Three years of free-air CO2 enrichment (POPFACE) only slightly affect profiles of light and leaf characteristics in closed canopies of Populus

Modelling is used to predict long‐term forest responses to increased atmospheric CO₂ concentrations. Although productivity models are based on light intercepted by the canopy, very little experimental data are available for closed forest stands. Nevertheless, the relationships between light inside a canopy, leaf area, canopy structure, and individual leaf characteristics may be affected by elevated CO₂, affecting in turn carbon gain. Using a free‐air CO₂ enrichment (FACE) design in a high‐density plantation of Populus spp., we studied the effects of increased CO₂ concentrations on transmittance (τ) of photosynthetic photon flux density (Q_p), on ratios of red/far‐red light (R/FR), on leaf area index (LAI), on leaf inclination, on leaf chlorophyll (chl) and nitrogen (N) concentrations, and on specific leaf area (SLA) in the 2nd and 3rd years of treatment. Continuous measurements of τ were made in addition to canopy height profiles of light and leaf characteristics. Two years of Q_p measurements showed an average decrease of canopy transmittance in the FACE treatment, with very small differences at canopy closure. Results were explained by an unaffected LAI in closed canopies, without a FACE‐induced stimulation of relative crown depth. In agreement, leaf inclination and extinction coefficients for light were similar in control and FACE conditions. Ratios of R/FR were not significantly affected by the FACE treatment, neither were leaf characteristics, with the exception of leaf N, which allows speculation about N limitation. In general, treatment differences in canopy profiles resulted from an initial stimulation of height growth in the FACE treatment. P. × euramericana differed from P. alba and P. nigra, but species did not differ significantly in their response to the FACE treatment. By the time fast‐growing high‐density forest plantations have passed the exponential growth phase and reached canopy closure, the likely effects of elevated atmospheric CO₂ concentration on canopy architecture and absorption of Q_p are minor.     

Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO2 across four free-air CO2 enrichment experiments in forest, grassland and desert

The magnitude of changes in carboxylation capacity in dominant plant species under long‐term elevated CO₂ exposure (elevated pC_a) directly impacts ecosystem CO₂ assimilation from the atmosphere. We analyzed field CO₂ response curves of 16 C₃ species of different plant growth forms in favorable growth conditions in four free‐air CO₂ enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO₂ assimilation (A) by +40±5% in elevated pC_a (49.5–57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC_a in some species. Photosynthesis at a common pC_a (A_a) was significantly reduced in five species growing under elevated pC_a, while leaf carboxylation capacity (V_cmax) was significantly reduced by elevated pC_a in seven species (change of ?19±3% among these species) across different growth forms and FACE sites. Adjustments in V_cmax with elevated pC_a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC_a treatment did not affect the mass‐based relationships between A or V_cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC_a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC_a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC_a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO₂ responses can only be measured experimentally on a small number of species, understanding elevated CO₂ effects on canopy N_m and N_a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO₂ in different species and plant growth forms.     

The influence of elevated CO2 on non-structural carbohydrate distribution and fructan accumulation in wheat canopies

We grew 2.4 m² wheat canopies in a large growth chamber under high photosynthetic photon flux (1000 μmol m⁻² s⁻¹) and using two CO₂ concentrations, 360 and 1200 μmol mol⁻¹. Photosynthetically active radiation (400–700 nm) was attenuated slightly faster through canopies grown in 360μmol mol⁻¹ than through canopies grown in 1200μmol mol⁻¹, even though high-CO₂ canopies attained larger leaf area indices. Tissue fractions were sampled from each 5-cm layer of the canopies. Leaf tissue sampled from the tops of canopies grown in 1200μmol mol⁻¹ accumulated significantly more total non-structural carbohydrate, starch, fructan, sucrose, and glucose (p≤ 0.05) than for canopies grown in 360μmol mol⁻¹. Non-structural carbohydrate did not significantly increase in the lower canopy layers of the elevated CO₂ treatment. Elevated CO₂ induced fructan synthesis in all leaf tissue fractions, but fructan formation was greatest in the uppermost leaf area. A moderate temperature reduction of 10 °C over 5d increased starch, fructan and glucose levels in canopies grown in 1200μmol mol⁻¹, but concentrations of sucrose and fructose decreased slightly or remained unchanged. Those results may correspond with the use of fructosyl-residues and release of glucose when sucrose is consumed in fructan synthesis.     

