Nocturnal CO2 Research Articles

Comparing CO2 Storage and Advection Conditions at Night at Different Carboeuroflux Sites

Anemometer and CO2 concentration data from temporary campaigns performed at six CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulent fluxes in nighttime conditions. While storage was observed to be significant only during periods of both low turbulence and low advection, the advective fluxes strongly influence the nocturnal CO2 balance, with the exception of almost flat and highly homogeneous sites. On the basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’, which takes net radiation and local topography into account, was introduced as a criterion to characterise the conditions of storage enrichment/depletion. Comparative analyses of the six sites showed several common features of the advective fluxes but also some substantial differences. In particular, all sites where advection occurs show the onset of a boundary layer characterised by a downslope flow, negative vertical velocities and negative vertical CO2 concentration gradients during nighttime. As a consequence, vertical advection was observed to be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. The main differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO2 concentration gradients, which is probably determined by the source/sink distribution. As a consequence, both positive and negative horizontal advective fluxes (corresponding respectively to CO2 removal from the ecosystem and to CO2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO2 exchange rates.

The effects of salinity, crassulacean acid metabolism and plant age on the carbon isotope composition of Mesembryanthemum crystallinum L., a halophytic C3-CAM species

The carbon isotope composition of the halophyte Mesembryanthemum crystallinum L. (Aizoaceae) changes when plants are exposed to environmental stress and when they shift from C(3) to crassulacean acid metabolism (CAM). We examined the coupling between carbon isotope composition and photosynthetic pathway by subjecting plants of different ages to salinity and humidity treatments. Whole shoot delta(13)C values became less negative in plants that were exposed to 400 mM NaCl in the hydroponic solution. The isotopic change had two components: a direct NaCl effect that was greatest in plants still operating in the C(3) mode and decreased proportionally with increasing levels of dark fixation, and a second component related to the degree of CAM expression. Ignoring the presumably diffusion-related NaCl effect on carbon isotope ratios results in an overestimation of nocturnal CO(2) gain in comparison to an isotope versus nocturnal CO(2) gain calibration established previously for C(3) and CAM species grown under well-watered conditions. It is widely taken for granted that the shift to CAM in M. crystallinum is partially under developmental control and that CAM is inevitably expressed in mature plants. Plants, cultivated under non-saline conditions and high relative humidity (RH) for up to 63 days, maintained diel CO(2) gas-exchange patterns and delta(13)C values typical of C(3) plants. However, a weak CAM gas-exchange pattern and an increase in delta(13)C value were observed in non-salt-treated plants grown at reduced RH. These observations are consistent with environmental control rather than developmental control of the induction of CAM in mature M. crystallinum under non-saline conditions.

Effect of elevated atmospheric CO2 on oviposition behavior in Manduca sexta moths

AbstractMoths can detect changes in environmental carbon dioxide (CO2) with extremely high sensitivity, but the role of CO2 in the biology of these and other insects is not well understood. Although CO2 has been demonstrated to influence egg‐laying (oviposition) behavior of the pyralid moth Cactoblastis cactorum and nectar foraging of the sphingid moth Manduca sexta, information about the generalized role of CO2 in the behavioral biology of these species is lacking. Comparative data are necessary to properly assess how the behaviors of different species may be modified by steadily rising levels of greenhouse gases in the environment. Experiments carried out in Biosphere 2 addressed whether changes in ambient CO2 levels play a role in the oviposition behaviors of M. sexta moths. In the first series of experiments, oviposition was measured inside a flight cage with different levels of nearly ambient or elevated CO2 (400, 800 or 1200 ppm). For each concentration, hostplants used as oviposition sites were grown from seed at a CO2 level that matched the environment inside the flight cage. Under homogenous levels of CO2, we observed no significant difference in oviposition behavior at the concentrations tested. In a second series of experiments, two groups of hostplants, each surrounded by a mini free‐air CO2 enrichment (FACE) ring, were assembled inside a flight cage. In this choice test, a dynamic plume of artificially high CO2 was generated around one group of test plants, while ambient CO2 was released around the second (control) group. After eggs were counted on both plant groups, M. sexta females showed a small preference for ovipositing on the control plants. Therefore, in contrast to C. cactorum females tested under similar dynamic flow conditions, M. sexta female oviposition was not strongly inhibited by elevated CO2. To investigate this phenomenon further, we used electrophysiological recording and found that the CO2 receptor cells in M. sexta, unlike those in C. cactorum, are not readily affected by elevated levels of ambient CO2. These findings therefore suggest that elevated background levels of CO2 affect the physiology of the CO2 detection system of M. sexta to a lesser extent than that of C. cactorum, and this correlates well with the observed differences in oviposition behavior between the two species under elevated levels of environmental CO2. Hostplants of C. cactorum are crassulacean acid metabolism plants that generate nocturnal CO2 sinks on the cladode surfaces, whereas, M. sexta hostplants are nocturnal sources of respiratory CO2. We hypothesize that the abrupt and continuing increase in global ambient CO2 levels will differentially alter the behavior and physiology of moths that use CO2 sinks and sources as sensory cues to find hostplants.

