Effects of elevated atmospheric CO2 on fine root production and activity in an intact temperate forest ecosystem

We investigated the effects of elevated atmospheric CO₂ concentrations (ambient + 200 ppm) on fine root production and soil carbon dynamics in a loblolly pine (Pinus taeda) forest subject to free‐air CO₂ enrichment (FACE) near Durham, NC (USA). Live fine root mass (LFR) showed less seasonal variation than dead fine root mass (DFR), which was correlated with seasonal changes in soil moisture and soil temperature. LFR mass increased significantly (by 86%) in the elevated CO₂ treatment, with an increment of 37 g(dry weight) m^?2 above the control plots after two years of CO₂ fumigation. There was no long‐term increment in DFR associated with elevated CO₂, but significant seasonal accumulations of DFR mass occurred during the summer of the second year of fumigation. Overall, root net primary production (RNPP) was not significantly different, but annual carbon inputs were 21.7 gC m^?2 y^?1 (68%) higher in the elevated CO₂ treatment compared to controls. Specific root respiration was not altered by the CO₂ treatment during most of the year; however, it was significantly higher by 21% and 13% in September 1997 and May 1998, respectively, in elevated CO₂. We did not find statistically significant differences in the C/N ratio of the root tissue, root decomposition or phosphatase activity in soil and roots associated with the treatment. Our data show that the early response of a loblolly pine forest ecosystem subject to CO₂ enrichment is an increase in its fine root population and a trend towards higher total RNPP after two years of CO₂ fumigation.     

Root dynamics and demography in shortgrass steppe under elevated CO2, and comments on minirhizotron methodology

The dynamics and demography of roots were followed for 5 years that spanned wet and drought periods in native, semiarid shortgrass steppe grassland exposed to ambient and elevated atmospheric CO₂ treatments. Elevated compared with ambient CO₂ concentrations resulted in greater root‐length growth (+52%), root‐length losses (+37%), and total pool sizes (+41%). The greater standing pool of roots under elevated compared with ambient CO₂ was because of the greater number of roots (+35%), not because individuals were longer. Loss rates increased relatively less than growth rates because life spans were longer (+41%). The diameter of roots was larger under elevated compared with ambient CO₂ only in the upper soil profile. Elevated CO₂ affected root architecture through increased branching. Growth‐to‐loss ratio regressions to time of equilibrium indicate very long turnover times of 5.8, 7.0, and 5.3 years for control, ambient, and elevated CO₂, respectively. Production was greater under elevated compared with ambient CO₂ both below‐ and aboveground, and the above‐ to belowground ratios did not differ between treatments. However, estimates of belowground production differed among methods of calculation using minirhizotron data, as well as between minirhizotron and root‐ingrowth methods. Users of minirhizotrons may need to consider equilibration in terms of both new growth and disappearance, rather than just growth. Large temporal pulses of root initiation and termination rates of entire individuals were observed (analogous to birth–death rates), and precipitation explained more of the variance in root initiation than termination. There was a dampening of the pulsing in root initiation and termination under elevated CO₂ during both wet and dry periods, which may be because of conservation of soil water reducing the suddenness of wet pulses and duration and severity of dry pulses. However, a very low degree of synchrony was observed between growth and disappearance (production and decomposition).     

Elevated atmospheric CO2 in open top chambers increases net nitrification and potential denitrification

The control of soil nitrogen (N) availability under elevated atmospheric CO₂ is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO₂ on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO₂ (ambient + 400 µmol mol^?1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N₂O emissions were significantly higher in soils from the elevated CO₂ treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO₃^– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO₂ treatment in the field when incubated in the laboratory under elevated CO₂ (increase of 100%), but there was no effect when incubated under ambient CO₂. Net nitrate production of the soil originating from the ambient CO₂ treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO₂ on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO₂ through higher soil CO₂ concentrations. Alternatively, lower microbial NH₄ assimilation under elevated CO₂ might explain the higher net nitrification. We conclude that elevated atmospheric CO₂ has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO₂ concentrations compared to atmospheric concentrations.     

