B51A-0922 0800h
HYDRA: A Programmable Portable Trace-Gas Measuring System
A multi-inlet, portable, air-sampling system (called ``HYDRA'') has been designed and constructed by the NCAR Atmospheric Technology Division (ATD). Since 2002 it has been deployed in field projects on both sides of the Atlantic Ocean (in Colorado and Germany). In the summer of 2002 HYDRA was used in a NCAR pilot project to study CO$_2$ advective transport in a sub-alpine forest below Niwot Ridge, Colorado, and, in the winter of 2003-2004, it was used to study undersnow horizontal and vertical gradients of CO$_2$ in the sub-alpine forest snowpack. In these experiments HYDRA was used in conjunction with a LICOR-7000 sensor (for measuring CO$_2$ and water vapor). Accuracy of the LICOR-7000 is enhanced by the use of three different calibration gases. A physical description of HYDRA, calibration and data reduction procedures, and preliminary results from the two above experiments are presented.
B51A-0923 0800h
Assimilation Modeling of CO$_{2}$ Fluxes at Niwot Ridge, Colorado, and Strategy for Scaling up to the Region
The net ecosystem exchange of CO$_{2}$ (NEE) is the small difference between two large fluxes: photosynthesis and ecosystem respiration. Consequently, separating NEE into its component fluxes, and determining the process-level controls over these fluxes, is a difficult problem. In this study, we used a data assimilation approach with the SIPNET flux model to extract process-level information from five years of eddy covariance data at an evergreen forest in the Colorado Rocky Mountains. SIPNET runs at a half-daily time step, and has two vegetation carbon pools, a single aggregated soil carbon pool, and a soil moisture sub-model that models both evaporation and transpiration. By optimizing the model parameters before evaluating model-data mismatches, we were able to probe the model structure independently of any arbitrary parameter set. In doing so, we were able to learn about the primary controls over NEE in this ecosystem. We also used this parameter optimization, coupled with a formal model selection criterion, to investigate the effects of making hypothesis-driven changes to the model structure. These experiments lent support to the hypotheses that (1) photosynthesis, and possibly foliar respiration, are down-regulated when the soil is frozen, and (2) the metabolic processes of soil microbes vary in the summer and winter, possibly because of the existence of distinct microbial communities at these two times. Finally, we present a strategy for scaling the modeled fluxes up to the region. This scaling approach incorporates data from multiple eddy covariance flux towers and from the satellite-based MODIS sensor to derive NEE estimates for the entire coniferous forest biome of Colorado.
B51A-0924 0800h
Determination of Forest Canopy Carbon Respiration From FTIR Measurements of Carbon Dioxide Isotopic Ratios
Isotopic analysis of atmospheric carbon dioxide provides a means to understand the complex carbon exchange between the biosphere and the atmosphere. Measurements of isotopic concentrations have traditionally been done by laboratory mass spectrometry analysis of grab samples taken from distant field sites with consequent limits on the number and frequency of measurements. In this work, we describe the development of an insitu carbon dioxide isotope analysis method based on Fourier transform infrared (FTIR) spectroscopy. Using molecular absorption in a 4-pass 1-meter length cell in the 2100 to 2600 cm$^{-1}$ region of the $^{12}$CO$_{2}$ and $^{13}$CO$_{2}$ isotopic vibration-rotation bands, concentrations of both isotopes are measured, and from their ratio, \delta$^{13}$CO$_{2}$ is determined. Coupled with the disjunct eddy covariance technique, CO$_{2}$ fluxes can also be measured. We demonstrate the capabilities of this technique using measurements in a poplar forest plantation near Boardman, Oregon. Beer's law and a nonlinear least squares analysis algorithm were used to analyze the data. Careful attention was paid to avoid analysis in spectral regions of nonlinear absorption. Sequential measurements were made at seven levels on a 25-meter tower erected in an 11-m canopy. Carbon dioxide concentrations are calculated to a precision of about 0.5 ppmv, and \delta$^{13}$CO$_{2}$ to about 0.2$\permil$ precision. In this work, we present initial results of temporal and vertical variations of CO$_{2}$ concentrations and \delta$^{13}$CO$_{2}$ in this forest.