Respiratory carbon losses and the carbon‐use efficiency of a northern hardwood forest, 1999–2003

Quantitative assessment of carbon (C) storage by forests requires an understanding of climatic controls over respiratory C loss. Ecosystem respiration can be estimated biometrically as the sum (R Sigma) of soil (Rs), leaf (Rl) and wood (Rw) respiration, and meteorologically by measuring above-canopy nocturnal CO2 fluxes (Fcn). Here we estimated R Sigma over 5 yr in a forest in Michigan, USA, and compared R Sigma and Fcn on turbulent nights. We also evaluated forest carbon-use efficiency (Ec = P(NP)/P(GP)) using biometric estimates of net primary production (P(NP)) and R Sigma and Fcn-derived estimates of gross primary production (P(GP)). Interannual variation in R Sigma was modest (142 g C m(-2) yr(-1)). Mean annual R Sigma was 1425 g C m(-2) yr(-1); 71% from Rs, 18% from Rl, and 11% from Rw. Hourly R Sigma was well correlated with Fcn, but 11 to 58% greater depending on the time of year. Greater R Sigma compared with Fcn resulted in higher estimated annual P(GP) and lower annual Ec (0.42 vs 0.54) using biometric and meteorological data, respectively. Our results provide one of the first multiyear estimates of R Sigma in a forested ecosystem, and document the responses of component respiratory C losses to major climatic drivers. They also provide the first assessment of Ec in a deciduous forest using independent estimates of P(GP).

Carbon isotope composition and water-use efficiency in plants with crassulacean acid metabolism.

The relationship between water-use efficiency, measured as the transpiration ratio (g H2O transpired g-1 above- plus below-ground dry mass accumulated), and 13C / 12C ratio (expressed as δ13C value) of bulk biomass carbon was compared in 15 plant species growing under tropical conditions at two field sites in the Republic of Panama. The species included five constitutive crassulacean acid metabolism (CAM) species [Aloe vera (L.) Webb & Berth., Ananas comosus (L.) Merr., Euphorbia tirucalli L., Kalanchoë daigremontiana Hamet et Perr., Kalanchoë pinnata (Lam.) Pers.], two species of tropical C3 trees (Tectona grandis Linn. f. and Swietenia macrophylla King), one C4 species (Zea mays L.), and seven arborescent species of the neotropical genus Clusia, of which two exhibited pronounced CAM. The transpiration ratios of the C3 and CAM species, which ranged between 496 g H2O g-1 dry mass in the C3-CAM species Clusia pratensis Seeman to 54 g H2O g-1 dry mass in the constitutive CAM species Aloe vera, correlated strongly with δ13C values and nocturnal CO2 gain suggesting that δ13C value can be used to estimate both water-use efficiency and the proportion of CO2 gained by CAM species during the light and the dark integrated over the lifetime of the tissues.

Welwitschia mirabilis: CAM or not CAM — what is the answer?