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 elevated atmospheric CO2, cutting frequency, and differential day/night atmospheric warming on root growth and turnover of Phalaris swards

We investigated seasonal root production and root turnover of fertilized and well‐watered monocultures of Phalaris for 2 years using minirhizotrons installed in six newly designed temperature gradient tunnels, combined with sequential soil coring. Elevated atmospheric CO₂ treatments were combined with two cutting frequencies and three warming scenarios: no warming, +3.0/+3.0 and +2.2/+4.0°C (day/night) atmospheric warming. The elevated CO₂ treatment increased both new and net root length production primarily when combined with atmospheric warming, where the constant warming treatment had a greater positive effect than the increased night‐time warming treatment. Responses to elevated CO₂ were greater when the swards were cut more frequently and responsiveness varied with season. For Phalaris swards, 17% of total net primary productivity went belowground. On account of root turnover, only one‐third of the new roots produced in the year following establishment could be expected, on average, to be recovered from soil cores. The interaction between the effects of CO₂ and warming, combined with the differential effects of the two warming treatments, has important implications for modelling belowground responses to projected climate change.     

Production, turnover and mycorrhizal colonization of root systems of three Populus species grown under elevated CO2 (POPFACE)

A fast growing high density Populus plantation located in central Italy was exposed to elevated carbon dioxide for a period of three years. An elevated CO₂ treatment (550 ppm), of 200 ppm over ambient (350 ppm) was provided using a FACE technique. Standing root biomass, fine root turnover and mycorrhizal colonization of the following Populus species was examined: Populus alba L., Populus nigra L., Populus x euramericana Dode (Guinier). Elevated CO₂ increased belowground allocation of biomass in all three species examined, standing root biomass increased by 47–76% as a result of FACE treatment. Similarly, fine root biomass present in the soil increased by 35–84%. The FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. x euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO₂. Response of the mycorrhizal community to elevated CO₂ was more varied, the rate of infection increased only in P. alba for both ectomycorrhizal (EM) and arbuscular mycorrhizas (AM). The roots of P. nigra showed greater infection only by AM and the colonization of the root system of P. x euramericana was not affected by FACE treatment. The results suggest that elevated atmospheric CO₂ conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This may have implications for C sequestration and must be taken into account when considering long‐term C storage in the soil.     

Rapid root closure after fire limits fine root responses to elevated atmospheric CO2 in a scrub oak ecosystem in central Florida, USA

Elevated atmospheric carbon dioxide (CO₂) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long‐term CO₂ exposure. We investigated the effects of elevated CO₂ on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO₂ enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO₂ would stimulate root growth; and (3) elevated CO₂ would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm^?2) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO₂ significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO₂ stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO₂ had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO₂ hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient‐poor soil – may explain the transient response to elevated CO₂.     

Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

We measured the short‐term direct and long‐term indirect effects of elevated CO₂ on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 µmol mol^?1 atmospheric CO₂ using free‐air carbon dioxide enrichment (FACE) technology. After correcting for measurement artefacts, a short‐term 200 µmol mol^?1 increase in CO₂ reduced leaf respiration by 7–14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO₂ concentration under which the trees were grown. Growth under elevated CO₂ did not appear to have any long‐term indirect effects on leaf maintenance respiration rates or the response of respiration to changes in temperature (Q₁₀, R₀). Also, we found no relationship between mass‐based respiration rates and leaf total nitrogen concentrations. Leaf construction costs were unaffected by growth CO₂ concentration, although leaf construction respiration decreased at elevated CO₂ in both species for leaves at the top of the canopy. We conclude that elevated CO₂ has little effect on leaf tissue respiration, and that the influence of elevated CO₂ on plant respiratory carbon flux is primarily through increased biomass.     