B51A-0925 0800h
Impact of vegetation cover and stand age on scaling carbon fluxes in the upper Midwest: a multiple eddy flux site study
Eight permanent and three roving eddy flux towers were used to observe the exchange of carbon dioxide between the ecosystem and atmosphere at fourteen different sites in northern Wisconsin and Michigan (USA) during the growing seasons (May-Sept) of 2002 and 2003. These towers were part of the Chequamegon Ecosystem-Atmosphere Study (ChEAS), the University of Michigan Biological Station (UMBS), and the Michigan Technical University. The sites spanned a range of vegetation types typical of the region (northern hardwood, hemlock-hardwood, mixed forest, red pine, jack pine, pine barrens and shrub wetland). The hardwood and red pine sites also spanned a range of forest stand age (young, intermediate, mature and old). All sites experienced roughly similar climate; thus, comparisons among the sites allow for an examination of the impact of heterogeneous vegetation cover and stand age across a regional landscape. Carbon fluxes at different sites generally reacted similarly in response to variability in climate. Results suggest that both cover type and stand age are important variables for modeling and predicting fluxes in this region. These results have implications for developing methods of scaling carbon dioxide fluxes from sites to regions. These results will be contrasted to a flux decomposition at the WLEF tall tower.
http://cheas.psu.edu
B51A-0926 0800h
Can the Variability in DMS Transfer Velocities be Explained by Wind Speed?
We measured the sea/air flux of DMS by eddy correlation (EC) on an sub-hourly time scale in the Eastern Equatorial Pacific from the NOAA ship Ronald H. Brown in October and November of 2003. We used an atmospheric pressure ionization mass spectrometer (APIMS) with an internal isotopically-labeled standard (D3-DMS) to measure atmospheric DMS concentrations. Lab tests suggest that a Nafion drier limited our effective frequency response to about 1 Hz. Comparisons with water vapor power spectra suggest that this response was adequate to capture more than 90 percent of the flux. The fluxes often responded on a time scale of 10 minutes or less to changes in wind speed, u. We measured seawater DMS concentrations with a purge and trap system once each half hour, so that we could compute the DMS transfer velocity (Vt, the EC-derived flux divided by the interfacial concentration difference) on an hourly basis. A plot of Vt vs u shows that hourly values of Vt ranged from less than the Liss and Merlivat model to more than the Wanninkhof model's value. When binned by wind speed, the average values of Vt lay between the two theories, with standard deviations of 15 to 40 percent. This large variability demonstrates that factors other than wind also affect the exchange velocity: surface roughness, lipid films, bubble spectra, and mean-square wave slope are likely candidates. The APIMS technology for making rapid sea/air gas flux and exchange velocity measurements worked very well, producing results that agree with accepted theories on its first ship-borne trial. It can now be used to address the functionalities of the other controlling factors.
B51A-0927 0800h
Environmental Control of Net Ecosystem Carbon Dioxide Exchange in Contrasting Peatlands in northern Alberta, Canada
Peatlands cover about 21 per cent of the landscape and contain about 80 per cent of the soil carbon stock in western Canada. However, the current rates of carbon accumulation and the environmental controls on ecosystem photosynthesis and respiration in peatland ecosystems is poorly understood. As part of Fluxnet-Canada, we continuously measured net ecosystem carbon dioxide exchange (NEE) using the eddy covariance technique in a treed fen (main site) dominated by stunted black spruce and larch trees during August 2003 through July 2004. Additional NEE measurements were made at two auxiliary sites during intervals in the active growing season (May through September 2004). One auxiliary site was dominated by Sphagnum moss, while the dominant species at other site were Carex and brown mosses. The NEE measurements were used to develop statistical models to assess temporal variation in physiological parameters for ecosystem photosynthesis and respiration. Large seasonal changes occurred in maximum photosynthetic capacity and standardized ecosystem respiration rate at 10 degrees C (R$_{10}$). The mid-day NEE uptake rate during July averaged 10 $\mu$mol m$^{-2}$ s$^{-1}$ at the main site, while lower values of approximately 6 $\mu$mol m$^{-2}$ s$^{-1}$ were observed at the two auxiliary sites. No photosynthetic activity was observed during mid-November through mid-March. On an annual basis R$_{10}$ varied from less than 0.5 $\mu$mol m$^{-2}$ s$^{-1}$ in the winter to approximately 3 $\mu$mol m$^{-2}$ s$^{-1}$ during August at the main site. During much of the growing season, a distinct hysteresis was observed in the light (photon flux density, PFD) response curves for NEE between morning and afternoon periods. This was caused by large diurnal changes in temperature, which at times resulted in the light compensation point for NEE shifting from a PFD of 100 $\mu$mol m$^{-2}$ s$^{-1}$ in the morning to 350 $\mu$mol m$^{-2}$ s$^{-1}$ in the afternoon. The main site recorded a net annual gain of 160 g C m$^{-2}$ yr$^{-1}$, the result of a difference between gross photosynthesis of 648 g C m$^{-2}$ yr$^{-1}$ and total ecosystem respiration of 488 g C m$^{-2}$ yr$^{-1}$.
