclimation is possible.     

The influence of elevated CO2 and O3 on fine roots and mycorrhizas of naturally growing young Scots pine trees during three exposure years

Young Scots pine trees naturally established at a pine heath were exposed to two concentrations of CO₂ (ambient and doubled ambient) and two O₃ regimes (ambient and doubled ambient) and their combination in open-top field chambers during growing seasons 1994, 1995 and 1996 (late May to 15 September). Filtered ozone treatment and chamberless control trees were also included in the treatment comparisons. Root ingrowth cores were inserted to the undisturbed soil below the branch projection of each tree at the beginning of the fumigation period in 1994 and were harvested at the end of the fumigation periods in 1995 and 1996. Root biomasses were determined from different soil layers in the ingrowth cores, and the infection levels of different mycorrhizal types were calculated. Elevated O₃ and CO₂ did not have significant effects on the biomass production of Scots pine coarse (Ø > 2 mm) or fine roots (Ø < 2 mm) and roots of grasses and dwarf shrubs. Elevated O₃ caused a transient stimulation, observable in 1995, in the proportion of tuber-like mycorrhizas, total mycorrhizas and total short roots but this stimulation disappeared during the last study year. Elevated CO₂ did not enhance carbon allocation to root growth or mycorrhiza formation, although a diminishing trend in the mycorrhiza formation was observed. In the combination treatment increased CO₂ inhibited the transient stimulating effect of ozone, and a significant increase of old mycorrhizas was observed. Our conclusion is that doubled CO₂ is not able to increase carbon allocation to growth of fine roots or mycorrhizas in nutrient poor forest sites and realistically elevated ozone does not cause a measurable limitation to roots within a period of three exposure years.     

Effects of atmospheric CO2 enrichment on regrowth of sour orange trees (Citrus aurantium; Rutaceae) after coppicing

Sixteen sour orange tree (Citrus aurantium L.) seedlings were grown out-of-doors at Phoenix, Arizona, in eight clear-plastic-wall open-top enclosures maintained at four different atmospheric CO₂ concentrations for a period of 2 years. Over the last year of this period, the trees were coppiced five times. The amount of dry matter harvested at each of these cuttings was a linear function of the atmospheric CO₂ concentration to which the trees were exposed. For a 75% increase in atmospheric CO, from 400 to 700 microliter per liter (μL liter ¹), total aboveground biomass rose, in the mean, by a factor of 3.19; while for a 400 to 800 μL liter ¹ doubling of the air's CO₂ content, it rose by a factor of 3.92. The relative summer (mean air temperature of 32.8 C) response to CO₂ was about 20% greater than the relative winter (mean air temperature of 16.4 C) response.