Peter M. Anthoni

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.

Carbon and water vapor exchange of an open-canopied ponderosa pine ecosystem

Eddy covariance measurements of carbon dioxide and water vapor exchange were made above a ponderosa pine (Pinus ponderosa Dougl. ex P. and C. Laws.) forest located in a semiarid environment in central Oregon. The stand is a mixture of old-growth and young trees. Annual net carbon gain by the ecosystem (NEE) was 320 170 gC m ˇ2 year ˇ1 in 1996 and 270 180 gC m ˇ2 year ˇ1 in 1997. Compared to boreal evergreen forest at higher latitudes, the pine forest has a substantial net carbon gain (150 80 gC m ˇ2 year ˇ1 in 1996 and 180 80 gC m ˇ2 year ˇ1 in 1997) outside the traditionally defined

Spatial and temporal variation in respiration in a young ponderosa pine forest during a summer drought

Respiration rates of heterogeneous forest canopies arise from needles, stems, roots and soil microbes. To assess the temporal and spatial variation in respiration rates of these components in a heterogeneous ponderosa pine forest canopy, and the processes that control these fluxes, we conducted an intensive field study during the summer of 2000. We employed a combination of biological and micrometeorological measurements to assess carbon respiratory fluxes at the soil surface, within and above a 4-m-tall ponderosa pine forest. We also conducted manipulation studies to examine the carbon fluxes from the roots and heteorotrophs. Spatial variation in soil CO2 efflux was large, averaging 40% of the mean, which varied by nearly a factor of two between minima for bare soil to maxima beneath dense patches of understorey vegetation. The estimated vertical profile of respiration from chamber data, and the profile of nocturnal fluxes measured by the three eddy flux systems suggested that >70% of the ecosystem respiration was coming from below the 1.75-m measurement height of one of the flux systems, and 71% of photosynthetic carbon uptake in July was released by soil processes, thus there was a strong vertical gradient in respiration relatively close to the soil surface in this young forest. These results stress the importance of understanding spatial and temporal variation in soil processes when interpreting nocturnal eddy covariance data.

Below-canopy and soil CO2 fluxes in a ponderosa pine forest

Below-canopy eddy covariance measurements of CO2 flux (Fcb) and soil surface CO2 flux measurements (Fs) were made seasonally in a ponderosa pine forest in central Oregon in 1996 and 1997. The forest ecosystem has a very open canopy, and it is subject to drought and high vapor pressure deficits in summer. Below-canopy flux measurements in March, May, and August 1997 showed increasing effluxes from the forest floor as soils warmed. In July 1996, daytime Fcb measurements appeared to have been influenced by photosynthetic uptake of CO2 by ground vegetation. We did not see a similar diurnal trend in Fcb data in August 1997, probably because photosynthesis may have decreased with senescence of ∼1/3 of the pine canopy and the herbaceous species. On 4 days in August 1997, the mean nocturnal Fs (2.6 ± 0.08 μmol m−2 s−1) was lower than nocturnal Fcb (3.5 ± 0.28 μmol m−2 s−1) by 26%, and daytime Fs was lower than nocturnal Fcb by 18%, possibly because Fcb includes respiration by understory and the lower portions of trees. The mean nocturnal NEE calculated from above-canopy flux and storage in the canopy airspace (Fca + Fstor) at this time was 2.8 ± 0.40 μmol m−2 s−1, 23% lower than ecosystem respiration calculated from chamber measurements on soils, wood, and foliage. The largest difference was observed on a more a turbulent night (u* = 0.30 m s−1) when Fca + Fstor was even significantly less than Fcb and Fs. Our hypothesis is that under calm conditions (e.g. u* < 0.15 m s−1 as observed on three of the nights), Fca is negligible and has no impact on the CO2 budget. Under weak wind conditions (e.g. u* = 0.30 m s−1), Fca begins to become significant and fluxes missed by the above-canopy eddy correlation system degrade the CO2 budget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO2 budget improves again. Below-canopy flux measurements provided useful temporal information for understanding seasonal differences in diel patterns, while the chambers allowed us to characterize spatial variation in CO2 fluxes. It is important to measure below-canopy fluxes along with above-canopy fluxes throughout the year to understand CO2 exchange components and annual contributions to the carbon budget of open canopy forest systems.

