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Studying the microclimate of plant canopies has long motivated scientists in various research fields such as agronomy, ecology or silviculture, and almost a century has passed since the first measurements of wind speed in a forest stand were published in the scientific literature. The behaviour of wind in canopies is an essential component of their microclimate, which largely conditions the rate of exchange of heat, water vapour, and other gases and particles of interest with the atmosphere. This review examines the evolution of our understanding of turbulent flow in plant canopies, focussing on the period that covers the last fifty years (1970-2020). We first describe how our knowledge and ideas have evolved since canopy flow became a topic of interest, and show how the 1970s was a pivotal decade in this field. Until then, canopy turbulence was considered to result from the superposition of standard surface-layer turbulence and small-scale turbulence generated in the wakes of plant elements. However, it was progressively found that the flow in plant canopies is dominated by large coherent structures, giving canopy turbulence unique characteristics. We thus describe the particular nature and structure of canopy flows, based on experimental observations accumulated over several decades. We show how canopy turbulence was reconsidered on the basis of a now widely-accepted analogy with a plane mixing layer, and we examine the significance of a key parameter, the "canopy-shear length scale". Investigating the effects of canopy density and atmospheric stability, we then discuss the extent of the mixing-layer analogy and the limits of our current understanding of canopy turbulence. Finally, we review the modelling tools used in this field and show how their development has evolved to date to meet our needs. In conclusion, we present a historical summary of the evolution of this research field and suggest future directions.

International audience; A key issue for predicting and mitigating the risk of wind damage in forests is to understand the mechanics of damage propagation during windstorms. Until now the acting processes have been poorly understood due to the difficulty of performing measurements in such extreme conditions. Here we use an innovative wind–tree interaction model, which allows for large deflection and tree breakage, to unravel for the first time the mechanisms of damage propagation at forest scale. We find that damage propagation involves two stages. Firstly, initial damage is caused by the impact of strong downward wind gusts. Trees break preferentially at the end of such critical passing sweeps, as the tree motion decelerates. The second stage starts when the damaged areas reach about 5 canopy heights in length and 1 canopy height in width: as the flow accelerates within the damaged areas the mean wind load becomes sufficient to break newly-created edge trees. From this bifurcation point tree damage increases drastically, irrespective of the tree motion state and the type of passing gusts. In addition to demonstrating the possibility of simulating wind damage propagation, these results have considerable potential for improving wind risk models.

Landscape discontinuities such as forest edges play an important role in determining the characteristics of the atmospheric flow by generating increased turbulence and triggering the formation of coherent tree-scale structures. In a fragmented landscape, consisting of surfaces of different heights and roughness, the multiplicity of edges may lead to complex patterns of flow and turbulence that are potentially difficult to predict. Here, we investigate the effects of different levels of forest fragmentation on the airflow. Five gap spacings (of length approximately 5h, 10h, 15h, 20h, 30h, where h is the canopy height) between forest blocks of length 8.7h, as well as a reference case consisting of a continuous forest after a single edge, were investigated in a wind tunnel. The results reveal a consistent pattern downstream from the first edge of each simulated case, with the streamwise velocity component at tree top increasing and turbulent kinetic energy decreasing as gap size increases, but with overshoots in shear stress and turbulent kinetic energy observed at the forest edges. As the gap spacing increases, the flow appears to change monotonically from a flow over a single edge to a flow over isolated forest blocks. The apparent roughness of the different fragmented configurations also decreases with increasing gap size. No overall enhancement of turbulence is observed at any particular level of fragmentation.

