Publications (32)50.45 Total impact
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Article: Linking flux network measurements to continental scale simulations: ecosystem carbon dioxide exchange capacity under non‐water‐stressed conditions
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ABSTRACT: This paper examines long-term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under non-water-stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light-class carboxylase-based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process-based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange.Global Change Biology 03/2007; 13(4):734 - 760. · 6.86 Impact Factor -
Article: Estimating parameters in a land‐surface model by applying nonlinear inversion to eddy covariance flux measurements from eight FLUXNET sites
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ABSTRACT: Flux measurements from eight global FLUXNET sites were used to estimate parameters in a process-based, land-surface model (CSIRO Biosphere Model (CBM), using nonlinear parameter estimation techniques. The parameters examined were the maximum photosynthetic carboxylation rate () the potential photosynthetic electron transport rate (jmax, 25) of the leaf at the top of the canopy, and basal soil respiration (rs, 25), all at a reference temperature of 25°C. Eddy covariance measurements used in the analysis were from four evergreen forests, three deciduous forests and an oak-grass savanna. Optimal estimates of model parameters were obtained by minimizing the weighted differences between the observed and predicted flux densities of latent heat, sensible heat and net ecosystem CO2 exchange for each year. Values of maximum carboxylation rates obtained from the flux measurements were in good agreement with independent estimates from leaf gas exchange measurements at all evergreen forest sites. A seasonally varying and jmax, 25 in CBM yielded better predictions of net ecosystem CO2 exchange than a constant and jmax, 25 for all three deciduous forests and one savanna site. Differences in the seasonal variation of and jmax, 25 among the three deciduous forests are related to leaf phenology. At the tree-grass savanna site, seasonal variation of and jmax, 25 was affected by interactions between soil water and temperature, resulting in and jmax, 25 reaching maximal values before the onset of summer drought at canopy scale. Optimizing the photosynthetic parameters in the model allowed CBM to predict quite well the fluxes of water vapor and CO2 but sensible heat fluxes were systematically underestimated by up to 75 W m−2.Global Change Biology 02/2007; 13(3):652 - 670. · 6.86 Impact Factor -
Article: On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm
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ABSTRACT: This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems.We show that the temperature sensitivity of Reco, derived from long-term (annual) data sets, does not reflect the short-term temperature sensitivity that is effective when extrapolating from night- to daytime. Specifically, in summer active ecosystems the long-term temperature sensitivity exceeds the short-term sensitivity. Thus, in those ecosystems, the application of a long-term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from half-hourly to annual time-scales, which can reach >25% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long-term temperature sensitivity is lower than the short-term temperature sensitivity resulting in underestimation of annual sums of respiration.We introduce a new generic algorithm that derives a short-term temperature sensitivity of Reco from eddy covariance data that applies this to the extrapolation from night- to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and Reco, we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.Global Change Biology 07/2005; 11(9):1424 - 1439. · 6.86 Impact Factor -
Article: DenNit – Experimental analysis and modelling of soil N2O efflux in response on changes of soil water content, soil temperature, soil pH, nutrient availability and the time after rain event
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ABSTRACT: To quantify the effects of soil temperature (Tsoil), and relative soil water content (RSWC) on soil N2O emission we measured N2O soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity, ammonium, and nitrate concentration in the field. The analysis was performed on three meadows, two bare soils and in one forest. We identified soil water content, soil temperature, soil nitrogen content, and pH as the main parameters influencing soil N2O emission. The response of N2O emission to soil temperature and relative soil water content was analysed for the field and climate chamber measurements. A non-linear regression model (DenNit) was developed for the field data to describe soil N2O efflux as a function of soil temperature, soil moisture, pH value, and ammonium and nitrate concentration. The model could explain 81% of the variability in soil N2O emission of all individual field measurements, except for data with short-term soil water changes, namely during and up to 2 h after rain stopped. We validated the model with an independent dataset. For this additional meadow site 73% of the flux variation could be explained with the model.Plant and Soil 04/2005; 272(1):349-363. · 2.73 Impact Factor -
