Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂. An erratum to this article can be found at     

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.     

Growth and phenology of mature temperate forest trees in elevated CO2

Are mature forest trees carbon limited at current CO₂ concentrations? Will ‘mid‐life’, 35 m tall deciduous trees grow faster in a CO₂‐enriched atmosphere? To answer these questions we exposed ca. 100‐year‐old temperate forest trees at the Swiss Canopy Crane site near Basel, Switzerland to a ca. 540 ppm CO₂ atmosphere using web‐FACE technology. Here, we report growth responses to elevated CO₂ for 11 tall trees (compared with 32 controls) of five species during the initial four treatment years. Tested across all trees, there was no CO₂ effect on stem basal area (BA) increment (neither when tested per year nor cumulatively for 4 years). In fact, the 4th year means were almost identical for the two groups. Stem growth data were standardized by pretreatment growth (5 years) in order to account for a priori individual differences in vigor. Although this experiment was not designed to test species specific effects, one species, the common European beech, Fagus sylvatica, showed a significant growth enhancement in the first year, which reoccurred during a centennial drought in the third year. None of the other dominant species (Quercus petraea, Carpinus betulus) showed a growth response to CO₂ in any of the 4 years or for all years together. The inclusion or exclusion of single individuals of Prunus avium and Tilia platyphyllos did not change the picture. In elevated CO₂, lateral branching in terminal shoots was higher in Fagus in 2002, when shoots developed from buds that were formed during the first season of CO₂ enrichment (2001), but there was no effect in later years and no change in lateral branching in any of the other species. In Quercus, there was a steady stimulation of leading shoot length in high‐CO₂ trees. Phenological variables (bud break, leaf fall, leaf duration) were highly species specific and were not affected by elevated CO₂ in any consistent way. Our 4‐year data set reflects a very dynamic and species‐specific response of tree growth to a step change in CO₂ supply. Stem growth after 4 years of exposure does not support the notion that mature forest trees will accrete wood biomass at faster rates in a future CO₂‐enriched atmosphere.     

Gypsy moth feeding in the canopy of a CO2-enriched mature forest

Rising atmospheric carbon dioxide (CO₂) concentration is expected to change plant tissue quality with important implications for plant–insect interactions. Taking advantage of canopy access by a crane and long‐term CO₂ enrichment (530 μ mol mol^?1) of a natural old‐growth forest (web‐free air carbon dioxide enrichment), we studied the responses of a generalist insect herbivore feeding in the canopy of tall trees. We found that relative growth rates (RGR) of gypsy moth (Lymantria dispar) were reduced by 30% in larvae fed on high CO₂‐exposed Quercus petraea, but increased by 29% when fed on high CO₂‐grown Carpinus betulus compared with control trees at ambient CO₂ (370 μ mol mol^?1). In Fagus sylvatica, there was a nonsignificant trend for reduced RGR under elevated CO₂. Tree species‐specific changes in starch to nitrogen ratio, water, and the concentrations of proteins, condensed and hydrolyzable tannins in response to elevated CO₂ were identified to correlate with altered RGR of gypsy moth larvae. Our data suggest that rising atmospheric CO₂ will have strong species‐specific effects on leaf chemical composition of canopy trees in natural forests leading to contrasting responses of herbivores such as those reported here. A future change in host tree preference seems likely with far‐ranging consequences for forest community dynamics.     