After more than 20 years of extensive study we found clear evidence that Welwitschia mirabilis Hook.f. is able to take up CO2 at night in both of its natural ecosystems, the Namib desert and the Mopane savannah, and hence should be classified a crassulacean acid metabolism (CAM) plant. At six different sites, 85 W. mirabilis plants were marked and the growth rate of their leaves and leaf ribbons were measured over a period of 2.5 years. The slowest and the fastest growing plant of these 85 plants were from the Mopane savannah and from the north-west of the Brandberg massif, respectively. These were selected for the gas-exchange measurements of this study. Within the course of a year nocturnal CO2 uptake was found only in December and January when the nights were shortest and plants were flowering. CO2 uptake during the night was not pronounced and never accounted for more than 4% of the total CO2 uptake over 24 h. Maximum rates of nocturnal CO2 uptake never exceeded 0.1 µmol m-2 s-1 for the slowest and 0.2 µmol m-2 s-1 for the fastest growing plant. Neither water availability in the soil nor night temperature was found to determine nocturnal CO2 uptake in terms known for CAM plants. Regardless of the growing site all leaves of W. mirabilis contained high amounts of malic and citric acid. Small increases of acids over night as calculated from the gas exchange measurements are masked by the extremely uneven distribution of these acids in the leaves, making the feature of an overnight malic or citric acid accumulation an unsuited test for CAM in W. mirabilis. An increase in 13C discrimination with increasing distance from the coast was confirmed. Photorespiration was extremely high and followed air temperature around the leaf. Although the debate whether or not W. mirabilis is a CAM plant can be closed, no answer could be given why W. mirabilis makes so little use of CAM.

Association of central sleep apnea and nocturnal CO 2 exhalation volume under oxygen inhalation in patients with chronic heart failure

We investigated impacts of O 2 on sleep apnea and nocturnal CO 2 exhalation volume in stable/severe CHF.Subjects were 7 patients with stable CHF (LVEF = 23.2 ± 7.6%, NYHA class = III). Nocturnal SaO 2 and sleep data were analyzed using PSG, with single breath CO 2 exhalation volume(SBCO 2) using a monitoring device.1. All of the patients had central sleep apnea (CSA). 2. SaO 24%ODI decreased with O 2 inhalation (7.9 ± 3.8 vs. 0.6 ± 0.5 times/hour, p < 0.01). 3. AHI was reduced with O 2 (19.7 ± 2.3 vs. 2.4 ± ;3.6 times/hour, p < 0.01). 4. SBCO 2 during sleep were: in room air, 44.0 ± 4.8ml during regular respiration and 119 ± 9.8ml at onset of CSA; in O 2, 90.8 ± 4.0ml during regular respiration. An increased level of the SBCO2 was attained with O 2.In CHF with CSA, a low level of SBCO 2 was observed. It was suggested that O 2 improved this low value and suppressed CSA.

Observing subcanopy CO 2 advection

Underestimation of nocturnal CO 2 respiration under calm conditions remains an unsolved problem at many forest flux stations. In this paper, the hypothesis is tested that horizontal mean transport, not previously measured, may account for the missing CO 2. A systematic methodology was developed that comprises characterizing the subcanopy motions, determining the appropriate size of the subcanopy network required to make the measurements, developing a method of integrating the measurements in the vertical, and determining the required averaging time. Measurements were performed at the Harvard Forest (Petersham, MA), over 4 years. The appropriate size of the network of wind and CO 2 sensors was shown to be on the order of 100 m, ensuring that sensors were generally observing coherent processes on this scale or larger and thus displayed some correlation. Horizontal transport of CO 2 at Harvard Forest was found to be restricted to the bottom ∼10 m of the forest. The fraction of the negative buoyancy force in the sum of dynamic driving forces described nights with missing flux problems (“deficit nights”) significantly better than the commonly used friction velocity criterion. Including the measured horizontal transport terms did not on average fully account for the observed difference in NEE of 1.2±0.3 μmol m −2 s −1 between deficit and non-deficit nights, but decreased the difference to 0.7±0.5 μmol m −2 s −1. Horizontal transport did account for the difference, to within measurement error, during summer months, but not during spring or fall.

Are tropical lowlands a marginal habitat for macrolichens? Evidence from a field study with Parmotrema endosulphureum in Panama

Summary There is a striking increase in the abundance of non-vascular organisms (mosses, liverworts, and macrolichens) with altitude in the tropics. The proposed mechanism behind this altitudinal gradient still awaits experimental verification. This study addresses this question using diel (= 24-h) time courses of CO 2 exchange, thallus water relations, and microclimate of the foliose lichen, Parmotrema endosulphureum , under natural tropical lowland conditions. In addition, we studied the responses of lichen gas exchange to experimental manipulations of incident light, temperature and thallus water content. Similar to the results of the only other field study on CO 2 exchange in lowland lichens, 24-h carbon gain was strongly limited by nocturnal CO 2 loss: the cumulative dark respiration reached almost 90% of diurnal carbon gain. Our estimates of long-term carbon gain are consistent with direct measurements of growth, and are substantially lower than those for lichens from montane sites in the tropics.

