Leaf and Stem CO(2) Uptake in the Three Subfamilies of the Cactaceae

Net CO₂ uptake over 24-hour periods was examined for the leaves and for the stems of 11 species of cacti representing all three subfamilies. For Pereskia aculeata, Pereskia grandifolia, and Maihuenia poeppigii (subfamily Pereskioideae), all the net shoot CO₂ uptake was by the leaves and during the daytime. In contrast, for the leafless species Carnegiea gigantea, Ferocactus acanthodes, Coryphantha vivipara, and Mammillaria dioica (subfamily Cactoideae), all the shoot net CO₂ uptake was by the stems and at night. Similarly, for leafless Opuntia ficus-indica (subfamily Opuntioideae), all net CO₂ uptake occurred at night. For leafy members of the Opuntioideae (Pereskiopsis porteri, Quiabentia chacoensis, Austrocylindropuntia subulata), at least 88% of the shoot CO₂ uptake over 24 hours was by the leaves and some CO₂ uptake occurred at night. Leaves responded to the instantaneous level of photosynthetically active radiation (PAR) during the daytime, as occurs for C₃ plants, whereas nocturnal CO₂ uptake by stems of O. ficus-indica and F. acanthodes responded to the total daily PAR, as occurs for Crassulacean acid metabolism (CAM) plants. Thus, under the well-watered conditions employed, the Pereskioideae behaved as C₃ plants, the Cactoideae behaved as CAM plants, and the Opuntioideae exhibited characteristics of both pathways.     

RELATION BETWEEN MONTHLY GROWTH OF FEROCACTUS ACANTHODES AND AN ENVIRONMENTAL PRODUCTIVITY INDEX

Most productivity studies use destructive harvesting methods, prohibiting continuous plant monitoring under various environmental conditions. Here a nondestructive procedure involving the displacement of areoles from the apex was utilized to study the growth of Ferocactus acanthodes, a common barrel cactus of the Sonoran Desert. Net CO₂ uptake measured under controlled conditions of temperature, water, and photosynthetically active radiation in the laboratory was used to indicate how F. acanthodes would respond to field values of these parameters. For example, net CO₂ uptake over 24 hr was maximal at day/night air temperatures of 23 C/14 C, the mean annual values in the field, and was approximately halved at 11 C/5 C and 32 C/23 C, the monthly extreme values in the field. An environmental productivity index (EPI), constructed as the product of indices for the three environmental variables, indicated the fraction of maximal CO₂ uptake expected. The monthly production of areoles on 33 plants was highly correlated with EPI (r² = 0.81). Areole production for individual plants, however, tended to be in pulses represented by Fibonacci numbers. EPI predicted an annual stem growth of 8% compared with 9 ± 3% measured previously in the field. Thus, morphological and physiological studies can be usefully combined and applied to indicate field productivity of F. acanthodes and, by extension, of other plants.     

Temperature,water, and PAR influences on predicted and measured productivity of Agave deserti at various elevations

Summary To measure productivity of Agave deserti over its elevational range in the northwestern Sonoran Desert, leaf unfolding from its basal rosette was monitored on groups of 10 plants at 13 sites. Based on data from an intermediate elevation (840 m), leaf unfolding proved to be highly correlated (r ²=0.88) with an environmental productivity index (EPI) formed as the product of indices for water status, temperature, and photosynthetically active radiation (PAR); each of these latter indices indicated the fraction of maximum net CO₂ uptake expected for that parameter based on laboratory measurements of gas exchange and field microclimatic data. At 840 m, the main environmental variable influencing leaf unfolding for A. deserti over a 2-y period was soil water potential. On steep slopes, however, leaf unfolding during the winter ranged from 0.7 leaves per 10 plants for north-facing slopes to 7.3 for south-facing slopes, reflecting the importance of PAR. Summer and winter rainfall increased 3-fold from elevations of 300 m to 1,200 m. Temperatures were more optimal for net CO₂ uptake at high elevations in the summer and at low elevations in the winter. Hence EPI increased with elevation in the summer but was maximal at an intermediate elevation in the winter. Moreover, measured leaf unfolding in both the summer and the winter closely followed the changes in EPI with elevation, indicating that productivity could be closely predicted for A. deserti based on physiological CO₂ responses and changes in environmental conditions with elevation.     