International audience; Forested landscapes often exhibit large spatial variability in vertical and horizontal foliage distributions. This variability may impact canopy-atmosphere ex- changes through its action on the development of turbulent structures. Here we inves- tigate in neutral stratification the turbulent structures encountered in a maritime pine forest characterised by a high, dense foliated layer associated with a deep and sparse trunk space. Both stand and edge regions are considered. In situ measurements and the outputs of large-eddy simulations are used and analysed together. In stand conditions, far from the edge, canopy-top structures appear stronlgy damped by the dense crown layer. Turbulent wind fluctuations within the trunk space, where the momentum flux vanishes, are closely related to these canopy-top structures through pressure diffusion. Consequently, auto-correlation and spectral analyses are not quite appropriate to characterise the vertical scale of coherent structures in this type of canopy, as pressure diffusion enhances the actual scale of structures. At frequencies higher than those associated with canopy-top structures, wind fluctuations related to wake structures developing behind tree stems are observed within the trunk space. They manifest themselves in wind velocity spectra as secondary peaks in the inertial subrange re- gion, confirming the hypothesis of spectral short-cuts in vegetation canopies. In the edge region specific turbulent structures develop just below the crown layer, in addi- tion to canopy-top structures. They are generated by the wind shear induced by the sub-canopy wind jet that forms at the edge. These structures provide a momentum exchange mechanism similar to that observed at the canopy top but in the opposite di- rection and with a lower magnitude. They may develop as in plane mixing-layer flows, with some perturbations induced by canopy-top structures. Wake structures are also observed within the trunk space in the edge region.

During the last few decades, many studies have been performed to determine water and carbon budgets of broadleaf and deciduous forests, crops and grasslands. However, since most measurements have been made in different regions and at different periods, it is difficult to compare the results directly. In order to evaluate accurately the respective contribution of various agroecosystems to global water and carbon exchanges, it is necessary to compare data obtained in similar climatic and weather conditions. To address this question, we present the results from simultaneous measurements carried out during one year over three typical agroecosystems of the Les Landes region in South-West France:~an agricultural field with maize from 29 May to 18 October, a young (5 year-old) pine forest and a mature (37 year-old) pine forest. All measurements were collected as part of the Regional Experiment component of the CarboEurope-IP project. During most of the year, the agricultural field without vegetation is a source of CO2, but from late June to early September the maize crop becomes a stronger carbon sink than the forests. Over the whole measurement period the three agroecosystems behave as CO2 sinks with carbon storage of about 335, 210 and 160 g C m−2 for the young forest, the mature forest and the agricultural field, respectively. We investigated the influence of climatic conditions on Gross Primary Production (GPP) of the three ecosystems and observed a predominant role of vapour pressure deficit (VPD) for forests and of photosynthetic photon flux density (FPP) for maize. Daily Water Use Efficiencies (WUE) of the three ecosystems were evaluated and expressed as functions of the mean daily vapour pressure deficit (VPD). Similar trends were observed for the two forests, which suggests that for a given species WUE is independent of stand age. The WUE of the maize crop at maturity was also found to depend upon VPD, but it is about twice as large as for the forests, owing to the physiological advantages of C4 species.

The Advanced Regional Prediction System (ARPS) has been modified so as to simulate turbulent flow at very fine scale within and above vegetation canopies using a large-eddy simulation (LES) approach. It is first shown that this new version of ARPS is able to reproduce accurately all essential features of turbulent flow over homogeneous canopies. A sensitivity study of the flow to the morphology of the canopy, i.e. the density and vertical leaf-area (LAI) distribution, is then performed numerically over three types of canopies with five levels of leaf-area index, from 1 to 5. This study confirms the universal characteristics of turbulent flow over vegetation canopies, as previously observed from wind-tunnel and in situ experiments. It shows that the typical features of canopy flow become more pronounced as canopy density increases, and quantifies the extent to which differences in canopy morphology can explain the experimental variability observed between canopies. This variability in turbulent characteristics is mostly visible in the subcanopy space, and depends significantly on the density of the upper foliated layers. The mean longitudinal separation Λ w between adjacent coherent structures has also been computed for each canopy using the wavelet transform and approximating the convection velocity of coherent structures as 1.8 times the average wind velocity at canopy top. Except for the sparsest canopies, the analogy between the atmospheric flow near the top of a vegetation canopy and a plane mixing layer is well verified: Λ w is directly related to the shear length scale L s = U ( h ) / U ′ ( h ) , where h is canopy height, U mean velocity and U ′ is the vertical gradient d U / d z , in a way close to the prediction Λ w / h = 8.1 L s / h .