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO₂ Enrichment experiment to determine the effects of elevated atmospheric CO₂ concentration ([CO₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO₂], although L senescence for hardwoods was slowed. Elevated [CO₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P<0.050); on average, enhancement following canopy closure was ～16% and 14% respectively. However, variation in pine L and its response to elevated [CO₂] was not random. Each year pine L under ambient and elevated [CO₂] was spatially correlated to the variability in site nitrogen availability (e.g. r²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO₂]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO₂] in moderate fertility sites, but are unable to respond to [CO₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (～3 g N inputs m^?2 yr^?1) may be sufficiently enhanced under elevated [CO₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (～14%).     

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 1. Leaf position and phenology determine acclimation response

We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO₂ concentration, and ample and limiting N supplies, within a field experiment using free-air CO₂ enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO₂ concentration (C_i) was used to estimate the in vivo carboxylation capacity (Vc_max) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (A _sat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO₂ did not alter Vc_max in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vc_max and A _sat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO₂ was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO₂ was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO₂ and nutrient stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic acclimation to elevated CO2 in Phaseolus vulgaris L. is altered by growth response to nitrogen supply

Abstract Long‐term exposure of plants to elevated CO₂ often leads to downward photosynthetic acclimation. Nitrogen (N) deficiency could potentially exacerbate this response by reducing growth rate and the sink for photosynthates, but this has not always been observed. Experimentally, the interpretation of N effects on CO₂ responses can be confounded by increasing severity of tissue N deficiency over time when N supply is not adjusted as demand increases. In this study, N supply ranged from sub‐ to supra‐optimal (20–540 kgN ha^–l equivalent), and relatively stable levels of tissue N concentration were obtained in all treatments by varying twice‐weekly application rates in proportion to plant growth. The effects of N on photosynthesis and growth of beans (Phaseolus vulgaris L.) raised at ambient (35 Pa) and three elevated (70, 105, 140 Pa) CO₂ partial pressures (pCO₂) were evaluated. Averaging across N treatments, leaf total non‐structural carbohydrates (TNC) were 2.5‐ to 3‐fold higher and leaf N concentrations were 31–35% lower at elevated compared to ambient pCO₂. Light‐saturated net CO₂ assimilation rates measured at growth pCO₂ (A_satg) were significantly higher (26–40% depending on N supply) in plants grown at elevated compared to ambient pCO₂. When measured at a common pCO₂ of 35 Pa, the A_sat of plants grown at elevated CO₂ was 15–29% less than that of plants grown at 35 Pa, indicative of downward photosynthetic acclimation. The magnitude of downward photosynthetic acclimation to elevated CO₂ was greater in plants grown at high (180 and 540 kgN ha^–l) compared to low (20 and 60 kgN ha^–l) N supply, and this was associated with a higher A_sat at growth pCO₂, higher leaf area ratio (leaf area/total biomass), and higher TNC in leaves of high‐N plants. Our results indicate that the effect of N on acclimation to CO₂ will depend on the balance between supply and demand for N during the growing period, and the effect this has on biomass allocation and source‐sink C balance at the whole‐plant level.     

Acclimation of photosynthesis to elevated CO2 in onion (Allium cepa) grown at a range of temperatures

Onion (Allium cepa) was grown in the field within temperature gradient tunnels (providing about ‐2.5°C to +2.5°C from outside temperatures) maintained at either 374 or 532 μmol mol^?1 CO₂. Plant leaf area was determined non‐destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO₂ concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO₂. The light‐saturated rates of leaf photosynthesis (A_sat) were greater in plants grown at normal than at elevated CO₂ concentrations at the same measurement CO₂ concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO₂. The ratio of intercellular to atmospheric CO₂ (C₁/C₃ ratio) was 7.4% less for plants grown at elevated compared with normal CO₂. A_sat in plants grown at elevated CO₂ was less than in plants grown at normal CO₂ when compared at the same C₁. Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO₂ fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO₂. In contrast, carbohydrate content was less at elevated compared with normal CO₂ in the leaf sections in which CO₂ exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field‐grown crops.     