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO₂ uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m^-2 day^-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m^-2 day^-1). Maximum net CO₂ uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO₂ diffusion conductances and the mesophyll surface available for CO₂ absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m^-2 day^-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m^-2 day^-1).     

Carbon balance during CAM: an assessment of respiratory CO2 recycling in the epiphytic bromeliads Aechmea nudicaulis and Aechmea fendleri

Abstract The regulation of crassulacean acid metabolism (CAM) under controlled environmental conditions has been investigated for two tropical epiphytes, relating plant water and carbon balance to growth form and habitat preference under natural conditions. Aechmea fendleri is restricted to wet, upper montane regions of Trinidad, while A. nudicaulis has a wider distribution extending into more arid regions of the island. Morphological characteristics of these plants are related to habitat preference in terms of leaf succulence (0.44 and 0.94 kg m^?2 for the two species respectively) and a distinct layer of water storage parenchyma in A. nudicaulis In contrast, the thinner leaves of A. fendleri contain little water-storage parenchyma and less chlorenchyma per unit area, but the plants have a more open leaf rosette. The two species differ in expression of CAM, since the proportion of respiratory CO₂ recycled as part of CAM had been found to be much lower in A. fendleri This study compared the efficiency of water use and role of respiratory CO₂ recycling under two PAR regimes (300 and 120 μnol m^?2 s^?1) and three night temperatures (12, 18 and 25 °C). Dark CO₂ uptake rates for both species were comparable to plants in the field (maximum of 2.3 ± 0.2 μmol m^?2s^?1± SD, n= 3). Total net CO₂ uptake at night increased on leaf area basis with temperature for both species under higher PAR, although under the low PAR regime CO₂ uptake was maximal at 18 °C. Water-use efficiency (WUE) increased at 18 °C and 25 °C during dark CO₂ uptake (Phase I) and also during late afternoon photosynthesis (Phase IV) in both species. For A. fendleri, dawn to dusk changes in titrable acidity (ΔH ⁺) were similar under high and low PAR, although ΔH⁺ was correlated to night temperature and PAR in A. nudicaulis. The proportion of ΔH⁺ derived from respiratory CO₂ also varied with experimental conditions. Thus percentage recycling was lower in A. fendleri under high PAR (0–10%), but was only reduced at 18 °C under low PAR. Recycling by A. nudicaulis ranged from 32–42% under high PAR, but was also reduced to 6% under low PAR at 18 °C; at 12 °C and 25 °C, recycling was 37% and 52% respectively. Previous studies have suggested a relationship between the proportion of recycling and degree of water stress. This study indicated that CAM as a CO₂ concentrating mechanism regulates both water-use efficiency and plant carbon balance in these epiphytes, in response to PAR and night temperature. However, the precise relationship between respiratory processes and the balance between external and internal sources of CO₂ is as yet unresolved.     

Water relations and photosynthesis of a barrel cactus,Ferocactus acanthodes,in the Colorado desert

Summary The structural characteristics, water relations, and photosynthesis of Ferocactus acanthodes (Lemaire) Britton and Rose, a barrel cactus exhibiting Crassulacean acid metabolism (CAM), were examined in its native habitat in the western Colorado desert. Water storage in its succulent stem permitted nighttime stomatal opening ot continue for about 40 days after the soil water potential became less than that of the stem, a period whe the plant would be unable to extract water from the soil. After 7 months of drought and consequent unreplenished water loss from a plant, diurnal stomatal activity was not observed and the stem osmotic pressure was 6.4 bars, more than double the value measured during wet periods with nighttime stomatal opening. F. acanthodes had a shallow root system (mean depth of 8 cm) which responded within 24 h to rainfall.When the nocturnal stem surface temperature was raised from 8.0° C to 35.0° C, the stomatal resistance increased 4-fold, indicating that cool nighttime temperatures are advantageous for gas exchange by F. acanthodes. Moreover, the optimal temperature for CO₂ uptake in the dark was only 12.6° C. CO₂ uptake at night became maximal for 3.0 mEinsteins cm^-2 of photosynthetically active radiation incident during the preceding day, and the minimum number of incident quanta absorbed per CO₂ fixed was 68. The transpiration ratio (mass of water transpired/mass of CO₂ fixed) had the relatively low value of 70 for an entire year, consistent with values obtained for other CAM plants. The total amount of water annually diverted to the floral structures was about 6% of the stem wet weight. The annual growth increment estimated from the net CO₂ assimilation corresponded to about 10% of the stem mass for barrel cacti 34 cm tall, in agreement with measured dimension changes, and indicated that such plants were about 26 years old.     