Physiological responses of the CAM epiphyte Tillandsia usneoides L. (Bromeliaceae) to variations in light and water supply

In an effort to understand the mechanisms that sustain rootless atmospheric plants, the modulation of Crassulacean acid metabolism (CAM) in response to variations in irradiance and water supply was investigated in the epiphyte Tillandsia usneoides. Plants were acclimated to three light regimes, i.e. high, intermediate and low, with integrated photon flux densities (PFD) of 14.40, 8.64 and 4.32 mol m-2 d-1 equivalent to an instantaneous PFD of 200, 100, and 50 mumol m-2 s-1, respectively. Daily watering was then withdrawn from half of the plants at each PFD for 7 d prior to sampling. In response to the three PFD treatments, chlorophyll content increased in plants acclimated to lower irradiances. Light response curves using non-invasive measurements of chlorophyll fluorescence demonstrated that photosystem II efficiency (phi PSII) was maintained in high PFD acclimated plants, as they exhibited a larger capacity for non-photochemical dissipation (NPQ) of excess light energy than low PFD acclimated plants. Net CO2 uptake increased in response to higher PFD, reflecting enhanced carboxylation capacity in terms of phosphoenolpyruvate carboxylase (PEPc) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) activities. After water was withdrawn, nocturnal net CO2 uptake and accumulated levels of acidity declined in all PFD treatments, concomitant with increased respiratory recycling of malate. Examining the strategies employed by epiphytes such as T. usneodies to tolerate extreme light and water regimes has demonstrated the importance of physiological mechanisms that allow flexible carboxylation capacity and continued carbon cycling to maintain photosynthetic integrity.

Measuring nighttime CO 2 flux over terrestrial ecosystems using eddy covariance and nocturnal boundary layer methods

The respiration from plants and soil is an important component of the carbon balance of ecosystems. Its measurement is challenging due to the relatively small size of the carbon dioxide fluxes and because these fluxes occur under environmental conditions that are frequently unfavorable for flux measurements. Micrometeorological techniques based on turbulent transfer frequently underestimate CO 2 fluxes during nighttime conditions. An approach based on calculating the CO 2 budget in the nocturnal boundary layer (NBL) might be an alternative during light wind conditions to estimate nighttime CO 2 fluxes. This study presents typical net CO 2 efflux observations measured at night over agricultural crops for several years near Ottawa, Ont. and over an old black spruce stand near Candle Lake, Sask. during the intensive field campaigns of the Boreal Ecosystem Atmosphere Study (BOREAS). We used the eddy covariance technique for windy nights and the NBL budget approach for calm nights. Criteria for screening data into either windy or calm conditions were made using the friction velocity ( u ∗) and the standard deviation of the vertical wind speed ( σ W). The threshold at which the 30 min turbulent CO 2 flux observations were independent of u ∗ or σ W and had limited scatter were determined to be in the range 0.075–0.1 m s −1 for u ∗ and σ W, and about 1.5 m s −1 for horizontal wind speed ( U) for multiple years of corn and soybean data. σ W≥0.4 m s −1 was shown to be a good screening threshold over the black spruce canopy. Current methods for handling nocturnal CO 2 data involve systematically replacing data during calm conditions where eddy covariance is deficient with those from windy conditions. This can lead to an overestimation of the nocturnal CO 2 flux. We suggest a variation in this procedure which, through the screening of entire nights, allows the retaining of an acceptable proportion of calm periods within predominantly windy nights. The NBL budget method requires calm nights for measuring the respiration unless other budget terms are quantified. A good agreement was found between CO 2 flux measured using the NBL approach when the NBL was well developed and the eddy covariance technique, when restricted to windy nights. The NBL profiles integrate a larger area than eddy covariance, which means that high-emission spots can be included with this kind of approach.