International audience; As forest edges are often associated with wind damage, it may be of interest to modify the edge region in order to reduce wind-induced risks. To this purpose, this study investigates tree vulnerability to wind load downwind from leading edges designed with various treatments: sharp, tapered, sparse, dense, tall and small edges. Using a large-eddy simulation flow model, instantaneous wind and turbulence fields are simulated on either side of each edge. These fields are then used to compute mean and extreme tree bending moments as well as their ratio, the gust factor. The behaviour of these variables downwind from the edge agrees well with previous wind tunnel measurements. The gust factor increases at some distance behind the edge, due to the development of coherent eddy structures generated at the canopy-air interface. Unlike wind gusts in the vicinity of the edge, these structures penetrate deep within the canopy through sweep motions. Tree vulnerability is slightly reduced downwind from tapered, sparse and small edges and enhanced downwind from dense ones. Behind tall edges, the gust factor is reduced in the edge region but enhanced further downstream due to the interaction of the canopy with the wake of the edge treatment.

Virot et al. [E. Virot et al., Phys. Rev. E 93, 023001 (2016)] assert that the critical wind speed at which \ensuremath{\geqslant}50% of all trees in a population break is \ensuremath{\approx}42 m/s, regardless of tree characteristics. We show that empirical data do not support this assertion, and that the assumptions underlying the theory used by Virot et al. are inconsistent with the biomechanics of trees.

International audience; Sharp heterogeneities in forest structure, such as edges, are often responsible for wind damage. In order to better understand the behaviour of turbulent flow through canopy edges, large-eddy simulations (LES) have been performed at very fine scale (2 m) within and above heterogeneous vegetation canopies. A modified version of the Advanced Regional Prediction System (ARPS), previously validated in homogeneous conditions against field and wind-tunnel measurements, has been used for this purpose. Here it is validated in a simple forest-clearing-forest configuration. The model is shown to be able to reproduce accurately the main features observed in turbulent edge flow, especially the "enhanced gust zone" (EGZ) present around the canopy top at a few canopy heights downwind from the edge, and the turbulent region that develops further downstream. The EGZ is characterized by a peak in streamwise velocity skewness, which reflects the presence of intense intermittent wind gusts. A sensitivity study of the edge flow to the forest morphology shows that with increasing canopy density the flow adjusts faster and turbulent features such as the EGZ become more marked. When the canopy is characterized by a sparse trunk space the length of the adjustment region increases significantly due to the formation of a sub-canopy wind jet from the leading edge. It is shown that the position and magnitude of the EGZ are related to the mean upward motion formed around canopy top behind the leading edge, caused by the deceleration in the sub-canopy. Indeed, this mean upward motion advects low turbulence levels from the bottom of the canopy; this emphasises the passage of sudden strong wind gusts from the clearing, thereby increasing the skewness in streamwise velocity as compared with locations further downstream where ambient turbulence is stronger.

A modelling study is performed in order to quantify the relative effect of allowing for the physiological properties of an undergrowth grass sward on total canopy water and carbon fluxes of the Le-Bray forest (Les-Landes, South-western France). The Le-Bray forest consists of maritime pine and an herbaceous undergrowth (purple moor-grass), which is characterised by a low stomatal control of transpiration, in contrast to maritime pine. A CO2-responsive land surface model is used that includes responses of woody and herbaceous species to water stress. An attempt is made to represent the properties of the undergrowth vegetation in the land surface model Interactions between Soil, Biosphere, and Atmosphere, CO2-responsive, ISBA-A-gs. The new adjustment allows for a fairly different environmental response between the forest canopy and the understory in a simple manner. The model's simulations are compared with long term (1997 and 1998) micro-meteorological measurements over the Le-Bray site. The fluxes of energy, water and CO2, are simulated with and without the improved representation of the undergrowth vegetation, and the two simulations are compared with the observations. Accounting for the undergrowth permits one to improve the model's scores. A simple sensitivity experiment shows the behaviour of the model in response to climate change conditions, and the understory effect on the water balance and carbon storage of the forest. Accounting for the distinct characteristics of the undergrowth has a substantial and positive effect on the model accuracy and leads to a different response to climate change scenarios.