Soil CO2 concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO₂ concentration on root respiration rates during short-term CO₂ exposure, and on plant growth during long-term CO₂ exposure, have been reported. Here we examine the effects of both short- and long-term exposure to soil CO₂ on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. & Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO₂. Respiration rates of citrus roots were unaffected by the CO₂ concentration used during the respiration measurements (200 and 2000 μmol mol⁻¹), regardless of the soil CO₂, concentration during the previous month (600 and 20 000 μmol mol⁻¹). Bean plants were grown with their roots exposed to either a natural CO₂ diffusion gradient, or to an artificially maintained CO₂ concentration of 600 or 20 000 μmol mol⁻¹. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected by CO₂ pretreatment and the CO₂ concentration used during the respiration measurements (200–2000 μmol mol⁻¹). We conclude that soil CO₂ concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean.     

The effects of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth

Spring wheat [ Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO₂ concentrations, 550 μmol mol^–1 for high-CO₂ plots, and 370 μmol mol^–1 for control plots, using a Free-Air CO₂ Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO₂, there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split-plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in-row and inter-row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO₂ and soil water, in this agricultural ecosystem. Wheat root growth increased under elevated CO₂ conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO₂ enhancement until anthesis. During the early vegetative growth, root dry mass of the inter-row space was significantly higher for FACE than for Control treatments suggesting that elevated CO₂ promoted the production of first-order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter-row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO₂ and reduced soil water supports the hypothesis that plants grown in a high-CO₂ environment may better compensate soil-water-stress conditions.     

Effects of elevated CO2 on fine root dynamics in a Mojave Desert community: a FACE study

Fine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO₂ will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO₂ on production, mortality, and standing crops of fine root length over 2 years in a free‐air CO₂ enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (～370 μmol mol^?1) and elevated CO₂ (～550 μmol mol^?1) atmospheric concentrations. Twenty‐eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreen Larrea shrubs, drought‐deciduous Ambrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ～85% of the area). Seasonal dynamics were similar for ambient and elevated CO₂: fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO₂. Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year^?1, and was not significantly different among CO₂ treatments, except for community transect tubes where it was significantly lower for elevated CO₂. There were no differences in fine root responses to CO₂ between evergreen (Larrea) and drought‐deciduous (Ambrosia) shrubs. Combined with observations of increased leaf‐level water‐use efficiency and lack of soil moisture differences, these results suggest that under elevated CO₂ conditions, reduced root systems (compared with ambient CO₂) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO₂ were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO₂ treatment during periods of high water availability.     

The effects of elevated CO2 on root respiration rates of two Mojave Desert shrubs

Although desert ecosystems are predicted to be the most responsive to elevated CO₂, low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient (～375 ppm) and elevated (～517 ppm) CO₂ concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO₂ treatments to determine responses of primary and lateral roots to an increase in CO₂. In both field and greenhouse studies, root respiration was not significantly affected by elevated CO₂. However, respiration of A. dumosa roots <1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (<1.0 mm diameter) roots were significantly greater than those of fine (1–2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO₂ and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO₂. Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO₂, which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO₂ for these species does not appear to be constrained by the plant's root growth or activity.     

Responses of soil respiration to elevated CO2, air warming, and changing soil water availability in a model old-field grassland

