Net Ecosystem CO2 Research Articles

Ecosystem CO 2 flux over two growing seasons for a sub-Boreal clearcut 5 and 6 years after harvest

We measured the ecosystem-level growing season CO 2 fluxes for a 6-year-old vegetated sub-Boreal clearcut from 24 May to 20 September 2000, and compared the results to CO 2 fluxes from the same clearcut in 1999 (27 June–3 September). Two independent approaches were used to measure ecosystem CO 2 flux for both years. A Bowen ratio energy balance (BREB) method was contrasted with a second approach using component fluxes. The Component model approach was based on scaling up from regressions relating in situ CO 2 flux measurements for conifer seedlings ( Picea glauca × engelmannii), as well as representative herbaceous ( Chamerion angustifolium), woody ( Lonicera involucrata) plant species and soil surface CO 2 efflux to microclimate conditions. The BREB method and Component model estimated the clearcut to be a source of CO 2 6 years post-harvest (24 May–20 September 2000) in amounts of 142 g C m −2 (1.4 t ha −1) and 103 g C m −2 (1.0 t ha −1), respectively. The positive net ecosystem CO 2 fluxes over the growing season resulted from a large soil surface CO 2 efflux (686 g C m −2) that surmounted the photosynthetic CO 2 uptake for the clearcut. The photosynthetic CO 2 uptake partially compensated for the soil surface losses: the conifer seedlings, herbaceous plants and woody shrubs were estimated to uptake −96, −315, −172 g C m −2, respectively. The results of 2000 contrasted with those of 1999. Over a comparable period of measurement (27 June–3 September), the clearcut was a sink for carbon in 1999 (−22.4 g C m −2 using the BREB method and −85 g C m −2 using the Component model) and a source in 2000 (65 g C m −2 using the BREB method and 44 g C m −2 using the Component model). The growing seasons of 1999 and 2000 experienced similar photosynthetic uptake over this same interval (−423 and −422 g C m −2, respectively). The main difference between the two field seasons was an increase in the soil surface CO 2 efflux from 1999 to 2000. In 1999, the soil surface CO 2 efflux was 338 g C m −2 and in 2000 the flux was 38% higher (466 g C m −2) for the same period. The results indicate that while there was notable inter-annual variation in CO 2 fluxes, particularly the soil surface CO 2 effluxes, this young regenerating sub-Boreal forest (≤6 years after harvesting) is a net source of CO 2 when the entire growing season is considered.

FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem–Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S. FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite. Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities to controlling biotic and abiotic factors, and to validate a hierarchy of soil–plant–atmosphere trace gas exchange models. Findings so far include 1) net CO 2 exchange of temperate broadleaved forests increases by about 5.7 g C m −2 day −1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem CO 2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO 2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net CO 2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

Modeling CO 2 and water vapor exchange of a temperate broadleaved forest across hourly to decadal time scales

Fluxes of carbon dioxide, water and energy between a temperate deciduous forest and the atmosphere were quantified across time scales of hours, days, seasons, years and decades. This exercise was performed using stand-level eddy covariance flux measurements and a biophysical model, CANOAK. The CANOAK model was tested with measurements of carbon dioxide, water vapor and energy flux densities we have been collecting since October 1994. Model calculations reproduced 80% of CO 2 and water vapor flux variance that are contained in a year-long time series, when the model was forced with hourly weather data and seasonal information on plant structure and physiological capacity. Spectral analysis of measured and computed time series revealed that peak time scales of flux variance have periods of a day, half-week, season and year. We examined questions relating to inter-annual variability of mass and energy exchange by forcing the validated model with a decade-long meteorological record. Theoretical estimates of year-to-year variability of net ecosystem CO 2 exchange were on the order of ±200 gC m −2 year. We also deduced that significant variance of water vapor and CO 2 exchange occurs on the time scale of 5–6 years, the time scale associated with El Nino phenomena. Sensitivity tests performed with the model examined issues associated with model complex and parameterization issues. Of particular importance were the effects of leaf clumping and length of the growing season on canopy photosynthesis and net ecosystem CO 2 exchange. Ignoring the effects of leaf clumping caused an error as large as 50% in the estimation of annual biosphere–atmosphere net carbon exchange. Each incremental day change in the length of the growing season altered the net ecosystem CO 2 exchange by 5.9 gC m −2.

Seasonal variations in the net ecosystem CO2 exchange of a mature Amazonian transitional tropical forest (cerradão)

Summary Tower‐based eddy covariance and measurements of the vertical CO2 concentration gradient within the canopy were used to quantify the seasonal variations in the net ecosystem CO2 exchange (NEE) of a 28–30 m tall transitional tropical forest (cerradão). The study was conducted near the city of Sinop, Mato Grosso, Brazil (11°24·75′ S; 55°19·50′ W), which is located in the ecotone of two major regional ecosystem types of South America (tropical rainforest and savanna). The NEE during the dry season (August–September) was in balance, but during the transition period between the dry and wet seasons (October–November) the cerradão stand became a net source of 50–150 mmol m−2 day−1 CO2 to the atmosphere. Measurements during the wet season (February, April) indicate that the forest was a net sink of between −55 and −102 mmol m−2 day−1. The NEE of the transitional tropical forest was more similar to that of tropical rainforest during the wet season, but during the dry season the NEE of the transitional forest was more similar to that reported for tropical savanna. The data suggest that seasonal variations in rainfall have important implications for the seasonal pattern of NEE in cerradão.

