[Show abstract][Hide abstract] ABSTRACT: Understanding the climatic and biotic controls of interannual variability (IAV) in net ecosystem exchange (NEE) is important for projecting future uptake of CO2 in terrestrial ecosystems. In this study, a statistical modeling approach was used to partition climatic and biotic effects on the IAV in NEE, gross primary productivity (GPP) and ecosystem respiration (RE) at a subtropical evergreen plantation in China (QYZ), a deciduous forest (MOZ), and a grassland (DK1) in the USA. The climatic effects in the study are defined as the interannual anomalies in carbon (C) fluxes directly caused by climatic variations, whereas the biotic effects are those caused by the IAV in photosynthetic and respiratory traits. The results showed that the contribution of biotic effects to the IAV in NEE increased significantly as the temporal scale got longer from daily to annual scales. At the annual scale, the contribution of biotic effects to the IAV in NEE was 47, 69, and 77% at QYZ, MOZ, and DK1, respectively. However, the IAV in NEE was mainly controlled by GPP at QYZ, and by RE at DK1, whereas the contributions of GPP and RE to the IAV in NEE were similar at MOZ, indicating different mechanisms regulating the IAV in NEE among ecosystems. Interestingly, there was a strong negative correlation between the climatic and biotic effects at the annual scale from 2003 to 2009 at QYZ (r 2 = 0.80, P NEE and contributed to our understanding of their underlying mechanisms.
[Show abstract][Hide abstract] ABSTRACT: In C3 plants, CO<sub>2</sub> concentrations drop considerably along mesophyll diffusion pathways from substomatal cavities to chloroplasts where CO<sub>2</sub> assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdown and therefore overestimate CO<sub>2</sub> available for carboxylation and underestimate photosynthetic responsiveness to atmospheric CO<sub>2</sub>. An explicit consideration of mesophyll diffusion increases the modeled cumulative CO<sub>2</sub> fertilization effect (CFE) for global gross primary production (GPP) from 915 to 1,057 PgC for the period of 1901–2010. This increase represents a 16% correction, which is large enough to explain the persistent overestimation of growth rates of historical atmospheric CO<sub>2</sub> by Earth system models. Without this correction, the CFE for global GPP is underestimated by 0.05 PgC/y/ppm. This finding implies that the contemporary terrestrial biosphere is more CO<sub>2</sub> limited than previously thought.
Proceedings of the National Academy of Sciences of the United States of America. 10/2014;
[Show abstract][Hide abstract] ABSTRACT: Peatlands harbor more than one-third of terrestrial carbon leading to the argument that the bryophytes, as major components of peatland ecosystems, store more organic carbon in soils than any other collective plant taxa. Plants of the genus Sphagnum is an important component of peatland ecosystems and are potentially vulnerable to changing climatic conditions. However, the response of Sphagnum to rising temperatures, elevated CO2 and shifts in local hydrology have yet to be fully characterized. In this review, we examine Sphagnum biology and ecology and explore the role of this keystone species and its associated microbiome in carbon and nitrogen cycling using literature review and model simulations. Several issues are highlighted including the consequences of a variable environment on plant-microbiome interactions, uncertainty associated with CO2 diffusion resistances and the relationship between fixed N and that partitioned to the photosynthetic apparatus. We note that the Sphagnum fallax genome is currently being sequenced and outline potential applications of population-level genomics and corresponding plant photosynthesis and microbial metabolic modeling techniques. We highlight Sphagnum as a model organism to explore ecosystem response to a changing climate, and to define the role that Sphagnum can play at the intersection of physiology, genetics and functional genomics.
