摘要:

AbstractContinuous and direct measurements of ecosystem carbon dioxide and water vapour fluxes can improve our ability to close regional and global carbon and hydrological budgets. On this behalf, an international and multidisciplinary group of scientists (micrometeorologists, ecophysiologists and biogeochemists) assembled at La Thuile, Italy to convene a workshop on ‘Strategies for Monitoring and Modelling CO2and Water Vapour Fluxes over Terrestrial Ecosystems’. Over the course of the week talks and discussions focused on: (i) the results from recent field studies on the annual cycle of carbon dioxide and water vapour fluxes over terrestrial ecosystems; (ii) the problems and pitfalls associated with making long‐term flux measurements; (iii) alternative methods for assessing ecosystem carbon dioxide and water vapour fluxes; (iv) how direct and continuous carbon dioxide and water vapour flux measurements could be used by the ecological and biogeochemical modelling communities; and (v) if, how and where to proceed with establishing a network of long‐term flux measurement sites. This report discusses the purpose of the meeting and summarizes the conclusions drawn from the discussions by the attending scientists.There was a consensus that recent advances in instrumentation and software make possible long‐term measurements of carbon dioxide and water vapour fluxes over terrestrial ecosystems. At this writing, eight research teams have conducted long‐term carbon dioxide and water vapour flux experiments and more long‐term studies are anticipated. The participants advocated an experimental design that would make long‐term flux measurement valuable to a wider community of modelers, biogeochemists and ecologists. A network of carbon dioxide and water vapour flux measurement stations should include ancillary measurements of meteorological, ecological and biological variables. To assess spatial representativeness of the long term and tower‐based flux measurements, periodic aircraft‐based flux experiments and satellite‐based assessments of land cover were recommended. Occasional cuvette‐based measurements of leaf‐level carbon dioxide and water vapour fluxes were endorsed to provide information on the biological control of surface fluxes. They can also provide data to parameterize ecophysiological models. Flask sampling of stable carbon isotopes was advocated to extend the flux measure

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00069.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe turbulent exchanges of CO2and water vapour between an aggrading deciduous forest in the north‐eastern United States (Harvard Forest) and the atmosphere were measured from 1990 to 1994 using the eddy covariance technique. We present a detailed description of the methods used and a rigorous evaluation of the precision and accuracy of these measurements. We partition the sources of error into three categories: (1)uniform systematic errorsare constant and independent of measurement conditions (2)selective systematic errorsresult when the accuracy of the exchange measurement varies as a function of the physical environment, and (3)sampling uncertaintyresults when summing an incomplete data set to calculate long‐term exchange.Analysis of the surface energy budget indicates a uniform systematic error in the turbulent exchange measurements of ‐20 to 0%. A comparison of nocturnal eddy flux with chamber measurements indicates a selective systematic underestimation during calm (friction velocity<0.17 m s−1) nocturnal periods. We describe an approach to correct for this error. The integrated carbon sequestration in 1994 was 2.1 t C ha−1y−1with a 90% confidence interval due to sampling uncertainty of ±0.3 t C ha−1y−1determined by Monte Carlo simulation. Sampling uncertainty may be reduced by estimating the flux as a function of the physical environment during periods when direct observations are unavailable, and by minimizing the length of intervals without flux data. These analyses lead us to place an overall uncertainty on the annual carbon sequestration in 1994 of ‐0.3 t

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00070.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractLong‐term and direct measurements of CO2and water vapour exchange are needed over forested ecosystems to determine their net annual fluxes of carbon dioxide and water. Such measurements are also needed to parameterize and test biogeochemical, ecological and hydrological assessment models. Responding to this need, eddy covariance measurements of CO2and water vapour were made ever a deciduous forest growing near Oak Ridge, TN, between April 1993 and April 1994. Periodic measurements were made of leaf area index, stomatal resistance, soil moisture and pre‐dawn leaf water potential to characterize the gas exchange capacity of the canopy.Four factors had a disproportionate influence on the seasonal variation of CO2flux densities. These factors were photon flux densities (during the growing season), temperature (during the dormant season), leaf area index and the occurrence of drought The drought period occurred during the peak of the growing season and caused a significant decline in daily and hourly CO2flux densities, relative to observations over the stand when soil moisture was plentiful.The annual net uptake of carbon was calculated by integrating flux measurements and filling missing and spurious data with the relations obtained between measured CO2fluxes and environmental forcing variables. The net flux of carbon for the period between April 1993 and April 1994 was ‐525 g C m−2y−1. This value represents a net flux of carbon from the atmosphere and into the forest. The net annual carbon exchange of this southern temperate broadleaved forest exceeded values measured over a northern temperate forest (which experiences a shorter growing season and has less leaf area) by 200 g C m−2y−1(cf. Wofsy et al 1993).The seasonal variation of canopy evaporation (latent heat flux) was controlled mostly by changes in leaf area and net radiation. A strong depression in evaporation rates was not observed during the drought Over a broadleaved forest large vapour pressure deficits promote evaporation and trees in a mixed stand are able to tap a variety of deep and shallow