Winter wheat carbon exchange in Thuringia, Germany

Eddy covariance measurements and estimates of biomass net primary production (NPP) in combination with soil carbon turnover modelled by the Roth-C model were used to assess the ecosystem carbon balance of an agricultural ecosystem in Thuringia, Germany, growing winter wheat in 2001. The eddy CO2 flux measurements indicate an annual net ecosystem exchange (NEE) uptake in the range from −185 to −245 g C m −2 per year. Main data analysis uncertainty in the annual NEE arises from night-time u ∗ screening, other effects (e.g. coordinate rotation scheme) have only a small influence on the annual NEE estimate. In agricultural ecosystems the fate of the carbon removed during harvest plays a role in the net biome production (NBP) of the ecosystem, where NBP is given by net ecosystem production (NEP =− NEE) minus non-respiratory losses of the ecosystem (e.g. harvest). Taking account of the carbon removed by the wheat harvest (290 g C m −2 ), the agricultural field becomes a source of carbon with a NBP in the order of −45 to −105 g C m −2 per year. Annual carbon balance modelled with the Roth-C model also indicated that the ecosystem was a source for carbon (NBP −25 to −55 g C m −2 per year). Based on the modelling most of carbon respired resulted from changes in the litter and fast soil organic matter pool. Also, the crop and management history, particularly the C input to soil in the previous year, significantly affect next year’s CO 2 exchange.

On measuring and modeling energy fluxes above the floor of a homogeneous and heterogeneous conifer forest

Information on mass and energy exchange at the soil surface under vegetation is a critical component of micrometeorological, climate, biogeochemical and hydrological models. Under sparse boreal and western conifer forests as much as 50% of incident solar energy reaches the soil surface. How this energy is partitioned into evaporating soil moisture, heating the air and soil remains a topic of scientific inquiry, as it is complicated by such factors as soil texture, litter, soil moisture, available energy, humidity deficits and turbulent mixing. Fluxes of mass and energy near the forest floor of a temperate ponderosa pine and a boreal jack pine stand were evaluated with eddy covariance measurements and a micrometeorological soil/plant/atmosphere exchange model. Field tests showed that the eddy covariance method is valid for studying the mean behavior of mass and energy exchange below forest canopies. On the other hand, large shade patches and sunflecks, along with the intermittent nature of atmospheric turbulence, cause run-to-run variability of mass and energy exchange measurements to be large. In general, latent heat flux densities are a non-linear function of available energy when the forest floor is dry. Latent heat flux densities ( E) are about one-quarter of available energy, when this energy is below 100 W m 2 . Latent heat flux density (E) peaks at about 35 W m 2 when available energy exceeds this threshold. A diagnosis of measurements with a canopy micrometeorological model indicates that the partitioning of solar energy into sensible, latent and soil heat flux is affected by atmospheric thermal stratification, surface wetness and the thickness of the litter layer.

Seasonal differences in carbon and water vapor exchange in young and old-growth ponderosa pine ecosystems

Eddy covariance measurements of carbon dioxide and water vapor exchange were made above a young and an old-growth ponderosa pine (Pinus ponderosaDougl. ex P. & C. Laws) ecosystem located in a semiarid environment in central Oregon. The old-growth stand (O site) is a mixture of 250- and 50-year-old ponderosa pine trees with no significant understory (summer maximum leaf area index (LAI) (m 2 half-surface area foliage per m 2 ground) is 2.1). The young stand (Y site; 15 years old in 2000), about 10 km southeast of the old stand, is naturally regenerating following the clear-cut of an old stand in 1978 and has at present about 40% of its LAI in understory shrubs (summer maximum LAI of 1.0). Even though climatic conditions at both sites were very similar, ecosystem carbon exchange differed substantially between the two ecosystems. The old-growth forest with about two times the LAI of the young site, had higher carbon assimilation rates per unit ground area than the young forest, with trends similar between the two forests in spring and fall. Deviations from the trend occurred during summer when water stress in trees at the young site led to a significant reduction in transpiration, and consequently carbon assimilation due to stomatal limitations. Throughout the year, ecosystem respiration ( Re) and gross ecosystem production (GEP) were generally greater at the O site than Y site, and the net of these two processes resulted in a lower net carbon uptake at the Y site.

Response of the carbon isotopic content of ecosystem, leaf, and soil respiration to meteorological and physiological driving factors in a Pinus ponderosa ecosystem