Elevated CO<Subscript>2</Subscript> reduces sap flux in mature deciduous forest trees

We enriched in CO₂ the canopy of 14 broad-leaved trees in a species-rich, ca. 30-m-tall forest in NW Switzerland to test whether elevated CO₂ reduces water use in mature forest trees. Measurements of sap flux density (J_S) were made prior to CO₂ enrichment (summer 2000) and throughout the first whole growing season of CO₂ exposure (2001) using the constant heat-flow technique. The short-term responses of sap flux to brief (1.5–3 h) interruptions of CO₂ enrichment were also examined. There were no significant a priori differences in morphological and physiological traits between trees which were later exposed to elevated CO₂ (n=14) and trees later used as controls (n=19). Over the entire growing season, CO₂ enrichment resulted in an average 10.7% reduction in mean daily J_S across all species compared to control trees. Responses were most pronounced in Carpinus, Acer, Prunus and Tilia, smaller in Quercus and close to zero in Fagus trees. The J_S of treated trees significantly increased by 7% upon transient exposure to ambient CO₂ concentrations at noon. Hence, responses of the different species were, in the short term, similar in magnitude to those observed over the whole season (though opposite because of the reversed treatment). The reductions in mean J_S of CO₂-enriched trees were high (22%) under conditions of low evaporative demand (vapour pressure deficit, VPD <5 hPa) and small (2%) when mean daily VPD was greater than 10 hPa. During a relatively dry period, the effect of elevated CO₂ on J_S even appeared to be reversed. These results suggest that daily water savings by CO₂-enriched trees may have accumulated to a significantly improved water status by the time when control trees were short of soil moisture. Our data indicate that the magnitude of CO₂ effects on stand transpiration will depend on rainfall regimes and the relative abundance of the different species, being more pronounced under humid conditions and in stands dominated by species such as Carpinus and negligible in mono-specific Fagus forests.     

Differential effects of elevated CO2 on acorn density, weight, germination, and predation among three oak species in a scrub-oak forest

Much research on the effects of elevated CO₂ on forest trees has focused on quantitative changes in photosynthesis, secondary chemistry, and plant biomass. However, plant fitness responses to rising CO₂ should also include quantitative measures of reproduction, since most forest systems are recruitment limited. Until now, it has proved very difficult to grow forest trees to sexual maturity in a CO₂‐enriched environment. This paper is the first of its kind to address the effects of elevated CO₂ on the reproduction of hardwood trees in a natural forest. Beginning in 1996, scrub‐oak vegetation, predominantly three species of scrub‐oaks, Quercus myrtifolia, Q. chapmanii, and Q. geminata, were grown inside eight chambers with elevated CO₂ (704 parts per million (ppm)) and eight with ambient CO₂ (379 ppm) at Kennedy Space Center, Florida. In elevated CO₂, acorn production increased significantly for the dominant species Q. myrtifolia and for Q. chapmanii, but it did not increase for the subdominant, Q. geminata. Acorn weight, germination rate, and predation by weevils were unaffected by CO₂. Thus, recruitment of some forest tree species into the Florida scrub‐oak community is likely to be accelerated in an atmosphere of increased CO₂. However, because the acceleration of recruitment differs among species, over the long term, Q. myrtifolia and Q. chapmanii will be favored over Q. geminata and this is likely to change patterns of species diversity.     

Enriched atmospheric CO2 and defoliation: effects on tree chemistry and insect performance

We examined the effects of CO₂ and defoliation on tree chemistry and performance of the forest tent caterpillar, Malacosoma disstria. Quaking aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees were grown in open-top chambers under ambient or elevated concentrations of CO₂. During the second year of growth, half of the trees were exposed to free-feeding forest tent caterpillars, while the remaining trees served as nondefoliated controls. Foliage was collected weekly for phytochemical analysis. Insect performance was evaluated on foliage from each of the treatments. At the sampling date coincident with insect bioassays, levels of foliar nitrogen and starch were lower and higher, respectively, in high CO₂ foliage, and this trend persisted throughout the study. CO₂-mediated increases in secondary compounds were observed for condensed tannins in aspen and gallotannins in maple. Defoliation reduced levels of water and nitrogen in aspen but had no effect on primary metabolites in maple. Similarly, defoliation induced accumulations of secondary compounds in aspen but not in maple. Larvae fed foliage from the enriched CO₂ or defoliated treatments exhibited reduced growth and food processing efficiencies, relative to larvae on ambient CO₂ or nondefoliated diets, but the patterns were host species-specific. Overall, CO₂ and defoliation appeared to exert independent effects on foliar chemistry and forest tent caterpillar performance.     