Plant Cell and Environment 09/2014; · 5.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The exchange of carbon dioxide is a key measure of ecosystem metabolism and a critical intersection between the terrestrial biosphere and the Earth's climate. Despite the general agreement that the terrestrial ecosystems in North America provide a sizeable carbon sink, the size and distribution of the sink remain uncertain. We use a data-driven approach to upscale eddy covariance flux observations from towers to the continental scale by integrating flux observations, meteorology, stand age, aboveground biomass, and a proxy for canopy nitrogen concentrations from AmeriFlux and Fluxnet-Canada Research Network as well as a variety of satellite data streams from the MODIS sensors. We then use the resulting gridded flux estimates from March 2000 to December 2012 to assess the magnitude, distribution, and interannual variability of carbon fluxes for the U.S. and Canada. The mean annual gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP) of the U.S. over the period 2001–2012 were 6.84, 5.31, and 1.10 Pg C yr−1, respectively; the mean annual GPP, ER, and NEP of Canada over the same 12-year period were 3.91, 3.26, and 0.60 Pg C yr−1, respectively. The mean nationwide annual NEP of natural ecosystems over the period 2001–2012 was 0.53 Pg C yr−1 for the U.S. and 0.49 Pg C yr−1 for the conterminous U.S. Our estimate of the carbon sink for the conterminous U.S. was almost identical with the estimate of the First State of the Carbon Cycle Report (SOCCR). The carbon fluxes exhibited relatively large interannual variability over the study period. The main sources of the interannual variability in carbon fluxes included drought and disturbance. The annual GPP and NEP were strongly related to annual evapotranspiration (ET) for both the U.S. and Canada, showing that the carbon and water cycles were closely coupled. Our gridded flux estimates provided an independent, alternative perspective on ecosystem carbon exchange over North America.
Agricultural and Forest Meteorology 07/2014; 197:142–157. · 3.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Great uncertainty exists in the global exchange of carbon between the atmosphere and the terrestrial biosphere. An important source of this uncertainty lies in the dependency of photosynthesis on the maximum rate of carboxylation (Vcmax) and the maximum rate of electron transport (Jmax). Understanding and making accurate prediction of C fluxes thus requires accurate characterization of these rates and their relationship with plant nutrient status over large geographic scales. Plant nutrient status is indicated by the traits: leaf nitrogen (N), leaf phosphorus (P), and specific leaf area (SLA). Correlations between Vcmax and Jmax and leaf nitrogen (N) are typically derived from local to global scales, while correlations with leaf phosphorus (P) and specific leaf area (SLA) have typically been derived at a local scale. Thus, there is no global-scale relationship between Vcmax and Jmax and P or SLA limiting the ability of global-scale carbon flux models do not account for P or SLA. We gathered published data from 24 studies to reveal global relationships of Vcmax and Jmax with leaf N, P, and SLA. Vcmax was strongly related to leaf N, and increasing leaf P substantially increased the sensitivity of Vcmax to leaf N. Jmax was strongly related to Vcmax, and neither leaf N, P, or SLA had a substantial impact on the relationship. Although more data are needed to expand the applicability of the relationship, we show leaf P is a globally important determinant of photosynthetic rates. In a model of photosynthesis, we showed that at high leaf N (3 gm−2), increasing leaf P from 0.05 to 0.22 gm−2 nearly doubled assimilation rates. Finally, we show that plants may employ a conservative strategy of Jmax to Vcmax coordination that restricts photoinhibition when carboxylation is limiting at the expense of maximizing photosynthetic rates when light is limiting.
Ecology and Evolution 07/2014; · 1.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The widespread invasion of exotic cool-season grasses, such as Kentucky bluegrass and smooth brome, in the mixed-grass rangelands is diminishing the hope of bringing back the natural native plant-dominated communities. A best management strategy for these rangelands, according to several plant community studies, is perhaps to live with these cool-season exotics to some extent while reducing their cover in favor of a diverse plant community. However, there are uncertainties for the future of these invaded rangelands as the ecophysiological mechanisms generally are lacking to explain the relative competitiveness between the native and exotic species. Our research provides a broad comparison of leaf-level photosynthesis among 26 plant species co-occurring in the mixed-grass prairie near Streeter, ND USA. The highlight is on the correlations between photosynthetic potential and plant success, with special reference to Kentucky bluegrass and smooth brome. Photosynthetic potential is defined as a composite variable summarizing the intrinsic limiting factors for photosynthesis as obtained from the net assimilation (A) vs. internal CO2 (Ci) response curves from plants grown under well-watered greenhouse conditions. Plant success was defined as the average frequency measured over 25 years (1988-2012) in the over-flow ecological sites across five levels (no-grazing, moderate, light, heavy and extremely heavy) of grazing intensity. The correlation between photosynthetic potential and plant frequency was negative (n=26 species, p<0.05), suggesting that the two cool-season grasses, Kentucky bluegrass and smooth brome, do not rely on their superior leaf-level photosynthesis for competitive success. Instead, some other traits, such as early and prolonged growth, may be more important for them to gain dominance in the mixed-grass prairie. This is compatible with the management suggestions provided by plant community-based studies for controlling exotic grass dominance and encouraging plant diversity through proper use of early-season grazing and fire.