applications of isotope-based models of the global carbon budget as well as for understanding ecosystem-level variation in isotopic discrimination (D). Discrimination may be strongly dependent on synoptic-scale variation in environmental drivers that control canopy-scale stomatal conductance (Gc) and photosynthesis, such as atmospheric vapor pressure deficit (vpd) photosynthetically active radiation (PAR) and air temperature (Tair). These potential relationships are complicated, however, due to time lags between the period of carbon assimilation and ecosystem respiration, which may extend up to several days, and may vary with tissue (i.e., leaves versus belowground tissues). Our objective was to determine if relationships exist over a short-term period (2 weeks) between meteorological and physiological driving factors and d 13 CR and its components, soil-respired d 13 C( d 13 CR-soil) and foliage-respired d 13 C( d 13 CR-foliage). We tested for these hypothesized relationships in a 250-year-old ponderosa pine forest in central Oregon, United States. A cold front passed through the region 3 days prior to our first sample night, resulting in precipitation (total rainfall 14.6 mm), low vpd (minimum daylight average of 0.36 kPa) and near-freeze temperature (minimum air temperature of 0.18� C± 0.3� C), followed by a warming trend with relatively high vpd (maximum daylight average of 3.19 kPa). Over this 2-week period Gc was negatively correlated with vpd (P < 0.01) while net ecosystem CO2 exchange (NEE) was positively correlated with vpd (P < 0.01), consistent with a vpd limitation to conductance and net CO2 uptake. Consistent with a stomatal influence over D, a negative correlation was observed between d 13 CR and Gc measured 2 days prior (i.e., a 2-day time lag, P = 0.04); however, d 13 CR was not correlated with other measured variables. Also consistent with a stomatal influence over discrimination, d 13 CR-soil was negatively correlated with Gc (P < 0.01) and positively correlated with vpd and PAR measured one to 3 days prior (P = 0.01 and 0.04, respectively). In contrast, d 13 CR-foliage was not correlated with vpd or Gc, but was negatively correlated with minimum air temperature measured 5 days previously (P < 0.01) supporting the idea that cold air temperatures cause isotopic enrichment of respired CO2. The significant driving parameters differed for d 13 CR-foliage and d 13 CR-soil potentially due to different controls over the isotopic content of tissue-specific respiratory fluxes, such as differing carbon transport times from the site of assimilation to the respiring tissue or different reliance on recent versus old photosynthate. Consistent with Gc control over photosynthesis and D, both d 13 CR-soil and d 13 CR-foliage became enriched as net CO2 uptake decreased (more positive NEE, P < 0.01 for both). The d 13 C value of Pinus ponderosa foliage (� 27.1%, whole-tissue) was 0.5 to 3.0% more negative than any observed respiratory signature, supporting the contention that foliage d 13 C can be a poor proxy for the isotopic content of respiratory fluxes. The strong meteorological controls

Deployment and Evaluation of a System for Ground-Based Measurement of Cloud Liquid Water Turbulent Fluxes

Abstract Direct interception of windblown cloud water by forests has been dubbed “occult deposition” because it represents a hydrological input that is hidden from rain gauges. Eddy correlation studies of this phenomenon have estimated cloud water fluxes to vegetation yet have lacked estimates of error bounds. This paper presents an evaluation of instrumental and methodological errors for cloud liquid water fluxes to put such eddy correlation measurements in context. Procedures for data acquisition, processing (including correction factors), and calibration testing of the particulate volume monitor (PVM) and forward-scattering spectrometer probe (FSSP) are detailed. Nearly 200 h of in-cloud data are analyzed for intercomparison of these instruments. Three methods of coordinate system rotation are investigated; the flux shows little sensitivity to the method used, and the difference between fluxes at different stations is even less sensitive to this choice. Side-by-side intercomparison of two PVMs and one ...

Variation of net radiation over heterogeneous surfaces: measurements and simulation in a juniper-sagebrush ecosystem.

As part of a larger study of carbon dioxide and energy exchange, energy components in an open-canopied juniper‐sagebrush ecosystem located in the semi-arid region of Eastern Oregon were measured with the eddy covariance technique. Daytime net radiation averaged 20‐30% greater than the sum of sensible, latent and soil heat fluxes. On cloudless days several days after a rain event the imbalance was 200‐250 W m 2 . At such times, differences between the surface radiation temperatures of soil and foliage were large, and we investigated whether such differences may generate systematic errors in the measurement of net radiation. A point measurement of net radiation above an open-canopied forest ecosystem is uncertain, because vegetation structure around the measurement location can be highly variable. Depending on location, various fractions of the upwelling radiation from the soil are intercepted by vegetation and do not reach the radiometer. To determine the magnitude of this uncertainty, we measured tree locations and dimensions, and surface radiation temperature (Tr) and shortwave reflection coefficients ( ) of soils and vegetation in a 100 by 100 m area. Geometrical models generated by ray tracing and rendering software were used to calculate the upwelling radiation that would reach radiometers placed at random locations above the surface. In summer, under cloudless skies the measured radiative surface temperatures of soil and vegetation varied considerably, from a mean of 56C for sunlit soil to 25C for shaded soil, and 27‐29C for sunlit and shaded vegetation (trees and shrubs). The mean shortwave reflection coefficient varied little between components (with vD0.10 for vegetation and sD0.13 for soil). Spatial variability in upwelling radiation ( Ru) arises mainly from component variability at viewing angles from 30 to 60, where contributions to Ru are large and variation in fractional cover between radiometer locations is large. Our measurements and modeling suggest that a radiometer deployed from a tower in a small clearing will only be affected slightly by the clearing since only about 10% of Ru arises from viewing angles less than 15 (directly below the radiometer). The spatial variation in the upwelling radiation reaching a sensor above the canopy increases with increasing differences between the radiation temperatures and reflection coefficients of the various ecosystem components. For the radiative properties found at our site, where the radiative temperature of sunlit soil was 30C larger than the temperature of vegetation and shaded components, the spatial variability in the longwave upwelling radiation (Rlu) was less than 20 W m 2 . The spatial variation in the shortwave upwelling radiation (Rsu) for the small differences in the reflection coefficient of the ecosystem