Responses of soil respiration to atmospheric and climatic change will have profound impacts on ecosystem and global carbon (C) cycling in the future. This study was conducted to examine effects on soil respiration of the concurrent driving factors of elevated atmospheric CO₂ concentration, air warming, and changing precipitation in a constructed old‐field grassland in eastern Tennessee, USA. Model ecosystems of seven old‐field species were established in open‐top chambers and treated with factorial combinations of ambient or elevated (+300 ppm) CO₂ concentration, ambient or elevated (+3 °C) air temperature, and high or low soil moisture content. During the 19‐month experimental period from June 2003 to December 2004, higher CO₂ concentration and soil water availability significantly increased mean soil respiration by 35.8% and 15.7%, respectively. The effects of air warming on soil respiration varied seasonally from small reductions to significant increases to no response, and there was no significant main effect. In the wet side of elevated CO₂ chambers, air warming consistently caused increases in soil respiration, whereas in the other three combinations of CO₂ and water treatments, warming tended to decrease soil respiration over the growing season but increase it over the winter. There were no interactive effects on soil respiration among any two or three treatment factors irrespective of time period. Treatment‐induced changes in soil temperature and moisture together explained 49%, 44%, and 56% of the seasonal variations of soil respiration responses to elevated CO₂, air warming, and changing precipitation, respectively. Additional indirect effects of seasonal dynamics and responses of plant growth on C substrate supply were indicated. Given the importance of indirect effects of the forcing factors and plant community dynamics on soil temperature, moisture, and C substrate, soil respiration response to climatic warming should not be represented in models as a simple temperature response function, and a more mechanistic representation including vegetation dynamics and substrate supply is needed.     

Forest soil CO2 flux: uncovering the contribution and environmental responses of ectomycorrhizas

Forests play a critical role in the global carbon cycle, being considered an important and continuing carbon sink. However, the response of carbon sequestration in forests to global climate change remains a major uncertainty, with a particularly poor understanding of the origins and environmental responses of soil CO₂ efflux. For example, despite their large biomass, the contribution of ectomycorrhizal (EM) fungi to forest soil CO₂ efflux and responses to changes in environmental drivers has, to date, not been quantified in the field. Their activity is often simplistically included in the ‘autotrophic’ root respiration term. We set up a multiplexed continuous soil respiration measurement system in a young Lodgepole pine forest, using a mycorrhizal mesh collar design, to monitor the three main soil CO₂ efflux components: root, extraradical mycorrhizal hyphal, and soil heterotrophic respiration. Mycorrhizal hyphal respiration increased during the first month after collar insertion and thereafter remained remarkably stable. During autumn the soil CO₂ flux components could be divided into ∼60% soil heterotrophic, ∼25% EM hyphal, and ∼15% root fluxes. Thus the extraradical EM mycelium can contribute substantially more to soil CO₂ flux than do roots. While EM hyphal respiration responded strongly to reductions in soil moisture and appeared to be highly dependent on assimilate supply, it did not responded directly to changes in soil temperature. It was mainly the soil heterotrophic flux component that caused the commonly observed exponential relationship with temperature. Our results strongly suggest that accurate modelling of soil respiration, particularly in forest ecosystems, needs to explicitly consider the mycorrhizal mycelium and its dynamic response to specific environmental factors. Moreover, we propose that in forest ecosystems the mycorrhizal CO₂ flux component represents an overflow ‘CO₂ tap’ through which surplus plant carbon may be returned directly to the atmosphere, thus limiting expected carbon sequestration from trees under elevated CO₂.     

Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2

To determine the long-term impact of elevated CO₂ on primary production of native tallgrass prairie, we compared the responses of tallgrass prairie at ambient and twice-ambient atmospheric CO₂ levels over an 8-year period. Plots in open-top chambers (4.5 m diameter) were exposed continuously (24 h) to ambient and elevated CO₂ from early April to late October each year. Unchambered plots were monitored also. Above-ground peak biomass was determined by clipping each year in early August, and root growth was estimated by harvesting roots from root ingrowth bags. Plant community composition was censused each year in early June. In the last 2 years of the study, subplots were clipped on 1 June or 1 July, and regrowth was harvested on 1 October. Volumetric soil water content of the 0–100 cm soil layer was determined using neutron scattering, and was generally higher in elevated CO₂ plots than ambient. Peak above-ground biomass was greater on elevated CO₂ plots than ambient CO₂ plots with or without chambers during years with significant plant water stress. Above-ground regrowth biomass was greater under elevated CO₂ than under ambient CO₂ in a year with late-season water stress, but did not differ in a wetter year. Root ingrowth biomass was also greater in elevated CO₂ plots than ambient CO₂ plots when water stress occurred during the growing season. The basal cover and relative amount of warm-season perennial grasses (C4) in the stand changed little during the 8-year period, but basal cover and relative amount of cool-season perennial grasses (C3) in the stand declined in the elevated CO₂ plots and in ambient CO₂ plots with chambers. Forbs (C3) and members of the Cyperaceae (C3) increased in basal cover and relative amount in the stand at elevated compared to ambient CO₂. Greater biomass production under elevated CO₂ in C4-dominated grasslands may lead to a greater carbon sequestration by those ecosystems and reduce peak atmospheric CO₂ concentrations in the future.     