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.

International audience; Forest soils are often covered with a litter that influences the rate of mass and energy transfer between the soil and the air above, thereby modifying the temperature and moisture fields in the soil. The presence of a litter should therefore be accounted for in forest SVAT models, especially when long-term simulations are to be performed. A heat and moisture litter model has been developed by adding two dynamical equations to a force-restore type soil model. The experimental data used for the model validation was collected in a pine forest canopy in the South-West of France, that was part of the Euroflux network. The model is tested and validated over a two-year period. It is shown to provide a fairly good simulation of soil and litter moisture, soil and litter temperature and turbulent fluxes measured above the forest floor. It is also shown that simulations without the litter layer are unable to reproduce all these variables simultaneously. We then perform a sensitivity analysis to the parameters whose values are either uncertain or likely to be variable in time and space, such as the litter thickness, the rainfall fraction intercepted by the litter or the maximum value of the surface resistance. A threshold value of the litter moisture used in the surface resistance parameterisation turns out to be the most critical parameter. Further work is needed to investigate the possible relationships between the various parameters describing the litter, but the present litter model can already be used in combination with other forest SVAT models.

International programmes such as EUROFLUX focus on the analysis of long-term fluxes and energy budgets in the biosphere. Reliable estimates of hourly energy budgets require an accurate estimation of soil heat flux, that is often non-negligible even in a forest, and can be predominant during the night. Over long periods of time such as one to several months, its contribution can also be significant. The present work has been carried out to get good estimates of the soil heat flux in a maritime pine stand in the southwest of France, one of the 15 EUROFLUX sites. Using a whole year’s worth of data, soil heat flux was estimated by a two-step version of the null-alignment method using soil temperature, water content and bulk density measurements between the soil surface and a depth of 1 m. A data subset was firstly used to estimate and model the soil thermal conductivity at various depths. The full data set was then used with the modelled conductivity to estimate heat storage between the surface and a reference depth, and calculate the heat flux at the soil surface. Throughout the investigated year and at a 30 min time scale, the soil heat flux represents 5‐10% of the incident net radiation, i.e. 30‐50% of the net radiation over the understorey. Cumulative values from September 1997 to March 1998 reach a maximum of 70 MJ m 2 , which represents nearly 50% of the cumulative values of transmitted net radiation (140 MJ m 2 ) over the same period. These estimates of soil heat flux allowed the energy budgets of the whole stand and the understorey to be closed, and showed that the storage terms are significant not only at a 30 min time scale but also at longer time scales (a few weeks). An attempt was finally made to model soil heat flux from meteorological data, which has rarely been done for a forest soil and over a long-term data set. In most of the existing models, soil heat flux is taken as a fraction of net radiation or sensible heat flux. Here, the litter acts as a mulch at the soil surface so that the only significant terms of the energy balance at this level are soil heat flux, transmitted net radiation and turbulent sensible heat flux. Soil heat flux is shown to be a linear combination of (1) net radiation above the understorey with a clear dependence of the coefficient on the soil cover fraction, and (2) the difference between the air and litter temperatures, with little influence of soil water content or wind speed on the coefficient. © 2001 Elsevier Science B.V. All rights reserved.

Wind-flow dynamics has been extensively studied over horizontally uniform canopies, but agricultural plantations structured in rows such as vineyards have received less attention. Here, the wind flow over a vineyard is studied in neutral stratification from both large-eddy simulation (LES) and in situ measurements. The impact of row structure on the wind dynamics is investigated over a range of wind directions from cross-row to down-row, and a typical range of row aspect ratio (row separation/height ratio). It is shown that the mean flow over a vineyard is similar to that observed in uniform canopies, especially for wind directions from cross-row to diagonal. For down-row winds, the mean flow exhibits noticeable spatial variability across each elementary row-gap pattern, as the wind is channeled in the inter-row. This spatial variability increases with the aspect ratio. With down-row winds the turbulent structures are also more intermittent and generate larger turbulent kinetic energy and momentum flux. The displacement height and roughness length of the vineyard vary with the aspect ratio in a way similar to their variation with canopy density in uniform canopies. Both parameters take smaller values in down-row wind flow, for which the canopy appears more open. The analysis of velocity spectra and autocorrelation functions shows that vineyard canopies share similar features to uniform canopies in terms of turbulent coherent structures, with only minor changes with wind direction.