Leaf senescence and decline of end-of-season gas exchange in five temperate deciduous tree species grown in elevated CO2 concentrations

We measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO₂ concentrations and low- and high-nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO₂] generally had smaller canopies than their current [CO₂]-grown counterparts throughout most of the senescent period. This was a result of smaller pre-senescent canopies or accelerated rates of leaf drop. Leaf-level photosynthetic rates were higher under elevated [CO₂] for grey birch canopies and for low-nutrient red maple and high-nutrient ash canopies, but declined rapidly to values below those of their current [CO₂] counterparts by midway through the senescent period. CO₂ enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO₂]-grown seedlings had lower indices of whole canopy end-of-season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO₂] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end-of-season estimates of water use efficiency, however, were consistently lower under CO₂ enrichment, with few exceptions. These results suggest that whole canopy end-of-season gas exchange may be altered significantly in an elevated [CO₂] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO₂-enriched forests of the future.     

Seasonal changes in the effects of elevated CO2 on rice at three levels of nitrogen supply: a free air CO2 enrichment (FACE) experiment

Over time, the stimulative effect of elevated CO₂ on the photosynthesis of rice crops is likely to be reduced with increasing duration of CO₂ exposure, but the resultant effects on crop productivity remain unclear. To investigate seasonal changes in the effect of elevated CO₂ on the growth of rice (Oryza sativa L.) crops, a free air CO₂ enrichment (FACE) experiment was conducted at Shizukuishi, Iwate, Japan in 1998–2000. The target CO₂ concentration of the FACE plots was 200 µmol mol^?1 above that of ambient. Three levels of nitrogen (N) were supplied: low (LN, 4 g N m^?2), medium [MN, 8 (1998) and 9 (1999, 2000) g N m^?2] and high N (HN, 12 and 15 g N m^?2). For MN and HN but not for LN, elevated CO₂ increased tiller number at panicle initiation (PI) but this positive response decreased with crop development. As a result, the response of green leaf area index (GLAI) to elevated CO₂ greatly varied with development, showing positive responses during vegetative stages and negative responses after PI. Elevated CO₂ decreased leaf N concentration over the season, except during early stage of development. For MN crops, total biomass increased with elevated CO₂, but the response declined linearly with development, with average increases of 32, 28, 21, 15 and 12% at tillering, PI, anthesis, mid‐ripening and grain maturity, respectively. This decline is likely to be due to decreases in the positive effects of elevated CO₂ on canopy photosynthesis because of reductions in both GLAI and leaf N. Up to PI, LN‐crops tended to have a lower response to elevated CO₂ than MN‐ and HN‐crops, though by final harvest the total biomass response was similar for all N levels. For MN‐ and HN‐crops, the positive response of grain yield (ca. 15%) to elevated CO₂ was slightly greater than the response of final total biomass while for LN‐crops it was less. We conclude that most of the seasonal changes in crop response to elevated CO₂ are directly or indirectly associated with N uptake.     

Influence of carbon dioxide enrichment, ozone and nitrogen fertilization on cotton (Gossypium hirsutum L.) leaf and root composition

The objective of this study was to test whether elevated [CO₂], [O₃] and nitrogen (N) fertility altered leaf mass per area (LMPA), non‐structural carbohydrate (TNC), N, lignin (LTGA) and proanthocyanidin (PA) concentrations in cotton (Gossypium hirsutum L.) leaves and roots. Cotton was grown in 14 dm³ pots with either sufficient (0·8 g N dm ^{? 3}) or deficient (0·4 and 0·2 g N dm ^{? 3}) N fertilization, and treated in open‐top chambers with either ambient or elevated ( + 175 and + 350 μ mol mol ^{? 1}) [CO₂] in combination with either charcoal‐filtered air (CF) or non‐filtered air plus 1·5 times ambient [O₃]. At about 50 d after planting, LMPA, starch and PA concentrations in canopy leaves were as much as 51–72% higher in plants treated with elevated [CO₂] compared with plants treated with ambient [CO₂], whereas leaf N concentration was 29% lower in elevated [CO₂]‐treated plants compared with controls. None of the treatments had a major effect on LTGA concentrations on a TNC‐free mass basis. LMPA and starch levels were up to 48% lower in plants treated with elevated [O₃] and ambient [CO₂] compared with CF controls, although the elevated [O₃] effect was diminished when plants were treated concurrently with elevated [CO₂]. On a total mass basis, leaf N and PA concentrations were higher in samples treated with elevated [O₃] in ambient [CO₂], but the difference was much reduced by elevated [CO₂]. On a TNC‐free basis, however, elevated [O₃] had little effect on tissue N and PA concentrations. Fertilization treatments resulted in higher PA and lower N concentrations in tissues from the deficient N fertility treatments. The experiment showed that suppression by elevated [O₃] of LMPA and starch was largely prevented by elevated [CO₂], and that interpretation of [CO₂] and [O₃] effects should include comparisons on a TNC‐free basis. Overall, the experiment indicated that allocation to starch and PA may be related to how environmental factors affect source–sink relationships in plants, although the effects of elevated [O₃] on secondary metabolites differed in this respect.     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996     