B51A-0928 0800h
Intercomparison of Photosynthetically Active Radiation Measurements Obtained With Different Sensor Types at Hokkaido, Japan
Field and simulation studies have indicated that accurate modeling of terrestrial photosynthesis and ecosystem-atmosphere carbon dioxide exchange at daily and sub-daily timescales requires information on the diffuse and beam fluxes of photosynthetically active radiation (PAR, 400-700 nm). Such data provide a critical basis for analyzing and monitoring the effects of clouds and aerosols on vegetation productivity and the terrestrial carbon cycle, and for validation of remotely sensed satellite estimates of PAR. In practice, diffuse PAR and global PAR are typically measured in situ, and beam PAR is calculated as the difference of these two fluxes. Whereas time-series data on instantaneous diffuse PAR for seasonal and longer periods were uncommon in the past, growing recognition of their importance in carbon cycle studies has led to increased deployment of PAR sensors capable of measuring both diffuse and global PAR fluxes. Improved information and understanding of the relative accuracy of PAR measurements obtained with different sensor types is important as a basis for determining the confidence limits of model results and to enable meaningful comparison of data sets. In this study, we examine measurements of diffuse and global PAR (photosynthetic photon flux density, PPFD) measured during 2004 at Hokkaido, Japan, with two relatively new and unique types of PAR sensors: the Multifilter Rotating Shadowband Radiometer (model MFR-7 from Yankee Environmental Systems, Inc., custom-configured for PAR) and the Sunshine Sensor (model BF3 from Delta-T Devices). As a reference data set, we employ measurements of the global flux of spectral solar irradiance obtained with a spectral radiometer (from Eko Instruments, Co., Ltd.), from which we calculate PPFD. The results provide insight into the potential uncertainty or error associated with measurements of diffuse and global PAR from two common sensor types and with application of the data to models of vegetation photosynthesis and ecosystem-atmosphere carbon dioxide exchange.
B51A-0929 0800h
Examining Advection Effects on Eddy Flux Measurements at the Niwot Ridge AmeriFlux Site in the Colorado Rocky Mountains
Neglecting CO2 advection in eddy flux measurements is questionable and measuring horizontal CO2 advection from a single tower is difficult. We estimate the contributions of horizontal and vertical advection to the net ecosystem exchange (NEE) of CO2 by using a 3-dimentional measure system that consists of four eddy flux towers in a complex terrain in the Rocky Mountain of Colorado. The measurements showed that horizontal advection components were stronger in the stronger carbon source/sink levels (tree crown and near soil levels) than the other levels. The magnitudes of both horizontal and vertical advective fluxes are similar and opposite in sign, and increase with decreasing of friction velocity (u*). We also examined the validity of the spatial similarity assumption for the vertical CO2 profiles that is usually used in estimating horizontal advection fluxes. The measurements showed that the spatial similarity method resulted in 36% underestimate of the horizontal advection fluxes as u*$<$0.22 m/s , 79% overestimate as 0.22 m/s $<$u* $<$0.66 m/s, and no significant difference as u*$<$0.66 m/s.