Energy balance closure at FLUXNET sites

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with a mean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO 2 fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated with the eddy covariance technique or from errors in calculating the available energy terms. Although it was not entirely possible to critically evaluate all the possible sources of the imbalance, circumstantial evidence suggested a link between the imbalance and CO 2 fluxes. For a given value of photosynthetically active radiation, the magnitude of CO 2 uptake was less when the energy imbalance was greater. Similarly, respiration (estimated by nocturnal CO 2 release to the atmosphere) was significantly less when the energy imbalance was greater.

Initial net CO2 uptake responses and root growth for a CAM community placed in a closed environment.

To help understand carbon balance between shoots and developing roots, 41 bare-root crassulacean acid metabolism (CAM) plants native to the Sonoran Desert were studied in a glass-panelled sealable room at day/night air temperatures of 25/15 degrees C. Net CO(2) uptake by the community of Agave schottii, Carnegia gigantea, Cylindropuntia versicolor, Ferocactus wislizenii and Opuntia engelmannii occurred 3 weeks after watering. At 4 weeks, the net CO(2) uptake rate measured for south-east-facing younger parts of the shoots averaged 1.94 micro mol m(-2) s(-1) at night, considerably higher than the community-level nocturnal net CO(2) uptake averaged over the total shoot surface, primarily reflecting the influences of surface orientation on radiation interception (predicted net CO(2) uptake is twice as high for south-east-facing surfaces compared with all compass directions). Estimated growth plus maintenance respiration of the roots averaged 0.10 micro mol m(-2) s(-1) over the 13-week period, when the community had a net carbon gain from the atmosphere of 4 mol C while the structural C incorporated into the roots was 23 mol. Thus, these five CAM species diverted all net C uptake over the 13-week period plus some existing shoot C to newly developing roots. Only after sufficient roots develop to support shoot water and nutrient requirements will the plant community have net above-ground biomass gains.

High light-induced switch from C3-photosynthesis to Crassulacean acid metabolism is mediated by UV-A/blue light

The high light-induced switch in Clusia minor from C(3)-photosynthesis to Crassulacean acid metabolism (CAM) is fast (within a few days) and reversible. Although this C(3)/CAM transition has been studied intensively, the nature of the photoreceptor at the beginning of the CAM-induction signal chain is still unknown. Using optical filters that only transmit selected wavelengths, the CAM light induction of single leaves was tested. As controls the opposite leaf of the same leaf pair was studied in which CAM was induced by high unfiltered radiation (c. 2100 micromol m(-2) s(-1)). To evaluate the C(3)-photosynthesis/CAM transition, nocturnal CO(2) uptake, daytime stomatal closure and organic acid levels were monitored. Light at wavelengths longer than 530 nm was not effective for the induction of the C(3)/CAM switch in C. minor. In this case CAM was present in the control leaf while the opposite leaf continued performing C(3)-photosynthesis, indicating that CAM induction triggered by high light conditions is wavelength-dependent and a leaf internal process. Leaves subjected to wavelengths in the range of 345-530 nm performed nocturnal CO(2) uptake, (partial) stomatal closure during the day (CAM-phase III), and decarboxylation of citric acid within the first 2 d after the switch to high light conditions. Based on these experiments and evidence from the literature, it is suggested that a UV-A/blue light receptor mediates the light-induced C(3)-photosynthesis/CAM switch in C. minor.

Carbon isotopic discrimination and control of nighttime canopy δ18O‐CO2 in a pine forest in the southeastern United States

Canopy and soil CO2 concentration and isotopic measurements were conducted in a slash pine forest during a progressive drought in the southeastern United States (1) to determine and compare variations in δ13C of foliage (δCf), soil (δCs), and ecosystem‐respired CO2 (δCr) and (2) to evaluate the usefulness of a two end‐member oxygen isotope ratio (δ18O of CO2) approach to partition nighttime ecosystem respiration into soil and plant components at different heights within the canopy. The δCf was enriched by 2.2 ± 0.3 (standard error) ‰ during the extreme drought in May relative to September when precipitation was above normal. The enrichment in δCf exceeded changes in δCr and δCs for the same time period (1.6 ± 0.5 and 1.0 ± 0.4‰, respectively). Lower variations in the 13C of soil‐respired CO2 relative to the variations of the autotrophic component can buffer changes in δCr, which integrates the 13C signature of canopy and soil respired CO2. Application of a two end‐member model to canopy δ18O‐CO2 indicated that soil CO2 contribution to nighttime CO2 buildup within the canopy decreased from 100% at the soil surface to 0% within the canopy in September. In May, during the extreme drought period, soil respiration rate was 2.7 times lower than the rate in September despite similar soil temperatures (23° in May versus 19°C in September). Soil and aboveground respiration equally contributed to the nocturnal CO2 buildup within the canopy in May. Our model was limited in that it produced an upper limit value for soil respiration and neglected respiration by woody tissue.