Modelling the interannual variability of net ecosystem CO2 exchange at a subarctic sedge fen

AbstractThis paper presents an empirical model of net ecosystem CO2 exchange (NEE) developed for a subarctic fen near Churchill, Manitoba. The model with observed data helps explain the interannual variability in growing season NEE. Five years of tower‐flux data are used to test and examine the seasonal behaviour of the model simulations. Processes controlling the observed interannual variability of CO2 exchange at the fen are examined by exploring the sensitivity of the model to changes in air temperature, precipitation and leaf area index. Results indicate that the sensitivity of NEE to changing environmental controls is complex and varies interannually depending on the initial conditions of the wetland. Changes in air temperature and the timing of precipitation events have a strong influence on NEE, which is largely manifest in gross ecosystem photosynthesis (GEP). Climate change scenarios indicate that warmer air temperatures will increase carbon acquisition during wet years but may act to reduce wetland carbon storage in years that experience a large water deficit early in the growing season. Model simulations for this subarctic sedge fen indicate that carbon acquisition is greatest during wet and warm conditions. This suggests therefore that carbon accumulation was greatest at this subarctic fen during its early developmental stages when hydroclimatic conditions were relatively wet and warm at approximately 2500 years before present.

Interannual variability of net ecosystem CO2 exchange at a subarctic fen

Landscape‐scale net ecosystem CO2 exchange (NEE) and the energy balance of a subarctic fen were studied during five growing seasons near Churchill, Manitoba. Interannual variability in NEE was large and ranged from a net sink of −235 g CO2 m−2 in 1996 to a net source of +76 g CO2 m−2 in 1994. Annual estimates of CO2exchange indicate that during the present period the fen is losing carbon nearly 3 times faster than its long‐term historical gain of about −11 g CO2 m−2 yr−1. Our estimates suggest that gross ecosystem photosynthesis may be more variable than ecosystem respiration on diurnal, seasonal, and interannual timescales. Our data strongly indicate that an early snowmelt combined with wet and warm conditions during the spring period lead to large carbon acquisition even when drier conditions were experienced over the majority of the growing season. The phenological stage of the vegetation relative to the climatic conditions experienced is an important cause of the interannual variability in NEE. An accurate representation of phenology in climate models is, therefore, critical to the success of forecasting the carbon budgets of northern wetlands.

Scaling net ecosystem CO2 exchange from the community to landscape‐level at a subarctic fen

SummaryLandscape‐ and community‐level CO2 measurements were made at a subarctic sedge fen near Churchill Manitoba during the 1997 growing season. Climatic conditions were warmer and drier than the 30‐y normal. Landscape‐scale micrometeorological measurements indicated that the wetland gained 49 g CO2 m−2 during the growing season. Chamber‐scale measurements from the main vegetation community types showed that small hummocks (Carex spp. sites) dominated the CO2 exchange, yielding an effective scaling factor of 70%. Scaled parameters of two algorithms describing photosynthesis and respiration for each community type show strong similarity to those derived at the landscape level. Scaling photosynthesis, respiration, and net ecosystem CO2 exchange from the community to landscape‐level over the season is within the maximum probable error of each methodological approach and helps substantiate the 1997 CO2 budget. We explore the equilibrium response of net ecosystem CO2 exchange of this fen to climatic change by examining the feedback of water table position on vegetation distribution and nitrogen availability. Based on the effective scaling factors computed for each community type, we hypothesize that a small decrease in mean water table position could nearly triple the net uptake of CO2 at this wetland.

Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July‐August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) < poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) < intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) < rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May‐June) and fall (September‐October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub‐dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze‐up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3.

Forest–atmosphere carbon dioxide exchange in eastern Siberia

We investigated the daily exchange of CO 2 between undisturbed Larix gmelinii (Rupr.) Rupr. forest and the atmosphere at a remote Siberian site during July and August of 1993. Our goal was to measure and partition total CO 2 exchanges into aboveground and belowground components by measuring forest and understory eddy and storage fluxes and then to determine the relationships between the environmental factors and these observations of ecosystem metabolism. Maximum net CO 2 uptake of the forest ecosystem was extremely low compared to the forests elsewhere, reaching a peak of only ∼5 μmol m −2 s −1 late in the morning. Net ecosystem CO 2 uptake increased with increasing photosynthetically active photon flux density (PPFD) and decreased as the atmospheric water vapor saturation deficit ( D) increased. Daytime ecosystem CO 2 uptake increased immediately after rain and declined sharply after about six days of drought. Ecosystem respiration at night averaged ∼2.4 μmol m −2 s −1 with about 40% of this coming from the forest floor (roots and heterotrophs). The relationship between the understory eddy flux and soil temperature at 5 cm followed an Arrhenius model, increasing exponentially with temperature (Q 10∼2.3) so that on hot summer afternoons the ecosystem became a source of CO 2. Tree canopy CO 2 exchange was calculated as the difference between above and below canopy eddy flux. Canopy uptake saturated at ∼6 μmol CO 2 m −2 s −1 for a PPFD above 500 μmol m −2 s −1 and decreased with increasing D. The optimal stomatal control model of Mäkelä et al. (1996) was used as a `big leaf' canopy model with parameter values determined by the non-linear least squares. The model accurately simulated the response of the forest to light, saturation deficit and drought. The precision of the model was such that the daily pattern of residuals between modeled and measured forest exchange reproduced the component storage flux. The model and independent leaf-level measurements suggest that the marginal water cost of plant C gain in Larix gmelinii is more similar to values from deciduous or desert species than other boreal forests. During the middle of the summer, the L. gmelinii forest ecosystem is generally a net sink for CO 2, storing ∼0.75 g C m −2 d −1.