Canopy N and P dynamics of a southeastern US pine forest under elevated CO2

Forest production is strongly nutrient limited throughout the southeastern US. If nutrient limitations constrain plant acquisition of essential resources under elevated CO₂, reductions in the mass or nutrient content of forest canopies could constrain C assimilation from the atmosphere. We tested this idea by quantifying canopy biomass, foliar concentrations of N and P, and the total quantity of N and P in a loblolly pine (Pinus taeda) canopy subject to 4 years of free-air CO₂ enrichment. We also used N:P ratios to detect N versus P limitation to primary production under elevated CO₂. Canopy biomass was significantly higher under elevated CO₂ during the first 4 years of this experiment. Elevated CO₂ significantly reduced the concentration of N in loblolly pine foliage (5% relative to ambient CO₂) but not P. Despite the slight reduction foliage N concentrations, there were significant increases in canopy N and P contents under elevated CO₂. Foliar N:P ratios were not altered by elevated CO₂ and were within a range suggesting forest production is N limited not P limited. Despite the clear limitation of NPP by N under ambient and elevated CO₂ at this site, there is no evidence that the mass of N or P in the canopy is declining through the first 4 years of CO₂ fumigation. As a consequence, whole-canopy C assimilation is strongly stimulated by elevated CO₂ making this forest a larger net C sink under elevated CO₂ than under ambient CO₂. We discuss the potential for future decreases in canopy nutrient content as a result of limited changes in the size of the plant-available pools of N under elevated CO₂.     

Does Leaf Position within a Canopy Affect Acclimation of Photosynthesis to Elevated CO2? : Analysis of a Wheat Crop under Free-Air CO2 Enrichment

Previous studies of photosynthetic acclimation to elevated CO₂ have focused on the most recently expanded, sunlit leaves in the canopy. We examined acclimation in a vertical profile of leaves through a canopy of wheat (Triticum aestivum L.). The crop was grown at an elevated CO₂ partial pressure of 55 Pa within a replicated field experiment using free-air CO₂ enrichment. Gas exchange was used to estimate in vivo carboxylation capacity and the maximum rate of ribulose-1,5-bisphosphate-limited photosynthesis. Net photosynthetic CO₂ uptake was measured for leaves in situ within the canopy. Leaf contents of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting-complex (LHC) proteins, and total N were determined. Elevated CO₂ did not affect carboxylation capacity in the most recently expanded leaves but led to a decrease in lower, shaded leaves during grain development. Despite this acclimation, in situ photosynthetic CO₂ uptake remained higher under elevated CO₂. Acclimation at elevated CO₂ was accompanied by decreases in both Rubisco and total leaf N contents and an increase in LHC content. Elevated CO₂ led to a larger increase in LHC/Rubisco in lower canopy leaves than in the uppermost leaf. Acclimation of leaf photosynthesis to elevated CO₂ therefore depended on both vertical position within the canopy and the developmental stage.     

Altered physiological function,not structure,drives increased radiation-use efficiency of soybean grown at elevated CO<Subscript>2</Subscript>