Cool-Season Invasive Grasses of the Northern Great Plains Workshop, Fargo, North Dakota; 03/2014
[Show abstract][Hide abstract] ABSTRACT: In recent years, temperate bamboo species have been introduced in Europe for multiple uses such as renewable bio-based materials (wood, composites, fibres, biochemicals…) and numerous ecological functions (soil and water conservation, erosion control, phytoremediation…). Despite their interesting potential, little is known on the ecophysiology of these plants in their new habitat. Therefore, we studied gas exchange parameters on a full soil bamboo plantation of Phyllostachys humilis on a test field in Ireland (Europe). We evaluated the seasonal, diurnal and vertical variation of the parameters of two commonly used photosynthetic models, i.e. the light response curve (LRC) model and the model of Farquhar, von Caemmerer and Berry (FvCB). Furthermore, we tested if there were environmental effects on the photosynthetic parameters of these models and if a correlation between photosynthetic parameters and fluorescence parameters was present, fluorescence parameters can be easily and fast determined. Our results show that the gas exchange parameters do not vary diurnally or vertically. Only seasonal variations were found and should, therefore, be taken into account when using the LRC or FvCB model when modelling canopy growth. Therefore, a big-leaf model or a sunlit-shade model can be used for modelling bamboo growth in Western Europe. There is no straightforward relation between environmental variables and the photosynthetic parameters. Although fluorescence parameters showed a correlation with the photosynthetic parameters, application of such correlation may be limited.
Photosynthesis Research 03/2014; · 3.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ecosystem fluxes of isoprene emissions were measured during the majority of the 2011 growing season at the University of Missouri's Baskett Wildlife Research and Education Area in central Missouri, USA (38.7° N, 92.2° W). This broadleaf deciduous forest is typical of forests common in the Ozarks region of the central United States. The goal of the isoprene flux measurements was to test our understanding of the controls on isoprene emission from the hourly to the seasonal timescale using a state-of-the-art emission model, MEGAN (Model of Emissions of Gases and Aerosols from Nature). Isoprene emission rates from the forest were very high with a maximum of 53.3 mg m−2 h−1 (217 nmol m−2 s−1), which to our knowledge exceeds all other reports of canopy-scale isoprene emission. The fluxes showed a clear dependence on the previous temperature and light regimes, which was successfully captured by the existing algorithms in MEGAN. During a period of drought, MEGAN was unable to reproduce the time-dependent response of isoprene emission to water stress. Overall, the performance of MEGAN was robust and could explain 90% of the observed variance in the measured fluxes, but the response of isoprene emission to drought stress is a major source of uncertainty.
[Show abstract][Hide abstract] ABSTRACT: The realistic representation of key biogeophysical and biogeochemical functions is the fundamental of process-based ecosystem models. A functional test platform is designed to create direct linkages between site measurements and the process-based ecosystem model within the Community Earth System Models (CESM). The platform consists of three major parts: 1) interactive user interfaces, 2) functional test models and 3) observational datasets. It provides much needed integration interfaces for both field experimentalists and ecosystem modelers to improve the model's representation of ecosystem processes within the CESM framework without large software overhead.
Environmental Modelling and Software 01/2014; 55:25–31. · 3.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Studies with the variable J method have reported that mesophyll conductance (gm ) rapidly decreases with increasing intercellular CO2 partial pressures (Ci ) or decreasing irradiance. Similar responses have been suggested with the online isotope discrimination method, although with less consistency. Here we show that even when the true gm is constant, the variable J method can produce an artifactual dependence of gm on Ci or irradiance similar to those reported in previous studies for any of the following factors: day respiration and chloroplastic CO2 photocompensation point are estimated with Laisk method; Ci or electron transport rate is positively biased; net photosynthetic rate is negatively biased; insufficient NADPH is assumed while insufficient ATP limits RuBP regeneration. The isotopic method produces similar artifacts if fractionation of carboxylation or Ci are positively biased or Δ(13) negatively biased. A nonzero chloroplastic resistance to CO2 movement results in a qualitatively different dependence of gm on Ci or irradiance and this dependence is only sensitive at low Ci . We thus cannot rule out the possibility that previously reported dependence of gm on Ci or irradiance is a methodological artifact. Recommendations are made to take advantage of sensitivities of the variable J and isotopic methods for estimating gm .