B51A-0930 0800h
Inverse Estimation of Vc$_{max}$, LAI, and the Ball-Berry Parameter From Carbon and Energy Flux Measures.
Ecosystem level fluxes of CO$_2$ and energy are modelled with high fidelity using a small number of environmental signals and a small number of seasonally-variant ecosystem parameters. Although these ecosystem parameters are invaluable for modeling canopy fluxes, they are not measured with nearly the same intensity as ecosystem fluxes themselves. An algorithm was developed to estimate leaf area (LAI), maximum carboxylation velocity (Vc$_{max}$), the Ball-Berry parameter {\bf m}, and substrate-dependent ecosystem respiration rate (\beta$_A$) by inverting a commonly-used modeling paradigm of canopy-level CO$_2$ and energy flux. Because these ecosystem parameters have collinear effects on CO$_2$ fluxes, energy flux measures are used to isolate different ecosystem attributes. LAI was solved by fitting the model to measured outgoing turbulent energy (H+LE); Vc$_{max}$ and \beta$_A$ were solved simultaneously by fitting to the flux of CO$_2$; {\bf m} was solved by varying the partitioning of available energy between H and LE. The results of the experiment showed that LAI, Vc$_{max}$, ecosystem respiration, and {\bf m} can be solved so that the carbon and energy fluxes can be modeled with R$^{2}$ from 80 to 95% and non-significant bias at 20-minute and daily timescales. LAI ranged from 2.0 to 2.4 over the season; Vc$_{max}$ declined from 20 to 5 \mu mol C m$^{-2}$ s$^{-1}$; respiration partitioning ranged from 0.5 to 0.75 (as a percentage of assimilation); {\bf m} varied between 17 and 24. These ecosystem parameters were consistent with independent measurements of the seasonal dynamics of the shortgrass steppe where they were evaluated, as well as literature values. In particular, {\bf m} must vary to accommodate changing energy partitioning over the course of the season. The ecosystem parameters are closely linked to mean daily fluxes of CO$_2$, but are not dependent on the environmental drivers during the periods when they are measured. Therefore, process-model inversion has potential for facilitating intercomparison of CO$_2$ and energy flux data among different sites and seasons by extending analyses from phenomenological to phenological considerations of ecosystem dynamics. This can add to the utility of flux data to provide essential land parameters for studies of climate dynamics.
B51A-0931 0800h
Uncertainty Analysis of Vertical Wind Motion Measurement for Airborne Flux Measurements
Measurement of atmosphere-surface exchange is a fundamental part of a more quantitative understanding of climate change. Among the impacts are changes in the fluxes of Biogenic Volatile Organic Compounds (BVOCs) and net ecosystem exchange of carbon, in forest ecosystems. Flux measurements are most frequently conducted on a local scale using flux measurement towers, where the measurement of vertical air motion is correlated with the concentration of the scalar to be studied. To expand the understanding of fluxes to a regional scale, and enable the scaling-up of fluxes to ecosystem levels, we need complementary approaches to the flux-tower model. We are thus engaged in the development of a flexible and low cost aircraft platform for flux measurements, using both eddy covariance, for water vapor and CO2, and Disjunct Eddy Accumulation (DEA), for volatile organic compounds (VOCs). The most challenging aspect of airborne flux measurements is the measurement of vertical scale turbulence. The uncertainties inherent in the measurement of vertical air motion depend on the ability to sense the rotational and translation motion of the aircraft, and the sensed wind, on a 10Hz time scale. We approach this using a combination of a pressure sphere probe for wind measurements, and an integrated Inertial Navigation/Global Positioning System (INS/GPS) to measure the aircraft translation and rotation. This paper summarizes the results of a series of low-speed wind tunnel tests and in-flight calibration maneuvers to determine the uncertainties in vertical wind measurement. The paper summarizes the airspeeds and flight regimes at which different error sources are dominant or negligible. Finally, an error propagation is developed and discussed. This process will lead us to the ability to conduct reliable flux measurements from a low cost aircraft, for a variety of studies of air-surface exchange of gases.