Induction of Crassulacean acid metabolism by water limitation.

Crassulacean acid metabolism (CAM), a key adaptation of photosynthetic carbon fixation to limited water availability, is characterized by nocturnal CO2 fixation and daytime CO2 re-assimilation, which generally results in improved water-use efficiency. However, CAM plants display a remarkable degree of photosynthetic plasticity within a continuum of diel gas exchange patterns. Genotypic, ontogenetic and environmental factors combine to govern the extent to which CAM is expressed. The ecological diversity of CAM is mirrored by plasticity in a range of biochemical and physiological attributes. In C3/CAM-intermediate plants, limited water availability can induce or enhance the expression of CAM. CAM induction is controlled by a combination of transcriptional, post-transcriptional and post-translational regulatory events. Early events in CAM induction point to a requirement for calcium and calcium-dependent protein kinase activities. Gene discovery efforts, improved transformation technologies and genetic models for CAM plants, coupled with detailed physiological investigations, will lead to new insights into the molecular genetic basis of induction processes and the circadian oscillator that governs carbon flux during CAM. Future integration of genomic, biochemical and physiological approaches in selected CAM models promise to provide a detailed view of the complex regulatory dynamics involved in CAM induction and modulation by water deficit. Such information is expected to have broad significance as the ecological and agricultural importance of CAM species increases in the face of global warming trends and the associated expansion of desertification in semi-arid regions around the world.

The Occurrence and Phylogenetics of Crassulacean Acid Metabolism in the Portulacaceae

The plant family Portulacaceae is a relatively small family (∼19 genera and ∼425 species) with a widespread distribution. Species of the Portulacaceae range in size from small herbaceous plants to shrubs that measure up to 4 m in height. Most members of the family have leaves that range from fleshy to fully succulent and live in very diverse habitats ranging from shady understory plants to plants exposed to full sunlight. The Portulacaceae have members that utilize different photosynthetic pathways ranging from C3 to C4 to Crassulacean acid metabolism (CAM) to CAM‐cycling plants. We are interested in the prevalence of the CAM photosynthetic pathway in this family. We studied the nocturnal CO2 uptake, diurnal acid fluctuation, and phosphoenolpyruvate (PEP) carboxylase activity of different members of the Portulacaceae. Our results indicate the highest levels of CAM activity in the genera of Anacampseros, Portulacaria, Ceraria, and Talinopsis. Intermediate in their CAM activity were the genera Talinum, Port...

Net CO2 Uptake for Agave tequilana in a Warm and a Temperate Environment1

The main objective was to evaluate the effects of seasonal variations in air temperature, soil water content, and irradiation on the net rate of CO2 uptake for Agave tequilana growing in a warm subtropical environment (Amatitan, Jalisco, Mexico) and in a temperate subtropical environment (Arandas, Jalisco, Mexico). At Amatitan, the lowest value of positive daily net CO2 uptake (141 mmol/m2/d) occurred at the end of the summer rainy season and the highest values during the winter (578 and 921 mmol/m2/d); at Arandas, the lowest value (188 mmol/m2/d) occurred at midsummer and the highest values at the end of the summer (763 mmol/m2/d) and in late winter (572 mmol/m2/d). The highest values of daily net CO2 uptake in both localities reflected prolonged daily periods of net CO2 uptake that included both daytime and nighttime assimilation accompanied by high rates of nocturnal net CO2 uptake due to moderate nighttime air temperatures. High temperatures in the summer reduced daily net CO2 uptake at both localities but in late summer, the nighttime temperatures at Arandas were moderate, which lead to high daily net CO2 uptake. Leaf succulence allowed substantial daily net CO2 uptake when the soil water content was relatively low during the winter and the spring (January–March). Indeed, appreciable daily net CO2 uptake occurred throughout the year for A. tequilana at both localities, indicating that this CAM plant species can sequester carbon even during prolonged dry periods.