Previous studies of elevated carbon dioxide concentration ([CO₂]) on crop canopies have found that radiation-use efficiency is increased more than radiation-interception efficiency. It is assumed that increased radiation-use efficiency is due to changes in leaf-level physiology; however, canopy structure can affect radiation-use efficiency if leaves are displayed in a manner that optimizes their physiological capacity, even though the canopy intercepts the same amount of light. In order to determine the contributions of physiology and canopy structure to radiation-use and radiation-interception efficiency, this study relates leaf-level physiology and leaf display to photosynthetic rate of the outer canopy. We used a new imaging approach that delivers three-dimensional maps of the outer canopy during the growing season. The 3D data were used to model leaf orientation and mean photosynthetic electron transport of the outer canopy to show that leaf orientation changes did not contribute to increased radiation-use; i.e. leaves of the outer canopy showed similar diurnal leaf movements and leaf orientation in both treatments. Elevated [CO₂] resulted in an increased maximum electron transport rate (ETR_max) of light reactions of photosynthesis. Modeling of canopy light interception showed that stimulated leaf-level electron transport at elevated [CO₂], and not alterations in leaf orientation, was associated with stimulated radiation-use efficiency and biomass production in elevated [CO₂]. This study provides proof of concept of methodology to quantify structure–function relationships in combination, allowing a quantitative estimate of the contribution of both effects to canopy energy conversion under 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.     

Structural characteristics and chemical composition of birch (Betula pendula) leaves are modified by increasing CO2 and ozone

Impacts of ozone and CO₂ enrichment, alone and in combination, on leaf anatomical and ultrastructural characteristics, nutrient status and cell wall chemistry in two European silver birch (Betula pendula Roth) clones were studied. The young soil‐growing trees were exposed in open‐top chambers over three growing seasons to 2 × ambient CO₂ and/or ozone concentrations in central Finland. The trees were measured for changes in altogether 35 variables of leaf structure, nutrients and cell wall chemistry of green leaves, and 20 of the measured variables were affected by CO₂ and/or O₃. Elevated CO₂ increased the size of chloroplasts and starch grains, number of mitochondria, P : N ratio, and contents of cell wall hemicellulose. Elevated CO₂ decreased the total leaf thickness, specific leaf area, concentrations of N, K, Cu, S and Fe, and contents of cell wall α‐cellulose, uronic acids, acid‐soluble lignin and acetone‐soluble extractives. Elevated ozone led to thinner leaves, higher palisade to spongy ratio, increased number of peroxisomes and mitochondria, reduced content of Mn, Zn, Cu, hemicellulose and uronic acids, and lower Mn : N and Zn : N ratios. In the combined exposure, interactions were antagonistic. Ultrastructural changes became more evident towards the end of the exposure. Young leaves were tolerant against ozone‐caused oxidative stress, whereas oxidative H₂O₂ accumulation was found in older leaves. CO₂ enrichment improved ozone tolerance not only through increased photosynthesis rates, but also through changes in cell wall chemistry (hemicellulose, in particular). However, nutrient imbalances due to ozone and/or CO₂ may predispose the trees to other biotic and abiotic stresses. Down‐regulation and up‐regulation of photosynthesis under elevated CO₂ through anatomical changes is discussed.     

Water savings in mature deciduous forest trees under elevated CO2

Stomatal conductance of plants exposed to elevated CO₂ is often reduced. Whether this leads to water savings in tall forest‐trees under future CO₂ concentrations is largely unknown but could have significant implications for climate and hydrology. We used three different sets of measurements (sap flow, soil moisture and canopy temperature) to quantify potential water savings under elevated CO₂ in a ca. 35 m tall, ca. 100 years old mixed deciduous forest. Part of the forest canopy was exposed to 540 ppm CO₂ during daylight hours using free air CO₂ enrichment (FACE) and the Swiss Canopy Crane (SCC). Across species and a wide range of weather conditions, sap flow was reduced by 14% in trees subjected to elevated CO₂, yielding ca. 10% reduction in evapotranspiration. This signal is likely to diminish as atmospheric feedback through reduced moistening of the air comes into play at landscape scale. Vapour pressure deficit (VPD)‐sap flow response curves show that the CO₂ effect is greatest at low VPD, and that sap flow saturation tends to occur at lower VPD in CO₂‐treated trees. Matching stomatal response data, the CO₂ effect was largely produced by Carpinus and Fagus, with Quercus contributing little. In line with these findings, soil moisture at 10 cm depth decreased at a slower rate under high‐CO₂ trees than under control trees during rainless periods, with a reversal of this trend during prolonged drought when CO₂‐treated trees take advantage from initial water savings. High‐resolution thermal images taken at different heights above the forest canopy did detect reduced water loss through altered energy balance only at <5 m distance (0.44 K leaf warming of CO₂‐treated Fagus trees). Short discontinuations of CO₂ supply during morning hours had no measurable canopy temperature effects, most likely because the stomatal effects were small compared with the aerodynamic constraints in these dense, broad‐leaved canopies. Hence, on a seasonal basis, these data suggest a <10% reduction in water consumption in this type of forest when the atmosphere reaches 540% ppm CO₂.     