B51A-0932 0800h
Evaporation rates of pasture-mesquite vegetation in central Mexico
The semiarid highlands of Queretaro, in central Mexico, are characterized by booming urban and industrial developments with increasing demand for water. Agriculture takes place in the valleys and the surrounding hills have different types of xeric to subtropical rangeland. Hills are unfit for agriculture and usually are managed for cattle production and fuelwood. However, recent studies suggest that some hill areas are important for groundwater recharge and if they are not protected, important water shortages are envisioned. A critical question involves the effects of land management practices on rangeland hydrologic processes. Evaporation (E), which includes plant and soil evaporation is the largest water loss from rangelands and few data are available for central Mexico. The objective of this study was to estimate E from a mesquite (Prosopis sp.) dominated vegetation using the eddy correlation and the Pennman-Monteith models. Measurements were made during 24 summer days of 2004 at a piedmont site at Amascala, Queretaro (1919 m, 20° 41' N, 100° 16' W). Long term annual rainfall is 568 ± 137 mm. Shrub density was 770 plants per hectare and mean height was 1.8 m. The understory was composed by a mixture of annual and perennial grasses but their biomass was negligible. Agroforestry was the current land use of the site. Shrubs were pruned every 2 or 3 years to maintain its height and promote leafty regrowth. Goats usually browsed the mesquite canopy, but during the time of the study they were excluded from the area.The rainy season started on 15 May and measurements initiated on 1 June, five days after a severe hail storm. Although the mesquite canopy had a full developed canopy with leaf area index of 3.2 by this time, they lost approximately 70% of leaf area. May and June rainfall was 146 mm and 46 mm occurred during the measuring period. Throughout the measurement period E was coupled to global radiation and total evaporation was 73.8 mm. On cloudy days E ranged from 1.1 to 2.0 mm d-1, maximum E was 4.3 mm d-1 on sunny days and the average E was 3.1 mm d-1. Average daily E increased during the measuring period at a rate of 0.05 mm d-1 (r2=0.2, p<0.05). Data suggest that evaporation from a pasture-mesquite vegetation is an important component in the water balance considering the limited rainfall occurring.
B51A-0933 0800h
Decomposing NEE measured over a mixed forest area and upscaling in northern WI using footprint models and a vegetation map
The measured net ecosystem-atmosphere exchange of CO2 (NEE) in a mixed forest area in Northern Wisconsin is decomposed into contributions from six vegetation classifications using footprint models and a vegetation map. The large footprint of the WLEF tall tower is used as a regional flux measurement, encompassing several vegetation classifications.The vegetation classifications are mixed coniferous/deciduous, aspen, mixed deciduous, low-canopy wetland, forested wetland, and others. The functional parameters in the ecosystem models of respiration-Temperature and flux-PAR for the six types of ecosystems are inferred and compared. It appears that separate ecosystems respond differently to the same environmental conditions. The aspen and forested wetland ecosystems have the largest values of NEE when saturated with respect to incoming solar radiation in the day. The forested wetland ecosystem has the largest Q10 and diurnally-averaged respiration rate at night. Another interesting finding is that the derived respiration rates for the deciduous and lowland wetland ecosystems in the area around WLEF tower are larger than those measured at two nearby flux tower sites, Willow Creek (deciduous forest) and Lost Creek (low-canopy wetland). These results indicate that the measurements at Willow Creek and Lost Creek are not representative of all deciduous and low-canopy wetland ecosystems in the region possibly because variables such as stand age, density, species composition, or coarse woody debris content can vary among similar land cover classifications. As an application, the NEE models with derived parameters for each ecosystem are aggregated in a bottom-up approach to estimate the flux in a 40km x 40km region, and the results are compared with those from a top-down ABL budget method.