A simple tree swaying model, valid for windstorm conditions, has been developed for the purpose of simulating the effect of strong wind on the vulnerability of heterogeneous forest canopies. In this model the tree is represented as a flexible cantilever beam whose motion, induced by turbulent winds, is solved through a modal analysis. The geometric nonlinearities related to the tree curvature are accounted for through the formulation of the wind drag force. Furthermore, a breakage condition is considered at very large deflections. A variety of case studies is used to evaluate the present model. As compared to field data collected on three different tree species, and to the outputs of mechanistic models of wind damage, it appears to be able to predict accurately large tree deflections as well as tree breakage, using wind velocity at tree top as a forcing function. The instantaneous response of the modelled tree to a turbulent wind load shows very good agreement with a more complex tree model. The simplicity of the present model and its low computational time make it well adapted to future use in large-eddy simulation airflow models, aimed at simulating the complete interaction between turbulent wind fields and tree motion in fragmented forests.

International audience; A new method for estimating scalar fluxes, called surface renewal analysis, was developed and successfully tested with air temperature data from a maize crop, an orchard, and a forest canopy. The method employs the scalar trace and is based on the concept of turbulent coherent structures causing most of the scalar fluxes. It was generally more accurate than other scalar based methods such as the Tillman variance method, and can be applied to data gathered under stable stability conditions. Filtering the temperature trace, based on the wind speed at the canopy height, resulted in more accurate surface renewal estimates than when raw 10 Hz data were used.

As forest edges are a major source of heterogeneity in fragmented landscapes, the atmospheric flow over forested areas is often under their influence. Understanding how far the upstream edge has an impact on the turbulent wind flow in a forest canopy is important, in particular for scalar flux measurement. In this study, edge and stand flows over a maritime pine forest characterized by a dense crown layer located above a deep and sparse trunk space are analysed in detail from in situ measurements and large-eddy simulations (LES). The LES model used here appears to simulate remarkably well most characteristics of the turbulent wind flow for this particular canopy structure. It is shown that the main characteristics of the edge flow in this case differ from those usually observed in forests with a more uniform vertical foliage distribution. The main differences are (i) the development of turbulence above the canopy occurring closer to the edge, (ii) the absence of a well-defined enhanced gust zone around the top of the canopy, (iii) the presence of a large secondary wind maximum within the trunk space, and (iv) the development of a positive momentum flux layer below the crown layer. Most of these differences are related to the presence of a substantial sub-canopy wind jet induced by the wind flow through the trunk space at the edge. The secondary velocity maximum induced by this wind jet differs from that observed in homogeneous stand conditions, where it seems to be related to the mesoscale pressure gradient. The wind jet appears to decrease very slowly with distance from the edge, so that edge effects are still significant at 9 h from the edge (where h is the mean canopy height). The length of the adjustment region is shown to be greater than 10–15 h , and to depend on the depth of the trunk space. In very fragmented forested areas with deep and sparse trunk space, within-canopy flow may always be under the influence of edges.

International audience; In order to investigate the possibility of modelling plant motion at the landscape scale, an equation for crop plant motion, forced by an instantaneous velocity field, is introduced in a large-eddy simulation (LES) airflow model, previously validated over homogeneous and heterogeneous canopies. The canopy is simply represented as a poroelastic continuous medium, which is similar in its discrete form to an infinite row of identical oscillating stems. Only one linear mode of plant vibration is considered. Two-way coupling between plant motion and the wind flow is insured through the drag force term. The coupled model is validated on the basis of a comparison with measured movements of an alfalfa crop canopy. It is also compared with the outputs of a linear stability analysis. The model is shown to reproduce the well-known phenomenon of honami which is typical of wave-like crop motions on windy days. The wavelength of the main coherent waving patches, extracted using a bi-orthogonal decomposition (BOD) of the crop velocity fields, is in agreement with that deduced from video recordings. The main spatial and temporal characteristics of these waving patches exhibit the same variation with mean wind velocity as that observed with the measurements. However they differ from the coherent eddy structures of the wind flow at canopy top, so that coherent waving patches cannot be seen as direct signatures of coherent eddy structures. Finally, it is shown that the impact of crop motion on the wind dynamics is negligible for current wind speed values. No lock-in mechanism of coherent eddy structures on plant motion is observed, in contradiction with the linear stability analysis. This discrepancy may be attributed to the presence of a nonlinear saturation mechanism in LES. © 2010 Cambridge University Press.