Differential anatomical responses to elevated CO2 in saplings of four hardwood species

To determine whether an elevated carbon dioxide concentration ([CO₂]) can induce changes in the wood structure and stem radial growth in forest trees, we investigated the anatomical features of conduit cells and cambial activity in 4‐year‐old saplings of four deciduous broadleaved tree species – two ring‐porous (Quercus mongolica and Kalopanax septemlobus) and two diffuse‐porous species (Betula maximowicziana and Acer mono) – grown for three growing seasons in a free‐air CO₂ enrichment system. Elevated [CO₂] had no effects on vessels, growth and physiological traits of Q. mongolica, whereas tree height, photosynthesis and vessel area tended to increase in K. septemlobus. No effects of [CO₂] on growth, physiological traits and vessels were seen in the two diffuse‐porous woods. Elevated [CO₂] increased larger vessels in all species, except B. maximowicziana and number of cambial cells in two ring‐porous species. Our results showed that the vessel anatomy and radial stem growth of Q. mongolica, B. maximowicziana and A. mono were not affected by elevated [CO₂], although vessel size frequency and cambial activity in Q. mongolica were altered. In contrast, changes in vessel anatomy and cambial activity were induced by elevated [CO₂] in K. septemlobus. The different responses to elevated [CO₂] suggest that the sensitivity of forest trees to CO₂ is species dependent.     

Stomatal conductance in mature deciduous forest trees exposed to elevated CO<Subscript>2</Subscript>

Stomatal conductance (g _s) of mature trees exposed to elevated CO₂ concentrations was examined in a diverse deciduous forest stand in NW Switzerland. Measurements of g _s were carried out on upper canopy foliage before noon, over four growing seasons, including an exceptionally dry summer (2003). Across all species reductions in stomatal conductance were smaller than 25% most likely around 10%, with much variation among species and trees. Given the large heterogeneity in light conditions within a tree crown, this signal was not statistically significant, but the responses within species were surprisingly consistent throughout the study period. Except during a severe drought, stomatal conductance was always lower in trees of Carpinus betulus exposed to elevated CO₂ compared to Carpinus trees in ambient air, but the difference was only statistically significant on 2 out of 15 days. In contrast, stomatal responses in Fagus sylvatica and Quercus petraea varied around zero with no consistent trend in relation to CO₂ treatment. During the 2003 drought in the third treatment year, the CO₂ effect became reversed in Carpinus, resulting in higher g _s in trees exposed to elevated CO₂ compared to control trees, most likely due to better water supply because of the previous soil water savings. This was supported by less negative predawn leaf water potential in CO₂ enriched Carpinus trees, indicating an improved water status. These findings illustrate (1) smaller than expected CO₂-effects on stomata of mature deciduous forest trees, and (2) the possibility of soil moisture feedback on canopy water relations under elevated CO₂.     

Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE)

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free‐air CO₂ enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion‐exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the ¹³C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO₂ enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope ¹⁵N increased strongly. This indicates that the CO₂ enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO₂‐enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine‐root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO₂ concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.     