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00071.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe seasonal carbon dioxide exchange of a beech forest of Central Italy was studied by means of the eddy covariance technique. Additional measurements of biomass respiration with cuvettes and relationship of carbon dioxide exchanges with temperature and light were used to interpolate missing data during the dormant and part of the growing season.The net ecosystem production of the forest equals 472 g C m−2y−1while the gross ecosystem production 1016 g C m−2y−1and respiration 544 g C m−2y−1. These estimates are compared with the net primary production determined by direct biomass sampling which amounts to 802

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00072.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract1 Eddy covariance measurements of CO2flux, based on four and six week campaigns in Rondôdnia, Brazil, have been used in conjunction with a model to scale up data to a whole year, and thus estimate the carbon balance of the tropical forest ecosystem, and the changes in carbon balance expected from small interannual variations in climatological conditions.2 One possible source of error in this estimation arises from the difficulty in measuring fluxes under stably stratified meteorological conditions, such as occur frequently at night. Flux may be ‘lost’ because of low velocity advection, caused by nocturnal radiative cooling at sites on raised ground. Such effects may be detected by plotting the net ecosystem flux of CO2, Fecois a function of wind speed. If flux is ‘lost’ then Fecois expected to decline with wind speed. In the present data set, this did not occur, and Fecowas similar to the nocturnal flux estimated independently from chamber measurements.3 The model suggests that in 1992/3, the Gross Primary Productivity (GPP) was 203.3 mol C m−2y−1and ecosystem respiration was 194.8 mol C m−2y−1, giving an ecosystem carbon balance of 8.5 mol C m−2y−1, equivalent to a sink of 1.0 ton C ha−1y−1. However, the sign and magnitude of this figure is very sensitive to temperature, because of the strong influence of temperature on respiration.4 The model also suggests that the effect of temperature on the net carbon balance is strongly dependen

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00073.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWater vapour and CO2fluxes were measured using the eddy correlation method above and below the overstorey of a 21‐m tall aspen stand in the boreal forest of central Saskatchewan as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Measurements were made at the 39.5‐m and 4‐m heights using 3‐dimensional sonic anemometers (Kaijo‐Denki and Solent, respectively) and closed‐path gas analysers (LI‐COR 6262) with 6‐m and 4.7‐m long heated sampling tubing, respectively. Continuous measurements were made from early October to mid‐November 1993 and from early February to late‐September 1994. Soil CO2flux (respiration) was measured using a LI‐COR 6000‐09 soil chamber and soil evaporation was measured using Iysimetry.The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (E) rate was 5–6 mm d−1. Following leaf‐out the hazelnut and soil accounted for 22% of the forestE. The estimated totalEwas 403 mm for 1994. About 88% of the precipitation in 1994 was lost as evapotranspiration.During the growing season, the magnitude of half‐hourly eddy fluxes of CO2from the atmosphere into the forest reached 1.2 mg CO2m−2s−1(33 μmol C m−2s−1) during the daytime. Downward eddy fluxes at the 4‐m height were observed when the hazelnut was growing rapidly in June and July. Under well‐ventilated night‐time conditions, the eddy fluxes of CO2above the aspen and hazelnut, corrected for canopy storage, increased exponentially with soil temperature at the 2‐cm depth. Estimates of daytime respiration rates using these relationships agreed well with soil chamber measurements. During the 1994 growing season, the cumulative net ecosystem exchange (NEE) was ‐3.5 t C ha−1y−1(a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (R) and gross ecosystem photosynthesis (GEP = R ‐ NEE) were estimated to be ‐1.3, 8.9 and 10.2 t C ha−1y−

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00074.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractFor surface fluxes of carbon dioxide, the net daily flux is the sum of daytime and nighttime fluxes of approximately the same magnitude and opposite direction. The net flux is therefore significantly smaller than the individual flux measurements and error assessment is critical in determining whether a surface is a net source or sink of carbon dioxide. For carbon dioxide flux measurements, it is an occasional misconception that the net flux is measured as the difference between the net upward and downward fluxes (i.e. a small difference between large terms). This is not the case. The net flux is the sum of individual (half‐hourly or hourly) flux measurements, each with an associated error term. The question of errors and uncertainties in long‐term flux measurements of carbon and water is addressed by first considering the potential for errors in flux measuring systems in general and thus errors which are relevant to a wide range of timescales of measurement. We also focus exclusively on flux measurements made by the micrometeorological method of eddy covariance. Errors can loosely be divided into random errors and systematic errors, although in reality any particular error may be a combination of both types. Systematic errors can be fully systematic errors (errors that apply on all of the daily cycle) or selectively systematic errors (errors that apply to only part of the daily cycle), which have very different effects. Random errors may also be full or selective, but these do not differ substantially in their properties. We describe an error analysis in which these three different types of error are applied to a long‐term dataset to discover how errors may propagate through long‐term data and which can be used to estimate the range of uncertainty in the reported sink strength of the particular ecosystem