Eddy fluxes were measured over different ecosystems, winter and summer crops, a maritime pine forest at different stages of development and grassland, from 17 May to 26 June 2005 in the southwestern region of France. During the experiment, summer crops started growing whereas winter crops and grassland achieved their senescence. Comparatively, the other ecosystems had a much slower growth emphasized by soil water deficit at forest sites. The ten ecosystems showed different partitioning of available energy. Net radiation was the highest above the maritime pine forest, followed, in decreasing order, by the crops, the vineyard and the grassland. Over the whole campaign period, the Bowen ratio (β=H/LE) was larger above the forest sites than for the other sites. The various vegetation types also showed contrasting net ecosystem exchange (NEE) dynamics following their growth status and respective behaviour in response to drought. Both the clearcut and summer crops before irrigation and plant growth behaved as sources of CO2, whereas the vineyard, the mature forest and winter crops acted as sinks. However the maize crops became substantial sinks of CO2 after the start of irrigation and canopy growth, with fluxes twice as large as for the mature pine forest. Finally, throughout the experiment, forest, grassland and crops sequestrated from about 50 gC m−2 to 230 gC m−2, while the cleacut and the beans crop rejected about 30 gC m−2. These results support the idea that converting a mature forest to a clearcut or bare soil available to agricultural use enhances the sensible heat flux and shifts the ecosystem from a sink to a source of carbon.

Large coherent structures over vegetation canopies are responsible for a substantial part of the turbulent transfer of momentum, heat and mass between the canopy and the atmosphere. As forested landscapes are often fragmented, edge regions may be of importance in turbulent transfer. The development of coherent structures from the leading edge of a forest is investigated here for the first time. For this purpose, the turbulent flow over a clearing–forest pattern is simulated using the Advanced Regional Prediction System (ARPS). In previous studies the code has been modified so as to simulate turbulent flows at very fine scale (0.1h, where h is the mean canopy height) within and above heterogeneous vegetation canopies, using a large-eddy simulation (LES) approach. Validations have also been performed over homogeneous forest canopies and over a simple forest–clearing–forest pattern, against field and wind-tunnel measurements. Here, a schematic picture of the development of coherent eddies downstream from the leading edge of a forest is extracted from the mean vorticity components, the Q-criterion field, the cross-correlation of the wind velocity components and the length and separation length scales of coherent structures, determined by using a wavelet transform. This schematic picture shows strong similarities with the development of coherent structures observed in a mixing layer, with four different regions: (i) close to the edge, Kelvin–Helmholtz instabilities develop when a strong wind gust hits the canopy; (ii) these instabilities roll over to form transverse vortices from around 3h downstream from the edge, characterized by a length scale close to the depth of the internal boundary layer that develops from the canopy edge; (iii) secondary instabilities destabilize these rollers and increase the vertical and streamwise vorticity components from around 6h, and two counter-rotating streamwise vortices appear; (iv) at about 9h the initial rollers have become complex three-dimensional coherent structures, with spatially constant mean length and separation length scales. These four stages of development occur closer to the edge with increasing canopy density. While this average picture of the development of coherent structures is similar to that observed in a mixing layer, the analysis of instantaneous fields shows that coherent structures behind the leading edge appear as resulting from the ‘branching’ of tubes localized in regions of low pressure, where their cores are characterized by high values of enstrophy and Q-criterion.

