Regional interpretation of environmental gradients which influence Trachypogon savannas in the Orinoco Llanos

Multivariate structural data describing Trachypogon savanna relationships were used to characterize the sources of variation among savannas as a function of the environmental characteristics of the Orinoco Llanos. Twenty-six savannas including major physiognomic types of the Orinoco Llanos ranging from herbaceous to bush island savannas were ordinated on a regional scale. Results of Detrented Canonical Correspondence Analysis (DCCA) using floristics variables indicated that savannas were ordinated along two complex gradients. A soil physical and chemical gradient as expressed by changes in bulk density and magnesium concentration were evident after the analysis of the first DCCA axis. The second axis was a climatic gradient of decreasing annual precipitation and incresing monthly precipitation during the dry season. The impact of human disturbance on the savanna composition was also an explanatory variable of the second DCCA axis. Savanna sites overlapped considerably in composition and most species were widely distributed, with aboveground phytomass abundance depending upon the site.     

CO2 Exchange in soil algal crusts occurring in the trachypogon savannas of the Orinoco Llanos,Venezuela

CO₂ exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO₂ flux of 0.207 mg m^-2 s^-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m^-2 s^-1) the flux was similar to values reported for terrestrial algae. The CO₂ flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO₂ fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole^-1), whereas they were evident in the other crust studied. An optimum CO₂ incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.     

Chemical composition and nutrient loading by precipitation in the Trachypogon savannas of the Orinoco llanos, Venezuela

Samples of bulk precipitation were collected in the Trachypogon savanna, Calabozo, Venezuela, during three consecutive years. In the first year, rain samples were taken daily; in the following years the samples were grouped on a monthly basis. In addition, samples of dry deposition were collected during the dry seasons. All samples were analyzed for the following water soluble cations and anions: P0₄-P, S0₄-S, N0₃-N, NH₄-N, Ca⁺², Mg⁺², K⁺, Na⁺ and H⁺. The mean annual input rate of chemical constituents (Kg ha^-1 year^-1) was: PO₄-P (0.42); SO₄-S (2.62); NO₃-N (0.21); NH₄-N (2.03); Ca₊₂ (3.50); Mg⁺² (11.31); K⁺ (3.60); Na⁺ (5.93) and H⁺ (0.03). The total mean input of particulate material to the savanna during the dry season was 2.06 Kg ha^-1 year^-1, with a soluble fraction of 30%. Possible sources of nutrients input were analyzed.     

Carbon Dioxide Exchange Between an Old-growth Forest and the Atmosphere

Eddy-covariance and biometeorological methods show significant net annual carbon uptake in an old-growth Douglas-fir forest in southwestern Washington, USA. These results contrast with previous assumptions that old-growth forest ecosystems are in carbon equilibrium. The basis for differences between conventional biomass-based carbon sequestration estimates and the biometeorologic estimates are discussed. Annual net ecosystem exchange was comparable to younger ecosystems at the same latitude, as quantified in the AmeriFlux program. Net ecosystem carbon uptake was significantly correlated with photosynthetically active radiation and air temperature, as well as soil moisture and precipitation. Optimum ecosystem photosynthesis occurred at relatively cool temperatures (5°–10°C). Understory and soil carbon exchange always represented a source of carbon to the atmosphere, with a strong seasonal cycle in source strength. Understory and soil carbon exchange showed a Q₁₀ temperature dependence and represented a substantial portion of the ecosystem carbon budget. The period of main carbon uptake and the period of soil and ecosystem respiration are out of phase, however, and driven by different climatic boundary conditions. The period of strongest ecosystem carbon uptake coincides with the lowest observed values of soil and ecosystem respiration. Despite the substantial contribution of soil, the overall strength of the photosynthetic sink resulted in the net annual uptake. The net uptake estimates here included two correction methods, one for advection and the other for low levels of turbulence.     

Interactions Among Abiotic Drivers,Disturbance and Gross Ecosystem Carbon Exchange on Soil Respiration from Subtropical Pine Savannas

Globally, soil CO₂ efflux rates (F_s) have been linked to changes in soil water content (SWC), rainfall and temperature and/or productivity. However, within an ecosystem, F_s can vary based on site structure and function, which can be affected by a combination of abiotic and biotic factors. This becomes particularly important when an ecosystem is faced with disturbances, such as drought or fire. Site-specific compensatory responses to disturbances may therefore alter C mineralization, as well as root respiration. Hence, single location F_s estimates may not be a representative for ecosystems across their distributional ranges. We conducted a 6-year study along an edaphic moisture gradient of longleaf pine ecosystems that were maintained with prescribed fire, using eddy covariance and soil respiration measurements to address how F_s varies with changes in ecosystem structure and function, as well as disturbances. Lower air temperatures (T_air) decreased F_s at all sites, but that response was also affected by productivity and SWC. Productivity significantly altered F_s rates at all sites, especially when we accounted for changes in temperature and SWC. Plant regrowth post-fire temporarily increased F_s (10–40%), whereas drought reduced F_s at all sites. Our results show that site productivity, F_s and the degree to which ecosystems adapt to climate variations and disturbance can be site specific. Hence, model forecasting of carbon dynamics would strongly benefit from multi-location measurements of F_s across the distributional range of an ecosystem.     