The contribution of bryophytes to the carbon exchange for a temperate rainforest

Bryophytes blanket the floor of temperate rainforests in New Zealand and may influence a number of important ecosystem processes, including carbon cycling. Their contribution to forest floor carbon exchange was determined in a mature, undisturbed podocarp‐broadleaved forest in New Zealand, dominated by 100–400‐year‐old rimu (Dacrydium cupressimum) trees. Eight species of mosses and 13 species of liverworts contributed to the 62% cover of the diverse forest floor community. The bryophyte community developed a relatively thin (depth <30 mm), but dense, canopy that experienced elevated CO₂ partial pressures (median 46.6 Pa immediately below the bryophyte canopy) relative to the surrounding air (median 37.6 Pa at 100 mm above the canopy). Light‐saturated rates of net CO₂ exchange from 14 microcosms collected from the forest floor were highly variable; the maximum rate of net uptake (bryophyte photosynthesis – whole‐plant respiration) per unit ground area at saturating irradiance was 1.9 μmol m^?2 s^?1 and in one microcosm, the net rate of CO₂ exchange was negative (respiration). CO₂ exchange for all microcosms was strongly dependent on water content. The average water content in the microcosms ranged from 1375% when fully saturated to 250% when air‐dried. Reduction in water content across this range resulted in an average decrease of 85% in net CO₂ uptake per unit ground area. The results from the microcosms were used in a model to estimate annual carbon exchange for the forest floor. This model incorporated hourly variability in average irradiance reaching the forest floor, water content of the bryophyte layer, and air and soil temperature. The annual net carbon uptake by forest floor bryophytes was 103 g m^?2, compared to annual carbon efflux from the forest floor (bryophyte and soil respiration) of ?1010 g m^?2. To put this in perspective of the magnitude of the components of CO₂ exchange for the forest floor, the bryophyte layer reclaimed an amount of CO₂ equivalent to only about 10% of forest floor respiration (bryophyte plus soil) or ～11% of soil respiration. The contribution of forest floor bryophytes to productivity in this temperate rainforest was much smaller than in boreal forests, possibly because of differences in species composition and environmental limitations to photosynthesis. Because of their close dependence on water table depth, the contribution of the bryophyte community to ecosystem CO₂ exchange may be highly responsive to rapid changes in climate.     

First-year growth response of trees in an intact forest exposed to elevated CO2

Although elevated atmospheric CO₂ has been shown to increase growth of tree seedlings and saplings, the response of intact forest ecosystems and established trees is unclear. We report results from the first large-scale experimental system designed to study the effects of elevated CO₂ on an intact forest with the full complement of species interactions and environmental stresses. During the first year of exposure to ^ 1.5 Ë ambient CO₂, canopy loblolly pine (Pinus taeda, L.) trees increased basal area growth rate by 24% but understorey trees of loblolly pine, sweetgum (Liquidambar styraciflua L.), and red maple (Acer rubrum L.) did not respond. Winged elm (Ulmus alata Michx.) had a marginally significant increase in growth rate (P = 0.069). These data suggest that this ecosystem has the capacity to respond immediately to a step increase in atmospheric CO₂; however, as exposure time increases, nutrient limitations may reduce this initial growth stimulation.     

Thirty years of in situ tree growth under elevated CO2: a model for future forest responses?

Rising concentrations of atmospheric carbon dioxide have been predicted to stimulate the growth of forest trees. However, long-term effects on trees growing to maturity and to canopy closure while exposed to elevated CO₂ have never been examined. We compared tree ring chronologies of Mediterranean Quercus ilex which have been continuously exposed to elevated CO₂ (around 650 μmol mol^–1) since they were seedlings, near two separate natural CO₂ springs with those from trees at nearby ambient-CO₂‘control’ sites. Trees grown under high CO₂ for 30 years (1964–93) showed a 12% greater final radial stem width than those growing at the ambient-CO₂ control sites. However, this stimulation was largely due to responses when trees were young. By the time trees were 25–30 y old the annual difference in tree ring width between low and high CO₂ grown trees had disappeared. At any given tree age, elevated CO₂ had a relatively greater positive effect on tree ring width in years with a dry spring compared to years with more rainfall between April and May. This indicates a beneficial effect of elevated CO₂ on tree water relations under drought stress. Our data suggest that the early regeneration phase of forest stands can be accelerated in CO₂-enriched atmospheres and that maximum biomass per land area may be reached sooner than under lower CO₂ concentrations. In our study, high CO₂ grown Q. ilex trees reached the same stem basal area at the age of 26 y as control trees at 29 y, i.e. three years earlier (faster turnover of carbon?). Reliable predictions of the future development of forests need to account for the variable responses of trees over their entire lifetime. Such responses to elevated CO₂ can presently only be assessed at such unique field sites.     

Greater seed production in elevated CO2 is not accompanied by reduced seed quality in Pinus taeda L.

For herbaceous species, elevated CO₂ often increases seed production but usually leads to decreased seed quality. However, the effects of increased atmospheric CO₂ on tree fecundity remain uncertain, despite the importance of reproduction to the composition of future forests. We determined how seed quantity and quality differed for pine trees grown for 12 years in ambient and elevated (ambient+200 μL L^?1) CO₂, at the Duke Forest free‐air CO₂ enrichment (FACE) site. We also compared annual reproductive effort with yearly measurements of aboveground net primary productivity (ANPP), precipitation (P), potential evapotranspiration (PET) and water availability [precipitation minus potential evapotranspiration (P?PET)] to investigate factors that may drive interannual variation in seed production. The number of mature, viable seeds doubled per unit basal area in high‐CO₂ plots from 1997 to 2008 (P<0.001), but there was no CO₂ effect on mean seed mass, viability, or nutrient content. Interannual variation in seed production was positively related to ANPP, with a similar percentage of ANPP diverted to reproduction across years. Seed production was negatively related to PET (P<0.005) and positively correlated with water availability (P<0.05), but showed no relationship with precipitation (P=0.88). This study adds to the few findings that, unlike herbaceous crops, woody plants may benefit from future atmospheric CO₂ by producing larger numbers of seeds without suffering degraded seed quality. Differential reproductive responses between functional groups and species could facilitate woody invasions or lead to changes in forest community composition as CO₂ rises.     

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.     

A free-air system for long-term stable carbon isotope labeling of adult forest trees

Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO₂ with a δ¹³C of −46.9‰. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO₂ gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO₂ concentration ([CO₂]) increase was ambient +100 μmol mol⁻¹. Canopy [CO₂] stayed within 10% of the target during more than 57% of the time and resulted in a drop of δ¹³C in canopy air by 7.8‰. After 19 labeling days about 50% of C in phloem sugars and stem CO₂ efflux were turned over and 20–30% in coarse root CO₂ efflux and soil CO₂. The isoFACE system successfully altered δ¹³C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.     

Life-long growth of Quercus ilex L. at natural CO2 springs acclimates sulphur, nitrogen and carbohydrate metabolism of the progeny to elevated pCO2

The aim of the present study was to analyse whether offspring of mature Quercus ilex trees grown under life‐long elevated pCO₂ show alterations in the physiological response to elevated pCO₂ in comparison with those originating from mature trees grown at current ambient pCO₂. To investigate changes in C‐ (for changes in photosynthesis, biomass and lignin see Polle, McKee & Blaschke Plant, Cell and Environment 24, 1075–1083, 2001), N‐, and S‐metabolism soluble sugar, soluble non‐proteinogenic nitrogen compounds (TSNN), nitrate reductase (NR), thiols, adenosine 5′‐phosphosulphate (APS) reductase, and anions were analysed. For this purpose Q. ilex seedlings were grown from acorns of mother tree stands at a natural spring site (elevated pCO₂) and a control site (ambient pCO₂) of the Laiatico spring, Central Italy. Short‐term elevated pCO₂ exposure of the offspring of control oaks lead to higher sugar contents in stem tissues, to a reduced TSNN content in leaves, and basipetal stem tissues, to diminished thiol contents in all tissues analysed, and to reduced APS reductase activity in both, leaves and roots. Most of the components of C‐, N‐ and S‐metabolism including APS reductase activity which were reduced due to short‐term elevated pCO₂ exposure were recovered by life‐long growth under elevated pCO₂ in the offspring of spring oaks. Still TSNN contents in phloem exudates increased, nitrate contents in lateral roots and glutathione in leaves and phloem exudates remained reduced in these plants. The present results demonstrated that metabolic adaptations of Q. ilex mother trees to elevated pCO₂ can be passed to the next generation. Short‐ and long‐term effects on source‐to‐sink relation and physiological and genetic acclimation to elevated pCO₂ are discussed.     

Decomposition and nitrogen release from leaves of three hardwood species grown under elevated O3 and/or CO2

Elevated concentrations of O₃ and CO₂ have both been shown to affect structure, nutrient status, and deposition of secondary metabolites in leaves of forest trees. While such studies have produced robust models of the effects of such air pollutants on tree ecophysiology and growth, few have considered the potential for broader, ecosystem-level effects after these chemically and structurally altered leaves fall as leaf litter and decay. To determine the effects of elevated O₃ and/or CO₂ on the subsequent decomposition and nutrient release from the leaves grown in such altered atmospheres, we grew seedlings of three widespread North American forest trees, black cherry (Prunus serotina) (BC), sugar maple (Acer saccharum) (SM), and yellow-poplar (Liriodendron tulipifera) (YP) for two growing seasons in charcoal-filtered air (CF-air=approximately 25% ambient O₃), ambient O₃ (1X) or twice-ambient O₃ (2X) in outdoor open-top chambers. We then assayed the loss of mass and N from the litter derived from those seedlings through one year litterbag incubations in the forest floor of a neighboring forest stand. Mass loss followed linear functions and was not affected by the O₃ regime in which the leaves were grown. Instantaneous decay rates (i.e. k values) averaged SM:–0.707 y^-1, BC:–0.613 y^-1, and YP:–0.859 y^-1. N loss from ambient (1X) O₃-grown SM leaves was significantly greater than from CF-air leaves: N loss from BC leaves did not differ among treatments. Significantly less N was released from CF-air-grown YP leaves than from 1X or 2X O₃-treated leaves. YP leaves from plants grown in pots at 2X O₃ and 350 ppm supplemental CO₂ in indoor pollutant fumigation chambers (CSTRs or Continuously Stirred Tank Reactors) loss 40% as much mass and 27% as much N over one year as did leaves from YP grown in CF-air or 2X O₃. Thus, for leaves from plants grown in pots in controlled environment fumigation chambers, the concentrations of both O₃ and CO₂ can affect N release from litter incubated in the field whereas mass loss rate was affected only by CO₂. Because both mass loss and N release from leaves grown at elevated CO₂ were reduced significantly (at least for yellow-poplar), forests exposed to elevated CO₂ may have significantly reduced N turnover rates, thereby resulting in increased N limitation of tree growth, especially in forests which are already N-limited.     

Sustained enhancement of photosynthesis in mature deciduous forest trees after 8 years of free air CO2 enrichment

Carbon uptake by forests constitutes half of the planet’s terrestrial net primary production; therefore, photosynthetic responses of trees to rising atmospheric CO₂ are critical to understanding the future global carbon cycle. At the Swiss Canopy Crane, we investigated gas exchange characteristics and leaf traits in five deciduous tree species during their eighth growing season under free air carbon dioxide enrichment in a 35-m tall, ca. 100-year-old mixed forest. Net photosynthesis of upper-canopy foliage was 48% (July) and 42% (September) higher in CO₂-enriched trees and showed no sign of down-regulation. Elevated CO₂ had no effect on carboxylation efficiency (V _cmax) or maximal electron transport (J _max) driving ribulose-1,5-bisphosphate (RuBP) regeneration. CO₂ enrichment improved nitrogen use efficiency, but did not affect leaf nitrogen (N) concentration, leaf thickness or specific leaf area except for one species. Non-structural carbohydrates accumulated more strongly in leaves grown under elevated CO₂ (largely driven by Quercus). Because leaf area index did not change, the CO₂-driven stimulation of photosynthesis in these trees may persist in the upper canopy under future atmospheric CO₂ concentrations without reductions in photosynthetic capacity. However, given the lack of growth stimulation, the fate of the additionally assimilated carbon remains uncertain.     

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.