B51A-0934 0800h
Error Analysis of Estimated Means and Horizontal Gradients of Scalar Variables
While random sampling errors (RSE) for eddy-correlation fluxes are discussed in the literature, little attention has been paid to RSE for the mean or gradient of scalar variables. Accurate estimates of the mean and the gradient of certain scalar variables are important in evaluating and balancing budgets for these variables. In the present work, we evaluate the RSE for the estimated mean and horizontal gradient of air temperature under various atmospheric stabilities, using data from three field programs. Although air temperature is chosen as an economical scalar variable, our approach is applicable for error analysis of estimated advection and local budgets of CO$_2$. Similarities are expected between the results of the error analysis for air temperature and those for CO$_2$. For all atmospheric stabilities, significant energy occurs at mesoscale frequencies in the spectra of air temperature, which corresponds to significant non-stationarity of air temperature. On the other hand, little energy is present at mesoscale frequencies in the spectra of the horizontal gradient of air temperature, except for highly stable conditions. Low-frequency mesoscale fluctuations result in nonstationary records of horizontal gradient of air temperature, leading to large RSE in the estimated gradient. The nonstationarity effect is found to increase with increasing separation distance between the two air temperature measurements. The evaluated RSE of the horizontal gradient of air temperature is compared to the instrumentation-related uncertainties and the magnitude of the estimated horizontal gradients. An optimum separation distance between two points for air temperature measurements is discussed.
B51A-0935 0800h
The impact of wind on the soil respiration measurement
Soil respiration is a significant component of the carbon balance for an ecosystem, however, how environmental variables regulate the soil respiration remains poorly understood. This limits our ability to understand the carbon budget at the ecosystem level, thus making it uncertain to predict the impact of climate change on soil respiration and its feedback. One major reason for this poor understanding is that we are still lacking continuous long-term soil respiration data at a very high temporal and spatial resolution due to unavailable robust and reliable automated soil respiration instruments. To meet this need, Licor Inc has developed a new automated instrument, the LI-8100, for continuous long-term soil respiration measurements. The instrument also supports a survey chamber for studying the spatial variability of soil respiration. In this paper, we will brief the design, the capability, a new way to compute the respiration rate and other features of the LI-8100. We will then focus on the impact of wind gusts on the soil CO2 efflux. The experiment was carried out in a grassland forest-edge ecosystem near Lincoln, Nebraska in the summer of 2004. Soil respiration rate was measured with our automated soil respiration system (LI-8100). 3D wind speed at 0.5 m was monitored at 1 Hz with a RM Young sonic anemometer. Soil temperature, soil moisture and precipitation were also measured with various instruments and sensors. Results showed that, when there was a sudden change in wind speed, soil respiration rate increased, sometimes by 50% or more. The validity of soil respiration measurements under such windy conditions will be discussed in terms of the pressure difference between the inside and the outside of the respiration chamber.
B51A-0936 0800h
Looking within and looking beyond soil respiration measurements: observing intra-site variation and patterns on the landscape.
There are many complex and poorly understood vectors for carbon movement within the terrestrial component of the global carbon cycle . One of which is soil respiration, or the combined release of CO2 from autotrophic and heterotrophic metabolism within the soil profile. Soil respiration is controlled by soil climate and soil biota, which includes both the amount and the relative activity of respiring root and microbial tissues. Many inter-ecosystem trends have been observed and documented, but little is known about what patterns occur within a site or if trends are related, geographically, across the terrain. This research aims to determine the causes of and to quantify spatial and temporal variation of soil respiration within and among common forests of Northern Wisconsin that are largely influenced by topography. Our research takes a close look at the causes of variation between individual measurements within a site; we also expand our viewpoint by looking at topographic variation on soil respiration maps which help to visualize the effects of time, space and climatic variability. To address these tasks we ask: (1) are observed influences of below-ground biomass, and C:N on site soil respiration also visible within a site? (2) how does topography affect the drivers of soil respiration and can topography be used to model soil respiration in 3 dimensions? By addressing these objectives, it should be possible to further explore one critical aspect of the carbon cycle and perhaps learn more about future biosphere-atmosphere interactions.