COMPARATIVE CACTUS ARCHITECTURE AND PAR INTERCEPTION

Because CO₂ uptake by cacti can be limited by low levels of photosynthetically active radiation (PAR) and because plant form affects PAR interception, various cactus forms were studied using a computer model, field measurements, and laboratory phototropic studies. Model predictions indicated that CO₂ uptake by individual stems at an equinox was greatest when the stems were vertical, but at the summer and the winter solstice CO₂ uptake was greatest for stems tilted 30° away from the equator. Stem tilting depended on form and taxonomic group; four barrel cacti in Ferocactus and in Copiapoa and four cylindropuntias in Opuntia tilted toward a horizontal light beam by an average of 18°, 48°, and 52°, respectively, after growth periods of 1 to 4 yr. In contrast, three columnar species showed no significant phototropic response, perhaps because structural stability requires their massive stems to be erect. Field plants of the dense, multiple-stemmed shrub Opuntia echinocarpa had stems which tended to radiate outward from the plant base, and, although this would not influence the total PAR intercepted, it would result in a more uniform PAR distribution and hence higher CO₂ uptake. For O. echinocarpa and the even denser, mound-forming Echinocereus engelmannii, PAR and chlorophyll decreased approximately exponentially with depth into the canopy. The canopies of O. echinocarpa and other cylindropuntias did not extend to the ground; in certain species, such truncation apparently resulted from a combination of very low PAR levels just below the lower canopy edge and the light-dependent growth responses of individual stems. In addition, although the canopy surfaces of O. echinocarpa and O. acanthocarpa tilted toward the equator by about 30°, the canopies of other cylindropuntias tilted less or not at all; the computer model predicted that a 30° tilt would decrease interstem shading, increase daily PAR, and increase nocturnal CO₂ uptake by as much as 54, 26, and 24%, respectively. Not only can the shape of cacti be affected by PAR, but also shape influences PAR interception and hence CO₂ uptake.     

Productivity of Agave deserti: measurement by dry weight and monthly prediction using physiological responses to environmental parameters

Summary An environmental productivity index based on physiological responses to three environmental variables was used to predict the net productivity of a common succulent perennial of the Sonoran Desert, Agave deserti, on a monthly basis. Productivity was also independently measured in the field from dry weight changes. The index was based on soil water availability, day/night air temperatures, and photosynthetically active radiation (PAR), which were individually varied in the laboratory and the effect on net CO₂ uptake by the leaves determined. From monthly precipitation, temperature, and PAR at the field site together with the responses measured in the laboratory, an index (maximum value of unity) was assigned to each of these three environmental variables and their product was termed the environmental productivity index. This index indicates the fraction of maximal CO₂ uptake expected in the field for each month (well-watered A. deserti assimilated 285 mmol CO₂ m^-2 leaf area day^-1 at PAR saturation and optimal day/night temperatures of 25° C/15° C). The dry weight analysis was based on the monthly unfolding of new leaves from the central spike of the rosette and their seasonal increase in dry weight, which were determined in the field. The production of new leaves was highly correlated with the environmental productivity index (r²=0.93), which in turn was highly correlated with the water status index (r²=0.97). After correction for respiration by folded leaves, stem, and roots, plant productivity predicted by the average environmental productivity index (0.36) over a wet June-to-October period agreed within 4% with the productivity based on the conventional dry weight analysis. The net productivity of A. deserti over this 5-month period was 0.57 kg m^-2 ground area (5.7 Mg ha^-1), a large value for a desert CAM plant. The environmental productivity index proposed here may provide a reliable means for predicting net productivity on a monthly basis, which may be particularly useful for species in relatively variable environments such as deserts.     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