Spatial and temporal variations in hectare‐scale net CO2 flux, respiration and gross primary production of Arctic tundra ecosystems

Abstract1. Eddy covariance was used to measure the net CO2 flux of Alaskan moist‐tussock and wet‐sedge tundra ecosystems between 1 June and 31 August 1995. The sites were located within 2·5 km of each other and, depending on wind speed and thermal stability, the eddy‐covariance measurements integrated surface fluxes over 0·8–26·5 ha (mean 3·8 ha) at the moist‐tussock site and 0·1–4·2 ha (mean 0·6) at the wet‐sedge site.2. Both sites were net sinks for atmospheric CO2 during the 92‐day measurement period but wet‐sedge tundra accumulated 1·5 times more CO2 than moist‐tussock tundra. Wet‐sedge tundra was a net CO2 sink of −6·4 mol m−2 and moist‐tussock tundra was a net sink of −4·6 mol m−2 over the June–August measurement period.3. Estimates of whole‐ecosystem respiration (R) were made using multiple non‐linear regression by quantifying the response of measured nocturnal CO2 efflux (Fn) to fluctuations in air temperature and water‐table depth (r2 = 0·68). Partial regression analysis revealed that water‐table depth explained relatively more of the variance in Fn (45%) than temperature (11%). R was estimated to be 14·9 mol m−2 for moist‐tussock tundra and 5·4 mol m−2 for wet‐sedge tundra over the 92‐day measurement period. The large difference in R was apparently owing to a substantially higher water table in wet‐sedge tundra.4. Estimated rates of gross primary production (GPP), calculated from measured net CO2 flux and estimated R, were 40% lower for wet‐sedge tundra over the 92‐day measurement period. Rates of GPP integrated over diel (24 h) periods were substantially larger for moist‐tussock tundra ecosystems throughout June and July but during the month of August, total diel rates of GPP for moist‐tussock and wet‐sedge tundra were comparable.5. Our results demonstrate that R and GPP of moist‐tussock and wet‐sedge tundra ecosystems can be reliably estimated from eddy‐covariance measurements using functional relationships developed from plot‐scale studies. Given the large spatial differences in hydrology, the larger sink observed for wet‐sedge tundra was probably the result of relatively lower rates of R.

CO 2 exchange at the floor of a boreal forest

Net CO 2 exchange at the forest floor in a mixed boreal spruce and pine forest in central Sweden was studied during 1996. Forest floor CO 2 efflux was measured continuously by means of a ventilated open soil chamber, covering a surface area of 0.6 m 2. The chamber was transparent and thus measured soil respiration by night, and soil respiration reduced by photosynthetic uptake by forest floor vegetation by day. Maximum nocturnal efflux rates were 0.2–0.7 and daytime rates were 0.05–0.2 mg m −2 s −1. Measured efflux rates were higher than reported in other studies, but have earlier been found to agree with nocturnal CO 2 exchange of the forest ecosystem measured by eddy-covariance technique. Soil temperature at 5 cm explained 49% of the variation in nocturnal soil respiration, while moss and air temperature explained 29 and 17% of the variation, respectively. For the relationship with soil temperature at 5 cm, base respiration rate and effective Q 10, derived from data over the seasonal course, were 0.04 mg m −2 s −1 and 4.75, respectively. Corresponding figures for the relationship with air temperature were 0.11 mg m −2 s −1 and 1.89. Over the 6 months (May–October) covered by measurements, August had the largest CO 2 efflux, 0.89 kg m −2 and May the smallest efflux, 0.20 kg m −2. During daytime photosynthetic uptake by forest floor vegetation reduced potential soil efflux through respiration by ca. 20%. On an annual basis total forest floor respiration was estimated to be 4.5 kg CO 2 m −2 and gross photosynthesis to be 0.7 kg CO 2 m −2, resulting in a net efflux of 3.8 kg CO 2 m −2.