B51A-0937 0800h
Carbon Exchange of Central New England Deciduous Forests: Variability Related to Age and Topography
Forests in much of the northeastern U.S. occur in hilly or mountainous terrain and vary widely in age, due to forest harvesting and natural disturbances. Sites in the NE U.S. with relatively long-term C exchange records represent two very different major tree species associations (boreal coniferous forest and oak-maple dominated deciduous forest) but cover relatively little variation in topography and age. All of the forests measured are in somewhat low-lying areas and are fairly mature, ranging from 65 to $<$120 years in age. Data are needed from younger forests and forests with higher slope position in order to accurately estimate forest C storage in the NE U.S. In May 2002 we began the first eddy covariance (EC) measurements in a higher deciduous forest, about 1.1 km from the Harvard Forest Environmental Measurement Site (HFEMS), where C exchange has been measured since 1991. The higher site has similar tree species composition to HFEMS, but most trees within 300 m of the higher eddy covariance tower (and some beyond) originated after a fire in 1957. Wind direction and nocturnal turbulence strongly affect EC data at the higher site. With wind between 30 and 210 $^{o}$ from N, we observe large apparent C effluxes ($<$30 mol m$^{-2}$ s$^{-1}$) at night, and sometimes during the day. Such large C effluxes have very seldom been observed at HFEMS, and at the higher site we interpret them as artifacts generated by lee-slope turbulence, due to airflow over forest that is 20-30 m higher than the point of EC measurements. With other wind directions, nocturnal C flux at the higher site increases with increasing turbulence. We attribute this to cold air drainage on the long approximately 10% slope to the W and NW. This inference is supported by very low measured C fluxes when air 20 cm from the ground is $<$ 1.5 $^{o}$C colder than air above the canopy. Accordingly, at the higher site we only accept C flux data if wind direction is between 215 and 360$^{o}$ and u* $<$ 0.35 m/s. Under these conditions, nocturnal C fluxes measured at the higher site were within the range measured at the HFEMS throughout 2002 and 2003, except during NW winds. With NW winds, the footprint of HFEMS includes a bog, and measured C fluxes were significantly higher. When the wind direction and turbulence criteria were not met, we estimated C flux using statistical models derived from the acceptable EC data. A combination of valid data and models indicates that in the summers of 2002 and 2003 C storage at the higher site was nearly equal to C storage at HFEMS. Peak monthly C storage of 1.9-2.1 Mg/ha occurred in July each year. However, during December through March estimated monthly C loss at the higher site was only about half as great as at HFEMS (0.20-0.28 versus 0.38-0.53 Mg//ha), perhaps due to frequent NW winds in winter. In sum, our first two years of data show that the younger, higher deciduous forest had similar annual net ecosystem exchange (NEE) (about 2.0 to 2.5 Mg C ha$^{-1}$ y$^{-1}$) as the 65-100 year old forest at the HFEMS site. Lower average ecosystem respiration at the higher site, possibly caused by a lack of wetland areas, may allow the higher forest to maintain approximately the same NEE with a smaller annual GEE.
B51A-0938 0800h
Ecosystem Respiration and its Components in an Old-growth and a Mature Northern Forest
Ecosystem respiration is important in global carbon cycling and may be more sensitive than photosynthesis to environmental conditions in influencing net ecosystem productivity. Total ecosystem respiration and its component fluxes such as soil, stem and leaf respiration vary with vegetation age, species composition and local climate. We measured ecosystem respiration and its components using chambers in an old-growth and a mature northern forest in northern Wisconsin and Michigan in a similar climate condition. Chamber measurements were compared with continuous eddy covariance measurements. Total ecosystem respiration measured from chambers was about 1 kgC m$^{-2}$ year$^{-1}$ in the mature forest and 0.9 kgC m$^{-2}$ year$^{-1}$ in the old-growth in 2002. This result agreed well with eddy covariance measurements from the old-growth but not from the mature forest. Our results indicate that the old-growth forest had less respiration, less photosynthesis, and less net ecosystem exchange than the nearby mature forest. Forest succession significantly influences carbon dynamics.