Responses of CAM species to increasing atmospheric CO2 concentrations

Crassulacean acid metabolism (CAM) species show an average increase in biomass productivity of 35% in response to a doubled atmospheric CO₂ concentration. Daily net CO₂ uptake is similarly enhanced, reflecting in part an increase in chlorenchyma thickness and accompanied by an even greater increase in water‐use efficiency. The responses of net CO₂ uptake in CAM species to increasing atmospheric CO₂ concentrations are similar to those for C₃ species and much greater than those for C₄ species. Increases in net daily CO₂ uptake by CAM plants under elevated atmospheric CO₂ concentrations reflect increases in both Rubisco‐mediated daytime CO₂ uptake and phosphoenolpyruvate carboxylase (PEPCase)‐mediated night‐time CO₂ uptake, the latter resulting in increased nocturnal malate accumulation. Chlorophyll contents and the activities of Rubisco and PEPCase decrease under elevated atmospheric CO₂, but the activated percentage for Rubisco increases and the K_M(HCO₃ ^? ) for PEPCase decreases, resulting in more efficient photosynthesis. Increases in root:shoot ratios and the formation of additional photosynthetic organs, together with increases in sucrose‐Pi synthase and starch synthase activity in these organs under elevated atmospheric CO₂ concentrations, decrease the potential feedback inhibition of photosynthesis. Longer‐term studies for several CAM species show no downward acclimatization of photosynthesis in response to elevated atmospheric CO₂ concentrations. With increasing temperature and drought duration, the percentage enhancement of daily net CO₂ uptake caused by elevated atmospheric CO₂ concentrations increases. Thus net CO₂ uptake, productivity, and the potential area for cultivation of CAM species will be enhanced by the increasing atmospheric CO₂ concentrations and the increasing temperatures associated with global climate change.     

Short-Term and Long-Term Responses of Crassulacean Acid Metabolism Plants to Elevated CO(2)

For the leaf succulent Agave deserti and the stem succulent Ferocactus acanthodes, increasing the ambient CO₂ level from 350 microliters per liter to 650 microliters per liter immediately increased daytime net CO₂ uptake about 30% while leaving nighttime net CO₂ uptake of these Crassulacean acid metabolism (CAM) plants approximately unchanged. A similar enhancement of about 30% was found in dry weight gain over 1 year when the plants were grown at 650 microliters CO₂ per liter compared with 350 microliters per liter. Based on these results plus those at 500 microliters per liter, net CO₂ uptake over 24-hour periods and dry weight productivity of these two CAM succulents is predicted to increase an average of about 1% for each 10 microliters per liter rise in ambient CO₂ level up to 650 microliters per liter.     

CO<Subscript>2</Subscript> exchange in a temperate Japanese cypress forest compared with that in a cool-temperate deciduous broad-leaved forest

To examine the characteristics of carbon exchange in coniferous forests, we analysed the seasonal and diurnal patterns of CO₂ exchange, as measured using the eddy covariance method, in a Japanese cypress forest in the Kiryu Experimental Watershed (KEW) in central Japan. The net CO₂ exchange data during periods of low-friction velocity conditions and during periods of missing data were interpolated. The daily CO₂ uptake was observed throughout the year, with maximum values occurring in early summer. Periods of low carbon uptake were seen in late summer owing to high respiratory CO₂ efflux. The diurnal and seasonal patterns of daytime CO₂ exchange at KEW were compared with those in a cool-temperate deciduous forest of the Tomakomai Experimental Forest (TOEF) in Japan. The environmental differences between evergreen and deciduous forests affected the seasonal patterns of carbon uptake. Although there were great differences in the mean monthly air temperatures between the sites, the mean monthly daytime carbon uptake was almost equal at both sites during the peak growing period. The carbon-uptake values at the same PAR level were greater before noon than after noon, especially at TOEF, suggesting the stomatal regulation of carbon uptake.     

BRANCHING PATTERNS OF COLUMNAR CACTI: INFLUENCES ON PAR INTERCEPTION AND CO2 UPTAKE

Field measurements and a computer model were used to determine how stem shape and arrangement of stems in space affect interception of photosynthetically active radiation (PAR) and CO₂ uptake under otherwise optimal conditions for four species of columnar cacti (Carnegiea gigantea, Lophocereus schottii, Pachycereus pringlei, and Stenocereus thurberi). In simulations where the number of widely spaced stems was increased from 1 to 19 but plant volume remained constant, surface area and PAR interception increased, leading to 3-fold increases in whole-plant CO₂ uptake. Increasing the distance between stems from 0 cm to infinity decreased self-shading and increased predicted CO₂ uptake 4-fold. Stem length, diam, ribbing characteristics, and spine coverage also influenced PAR interception. The model indicated that the observed higher frequency of branches on the south side of the trunk of C. gigantea had only a slight, though positive, effect on CO₂ uptake for single-branched plants. Because of its greater surface area (A), a five-stemmed plant of C. gigantea typical for a field site near Tucson, Arizona was predicted to have 52% more CO₂ uptake than a single-stemmed plant of the same volume (V). Although large A/V decreases water storage per unit transpiring area, whole-plant CO₂ uptake can be increased when A/V is increased by branching for these constant-volume plants. However, the stems must be arranged to avoid excessive self-shading and thus keep the area below PAR compensation small.     