Plant Cell and Environment 11/2013; · 5.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Worldwide measurements of nearly 130 C3 species covering all major plant functional types are analyzed in conjunction with model simulations to determine the effects of mesophyll conductance (gm ) on photosynthetic parameters and their relationships estimated from A/Ci curves. We find that an assumption of infinite gm results in up to 75% underestimation for maximum carboxylation rate Vcmax , 60% for maximum electron transport rate Jmax , and 40% for triose phosphate utilization rate Tu . Vcmax is most sensitive, Jmax is less sensitive, and Tu has the least sensitivity to the variation of gm . Due to this asymmetrical effect of gm , the ratios of Jmax to Vcmax , Tu to Vcmax , and Tu to Jmax are all overestimated. An infinite gm assumption also limits the freedom of variation of estimated parameters and artificially constrains parameter relationships to stronger shapes. These findings suggest the importance of quantifying gm for understanding in-situ photosynthetic machinery functioning. We show that a nonzero resistance to CO2 movement in chloroplasts has small effects on estimated parameters. A nonlinear function with gm as input is developed to convert the parameters estimated under an assumption of infinite gm to proper values. This function will facilitate gm representation in global carbon cycle models.
Plant Cell and Environment 10/2013; · 5.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Micrometeorological measurements of nighttime ecosystem respiration can be systematically biased when stable atmospheric conditions lead to drainage flows associated with decoupling of air flow above and within plant canopies. The associated horizontal and vertical advective fluxes cannot be measured using instrumentation on the single towers typically used at micrometeorological sites. A common approach to minimize bias is to use a threshold in friction velocity, u*, to exclude periods when advection is assumed to be important, but this is problematic in situations when in-canopy flows are decoupled from the flow above. Using data from 25 flux stations in a wide variety of forest ecosystems globally, we examine the generality of a novel approach to estimating nocturnal respiration developed by van Gorsel et al. (van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u*-threshold filtering technique. Tellus 59B, 397–403, Tellus, 59B, 307-403). The approach is based on the assumption that advection is small relative to the vertical turbulent flux (FC) and change in storage (FS) of CO2 in the few hours after sundown. The sum of FC and FS reach a maximum during this period which is used to derive a temperature response function for ecosystem respiration. Measured hourly soil temperatures are then used with this function to estimate respiration RRmax. The new approach yielded excellent agreement with (1) independent measurements using respiration chambers, (2) with estimates using ecosystem light-response curves of Fc + Fs extrapolated to zero light, RLRC, and (3) with a detailed process-based forest ecosystem model, Rcast. At most sites respiration rates estimated using the u*-filter, Rust, were smaller than RRmax and RLRC. Agreement of our approach with independent measurements indicates that RRmax provides an excellent estimate of nighttime ecosystem respiration.
Agricultural and Forest Meteorology 05/2013; · 3.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: There is a continued need for models to improve consistency and agreement with observations [Friedlingstein et al., 2006], both overall and under more frequent extreme climatic events related to global environmental change such as drought [Trenberth et al., 2007]. Past validation studies of terrestrial biosphere models have focused only on few models and sites, typically in close proximity and primarily in forested biomes [e.g., Amthor et al., 2001; Delpierre et al., 2009; Grant et al., 2005; Hanson et al., 2004; Granier et al., 2007; Ichii et al., 2009; Ito, 2008; Siqueira et al., 2006; Zhou et al., 2008]. Furthermore, assessing model‐data agreement relative to drought requires, in addition to high‐quality observedCO2 exchange data, a reliable drought metric as well as a natural experiment across sites and drought conditions.