International audience; Carbon dioxide, water vapour and energy fluxes were measured above and within a maritime pine forest during an atypical year with long-lasting reduced soil water availibility. Energy balance closure was adequately good at both levels. As compared with what is usually observed at this site the ecosystem dissipated less energy via latent heat flux and more via sensible heat flux. The understorey canopy was responsible for a variable, significant component of the whole canopy fluxes of water vapour and carbon dioxide. The annual contribution of the understorey was 38% (154 mm) of the overall evaporation (399 mm) and 32% (89 mm) of the overall sensible heat flux (274 mm). The participation of the understorey reached 45% of the overall evaporation and 30% of the daytime overall assimilation during significant soil water deficit periods in summertime. Even during winter, understorey photosynthesis was consistent as it compensated soil and understorey respiration. The ecosystem behaved as a sink of carbon, with a negative annual carbon budget (-57 gC m-2). However, due to high soil water deficit, the annual ecosystem GPP was 40% less than usually observed at this site. This budget resulted from a sink of -131 gC m-2 for the overstorey and a source of +74 gC m-2 for the understorey. Moreover, on an annual basis the overstorey layer contributed to almost two thirds of the ecosystem respiration. Finally, the effect of long-lasting soil water deficit on the maritime pine forest was found more important than the effect of the heat wave and drought of summer 2003.

International audience; This study presents a predictive dynamic model developed to analyse the mechanical response of trees submitted to a turbulent airflow. This finite-element model integrates a three-dimensional description of tree architecture and is driven by fluctuating drag forces applied on all parts. For validation purposes, instantaneous wind velocities and wind-induced stem displacements of two trees were recorded in a mature Maritime pine stand (Pinus pinaster) at several heights. The tree geometrical and physical characteristics were measured to describe their architecture. No model parameter was adjusted. Tree motions appear to be driven by wind pulses reflecting turbulence intermittency. No evidence is found for resonant behaviour. In the mean wind direction, the simulated oscillations agree well with the measured time series. The underestimation of tree movement in the cross-stream direction outlines the importance of torque behaviour on the predictive accuracy of the model. The mechanical transfer functions of the modelled trees show vibration peak frequencies very similar to the measured ones. At higher frequencies, the simulated damping appears overestimated, with the set of parameters used. The model provides a sound basis to further investigate the influence of tree aerial architecture and turbulence structure on tree stability to wind.

This article takes stock of the empirical, phenomenological and mechanistic studies on forest stand resistance to wind, in particular mixed forests. Major factors in the resistance of pure species forests to wind are the characteristics of the climatic event, the prevailing species, stand height and the depth of soil explored by the roots. Extensive, recent thinning intensifies the risk. The internal structure of the stands (density, vertical structure, etc), which has little effect on the speed of wind and the intensity of the turbulence, plays a lesser role in explaining the magnitude of damage. A classification of single-species stand resistance is offered. Although the intermingling of the canopy and the root systems may somewhat alter resistance, the phenomenological studies conducted to date show that the rate of damage in mixed stands derives essentially from the intrinsic stability (as observed in pure stands) of the species involved and their relative proportions. Any particular mixture may therefore have either an overall positive or negative effect. However, combinations may help to contain large-scale windthrows. A number of silvicultural recommendations are offered that need to be incorporated into an overall economic and ecological analysis of the particular situation considered. Combining several resistant species is particularly desirable in conditions of significant vulnerability or where the interests at stake are high. The resistance of species can be assessed on the basis of the proposed classification and the local features. Finally, just as in the case of single-species stands, intensive silvicultural practises tend to diminish the risk.; Cet article propose un bilan des études empiriques, phénoménologiques et mécanistes sur la résistance aux vents forts des peuplements forestiers, notamment des mélanges d’espèces. La résistance des peuplements purs dépend en premier lieu des caractéristiques de l’événement climatique, de l’essence principale, de la hauteur des peuplements et de la profondeur de sol prospectée par les racines. Des éclaircies fortes et récentes aggravent le risque. La structure interne des peuplements (densité, structure verticale...), qui modifie relativement peu les champs de vitesse de vent et l’intensité des turbulences, intervient dans une moindre mesure dans l’explication des dégâts. Un classement de résistance des essences en peuplement pur est proposé. Bien que, en mélange, les processus d’imbrication des houppiers et des racines puissent moduler cette résistance, les études phénoménologiques réalisées jusqu’ici montrent que le taux de dégâts en peuplements mélangés s’explique avant tout par la stabilité intrinsèque (c’est-à-dire observée en peuplement pur) des essences en présence et par leur proportion relative. Le mélange peut donc avoir des effets positifs ou négatifs, selon les cas. En revanche, il permet de limiter la fréquence des grandes arasées. Quelques recommandations sylvicoles sont proposées, à intégrer dans une analyse globale économique et écologique. L’introduction d’essences résistantes en mélange est particulièrement recommandée dans les situations de forte vulnérabilité ou à forts enjeux. Un diagnostic de la résistance des essences devra être réalisé sur la base du classement proposé et des caractéristiques stationnelles. Enfin, tout comme pour les peuplements purs, une intensification de la sylviculture permettra de limiter les risques.