Leaf expansion,net CO2 uptake,Rubisco activity,and efficiency of long-term biomass gain for the common desert subshrub Encelia farinosa

Encelia farinosa is one of the most abundant and highly studied species of the Sonoran Desert, yet characteristics of its leaf development and long-term photosynthetic capacity are relatively unknown. The net CO₂ uptake rate and the Rubisco activity per unit leaf area for E. farinosa in a glasshouse increased in parallel for about 18 days after leaf emergence (leaf area was then 5 cm²), after which both were constant, suggesting that Rubisco levels controlled net CO₂ uptake. Instantaneous net CO₂ uptake rates at noon for well-watered E. farinosa in the glasshouse at different temperatures and light levels correctly predicted differences in daily net CO₂ uptake at four seasonally diverse times for transplanted plants under irrigated conditions in the field but overpredicted the daily means by 13%. After this correction, seasonally adjusted net CO₂ uptake per unit leaf area multiplied by the estimated monthly leaf area predicted that 42% of the net carbon gain was incorporated into plant dry weight over a 17-month period. The ecological success of E. farinosa apparently reflects an inherently high daily net CO₂ uptake and retention of a substantial fraction of its leaf carbon gain.     

CO2 uptake patterns depend on water current velocity and shoot morphology in submerged stream macrophytes

1. The influence of current velocity on the pattern of photosynthetic CO₂ uptake in three species of submerged stream macrophytes was described by analysing the grain density in autoradiographs of leaves exposed to ¹⁴CO₂. 2. In Elodea canadensis, the CO₂ uptake was approximately two‐fold higher near the leaf periphery compared with the midrib section at high current velocity, whereas at low current velocity the area of relatively high CO₂ uptake expanded from the leaf periphery towards the midrib and basal sections of the leaves. 3. In Potamogeton crispus and Callitriche stagnalis the CO₂ uptake was uniform throughout the leaves at low current velocity, whereas at high current velocity the CO₂ uptake appeared to increase randomly in some areas of the leaves. 4. The relationship between the photosynthetic CO₂ uptake pattern and the dynamics of flow surrounding submerged shoots at low and high current velocity is discussed in relation to shoot morphology. In E. canadensis, thick diffusive boundary layers may develop between leaves because of screening effects at high current velocity. Increased diffusion path for CO₂ may contribute to inhibitory effects on photosynthesis in this species.     

Influences of PAR, temperature and water vapor concentration on gas exchange by ferns

The effects of photosynthetically active radiation (PAR), leaf temperature and the leaf-to-air water vapor concentration drop on net CO₂ uptake and water vapor conductance were surveyed for 14 species of ferns. Most previous studies indicated that ferns have extremely low maximal rates of net CO₂ uptake, below 2 umol m^?2 s^?1, whereas the average maximal rate observed here at 25⁰ C was 7 umol m^?2 s^?1. Net CO₂ uptake reached 90% of saturation at an average PAR (400 to 700 nm) of only 240 umol m^?2 s^?1, consistent with the typically shaded habitats of most ferns. Maximal CO₂ uptake rates were positively correlated with the PAR for 90% saturation (r²=0.59), the chlorophyII per unit leaf area (r²=0.30), the water vapor conductance (r²=0.65), and the CO₂ residual conductance (r²=0.69). A higher water vapor conductance (gwv) was correlated with a greater fractional change in gwv as the leaf-to-air water vapor concentration drop (Δc_wv) was raised from 5to20 g m^?3 (r²=0.90). Specifically, for species with low g_wv of about I mm s^?1 the ratio of g_wv at Δc_wv= 5 g m^?3 to that at Δc_wv= 20 g m^?3 was near 1, but it was near 2 for species with g_wv of about 4 mm s^?1. Such a relationship, which can prevent excessive transpiration, has apparently not previously been pointed out in surveys of other plant groups.     