Journal of Geophysical Research 02/2013; 115. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The eddy-covariance method often underestimates fluxes under stable, low-wind conditions at night when turbulence is not well developed. The most common approach to resolve the problem of nighttime flux underestimation is to identify and remove the deficit periods using friction-velocity (u*) threshold filters (u*(Th)). This study modifies an accepted method for u*(Th) evaluation by incorporating change-point-detection techniques. The original and modified methods are evaluated at 38 sites as part of the North American Carbon Program (NACP) site-level synthesis. At most sites, the modified method produced u*(Th) estimates that were higher and less variable than the original method. It also provided an objective method to identify sites that lacked a u*(Th) response. The modified u*(Th) estimates were robust and comparable among years. Inter-annual u*(Th) differences were small, so that a single u*(Th) value was warranted at most sites. No variation in the u*(Th) was observed by time of day (dusk versus mid or late night), however, a few sites showed significant u*(Th) variation with time of year. Among-site variation in the u*(Th) was strongly related to canopy height and the mean annual nighttime u*. The modified u*(Th) estimates excluded a high fraction of nighttime data - 61% on average. However, the negative impact of the high exclusion rate on annual net ecosystem production (NEP) was small compared to the larger impact of underestimating the u*(Th). Compared to the original method, the higher u*(Th) estimates from the modified method caused a mean 8% reduction in annual NEP across all site-years, and a mean 7% increase in total ecosystem respiration (R-e). The modified method also reduced the u*(Th)-related uncertainties in annual NEP and R-e by more than 50%. These results support the use of u*(Th) filters as a pragmatic solution to a complex problem. (C) 2012 A.G. Barr. Published by Elsevier B.V. All rights reserved.
Agricultural and Forest Meteorology 01/2013; 171:31-45. · 3.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Community Land Model (CLM) uses a fixed-point iteration approach to
solve the coupled photosynthesis and stomatal conductance model (A-
gs). Here we demonstrate that this approach does not converge
in its iterative calculation of gross primary production (GPP) and
transpiration in large portions of land surface, because the coupled A-
gsmodel does not always comply with a condition (the
fixed-point theorem) required by the fixed-point approach for
convergence. This iteration fails more frequently in some regions of the
world than in others, leading to regionally varying uncertainty and
global biases in the estimated carbon and water fluxes. Moreover, CLM
applies an artificial constraint to the water vapor pressure of canopy
air in its calculations ofA- gs, with an intention to prevent
the `numerical instability' arising from the fixed-point approach. Our
results show that this constraint reduces but does not prevent the
occurrence of nonconvergence. Since this constraint is artificial, it
can bias GPP and transpiration simulations. We then propose a
Newton-Raphson iteration scheme to replace the fixed-point approach and
show that this new approach can ensure convergence, does not require an
artificial constraint on the atmospheric water vapor pressure, and is
computationally efficient. On the other hand, the default fixed-point
treatment in CLM leads to a ˜2.7 PgCyr-1overestimation
of GPP globally but with much higher regional biases (˜27%). We
suggest that the current fixed-point treatment in CLM be replaced with
the Newton-Raphson approach and that the artificial constraint on the
atmospheric water vapor pressure be removed.
Journal of Geophysical Research 11/2012; 117(D22):22103-. · 3.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: About 10% of China's forests were impacted by a destructive ice storm and subsequently subjected to poorly planned salvage logging in 2008. We used the remote-sensing products of Enhanced Vegetation Indexes (EVI) corroborated with information gathered from ground visits to examine the spatial patterns and temporal trajectories of greenness of these nearly 20 million hectares of forests. We found (1) the EVI of about 50% of the impacted forests returned to normal status (i.e., within the 95% confidence interval of the long-term mean) within five months, and about 80% within one year after the storm, (2) the higher the pre-storm EVI (relative to the long-term mean), the slower the rebound of post-storm EVI, and (3) the rebound of greenness was slowest in forests that were moderately impacted by the ice storm only (i.e. before the occurrences of logging), resulting in a nonlinear relationship between greenness rebound time (GRT) and ice storm impact severity (IS). Ground visits suggested a hypothesis that the region-wide rebound in greenness was a consequence of resprouting of physically damaged trees and growth of understory plants including shrub, herbaceous and epiphytic species. These processes were facilitated by the rapid increase in temperature and ample moisture after the ice storm. Gap-phase dynamics could be responsible for the counterintuitive relationship between IS and GRT that was obtained. However, a more parsimonious explanation appears to be biased salvage logging, which may have selectively targeted lightly to moderately impacted forests for economic and accessibility reasons and thus adversely affected the GRT of these forests. Although a purely natural disturbance may result in forest greenness patterns different than those reported here, we suggest that remote-sensing-based dynamic analyses of greenness can play a major role in evaluating disturbance theories and in developing testable hypotheses to guide ground-based studies of the integrated effects of large extreme events and human intervention on forest ecosystems.