Article: Comparison of surface energy exchange models with eddy flux data in forest and grassland ecosystems of Germany
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ABSTRACT: Latent heat, which plays a major role in the energy balance of ecosystems, is usually calculated in different types of models, e.g., water balance approaches, aggregated big-leaf models, multi-layer ecosystem process models, that can be generally catalogued as Soil-Vegetation-Atmosphere-Transport models (SVATs) in a broader sense, etc. In this study, four (multi-) layered models (PLant-ATmosphere INteraction model (PLATIN), MixFor-SVAT, SVAT-CN, PnET-N-DNDC) and an atmospheric boundary layer model including vegetation (HIgh Resolution Vegetation Atmosphere Coupler (HIRVAC)) were used to predict latent heat (LE) and sensible heat (H) exchange in the target area of VERTIKO, a research project on vertical transports of energy and trace gases and their spatial/temporal extrapolation under complex natural conditions. Investigated vegetation types were a Norway spruce forest, and two grassland sites in Brandenburg and Saxony (Germany) during field campaigns in 2001 and 2002. Four models had half-hoEcological Modelling 01/2005; 188(2-4):174-216. · 2.33 Impact Factor -
Article: The effect of soil water content, soil temperature, soil pH-value and the root mass on soil CO2 efflux – A modified model
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ABSTRACT: To quantify the effects of soil temperature (Tsoil), and relative soil water content (RSWC) on soil respiration we measured CO2 soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity and fine root mass in the field. The analysis was performed on three meadow, two bare fallow and one forest sites. The influence of soil temperature on CO2 emissions was highly significant with all land-use types, except for one field campaign with continuous rain. Where soil temperature had a significant influence, the percentage of variance explained by soil temperature varied from site to site from 13–46% in the field and 35–66% in the climate chamber. Changes of soil moisture influenced only the CO2 efflux on meadow soils in field and climate chamber (14–34% explained variance), whereas on the bare soil and the forest soil there was no visible effect. The spatial variation of soil CO2 emission in the field correlated significantly with the soil pH and fine root mass, explaining up to 24% and 31% of the variability. A non-linear regression model was developed to describe soil CO2 efflux as a function of soil temperature, soil moisture, pH-value and root mass. With the model we could explain 60% of the variability in soil CO2 emission of all individual field chamber measurements. Through the model analysis we highlight the temporal influence of rain events. The model overestimated the observed fluxes during and within four hours of the last rain event. Conversely, after more than 72h without rain the model underestimated the fluxes. Between four and 72 h after rainfall, the regression model of soil CO2 emission explained up to 91% of the variance.Plant and Soil 12/2004; 268(1):21-33. · 2.73 Impact Factor -
Article: Seasonality of ecosystem respiration and gross primary production as derived from FLUXNET measurements
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ABSTRACT: Differences in the seasonal pattern of assimilatory and respiratory processes are responsible for divergences in seasonal net carbon exchange among ecosystems. Using FLUXNET data (http://www.eosdis.ornl.gov/FLUXNET) we have analyzed seasonal patterns of gross primary productivity (FGPP), and ecosystem respiration (FRE) of boreal and temperate, deciduous and coniferous forests, Mediterranean evergreen systems, a rainforest, temperate grasslands, and C3 and C4 crops. Based on generalized seasonal patterns classifications of ecosystems into vegetation functional types can be evaluated for use in global productivity and climate change models. The results of this study contribute to our understanding of respiratory costs of assimilated carbon in various ecosystems. Seasonal variability of FGPP and FRE of the investigated sites increased in the order . Together with the boreal forest sites, the managed grasslands and crops show the largest seasonal variability. In the temperate coniferous forests, seasonal patterns of FGPP and FRE are in phase, in the temperate deciduous and boreal coniferous forests FRE was delayed compared to FGPP, resulting in the greatest imbalance between respiratory and assimilatory fluxes early in the growing season. FGPP adjusted for the length of the carbon uptake period decreased at the sampling sites across functional types in the order C4 crops, temperate and boreal deciduous forests conifers, C3 grassland and crops conifers (4.6 g C m−2 per day). Annual FGPP and net ecosystem productivity (FNEP) decreased across climate zones in the order tropical>temperate>boreal. However, the decrease in FNEP with latitude was greater than the decrease in FGPP, indicating a larger contribution of respiratory (especially heterotrophic) processes in boreal systems.Agricultural and Forest Meteorology 12/2002; 113(1):53-74. · 3.39 Impact Factor -
Article: Phase and amplitude of ecosystem carbon release and uptake potentials as derived from FLUXNET measurements