Growing Season Carbon Dioxide Exchange in Flooded Non-Mulching and Non-Flooded Mulching Cotton

There is much interest in the role that agricultural practices might play in sequestering carbon to help offset rising atmospheric CO₂ concentrations. However, limited information exists regarding the potential for increased carbon sequestration of different management strategies. The objective of this study was to quantify and contrast carbon dioxide exchange in traditional non-mulching with flooding irrigation (TF) and plastic film mulching with drip irrigation (PM) cotton (Gossypium hirsutum L.) fields in northwest China. Net primary productivity (NPP), soil heterotrophic respiration (R_h) and net ecosystem productivity (NEP) were measured during the growing seasons in 2009 and 2010. As compared with TF, PM significantly increased the aboveground and belowground biomass and the NPP (340 g C m⁻² season⁻¹) of cotton, and decreased the R_h (89 g C m⁻² season⁻¹) (p<0.05). In a growing season, PM had a higher carbon sequestration in terms of NEP of ∼ 429 g C m⁻² season⁻¹ than the TF. These results demonstrate that conversion of this type of land use to mulching practices is an effective way to increase carbon sequestration in the short term in cotton systems of arid areas.     

Carbon Dioxide Exchange in an Incompatible Barley/Powdery Mildew Combination

CO₂ exchanges in intact barley leaves were studied by infra red gas analysis after inoculation with an incompatible race of powdery mildew. Net photosynthesis was inhibited, and dark and photorespiration were stimulated, by inoculation. The increase in respiratory processes was approximately equal to the change in net photosynthesis indicating that CO₂ fixation, “gross photosynthesis”, was unaffected by inoculation. It is concluded that changes in net photosynthesis may be the primary cause of the reduced quantity and quality of grain in incompatible barley/mildew combinations.     

Carbon Dioxide and Water Vapour Exchange of Leaves on Field-Grown Citrus Trees

Carbon dioxide and water vapour exchange rates were measuredon attached leaves of field-grown citrus trees. The exchangerates were measured continuously during several weeks in thespring of two successive years. These data confirmed the ratherlow rates of maximum CO₂ exchange (6–11 µmol m^–2s^–1) by citrus leaves. However, the maximum rate was maintainedthrough the midday period on only about half the days. On theother days, characterized by high temperatures and high atmosphericwater vapour pressure deficits, pronounced midday depressionsin CO₂ exchange rates were observed. Since midday transpirationremained stable at a constant rate even with increasing vapourpressure deficit, these results indicate that stomatal closurewas occurring. In fact, the data suggest tfiat specific, maximumtranspiration rates were associated with differing rootstocks.Thus, the rate of water supply to the leaves may be an importantfactor in determining the maximum transpiration rate, and therebymediating control of stomatal conductance and the resultantmidday depression in CO² exchange rates.     

Precise Measurements of Carbon Dioxide Exchange by Illuminated Leaves near the Compensation Point

The results of precise measurements of rates of carbon dioxideassimilation at low external concentrations of carbon dioxideand of rates of carbon dioxide output into virtually carbondioxide-freeair show that linear extrapolations of the carbon dioxide intakecurve plotted against the external concentrations of carbondioxide give an exaggerated estimate of the rate of carbon dioxideoutput from illuminated leaves into carbon dioxide-free air. The shape of the exchange curve suggests that the rate of endogenousproduction of carbon dioxide changes at external concentrationsin the region of the carbon dioxide compensation point ().     

Aluminium concentration in the biomass of native species of the Morichals (swamp palm community) at the Orinoco Llanos,Venezuela

Aluminium concentration was analyzed in roots, stems and leaves of species growing in morichals with extreme flooded acid soils, deficient in K, Ca, Mg and P. Mean aluminium saturation was 73%. Results seem to indicate that species depending on seasonal changes in the Al concentration of plant compartments, could be grouped into “perennial aluminium accumulators”, “late aluminium accumulators” and “threshold aluminium accumulators”.     

Carbon Dioxide Exchange of Developing Apple (Malus pumila Mill.) Fruits

The carbon dioxide exchange of developing apple fruits was monitoredduring development. The results of measurements on detachedfruits in the laboratory were consistent with those made onattached fruit in the field. Respiration rate at 20 °C inthe dark declined from 120 ng CO₂ g^–1 fr. wt. s_–1on 5 June (4 weeks after full bloom) to less than 3 ng g^–1fr. wt. s^–1 by late September. In the light, net CO₂ evolutionwas much decreased, but on no occasion did photosynthesis exceedrespiration and no net CO₂ uptake was detected. The Q₁₀ fordark respiration over the interval from 15 to 25 °C changedfrom 2.8 in early June to 1.6 in early August     

Glycolate Excretion and the Oxygen to Carbon Dioxide Net Exchange Ratio during Photosynthesis in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii cells were grown in high (5% v/v) or low (0.03% v/v) CO₂ concentration in air. O₂ evolution, HCO₃⁻ assimilation, and glycolate excretion were measured in response to O₂ and CO₂ concentration. Both low- and high-CO₂-grown cells excrete glycolate. In low-CO₂-grown cells, however, glycolate excretion is observed only at much lower CO₂ concentrations in the medium, as compared with high-CO₂-adapted cells. It is postulated that the activity of the CO₂-concentrating mechanism in low-CO₂-grown cells is responsible for the different dependence of glycolate excretion on external CO₂ concentration in low- versus high-CO₂-adapted cells.     

Simultaneous Measurement of Oxygen, Carbon Dioxide, and Water Vapour Exchange in Intact Plants

A low flow of air is passed through a temperature-controlledplant chamber in order to obtain relatively large (300–500µl-1) differences in [O₂] between influx and efflux streams.These differences are measured with a stabilized O₂ electrodesystem incorporating elements of gas conditioning electroniczero suppression and signal amplification. Changes in [O₂] of400 µl l-1 can be detected at full scale recorder deflectionagainst a background concentration of 21% O₂. The concentrationsof CO₂ and H₂O within the chamber are held constant by con-trolled-flowCO₂-scrubbing and dehumidifying loops. Carbon dioxide, H₂O,and O₂ fluxes are measured and leaf diffusion resistance andinternal [CO₂] are calculated in essentially ‘real time’.     

Carbon Dioxide Exchange Rate and Chlorophyll Content of Turfgrasses Treated with Flurprimidol or Mefluidide

Plant growth regulators (PGRs) are being used increasingly for high maintenance turf production. A greenhouse and growth chamber study was conducted to determine the effect of two PGRs, mefluidide and flurprimidol, on the carbon dioxide exchange rate, chlorophyll content, and specific leaf weight of annual bluegrass and creeping bentgrass. Rate-response and time course studies were conducted. In the rate-response study, increasing flurprimidol rates caused a linear decrease in the carbon dioxide exchange rate (CER). Mefluidide had no effect on the CER in the rate-response experiment but did cause a significant drop in the CER at 4 and 8 days after treatment (DAT) in the time course study. Both PGRs increased chlorophyll content at 16 DAT in the time course study. In the rate-response study, chlorophyll content displayed a quadratic response to an increasing mefluidide rate. The 0.28 kg ha⁻¹ rate of mefluidide increased the chlorophyll content to 46 μg cm⁻² from 21.5 μg cm⁻² for control plants. Both PGRs increased specific leaf weight, although for flurprimidol the effect was significant only at the 0.1 level of probability. PGRs decreased the CER and increased the chlorophyll content and specific leaf weight for both species tested. The physiologic effects measured were short lived, and so the physiologic significance of these changes is difficult to determine without further research. Received September 19, 1996; accepted January 22, 1997     

Enzyme Activities Associated with Carbon Dioxide Exchange in Illuminated Leaves of Hordeum vulgare L.: II. Effects of External Concentrations of Carbon Dioxide and Oxygen

Plants grown in a high carbon dioxide environment (< 1 percent) were found to have increased levels of RuDP carboxylase,and suppressed activities of catalase, glycollate oxidase, andnitrate reductase, enzymes all associated with the peroxisome.Similarly, plants grown in low oxygen concentrations showedsuppressed activities of the peroxisomal enzymes. However, underthese conditions RuDP carboxylase activity was also suppressed.These results further suggest that nitrate reductase activityis associated with photorespiration.     