The influence of elevated O3 and CO2 concentrations on secondary metabolites of Scots pine (Pinus sylvestris L. ) seedlings

Terpene, resin acid and total phenolic concentrations in five‐year‐old Scots pine (Pinus sylvestris L.) seedlings were analysed after exposure to ambient and realistically elevated (2 × ambient) O₃ and CO₂ concentrations and their combination in open‐top chambers during two growing seasons. Under O₃ exposure, limonene concentration in needles and isopimaric concentration in stems decreased significantly. As a response to elevated CO₂, α‐pinene and total phenolic concentrations in needles increased significantly, while bornyl acetate concentration in needles and palustric + levopimaric and neoabietic acid concentrations in stems decreased significantly. Some terpenes and resin acids were found at lower concentrations in the combined O₃ and CO₂ treatment than in O₃ exposure or elevated CO₂. A negative chamber effect was found: seedlings growing inside the chambers with ambient air had significantly lower concentrations of some terpenes and resin acids than seedlings growing outside the chambers. There was a lot of between‐tree variation in terpene and resin acid concentrations, which is typical of open‐pollinated populations. The results of this study suggest that, at least in short‐term experiments, Scots pine secondary metabolites are relatively insensitive to climate change factors. Total phenolics in the needles were the most responsive group showing about 25% increase in elevated CO₂, and O₃ exposure did not mitigate this CO₂ effect. Terpenes and resin acids were less responsive, although some individual compounds showed notable responses, e.g. α‐pinene in needles, which increased about 50% in response to elevated CO₂. As a consequence, although there were only slight effects on total pools of needle secondary metabolites, considerable O₃ and CO₂ effects on certain individual compounds might have ecological significance via trophic amplification, e.g. in decomposing processes of needle litter.     

NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2

The effect of elevated atmospheric CO₂ on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (¹H) nuclear magnetic resonance (NMR) imaging. Exposure of 24-d-old plants to atmospheric CO₂-enriched air at 650 cm³ m^?3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short-term drying-stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO₂ (350 m³ m^?3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO₂ does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO₂ environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate.     

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.     

Effects of elevated atmospheric CO2 on fine root length and distribution in an oak-palmetto scrub ecosystem in central Florida

Atmospheric CO₂ concentration is rising and it has been suggested that a portion of the additional carbon is being sequestered in terrestrial vegetation and much of that in below-ground structures. The objective of the present study was to quantify the effects of elevated atmospheric CO₂ on fine root length and distribution with depth with minirhizotrons in an open-top chamber experiment in an oak-palmetto scrub ecosystem at Kennedy Space Centre, Florida, USA. Observations were made five times over a period of one and a half years in three ambient chambers (350 p.p.m. CO₂), three CO₂ enriched chambers (700 p.p.m. CO₂), and three unchambered plots. Greater root length densities were produced in the elevated CO₂ chambers (14.2 mm cm^?2) compared to the ambient chambers (8.7 mm cm^?2). More roots may presumably lead to more efficient acquisition of resources. Fine root abundance varied significantly with soil depth, and there appeared to be enhanced proliferation of fine roots near the surface (0–12 cm) and at greater depth (49–61 cm) in the elevated CO₂ chambers. The vertical root distribution pattern may be a response to availability of nutrients and water. More studies are needed to determine if increased root length under CO₂ enriched conditions actually results in greater sequestering of carbon below ground.