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ABSTRACT: As length and timing of the growing season are major factors explaining differences in carbon exchange of ecosystems, we analyzed seasonal patterns of net ecosystem carbon exchange (FNEE) using eddy covariance data of the FLUXNET data base (http://www-eosdis.ornl.gov/FLUXNET). The study included boreal and temperate, deciduous and coniferous forests, Mediterranean evergreen systems, rainforest, native and managed temperate grasslands, tundra, and C3 and C4 crops. Generalization of seasonal patterns are useful for identifying functional vegetation types for global dynamic vegetation models, as well as for global inversion studies, and can help improve phenological modules in SVAT or biogeochemical models. The results of this study have important validation potential for global carbon cycle modeling. The phasing of respiratory and assimilatory capacity differed within forest types: for temperate coniferous forests seasonal uptake and release capacities are in phase, for temperate deciduous and boreal coniferous forests, release was delayed compared to uptake. According to seasonal pattern of maximum nighttime release (evaluated over 15-day periods, Fmax) the study sites can be grouped in four classes: (1) boreal and high altitude conifers and grasslands; (2) temperate deciduous and temperate conifers; (3) tundra and crops; (4) evergreen Mediterranean and tropical forests. Similar results are found for maximum daytime uptake (Fmin) and the integral net carbon flux, but temperate deciduous forests fall into class 1. For forests, seasonal amplitudes of Fmax and Fmin increased in the order tropical<Mediterranean and temperate coniferous<temperate deciduous and boreal forests, and the pattern seems relatively stable for these groups. The seasonal amplitudes of Fmax and Fmin are largest for managed grasslands and crops. Largest observed values of Fmin varied between −48 and −2 μmol m−2 s−1, decreasing in the order C4-crops>C3-crops>temperate deciduous forests>temperate conifers>boreal conifers>tundra ecosystems. Due to data restrictions, our analysis centered mainly on Northern Hemisphere temperate and boreal forest ecosystems. Grasslands, crops, Mediterranean ecosystems, and rainforests are under-represented, as are savanna systems, wooded grassland, shrubland, or year-round measurements in tundra systems. For regional or global estimates of carbon sequestration potentials, future investigations of eddy covariance should expand in these systems.Agricultural and Forest Meteorology 12/2002; 113(1):75-95. · 3.39 Impact Factor -
Article: Energy balance closure at FLUXNET sites
Agricultural and Forest Meteorology. 01/2002; 113(1-4):223-243. -
Article: Phase and amplitude of ecosystem carbon release and uptake potentials as derived from FLUXNET measurements
Agricultural and Forest Meteorology 01/2002; 113 (1-4), 75-95. · 3.39 Impact Factor -
Article: Gap filling strategies for long term energy flux data sets
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ABSTRACT: At present a network of over 100 field sites are measuring carbon dioxide, water vapor and sensible heat fluxes between the biosphere and atmosphere, on a nearly continuous basis. Gaps in the long term measurements of evaporation and sensible heat flux must be filled before these data can be used for hydrological and meteorological applications. We adapted methods of gap filling for NEE (net ecosystem exchange of carbon) to energy fluxes and applied them to data sets available from the EUROFLUX and AmeriFlux eddy covariance databases. The average data coverage for the sites selected was 69% and 75% for latent heat (λE) and sensible heat (H). The methods were based on mean diurnal variations (half-hourly binned means of fluxes based on previous and subsequent days, MDV) and look-up tables for fluxes during assorted meteorological conditions (LookUp), and the impact of different gap filling methods on the annual sum of λE and H is investigated. The difference between annual λE filled by MDV and λE filled by LookUp ranged from −120 to 210 MJ m−2 per year, i.e. −48 to +86 mm per year, or −13 to +39% of the annual sum. For annual sums of H differences between −140 and +140 MJ m−2 per year or −12 to +19% of the annual sum were found.Agricultural and Forest Meteorology 04/2001; · 3.39 Impact Factor -
Article: Gap filling strategies for defensible annual sums of net ecosystem exchange
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ABSTRACT: Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange (FNEE) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of FNEE; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required.We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables (LookUp), and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of FNEE is investigated. The difference between annual FNEE filled by MDV compared to FNEE filled by Regr. ranged from −45 to +200 g C m−2 per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150 g C m−2 per year.We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities (u∗) shifted annual FNEE on average by +77 g C m−2 per year, but in certain cases as much as +185 g C m−2 per year.Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks.Agricultural and Forest Meteorology 04/2001; · 3.39 Impact Factor -
Article: FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities.
