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This paper argues that the active turbulence and coherent motions near the top of a vegetation canopy are patterned on a plane mixing layer, because of instabilities associated with the characteristic strong inflection in the mean velocity profile. Mixing-layer turbulence, formed around the inflectional mean velocity profile which develops between two coflowing streams of different velocities, differs in several ways from turbulence in a surface layer. Through these differences, the mixing-layer analogy provides an explanation for many of the observeddistinctive features of canopy turbulence. These include: (a) ratios between components of the Reynolds stress tensor; (b) the ratio KH/KM of the eddy diffusivities for heat and momentum; (c) the relative roles of ejections and sweeps; (d) the behaviour of the turbulent energy balance, particularly the major role of turbulent transport; and (e) the behaviour of the turbulent length scales of the active coherent motions (the dominant eddies responsible for vertical transfer near the top of the canopy). It is predicted that these length scales are controlled by the shear length scale L, = U(h)/U'(h) (where h is canopy height, U(z) is mean velocity as a function of height Z, and U' = dU/dz). In particular, the streamwise spacing of the dominant canopy eddies is A, = mL,, with m = 8.1. These predictions are tested against many sets of field and wind-tunnel data. We propose a picture of canopy turbulence in which eddies associated with inflectional instabilities are modulated by larger-scale, inactive turbulence, which is quasi-horizontal on the scale of the canopy.

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The current emphasis on global climate studies has led the scientific community to set up a number of sites for measuring the long‐term biosphere‐atmosphere net CO2 exchange (net ecosystem exchange, NEE). Partitioning this flux into its elementary components, net assimilation (FA), and respiration (FR), remains necessary in order to get a better understanding of biosphere functioning and design better surface exchange models. Noting that FR and FA have different isotopic signatures, we evaluate the potential of isotopic 13CO2 measurements in the air (combined with CO2 flux and concentration measurements) to partition NEE into FR and FA on a routine basis. The study is conducted at a temperate coniferous forest where intensive isotopic measurements in air, soil, and biomass were performed in summer 1997. The multilayer soil‐vegetation‐atmosphere transfer model MuSICA is adapted to compute 13CO2 flux and concentration profiles. Using MuSICA as a “perfect” simulator and taking advantage of the very dense spatiotemporal resolution of the isotopic data set (341 flasks over a 24‐hour period) enable us to test each hypothesis and estimate the performance of the method. The partitioning works better in midafternoon when isotopic disequilibrium is strong. With only 15 flasks, i.e., two 13CO2 nighttime profiles (to estimate the isotopic signature of FR) and five daytime measurements (to perform the partitioning) we get mean daily estimates of FR and FA that agree with the model within 15–20%. However, knowledge of the mesophyll conductance seems crucial and may be a limitation to the method.

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Abstract. We applied a site evaluation approach combining Lagrangian Stochastic footprint modeling with a quality assessment approach for eddy-covariance data to 25 forested sites of the CarboEurope-IP network. The analysis addresses the spatial representativeness of the flux measurements, instrumental effects on data quality, spatial patterns in the data quality, and the performance of the coordinate rotation method. Our findings demonstrate that application of a footprint filter could strengthen the CarboEurope-IP flux database, since only one third of the sites is situated in truly homogeneous terrain. Almost half of the sites experience a significant reduction in eddy-covariance data quality under certain conditions, though these effects are mostly constricted to a small portion of the dataset. Reductions in data quality of the sensible heat flux are mostly induced by characteristics of the surrounding terrain, while the latent heat flux is subject to instrumentation-related problems. The Planar-Fit coordinate rotation proved to be a reliable tool for the majority of the sites using only a single set of rotation angles. Overall, we found a high average data quality for the CarboEurope-IP network, with good representativeness of the measurement data for the specified target land cover types.

The current emphasis on global climate studies has led the scientific community to set up a number of sites for measuring long‐term biospheric fluxes, and to develop a wide range of biosphere–atmosphere exchange models. This paper presents a new model of this type, which has been developed for a pine forest canopy. In most coniferous species the canopy layer is well separated from the understorey and several cohorts of needles coexist. It was therefore found necessary to distinguish several vegetation layers and, in each layer, several leaf classes defined not only by their light regime and wetness status but also by their age. This model, named MuSICA, is a multilayer, multileaf process‐based model. Each submodel is first independently parameterized using data collected at a EUROFLUX site near Bordeaux (Southwestern France). Particular care is brought to identify the seasonal variations in the various physiological parameters. The full model is then evaluated using a two‐year long data set, split up into 12 day‐type classes defined by the season, the weather type and the soil water status. Beyond the good overall agreement obtained between measured and modelled values at various time scales, several points of further improvement are identified. They concern the seasonal variations in the stomatal response of needles and the soil/litter respiration, as well as their interaction with soil or litter moisture. A sensitivity analysis to some of the model features (in‐canopy turbulent transfer scheme, leaf age classes, water retention, distinction between shaded and sunlit leaves, number of layers) is finally performed in order to evaluate whether significant simplifications can be brought to such a model with little loss in its predictive quality. The distinction between several leaf classes is crucial if one is to compute biospheric fluxes accurately. It is also evidenced that accounting for in‐canopy turbulent transfer leads to better estimates of the sensible heat flux.

[1] Stable CO 2 isotope measurements are increasingly used to partition the net CO 2 exchange between terrestrial ecosystems and the atmosphere in terms of nonfoliar respiration (F R ) and net photosynthesis (F A ) in order to better understand the variations of this exchange. However, the accuracy of the partitioning strongly depends on the isotopic disequilibrium between these two gross fluxes, and a rigorous estimation of the errors on F A and F R is needed. In this study, we account for and propagate uncertainties on all terms in the mass balance and isotopic mass balance equations for CO 2 in order to get accurate estimates of the errors on F A and F R . We apply our method to a maritime pine forest in the southwest of France. Nighttime Keeling plots are used to estimate the 13 C and 18 O isotopic signature of F R (d R ), and for both isotopes the a priori uncertainty associated with this term is estimated to be around 2% at our site. Using d 13 C-CO 2 and [CO 2 ] measurements, we then show that the uncertainty on instantaneous values of F A and F R can be as large as 4 mmol m À2 s À1 . Even if we could get more accurate estimates of the net CO 2 flux, the isoflux, and the isotopic signatures of F A and F R , this uncertainty would not be significantly reduced because the isotopic disequilibrium between F A and F R is too small, around 2-3%. With d 18 O-CO 2 and [CO 2 ] measurements the uncertainty associated with the gross fluxes lies also around 4 mmol m À2 s À1 but could be dramatically reduced if we were able to get more accurate estimates of the CO 18 O isoflux and the associated discrimination during photosynthesis. This is because the isotopic disequilibrium between F A and F R is large, of the order of 12-17%. The isotopic disequilibrium between F A and F R and the uncertainty on d R vary among ecosystems and over the year. Our approach should help to choose the best strategy to study the carbon budget of a given ecosystem using stable isotopes.