Diurnal and Seasonal Variations in Carbon Dioxide Exchange in Ecosystems in the Zhangye Oasis Area,Northwest China

Quantifying carbon dioxide exchange and understanding the response of key environmental factors in various ecosystems are critical to understanding regional carbon budgets and ecosystem behaviors. For this study, CO₂ fluxes were measured in a variety of ecosystems with an eddy covariance observation matrix between June 2012 and September 2012 in the Zhangye oasis area of Northwest China. The results show distinct diurnal variations in the CO₂ fluxes in vegetable field, orchard, wetland, and maize cropland. Diurnal variations of CO₂ fluxes were not obvious, and their values approached zero in the sandy desert, desert steppe, and Gobi ecosystems. Additionally, daily variations in the Gross Primary Production (GPP), Ecosystem Respiration (R_eco) and Net Ecosystem Exchange (NEE) were not obvious in the sandy desert, desert steppe, and Gobi ecosystems. In contrast, the distributions of the GPP, R_eco, and NEE show significant daily variations, that are closely related to the development of vegetation in the maize, wetland, orchard, and vegetable field ecosystems. All of the ecosystems are characterized by their carbon absorption during the observation period. The ability to absorb CO₂ differed significantly among the tested ecosystems. We also used the Michaelis-Menten equation and exponential curve fitting methods to analyze the impact of Photosynthetically Active Radiation (PAR) on the daytime CO₂ flux and impact of air temperature on R_eco at night. The results show that PAR is the dominant factor in controlling photosynthesis with limited solar radiation, and daytime CO₂ assimilation increases rapidly with PAR. Additionally, the carbon assimilation rate was found to increase slowly with high solar radiation. The light response parameters changed with each growth stage for all of the vegetation types, and higher light response values were observed during months or stages when the plants grew quickly. Light saturation points are different for different species. Nighttime R_eco increases exponentially with air temperature. High Q₁₀ values were observed when the vegetation coverage was relatively low, and low Q₁₀ values occurred when the vegetables grew vigorously.     

Carbon Dioxide Exchange of Spring-wheat in Relation to Age and Photon-flux Density at Different Growth Temperatures

CO₂-exchange rates (CER) of the sixth and the flag leaves oftwo spring-wheat varieties, Kolibri and Famos, were comparedusing an open-circuit infrared gas analysing system. Measurementswere repeated every two weeks starting when leaf blades werefully expanded. Single plants were grown in a controlled environmenthaving a photopuiod of 15 h and a day/night temperature of 24/19°C(H), 18/13 °C (M), and 12/7 °C (L) respectively untilapprox. 2 weeks after anthesis and at 18/13 °C until maturity.The photosynthetic photon-flux density (PPFD) at the top ofthe plants was 500 µE m^–2 sec^–1. During themeasurements PPFD was gradually reduced from 2000 to 0 µEm^–2 sec^–1 whereas the temperature was maintainedat the respctive growth-temperatures during the light period.The CER of the sixth leaf declined fairly similarly for bothvarieties, except for Kolibri where a faster decline was observedduring the first two weeks after full leaf expansion. The CERof the flag leaf declined more slowly than that of the sixthleaf. With the flag leaf of Famos, the decline was nearly linear,whereas with Kolibri it was very slow during the first few weeksbut rapid as the leaves further senesced. This pattern becamemore pronounced as the growth temperature decreased. The declinein relation to leaf age was much smaller at low PPFD than athigh PPFD during the same period. At full leaf expansion Kolibrireached higher maximum CER than Famos except at H. As the PPFDwas reduced the difference became smaller and at very low PPFDsuch as 50 µE m^–2 sec^–1 was reversed for thesixth leaf. Under optimum growth conditions maximum values ofCER were greater than 50mg CO₂ dm^–2h^–1 and PPFDfor light saturation was close to 2000 µE m^–2 sec^–1.A comparison between the actual CER and a fitted curve widelyused, PN=(a+b/l)^–1–DR, showed that the goodnessof fit strongly depends on cultivar, treatment and leaf ageas well as on the number and the level of PPFD from which datafor calculations are taken. Triticum aestivum, L., wheat, photosynthesis, photon-flux density, light response, carbon, dioxide exchange     

Carbon Isotope Discrimination in Cyanobacteria of Rocks of Inselbergs and Soils of Savannas in the Neotropics*

An extensive set of data on carbon isotope ratios in samples of terrestrial cyanobacteria from the surface of rocks of inselbergs and soils of savannas in the neotropics is presented. A comparison of the data shows two surprising features. First, all values deviate from the ¹³C discrimination of carboxylation by ribulose-bis-phosphate carboxylase/oxygenase (RuBISCO) which is generally about ?27‰. They are less negative than that by between 0.62‰ and 10.46‰. Second, the values show a considerable variation, minima and maxima being ?26.38‰ and ?16.54‰, respectively. This variation cannot be explained by differences in dominant taxa in the samples. It must be related to microsite characteristics, most probably the extent in time of coverage by liquid water, where diffusion of CO₂ constitutes a limiting step with a lower discrimination against ¹³C than that of carboxylation via RuBISCO.