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ABSTRACT: FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of car-bon 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 con-tinents 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 infor-mation 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 ex-change 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.©2001 American Meteorological Society. 01/2001; 82. -
Article: On Measuring Net Ecosystem Carbon Exchange Over Tall Vegetation on Complex Terrain
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ABSTRACT: To assess annual budgets of CO2 exchange betweenthe biosphere and atmosphere over representativeecosystems, long-term measurements must be made overecosystems that do not exist on ideal terrain. How tointerpret eddy covariance measurements correctlyremains a major task. At present, net ecosystemCO2 exchange is assessed, by members of themicrometeorological community, as the sum of eddycovariance measurements and the storage of CO2 inthe underlying air. This approach, however, seemsunsatisfactory as numerous investigators are reportingthat it may be causing nocturnal respiration fluxdensities to be underestimated.A new theory was recently published by Lee (1998, Agricultural and Forest Meteorology 91: 39–50) for assessing net ecosystem-atmosphere CO2 exchange(Ne) over non-ideal terrain. Itincludes a vertical advection term. We apply thisequation over a temperate broadleaved forest growingin undulating terrain. Inclusion of the verticaladvection term yields hourly, daily and annual sums ofnet ecosystem CO2 exchange that are moreecologically correct during the growing season.During the winter dormant period, on the other hand,corrected CO2 flux density measurements of anactively respiring forest were near zero. Thisobservation is unrealistic compared to chambermeasurements and model calculations. Only duringmidday, when the atmosphere is well-mixed, domeasurements of Ne match estimatesbased on model calculations and chamber measurements. On an annual basis, sums of Newithout the advection correction were 40% too large,as compared with computations derived from a validatedand process-based model. With the inclusion of theadvection correction term, we observe convergencebetween measured and calculated values ofNe on hourly, daily and yearly time scales. We cannot, however, conclude that inclusion of aone-dimensional, vertical advection term into thecontinuity equation is sufficient for evaluatingCO2 exchange over tall forests in complexterrain. There is an indication that the neglected term,( c/ x), isnon-zero and that CO2 may be leakingfrom the sides of the control volume during the winter. In this circumstance, forest floor CO2 effluxdensities exceed effluxes measured above the canopy.Boundary-Layer Meteorology 07/2000; 96(1):257-291. · 1.74 Impact Factor -
Article: Temporal and spatial variation in transpiration of Norway spruce stands within a forested catchment of the Fichtelgebirge, Germany
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ABSTRACT: Tree transpiration was observed with sapflow methods in six Norway spruce (Picea abies) stands located in the Lehstenbach catchment, Fichtelgebirge, Germany, differing in age (40 years up to 140 years), structure, exposition and soil characteristics. The seasonal pattern in tree canopy transpiration, with the highest transpiration rates in July, was very similar among the stands. However, young dense stands had higher transpiration compared to older less dense stands. Because of forest management practices, stand density decreases with increasing stand age and provides the best predictor of canopy water use. Measured xylem sapflux density did not differ significantly among stands, e.g. vary in correlation with stand density. Thus, differences in canopy transpiration were related to differences in cumulative sapwood area, which decreases with age and at lower tree density. While both total sapwood area and individual tree sapwood area decrease in older less dense stands, leaf area index of the stands remains high. Thus, transpiration or physiological activity of the average individual needle must decrease. Simulations with a three-dimensional stand model suggest that stand structural changes influence light climate and reduce the activity of the average needle in the stands. Nevertheless, age and nutrition must be considered with respect to additional direct effects on canopy transpiration. (© Inra/Elsevier, Paris.) Variations spatiotemporelles de la transpiration de peuplements d'épicéas dans un bassin-versant du Fichtelgebirge (Allemagne). La transpiration des arbres a été évaluée au moyen de méthodes de mesure du flux de sève dans six peuplements d'épicéas (Picea abies), situés dans le bassin-versant du Lehstenbach, Fichtelgebirge (Allemagne), qui différaient en âge (40 à 140 ans), structure, exposition, et