Seasonal variations of the exchange of CO 2 and H 2O between a grassland and the atmosphere: An experimental study

Seasonal variations in the CO 2 and H 2O fluxes were investigated experimentally over a grassland in central Japan during the growing periods of 1993 and 1994. The CO 2 flux over the grassland was measured by an aerodynamic method. The surface heat budget was routinely measured and the data were used to analyze the CO 2 and H 2O exchange between the grassland and the atmosphere. Conditions were relatively cool and wet due to the prolonged rainy season from July to August 1993, being hot during the 1994 summer. The annual maximum value of sensible heat flux density was observed prior to the growing period on March 1993 and on April 1994, accounting for 40–50% of the net radiation. The annual maximum of latent heat flux density was observed in August, accounting for about 85% of the net radiation in both years. The latent heat flux observed in August 1994 was quite high, being 135 W m −2 (monthly mean, daily basis), while that in 1993 was 96 W m −2. The grassland absorbed CO 2 during the period from the beginning of May to October in 1994, and maximum of daily net ecosystem CO 2 exchange during the growing period amounted to values more than 40 g CO 2 m −2 day −1, in both 1993 and 1994. The dependence of daily net ecosystem CO 2 exchange on leaf area index (LAI) and incoming photosynthetic photon flux density Q p was investigated. When the Q p was higher than 45 mol m −2 day −1, the daily CO 2 exchange strongly increased with an increase in LAI. The daily canopy surface conductance was calculated based on the Penman-Monteith equation, and it showed a linear increase with LAI when the LAI had small values (typically less than one). With an increase in LAI from one to four, the maximum level of surface conductance became dominated by water vapor pressure deficits.

Annual CO 2 budget of spruce model ecosystems in the third year of exposure to elevated CO 2

Clones of 4-year-old spruce trees ( Picea abies) were grown in competition in model ecosystems with nutrient-poor natural forest soil and natural understory vegetation and were exposed to three CO 2 concentrations (280, 420 and 560 μmol mol −1) for three years. Diurnal net ecosystem CO 2 uptake (NEC d), nocturnal net ecosystem CO 2 loss (NEC n) and soil CO 2 efflux were measured repeatedly in the third year of CO 2 exposure and were used to estimate an annual ecosystem CO 2 budget. The CO 2-induced stimulation of NEC d varied over the year with no measurable stimulation in spring and fall but a high mid-season CO 2 stimulation. Respiratory losses of whole ecosystems and soil CO 2 efflux alone were both progressively increased with increasing CO 2, thus counteracting the CO 2 stimulation of photosynthesis per unit ground area. Consequently, the annual net ecosystem CO 2 uptake was only moderately and non-linearly stimulated by CO 2 (+8%=84 g C m −2 a −1 at 420 and +9%=90 g C m −2 a −1 at 560 compared to 280 μmol CO 2 mol −1). We conclude that the rising atmospheric CO 2 concentration may lead to an increase in annual net ecosystem carbon gain of rather nutrient-poor spruce communities. Our results further suggest that CO 2 fertilization effects may be greatest under current CO 2 concentration and that relative increases of net ecosystem CO 2 uptake will become relatively smaller as atmospheric CO 2 will continue to rise.

Effects of elevated CO 2 on gas exchange characteristics of alpine grassland

The ecosystem-level gas exchange characteristics of an alpine grassland treated with a combination of elevated CO 2 and moderate additions of NPK fertilizer during the third season of experimental treatments are described. Mid-season maximum daytime net ecosystem CO 2 flux (NEC max) increased significantly under elevated CO 2 (+45%), whereas nighttime NEC was unaffected by the CO 2 treatment. Since daytimke NEC under elevated CO 2 underwent a seasonal decline, only moderate carbon surpluses accumulated under elevated CO 2. The observed seasonal decline in daytime NEC may be due to reduced sink strength once maximum aboveground biomass is attained and appears to be a regulatory mechanism of ecosystem carbon accumulation. Moderate additions of NPK fertilizer stimulated both day-(+39%) and nighttime NEC (+29%) due to increased plant biomass, independent of CO 2 treatment. Yet there is no indication that enhanced mineral nutrient status will increase ecosystem responsiveness to elevated CO 2.