Laboratory incubations reveal potential responses of soil nitrogen cycling to changes in soil C and N availability in Mojave Desert soils exposed to elevated atmospheric CO2

Elevated atmospheric carbon dioxide (CO₂) has the potential to alter soil carbon (C) and nitrogen (N) cycling in arid ecosystems through changes in net primary productivity. However, an associated feedback exists because any sustained increases in plant productivity will depend upon the continued availability of soil N. We took soils from under the canopies of major shrubs, grasses, and plant interspaces in a Mojave Desert ecosystem exposed to elevated atmospheric CO₂ and incubated them in the laboratory with amendments of labile C and N to determine if elevated CO₂ altered the mechanistic controls of soil C and N on microbial N cycling. Net ammonification increased under shrubs exposed to elevated CO₂, while net nitrification decreased. Elevated CO₂ treatments exhibited greater fluxes of N₂O–N under Lycium spp., but not other microsites. The proportion of microbial/extractable organic N increased under shrubs exposed to elevated CO₂. Heterotrophic N₂‐fixation and C mineralization increased with C addition, while denitrification enzyme activity and N₂O–N fluxes increased when C and N were added in combination. Laboratory results demonstrated the potential for elevated CO₂ to affect soil N cycling under shrubs and supports the hypothesis that energy limited microbes may increase net inorganic N cycling rates as the amount of soil‐available C increases under elevated CO₂. The effect of CO₂ enrichment on N‐cycling processes is mediated by its effect on the plants, particularly shrubs. The potential for elevated atmospheric CO₂ to lead to accumulation of NH₄⁺ under shrubs and the subsequent volatilization of NH₃ may result in greater losses of N from this system, leading to changes in the form and amount of plant‐available inorganic N. This introduces the potential for a negative feedback mechanism that could act to constrain the degree to which plants can increase productivity in the face of elevated atmospheric CO₂.     

Host-specific aphid population responses to elevated CO2 and increased N availability

Sap-feeding insects such as aphids are the only insect herbivores that show positive responses to elevated CO₂. Recent models predict that increased nitrogen will increase aphid population size under elevated CO₂, but few experiments have tested this idea empirically. To determine whether soil nitrogen (N) availability modifies aphid responses to elevated CO₂, we tested the performance of Macrosiphum euphorbiae feeding on two host plants; a C₃ plant (Solanum dulcamara), and a C₄ plant (Amaranthus viridis). We expected aphid population size to increase on plants in elevated CO₂, with the degree of increase depending on the N availability. We found a significant CO₂× N interaction for the response of population size for M. euphorbiae feeding on S. dulcamara: aphids feeding on plants grown in ambient CO₂, low N conditions increased in response to either high N availability or elevated CO₂. No population size responses were observed for aphids infesting A. viridis. Elevated CO₂ increased plant biomass, specific leaf weight, and C : N ratios of the C₃ plant, S. dulcamara but did not affect the C₄ plant, A. viridis. Increased N fertilization significantly increased plant biomass, leaf area, and the weight : height ratio in both experiments. Elevated CO₂ decreased leaf N in S. dulcamara and had no effect on A. viridis, while higher N availability increased leaf N in A. viridis and had no effect in S. dulcamara. Aphid infestation only affected the weight : height ratio of S. dulcamara. We only observed an increase in aphid population size in response to elevated CO₂ or increased N availability for aphids feeding on S. dulcamara grown under low N conditions. There appears to be a maximum population growth rate that M. euphorbiae aphids can attain, and we suggest that this response is because of intrinsic limits on development time and fecundity.     

The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N₂ fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol^?1 CO₂ and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol^?1 CO₂. The daylength was 12 h and the photon flux density 500±25 μmol m^?2 s^?1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO₂ increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO₂ or temperature. In contrast, N₂ fixation increased in concert with the promoting effect of elevated CO₂ on biomass production. The increased yield of biomass harvested in 680 μmol mol^?1 CO₂ was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO₂ also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO₂. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO₂ in terms of a sustained increase in biomass production.