en caractéristiques de sol. L'allure des variations saisonnières de la transpiration des arbres, avec notamment un maximum en juillet, était très similaire entre ces peuplements. Néanmoins, les jeunes peuplements denses ont montré une plus forte transpiration que les peuplements âgés et moins denses. La densité du peuplement s'est avérée être la meilleure variable explicative de la transpiration, car les pratiques sylvicoles réduisent la densité des peuplements en fonction de l'âge. La densité de flux de sève n'a pas montré de différences significatives entre les peuplements. Ainsi, les différences de transpiration étaient seulement dues aux différences de surface de bois d'aubier, qui diminue avec l'âge et la densité. Alors que la surface de bois d'aubier à l'échelle du peuplement comme à celle de l'arbre diminuaient dans les peuplements âgés et peu denses, l'indice foliaire de tous les peuplements étudiés restait élevé. Ainsi, il est probable que la transpiration ou l'activité physiologique des aiguilles diminuent avec l'âge des arbres. Des simulations réalisées au moyen d'un modèle de couvert 3D suggèrent que les modifications de structure des peuplements influencent le microclimat lumineux et réduisent l'activité foliaire. Malgré tout, l'âge et la nutrition doivent être pris en compte dans leurs effets sur la transpiration des arbres. (© Inra/Elsevier, Paris.)http://dx.doi.org/10.1051/forest:19980107. -
Article: Vertical structure of evapotranspiration at a forest site (a case study)
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ABSTRACT: {textlessptextgreatertextless}br/textgreaterThe components of ecosystem evapotranspiration of a Norway spruce forest (Picea abies L.) as well as the vertical structure of canopy evapotranspiration were analyzed with a combination of measurements and models for a case study of 5 days in September 2007. Eddy-covariance and sap flux measurements were performed at several heights within the canopy at the FLUXNET site Waldstein-Weidenbrunnen (DE-Bay) in the Fichtelgebirge mountains in Germany. Within and above canopy fluxes were simulated with two stand-scale models, the 1D multilayer model ACASA that includes a third-order turbulence closure and the 3D model STANDFLUX. The soil and understory evapotranspiration captured with the eddy-covariance system in the trunk space constituted 10% of ecosystem evapotranspiration measured with the eddy-covariance system above the canopy. A comparison of transpiration measured with the sap flux technique and inferred from below and above canopy eddy-covariance systems revealed higher estimates from eddy-covariance measurements than for sap flux measurements. The relative influences of possible sources of this mismatch, such as the assumption of negligible contribution of evaporation from intercepted water, and differences between the eddy-covariance flux footprint and the area used for scaling sap flux measurements, were discussed. Ecosystem evapotranspiration as well as canopy transpiration simulated with the two models captured the dynamics of the measurements well, but slightly underestimated eddy-covariance values. Profile measurements and models also gave us the chance to assess in-canopy profiles of canopy evapotranspiration and the contributions of in-canopy layers. For daytime and a coupled or partly coupled canopy, mean simulated profiles of both models agreed well with eddy-covariance measurements, with a similar performance of the ACASA and the STANDFLUX model. Both models underestimated profiles for nighttime and decoupled conditions. During daytime, the upper half of the canopy contributed approximately 80% to canopy evapotranspiration, whereas during nighttime the contribution shifted to lower parts of the canopy.textless/ptextgreaterAgricultural and Forest Meteorology. 151(6):709-729. -
Article: Vertical structure of evapotranspiration at a forest site (a case study)
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ABSTRACT: The components of ecosystem evapotranspiration of a Norway spruce forest (Picea abies L.) as well as the vertical structure of canopy evapotranspiration were analyzed with a combination of measurements and models for a case study of 5 days in September 2007. Eddy-covariance and sap flux measurements were performed at several heights within the canopy at the FLUXNET site Waldstein-Weidenbrunnen (DE-Bay) in the Fichtelgebirge mountains in Germany. Within and above canopy fluxes were simulated with two stand-scale models, the 1D multilayer model ACASA that includes a third-order turbulence closure and the 3D model STANDFLUX. The soil and understory evapotranspiration captured with the eddy-covariance system in the trunk space constituted 10% of ecosystem evapotranspiration measured with the eddy-covariance system above the canopy. A comparison of transpiration measured with the sap flux technique and inferred from below and above canopy eddy-covariance systems revealed higher estimates from eddy-cAgricultural and Forest Meteorology. 151(6):709-729. -