B51A-0939 0800h
Characterizing Canopy Structure Information on Savanna Woodland Using Small-footprint Airborne Lidar Data
Modeling the carbon dynamics in a savanna ecosystem is a very challenging task due to its horizontal and vertical heterogeneity and it usually requires complex 3-D biogeochemical model. In turn, the application of 3D biogeochemical model at the landscape level requires detailed canopy structure information on individual trees, which is an impossible task until the recent advent of new remotely sensed data and information extraction technqiues. This study explores the utility of airborne small-footprint airborne Lidar (Light Detection and Ranging) to isolate individual trees and extract their heights, crown diameters, crown heights, and LAI. These parameters will be feed into a 3D model called MAESTRO to quantify carbon flux and will be validated with eddy covariance measurements.
B51A-0940 0800h
Quantification of large vertical tree roots with borehole radar
Ground-penetrating radar can be used to detect tree roots provided there is sufficient electromagnetic contrast to separate roots from soil. Forest researchers need root biomass, distribution and architecture data to assess the effects of forest management practices on productivity and resource allocation in trees. Ground-penetrating radar is a non-destructive alternative to laborious excavations that are commonly employed. Tree roots are not ideal subjects for radar studies; clutter from non-target materials can degrade the utility of GPR profiles. On amenable soils, rapid root biomass surveys provide valuable information in a short period time, though some destructive ground-truthing may be required. Surface-based GPR can provide excellent resolution of lateral roots. However, some forest trees have significant allocation to large vertical taproots roots (i.e. loblolly pine, {\it Pinus taeda} L., longleaf pine, {\it Pinus palustris} Mill.), which cannot be accurately assessed by surface measures. A collaborative project between the USDA Forest Service, Southern Research Station, Radarteam AB and the Swedish Experimental Forest system was undertaken in 2003 to assess the potential of high-frequency borehole radar to detect vertical near surface reflectors (0-2 m). A variety of borehole methods were assessed to identify the most promising technique to image large vertical roots. We used a 1000 mhz transducer (Radarteam tubewave-1000) along with a GSSI ground-penetrating radar unit (Sir-20) to collect reflective data in boreholes adjacent to trees as well as cross-hole travel time measurements. This research was conducted near Vindeln in northern Sweden in August 2003. Six trees ({\it Pinus sylvestris}) whose DBH ranged from approximately 20-60 cm were intensively measured to provide information on a variety of size classes. On either side of each tree a 5 cm diameter hole was excavated to a depth of 2 m with a soil auger. One antenna was configured as a transmitter (Tx), the other as a receiver (Rx) and they were lowered into the holes opposite each other. The Tx was operated in single shot mode, where an electromagnetic pulse was propagated and the time it took to penetrate the soil matrix and be detected by the Rx was measured. To allow for tomographic reconstruction of the vertical roots, a series of vectors were created by raising and lowering the antennas at intervals of 5 cm. Then the antennas were moved to opposite holes and the process was repeated creating 3200 unique travel-paths per tree. Borehole to surface measures were collected in a similar fashion, though the Rx was moved across the soil surface (10 cm interval) and the Tx was manipulated below ground (5 cm interval), generating 2400 unique travel-paths per tree. This is the first report of using borehole radar to study vertical tree roots. Cross-hole tomography provided excellent information on the depth of tree roots, but was less useful for imaging near surface features. Borehole to surface measures provided the best information on the near surface, where the bulk of roots are found (0-0.3 m). Cross-hole and borehole to surface data may be combined to further define vertical roots systems. Analysis of root mass and projected root mass is ongoing.
B51A-0941 0800h
Demonstration of a New Capability for Airborne Measurement of Carbon Dioxide Fluxes and Accurate Mixing Ratios: Preliminary Results from GOTEX and ACME
A new instrument was developed for airborne carbon dioxide measurements. Control of time response allows operation in two modes for application to either eddy covariance or highly precise mixing ratio measurements. Results of Pacific marine boundary layer flux observations during the Gulf of Tehuantepec Experiment (GOTEX) will be presented, including quantification of instrumental air motion sensitivity. Preliminary spectral analysis of flight data indicates a 5-Hz frequency response. The instrument was tuned for mixing ratio precision, with concomitant reduction in time response, during the Airborne Carbon in the Mountains Experiment. Performance will be assessed, including accuracy estimates derived from intercomparison activities.