Environmental Research Letters 09/2012; 7(3):035702. · 3.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: • It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.
New Phytologist 03/2012; 194(3):775-83. · 6.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The regular monitoring of evapotranspiration from satellites has been limited because of discontinuous temporal coverage, resulting in snapshots at a particular point in space and time. We developed a temporal upscaling scheme using satellite-derived instantaneous estimates of evapotranspiration to produce a daily-sum evapotranspiration averaged over an 8-day interval. We tested this scheme against measured evapotranspiration data from 34 eddy covariance flux towers covering seven plant functional types from boreal to tropical climatic zones. We found that the ratio of a half-hourly-sum of potential solar radiation (extraterrestrial solar irradiance on a plane parallel to the Earth’s surface) between 10:00 hh and 14:00 hh to a daily-sum of potential solar radiation provides a robust scaling factor to convert a half-hourly measured evapotranspiration to an estimate of a daily-sum; the estimated and measured daily sum evapotranspiration showed strong linear relation (r2 = 0.92) and small bias (−2.7%). By comparison, assuming a constant evaporative fraction (the ratio of evapotranspiration to available energy) during the daytime, although commonly used for temporal upscaling, caused 13 underestimation of evapotranspiration on an annual scale. The proposed temporal upscaling scheme requires only latitude, longitude and time as input. Thus it will be useful for developing continuous evapotranspiration estimates in space and time, which will improve continuous monitoring of hydrological cycle from local to global scales.
Agricultural and Forest Meteorology 01/2012; 152:212-222. · 3.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A fundamental equation of eddy covariance (FQEC) is derived that allows the net ecosystem exchange (NEE) of a specified atmospheric constituent s to be measured with the constraint of conservation of any other atmospheric constituent (e.g. N2, argon, or dry air). It is shown that if the condition is true, the conservation of mass can be applied with the assumption of no net ecosystem source or sink of dry air and the FQEC is reduced to the following equation and its approximation for horizontally homogeneous mass fluxes:Here w is vertical velocity, c molar density, t time, h eddy flux measurement height, z vertical distance and χs = cs/cd molar mixing ratio relative to dry air. Subscripts s, d and CO2 are for the specified constituent, dry air and carbon dioxide, respectively. Primes and overbars refer to turbulent fluctuations and time averages, respectively. This equation and its approximation are derived for non-steady state conditions that build on the steady-state theory of Webb, Pearman and Leuning (WPL; Webb et al., 1980. Quart. J. R. Meteorol. Soc. 106, 85–100), theory that is widely used to calculate the eddy fluxes of CO2 and other trace gases. The original WPL constraint of no vertical flux of dry air across the EC measurement plane, which is valid only for steady-state conditions, is replaced with the requirement of no net ecosystem source or sink of dry air for non-steady state conditions. This replacement does not affect the ‘eddy flux’ term but requires the change in storage to be calculated as the ‘effective change in storage’ as follows:Without doing so, significant diurnal and seasonal biases may occur. We demonstrate that the effective change in storage can be estimated accurately with a properly designed profile of mixing ratio measurements made at multiple heights. However further simplification by using a single measurement at the EC instrumentation height is shown to produce substantial biases. It is emphasized that an adequately designed profile system for measuring the effective change in storage in proper units is as important as the eddy flux term for determining NEE.When the EC instrumentation measures densities rather than mixing ratios, it is necessary to use:Here T is temperature and cv and c are the molar densities of water vapor and moist air, respectively.For some atmospheric gas species such as N2 and O2, the condition is not satisfied and additional information is needed in order to apply the EC technique with the constraint of conservation of dry air.Highlights► The fundamental equation of eddy covariance is derived. ► Misunderstandings about the eddy covariance theory are clarified. ► The steady-state theory is shown to cause substantial biases. ► The proper application of the non-steady state theory is presented.
Agricultural and Forest Meteorology 01/2012; 152:135-148. · 3.89 Impact Factor