Article: Modelling Age- and Density-Related Gas Exchange of Picea abies Canopies in the Fichtelgebirge, Germany
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ABSTRACT: Differences in canopy exchange of water and carbon dioxide that occur due to changes in tree structure and density in montane Norway spruce stands of Central Germany were analyzed with a three dimensional microclimate and gas exchange model STANDFLUX. The model was used to calculate forest radiation absorption, the net photosynthesis and transpiration of single trees, and gas exchange of tree canopies. Model parameterizations were derived for six stands of Picea abies (L.) Karst. differing in age from 40 to 140 years and in density from 1680 to 320 trees per hectare. Parameterization included information on leaf area distribution from tree harvests, tree positions and tree sizes. Gas exchange was modelled using a single species-specific set of physiological parameters and assuming no influence of soil water availability. For our humid montane stands, these simplifying assumptions appeared to be acceptable. Comparisons of modelled daily tree transpiration with water use estimates from xylem sapflow measurements provided a test of the model. Estimates for canopy transpiration rate derived from the model and via xylem sapflow measurements agreed within $\pm$20%, especially at moderate to high air vapor pressure deficits. The results suggest that age and density dependent changes in canopy structure (changes in clumping of needles) and their effect on light exposure of the average needle lead to shifts in canopy conductance and determine tree canopy transpiration in these managed montane forests. Modelled canopy net photosynthesis rates are presented, but have not yet been verified at the canopy level.Wildland Resources Faculty Publications. -
Article: Diurnal centroid of ecosystem energy and carbon fluxes at FLUXNET sites
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ABSTRACT: 1] Data from a network of eddy covariance stations in Europe and North America (FLUXNET) were analyzed to examine the diurnal patterns of surface energy and carbon fluxes during the summer period across a range of ecosystems and climates. Diurnal trends were quantified by assessing the time of day surface fluxes and meteorological variable reached peak values, using the ''diurnal centroid'' method; the diurnal centroid enabled us to discern whether the peak activity of the variable of interest is weighted more toward the morning or afternoon. In this paper, diurnal centroid estimates were used to diagnose which atmospheric and physiological processes controlled carbon dioxide, water vapor, and sensible heat fluxes across different ecosystems and climates. Sensitivity tests suggested that the diurnal centroids for latent (LE) and sensible (H) heat flux depend on atmospheric resistance, static stability in the free atmosphere, stomatal response to vapor pressure deficit, and advection. With respect to diurnal trends of surface energy fluxes at FLUXNET sites, maximum LE occurred later in the day relative to H at most tall forests with continental climates. The lag between LE and H was reduced or reversed at sites that were influenced by advection or by afternoon stomatal closure. The time of peak carbon uptake of temperate forests occurred earlier relative to the temporal peak of photosynthetically active radiation, as compared to boreal forests. The timing of this peak occurred earlier during periods with low soil water content, as it did during the summer in Mediterranean climates. In this case, the diurnal centroid for the CO 2 flux was influenced by the response of respiration and photosynthesis to increasing afternoon temperature and by afternoon stomatal closure.Journal of Geophysical Research. 108. -
Article: On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm
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ABSTRACT: This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (Reco). In particular, we analyse the effect of the extrapolation of night-time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long-term data sets. For this analysis, we used 16 one-year-long data sets of carbon dioxide exchange measurements from European and US-American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems.Global Change Biology. 11(9):1424-1439.
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Institutions
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2001–2007
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Universität Bayreuth
- Department of Plant Ecology
Bayreuth, Bavaria, Germany -
University of California, Berkeley
Berkeley, CA, USA
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