Allometric Root/Shoot Relationships and Predicted Water Uptake for Desert Succulents

Root morphology, shoot morphology, and water uptake for Agavedeserti and Ferocactus acanthodes of various sizes were studiedusing allometric relationships (y = ax^b) and a previously developedwater uptake model. Shoot surface area increased with shootvolume with an exponent b of 0.75 for both species. Root lengthand the ground area explored by the roots increased with shootsurface area with b's of 0.72 for A. deserti and 0.92 for F.acanthodes. Various sized individuals had about the same ratioof root length to explored ground area, with higher values occurringfor A. deserti. Predicted water uptake averaged over the exploredground area was approximately constant over a 10⁴-fold rangein shoot surface area, suggesting that shoot size confers nointraspecific competitive advantage for water uptake. For theroot lengths per explored ground area observed in the field,water uptake was predicted to be 85 per cent of maximal; wateruptake could be increased by the production of more rain roots.When differences in shoot volume were accounted for by allometry,small plants had relatively less shoot surface area and relativelymore root length per shoot volume than did large plants, whichmay be important for the water relations of seedling establishment. Agave deserti, Ferocactus acanthodes, allometry, desert succulents, root distribution, root length, seedling growth, seedling establishment, shoot surface area, shoot volume, water uptake     

Growing season net ecosystem CO2 exchange of two desert ecosystems with alkaline soils in Kazakhstan

Central Asia is covered by vast desert ecosystems, and the majority of these ecosystems have alkaline soils. Their contribution to global net ecosystem CO₂ exchange (NEE) is of significance simply because of their immense spatial extent. Some of the latest research reported considerable abiotic CO₂ absorption by alkaline soil, but the rate of CO₂ absorption has been questioned by peer communities. To investigate the issue of carbon cycle in Central Asian desert ecosystems with alkaline soils, we have measured the NEE using eddy covariance (EC) method at two alkaline sites during growing season in Kazakhstan. The diurnal course of mean monthly NEE followed a clear sinusoidal pattern during growing season at both sites. Both sites showed significant net carbon uptake during daytime on sunny days with high photosynthetically active radiation (PAR) but net carbon loss at nighttime and on cloudy and rainy days. NEE has strong dependency on PAR and the response of NEE to precipitation resulted in an initial and significant carbon release to the atmosphere, similar to other ecosystems. These findings indicate that biotic processes dominated the carbon processes, and the contribution of abiotic carbon process to net ecosystem CO₂ exchange may be trivial in alkaline soil desert ecosystems over Central Asia.     

Environmental productivity indices and productivity for Opuntia ficus-indica under current and elevated atmospheric CO2 levels*

Abstract. The productivity of the prickly-pear cactus Opuntia ficus-indica, which is cultivated worldwide for its fruits and stem segments, was predicted based on the responses of its net CO₂ uptake to soil water status, air temperature and photosynthetic photon flux density (PPFD). Each of these environmental factors was represented by an index with a maximum value of unity when that factor was not limiting net CO₂ uptake over a 24-h period. The water index, the temperature index, and the PPFD index were determined for 87 sites in the contiguous United States using data from 189 weather stations and for 148 sites worldwide using data from 1464 weather stations. The product of these three indices, the environmental productivity index (EPI), was used to predict the productivity of O. ficus-indica under current climatic conditions and under those accompanying a possible increase in the atmospheric CO₂ level to 650μumol mol^?1. Sites with temperatures always above -10°C and hence suitable for prickly-pear cultivation numbered 37 in the United States and 110 worldwide; such sites increased by 43 and 5%, respectively, for the global warming accompanying the elevated CO₂. Productivity of O. ficus-indica was at least 15 tonnes dry weight hectare^?1 year^?1, comparable to that of many agronomic crops, for 20 sites with temperatures always above -10°C in the contiguous United States and for 12 such sites worldwide under current climatic conditions; such sites increased by 85 and 117%, respectively, under the elevated CO₂ condition, mainly because of direct effects of the atmospheric CO₂ level on net CO₂ uptake. In summary, simulations based on EPI indicate that O. ficus-indica may presently be advantageously cultivated over a substantial fraction of the earth's surface, such regions increasing markedly with a future doubling in atmospheric CO₂ levels.     

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO₂, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO₂, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO₂, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO₂. Beech had a greater net daily C gain with CO₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO₂. Elevated CO₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO₂ enrichment when predicting potential change in species distribution with global climate change.