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00075.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractTheoretical and practical aspects of measuring eddy fluxes of trace gases using open‐and closed‐path analysers are presented. Trace gas fluxes measured with an open‐path analyser require the concurrent measurement of sensible and latent heat fluxes to correct for density fluctuations in trace gas concentration caused by these fluxes. A closed‐path analyser eliminates the corrections due to sensible heat flux, but not for water vapour, provided temperature fluctuations are completely removed without significantly reducing fluctuations in the trace gas mixing ratio. Theory for the design of heat exchangers and for the attenuation of concentration fluctuations during air flow through tubes is used to provide design criteria for closed‐path systems. Spectral transfer functions are used to estimate flux losses caused by flow through the sampling tube and gas analyser. Other factors considered include cross‐sensitivity of infrared CO2analysers to water vapour, and deterioration of system performance caused by contaminants on the walls of sampling tubes. Of two open‐path, infrared CO2analysers tested, one showed a strong interaction between CO2and water vapour, while the other showed little sensitivity to the presence of water vapour, other than caused by dilution. A commercial closed‐path CO2analyser also showed little cross‐sensitivity to water vapour. Compared to results for a clean sampling tube, the spectral bandwidth for water vapour fluctuations decreased significantly after several weeks of sampling. No such deterioration in bandwidth was observed for CO2. These findings are attributed to differential adsorption/desorption of water vapour by dust or salt on the tubing walls. Rain and dust must be removed from open‐path analysers to obtain satisfactory measurements. Careful system design and maintenance is required for both open‐ and closed‐path systems to ensure satisfactory long‐term measurement of trace gas fluxes. With these precautions, both approaches will provide satis

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00076.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe paper considers the theory and application of budget techniques for regional scalar flux estimation using the daytime convective boundary layer (CBL) and the nocturnal boundary layer (NBL). CBL techniques treat the well mixed layer of air between heights of, say, 100 m and 1000 m as an integrator of surface fluxes along the path of a column of air moving over the landscape. They calculate the average surface flux from the scalar concentration in and above the mixed layer, and the CBL height. The flux estimates are averaged over regions of 10–104km2extending 10 to 100 km upwind.An integral form of the CBL budget is used to estimate daily regional rates of CO2uptake and evaporation from three data sets. There was plausible agreement between the estimates and locally measured fluxes. CBL budgets have great potential for estimating regional scalar fluxes, but there is an urgent need for validation through direct measurements of fluxes and budget parameters.NBL budgets are useful when low‐level, radiative inversions inhibit vertical mixing. Surface scalar fluxes can then be estimated from the rate of concentration change below the inversion. An example application for estimating the nocturnal CO2flux is given. While simple in concept, NBL budgets are more difficult to apply in practice because of the unpredictability of the depth of the layer and sometimes, its absence altogether. On the other hand, the depth of the atmospheric mixing chamber is better defined, few assumptions are required and the concentration changes usually will be larger and hence more easily detectable than in CBL budgett

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00077.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractWe show that sapflow is a useful tool for studies of water fluxes in forest ecosystems, because (i) it gives access to the spatial variability within a forest stand, (ii) it can be used even on steep slopes, and (iii) when combined with eddy correlation measurements over forests, it allows separation of individual tree transpiration from the total water loss of the stand. Moreover, sapflow techniques are quite easy to implement.Four sapflow techniques currently coexist, all based on heat diffusion in the xylem. We found a good agreement between three of these techniques. Most results presented here were obtained using the radial flow meter (Granier 1985).Tree sapflow is computed as sap flux density times sapwood area. To scale up from trees to a stand, measurements have to be made on a representative sample of trees. Thus, a number of trees in each circumference class is selected according to the fraction of sapwood they represent in the total sapwood area of the stand. The variability of sap flux density among trees is usually low (CV. 10–15%) in close stands of temperate coniferous or deciduous forests, but is much higher (35–50%) in a tropical rain forest. It also increases after thinning or during a dry spell.A set of 5–10 sapflow sensors usually provides an accurate estimate of stand transpiration. Transpiration measured on two dense spruce stands in the Vosges mountains (France) and one Scot's pine plantation in the Rhine valley (Germany) showed that maximum rate was related to stand LAI and to local climate. Preliminary results comparing the sapflow of a stand ofPinus banksianato the transpiration of large branches, as part of the BOREAS programme in Saskachewan, Canada showed a similar trend.For modelling purposes, tree canopy conductance (gc) was calculated from Penman‐Monteith equation. In most experiments, calculated canopy conductance was dependent on global radiation (positive effect) and on vapour pressure deficit (negative effect) in the absence of other limiting factors. A comparison of the vapour pressure deficit response curves ofgcfor several tree species and sites showed only small differences among spruce, oak and pine forests when including understorey. Tropical rainforests exhibited a similar be

ISSN:1354-1013    

DOI:10.1111/j.1365-2486.1996.tb00078.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY