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1.
Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO2]. Net ecosystem CO2 exchange (NEE) was measured in the eighth year of CO2 enrichment at the Nevada Desert Free‐Air CO2 Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO2 m?2 s?1) of ecosystems exposed to elevated atmospheric CO2 were similar to those maintained at current ambient CO2 levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO2] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO2]. Mean daily NEE varied seasonally across both CO2 treatments, increasing from about 0.1 μmol CO2 m?2 s?1 in December to a maximum of 0.5–0.6 μmol CO2 m?2 s?1 in early spring. Maximum NEE in ecosystems exposed to elevated CO2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO2, with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO2 m?2 s?1), but then increasing to near peak levels in late October (0.36±0.08 μmol CO2 m?2 s?1), November (0.28±0.03 μmol CO2 m?2 s?1), and December (0.54±0.06 μmol CO2 m?2 s?1). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO2 uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO2 treatments were net CO2 sinks in 2004, but exposure to elevated CO2 reduced CO2 sink strength by 30% (positive net ecosystem productivity=127±17 g C m?2 yr?1 ambient CO2 and 90±11 g C m?2 yr?1 elevated CO2, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO2– along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.  相似文献   

2.
Net grassland carbon flux over a subambient to superambient CO2 gradient   总被引:2,自引:0,他引:2  
Increasing atmospheric CO2 concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol?1 to 550 µmol mol?1 CO2 gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO2 concentrations (200–360 µmol mol?1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol?1). Continuous CO2 gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO2 flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO2 fluxes were maximal in late May (c. day 150) in 1998 and in late August in 1999 (c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO2 flux in 1998 was 13 g CO2 m?2 day?1 in the subambient chamber and 20 g CO2 m?2 day?1 in the superambient chamber; comparable averages were 15 and 26 g CO2 m?2 day?1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO2 concentration; a finding likely to be explained by inherent differences in vegetation. Because C3 plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C3 cover. Adjusted fluxes were better correlated with CO2 concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO2 flux increased linearly with CO2 concentration. This trend was more evident at subambient than superambient CO2 concentrations.  相似文献   

3.
Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO2 mol?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO2 uptake, with 34% more total daily CO2 uptake under the doubled CO2 concentration and most of the increase occurring in the late afternoon. For both CO2 concentrations, 90% of the maximal daily CO2 uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m?2 day?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO2 uptake by doubling the CO2 concentration was greater under the highest PPFD (30 mol m?2 day?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO2 uptake under 370 μmol CO2 mol?1 was 25% of that under 740 μmol CO2 mol?1. The ratio of variable to maximal chlorophyll fluorescence (Fy/Fm) decreased as the PPFD was raised above 5 mol m?2 day?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. Fv/Fm was higher under the doubled CO2 concentration, indicating that the current CO2 concentration was apparently limiting for photosynthesis. Thus net CO2 uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO2 concentration, especially under stressful environmental conditions.  相似文献   

4.
CO2 exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m?2S?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO2 evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO2 (g dry weight)?1 S?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m?2 S?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O10 value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO2 exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO2 concentrations via CO2 refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m?2 S?1. Young peach fruits responded to increasing ambient CO2 concentrations with decreasing net CO2 exchange rates in light, but more mature fruits did not respond to increases in ambient CO2. Fruit CO2 exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO2 concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO2 response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO21?2. In CO2 free air, fruit photosynthesis was dependent on CO2 refixation since CO2 uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO2-free air and 370 μl CO2 1?1 indicated that young peach fruits were apparently able to take up CO2 from the external atmosphere. CO2 uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO2 refixation.  相似文献   

5.
Rates of atmospheric CH4 consumption of soils in temperate forest were compared in plots continuously enriched with CO2 at 200 µL L?1 above ambient and in control plots exposed to the ambient atmosphere of 360 µL CO2 L?1. The purpose was to determine if ecosystem atmospheric CO2 enrichment would alter soil microbial CH4 consumption at the forest floor and if the effect of CO2 would change with time or with environmental conditions. Reduced CH4 consumption was observed in CO2‐enriched plots relative to control plots on 46 out of 48 sampling dates, such that CO2‐enriched plots showed annual reductions in CH4 consumption of 16% in 1998 and 30% in 1999. No significant differences were observed in soil moisture, temperature, pH, inorganic‐N or rates of N‐mineralization between CO2‐enriched and control plots, indicating that differences in CH4 consumption between treatments were likely the result of changes in the composition or size of the CH4‐oxidizing microbial community. A repeated measures analysis of variance that included soil moisture, soil temperature (from 0 to 30 cm), and time as covariates indicated that the reduction of CH4 consumption under elevated CO2 was enhanced at higher soil temperatures. Additionally, the effect of elevated CO2 on CH4 consumption increased with time during the two‐year study. Overall, these data suggest that rising atmospheric CO2 will reduce atmospheric CH4 consumption in temperate forests and that the effect will be greater in warmer climates. A 30% reduction in atmospheric CH4 consumption by temperate forest soils in response to rising atmospheric CO2 will result in a 10% reduction in the sink strength of temperate forest soils in the atmospheric CH4 budget and a positive feedback to the greenhouse effect.  相似文献   

6.
Growth response of cotton to CO2 enrichment in differing light environments   总被引:1,自引:0,他引:1  
Experiments were conducted to examine the growth responses of cotton (Gossypium hirsutum L. cv. Coker 315) to CO2 enrichment under different light regimes. Plants were exposed to 350 or 700 μl l?1 CO2 and six light treatments differing in photosynthetic period length (8 or 16 h) and in photosynthetic photon flux density (PPFD) for 32 days of vegetative growth. Higher PPFD (1 100 μmol m?2 s?1) was provided by a combination of high intensity discharge and incandescent lamps (HID), and lower PPFD (550 μmol m?2 s?1) was provided by fluorescent and incandescent lamps (F) or HID and incandescent lamps with shade cloth (HIDs). Growth was generally much slower with the 8-h photosynthetic periods, but the growth stimulation by CO2 enrichment was larger than with 16-h photosynthetic periods. After 28 to 32 days of treatment, the growth enhancement with CO2 enrichment was 152 and 78% for 8- and 16-h photosynthetic periods, respectively, under HID; 100 and 77% in F, and 77 and 56% in HIDs. The higher PPFD of HID positively influenced the CO2 effect only at the slower growth rate in the 8-h light period. The stimulation of leaf area expansion by CO2 enrichment was also greater with the 8-h photosynthetic period for all light sources. These results, and others on net assimilation rate, shoot to root dry weight ratios and specific leaf weights, suggest that the growth response to CO2 enrichment with the longer photosynthetic period was depressed by limiting factors, perhaps nutritional, in the growth environment. The results also show that extensive variability in CO2 response can occur under light intensities which are often used in growth chamber experiments.  相似文献   

7.
The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO2 upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO2. Equal total daily photon flux density (PFD) (~7·7 mol m?2 d?1) was supplied as either uniform irradiance (~170 µmol m?2 s?1) or shade/fleck sequences (~30 µmol m?2 s?1/~525 µmol m?2 s?1). Photosynthesis and growth were enhanced by elevated CO2 treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO2 but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO2 were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO2 under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.  相似文献   

8.
Increased fire frequency in the Great Basin of North America's intermountain West has led to large‐scale conversion of native sagebrush (Artemisia tridentata Nutt.) communities to postfire successional communities dominated by native and non‐native annual species during the last century. The consequences of this conversion for basic ecosystem functions, however, are poorly understood. We measured net ecosystem CO2 exchange (NEE) and evapotranspiration (ET) during the first two dry years after wildfire using a 4‐m diameter (16.4 m3) translucent static chamber (dome), and found that both NEE and ET were higher in a postfire successional ecosystem (?0.9–2.6 µ mol CO2 m?2 s?1 and 0.0–1.0 mmol H2O m?2 s?2, respectively) than in an adjacent intact sagebrush ecosystem (?1.2–2.3 µ mol CO2 m?2 s?1 and ?0.1–0.8 mmol H2O m?2 s?2, respectively) during relatively moist periods. Higher NEE in the postfire ecosystem appears to be due to lower rates of above‐ground plant respiration while higher ET appears to be caused by higher surface soil temperatures and increased soil water recharge after rains. These patterns disappeared or were reversed, however, when the conditions were drier. Daily net ecosystem productivity (NEP; g C m?2 d?1), derived from multiple linear regressions of measured fluxes with continuously measured climate variables, was very small (close to zero) throughout most of the year. The wintertime was an exception in the intact sagebrush ecosystem with C losses exceeding C gains leading to negative NEP while C balance of the postfire ecosystem remained near zero. Taken together, our results indicate that wildfire‐induced conversion of native sagebrush steppe to ecosystems dominated by herbaceous annual species may have little effect on C balance during relatively dry years (except in winter months) but may stimulate water loss immediately following fires.  相似文献   

9.
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.  相似文献   

10.
The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO2 concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m?2 s?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO2 (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO2 enhanced photosynthesis but leaf respiration rates were not affected by either the CO2 or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO2, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO2. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO2 and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.  相似文献   

11.
An investigation to determine whether stomatal acclimation to [CO2] occurred in C3/C4 grassland plants grown across a range of [CO2] (200–550 µmol mol?1) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (gs) to a range of intercellular CO2 (a gsCi curve) at each growth [CO2] in the third and fourth growing seasons of the treatment. The gsCi response curves for Solanum dimidiatum (C3 perennial forb) differed significantly across [CO2] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO2] in this species was also found as maximum gs (gsmax; gs measured at the lowest Ci) increased with decreasing growth [CO2] only below 400 µmol mol?1. The substantial increase in gs at subambient [CO2] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (Vcmax; r2 = 0·27) and was not associated with CO2 saturated photosynthesis (Amax). The response of gs to Ci did not vary with growth [CO2] in Bromus japonicus (C3 annual grass) or Bothriochloa ischaemum (C4 perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO2] in both C3 species, was not correlated with gs. Larger stomatal size at subambient [CO2], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO2 and to understand their magnitudes relative to the past.  相似文献   

12.
It is estimated that more than 100 geothermal CO2 springs exist in central-western Italy. Eight springs were selected in which the atmospheric CO2 concentrations were consistently observed to be above the current atmospheric average of 354μmol mol-1. CO2 concentration measurements at some of the springs are reported. The springs are described, and their major topographic and vegetational features are reported. Preliminary observations made on natural vegetation growing around the gas vents are then illustrated. An azonal pattern of vegetation distribution occurs around every CO2 spring regardless of soil type and phytoclimatic areas. This is composed of pioneer populations of a Northern Eurasiatic species (Agrostis canina L.) which is often associated with Scirpus lacustris L. The potential of these sites for studying the long-term response of vegetation to rising atmospheric CO2 concentrations is discussed.  相似文献   

13.
This study investigated the spatial and temporal variation in soil carbon dioxide (CO2) efflux and its relationship with soil temperature, soil moisture and rainfall in a forest near Manaus, Amazonas, Brazil. The mean rate of efflux was 6.45±0.25 SE μmol CO2 m?2s?1 at 25.6±0.22 SE°C (5 cm depth) ranging from 4.35 to 9.76 μmol CO2 m?2s?1; diel changes in efflux were correlated with soil temperature (r2=0.60). However, the efflux response to the diel cycle in temperature was not always a clear exponential function. During period of low soil water content, temperature in deeper layers had a better relationship with CO2 efflux than with the temperature nearer the soil surface. Soil water content may limit CO2 production during the drying‐down period that appeared to be an important factor controlling the efflux rate (r2=0.39). On the other hand, during the rewetting period microbial activity may be the main controlling factor, which may quickly induce very high rates of efflux. The CO2 flux chamber was adapted to mimic the effects of rainfall on soil CO2 efflux and the results showed that efflux rates reduced 30% immediately after a rainfall event. Measurements of the CO2 concentration gradient in the soil profile showed a buildup in the concentration of CO2 after rain on the top soil. This higher CO2 concentration developed shortly after rainfall when the soil pores in the upper layers were filled with water, which created a barrier for gas exchange between the soil and the atmosphere.  相似文献   

14.
Abstract Increasing atmospheric CO2 may result in alleviation of salinity stress in salt-sensitive plants. In order to assess the effect of enriched CO2 on salinity stress in Andropogon glomeratus, a C4 non-halophyte found in the higher regions of salt marshes, plants were grown at 350, 500, and 650 cm3 m?3 CO2 with 0 or 100 mol m?3 NaCl watering treatments. Increases in leaf area and biomass with increasing CO2 were measured in salt-stressed plants, while decreases in these same parameters were measured in non-salt-stressed plants. Tillering increased substantially with increasing CO2 in salt-stressed plants, resulting in the increased biomass. Six weeks following initiation of treatments, there was no difference in photosynthesis on a leaf area basis with increasing CO2 in salt-stressed plants, although short-term increases probably occurred. Stomatal conductance decreased with increasing CO2 in salt-stressed plants, resulting in higher water-use efficiency, and may have improved the diurnal water status of the plants. Concentrations of Na+ and Cl? were higher in salt stressed-plants while the converse was found for K +. There were no differences in leaf ion content between CO2 treatments in the salt-stressed plants. Decreases in photosynthesis in salt-stressed plants occurred primarily as a result of decreased internal (non-stomatal) conductance.  相似文献   

15.
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.  相似文献   

16.
Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO2 levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C4 grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m−2 s−1 versus 350 μmol m−2 s−1 shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO2 when compared to plants at ambient CO2. This was due primarily to the 50% reduction in stomatal conductance at elevated CO2, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO2. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO2 enhanced plant water status. CO2-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO2 on plant water relations in all ecosystems.  相似文献   

17.
Native scrub‐oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO2] (approx. 350 μmol mol?1) and to high [CO2] (increased by 350 μmol mol?1). Stomatal and photosynthetic acclimation to high [CO2] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (gs) was approximately 40% lower in the high‐ compared to low‐[CO2]‐grown plants when measured at their respective growth concentrations. Reciprocal measurements of gs in both high‐ and low‐[CO2]‐grown plants showed that there was negative acclimation in the high‐[CO2]‐grown plants (9–16% reduction in gs when measured at 700 μmol mol?1), but these were small compared to those for net CO2 assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO2] (ci) which showed a reduction in stomatal sensitivity at low ci in the high‐[CO2]‐grown plants. Stomatal density showed no change in response to growth in high growth [CO2]. Long‐term stomatal and photosynthetic acclimation to growth in high [CO2] did not markedly change the 2·5‐ to 3‐fold increase in gas‐exchange‐derived water use efficiency caused by high [CO2].  相似文献   

18.
Currently, it is unknown what role tropical forest soils will play in the future global carbon cycle under higher temperatures. Many tropical forests grow on deeply weathered soils and although it is generally accepted that soil carbon decomposition increases with higher temperatures, it is not known whether subsurface carbon pools are particularly responsive to increasing soil temperatures. Carbon dioxide (CO2) diffusing out of soils is an important flux in the global carbon. Although soil CO2 efflux has been the subject of many studies in recent years, it remains difficult to deduct controls of this flux because of the different sources that produce CO2 and because potential environmental controls like soil temperature and soil moisture often covary. Here, we report results of a 5‐year study in which we measured soil CO2 production on two deeply weathered soil types at different depths in an old‐growth tropical wet forest in Costa Rica. Three sites were developed on old river terraces (old alluvium) and the other three were developed on old lava flows (residual). Annual soil CO2 efflux varied between 2.8–3.6 μmol CO2‐C m?2 s?1 (old alluvium) and 3.4–3.9 μmol CO2‐C m?2 s?1 (residual). More than 75% of the CO2 was produced in the upper 0.5 m (including litter layer) and less than 7% originated from the soil below 1 m depth. This low contribution was explained by the lack of water stress in this tropical wet forest which has resulted in very low root biomass below 2 m depth. In the top 0.5 m CO2 production was positively correlated with both temperature and soil moisture; between 0.6 and 2 m depth CO2 production correlated negatively with soil moisture in one soil and positively with photosynthetically active radiation in the other soil type. Below 2 m soil CO2 production strongly increased with increasing temperature. In combination with reduced tree growth that has been shown for this ecosystem, this would be a strong positive feedback to ecosystem warming.  相似文献   

19.
Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO2, ambient + 165 μmol mol?1 CO2 or ambient + 330 μmol mol?1 CO2 concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO2 treatments in all seasons. On the basis of A/Ci, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO2 treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO2 treatment, rates of net assimilation were ~1·6 times greater in the ambient + 165 μmol mol?1 CO2 treatment and 2·2 times greater in the ambient + 330 μmol mol?1 CO2 treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO2 concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO2 (+ 165 or + 330 μmol mol?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.  相似文献   

20.
N2 fixation by Acacia species increases under elevated atmospheric CO2   总被引:1,自引:0,他引:1  
In the present study the effect of elevated CO2 on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol?1) or elevated (700 µmol mol?1) CO2 concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO2. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO2, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO2 increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.  相似文献   

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