Modelling turbulent flow at a very fine scale within and above vegetation canopies over complex terrain is of great interest for many environmental applications. Over the last decade, it has been demonstrated that the large-eddy simulation (LES) technique is able to reproduce in detail many observed features of turbulent flow over homogeneous and heterogeneous vegetation canopies on flat terrain. For the first time, a nested LES of turbulent flow within and above a forested canopy on an isolated two-dimensional hill is analysed and validated against pressure and velocity data from a wind-tunnel experiment. The model is shown to be able to reproduce accurately the main features observed over a forested hill. The results also confirm recent observations on the intermittency of the recirculation region behind the hill, and on the domination of sweep motions in momentum transfer at the canopy top all along the hill. The main characteristics of turbulent structures have also been investigated from the analysis of vorticity fields and two-point velocity correlations across the hill. It is shown that the streamwise wind velocity upwind from the summit is not correlated with the flow within the wake region but only with the flow above it, while in the wake region the streamwise wind velocity at the canopy top is only correlated with the flow within the wake region. This result suggests that turbulence within the wake region results from the superposition of various turbulent structures: large turbulent structures induced by the elevated shear layer that may result from the rolling over of Kelvin-Helmotz instabilities, structures induced at the lee-side foot of the hill by an adverse pressure gradient and structures induced by the presence of the canopy. Implications of hilly terrain on canopy-atmosphere exchanges and tree vulnerability to windload are briefly discussed. Copyright © 2008 Royal Meteorological Society

We applied a site evaluation approach combining Lagrangian Stochastic footprint modelling 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.

Land-use practices such as deforestation or agricultural management may affect regional climate, ecosystems and water resources. The present study investigates the impact of surface heterogeneity on the behaviour of the atmospheric boundary layer (ABL), at a typical spatial scale of 1 km. Large-eddy simulations, using an interactive soil–vegetation–atmosphere surface scheme, are performed to document the structure of the three-dimensional flow, as driven by buoyancy forces, over patchy terrain with different surface characteristics (roughness, soil moisture, temperature) on each individual patch. The patchy terrain consists of striped and chessboard patterns. The results show that the ABL strongly responds to the spatial configuration of surface heterogeneities. The stripe configuration made of two patches with different soil moisture contents generates the development of a quasi- two-dimensional inland breeze, whereas a three-dimensional divergent flow is induced by chessboard patterns. The feedback of such small-scale atmospheric circulations on the surface fluxes appears to be highly non-linear. The surface sensible and latent heat fluxes averaged over the 25-km2 domain may vary by 5% with respect to the patch arrangement.

International audience; With the introduction of genetically modified (GM) crops, predictive tools modelling wind velocity and pollen concentration fields as well as pollen deposition rate over heterogeneous canopies are required to assess the cross-pollination rates between GM and conventional crops. Over the last decade several numerical flow models have been developed to simulate dynamic mean and turbulent fields within and above the vegetation layer. In this paper, an Eulerian advection–diffusion conservation equation for pollen particles has been incorporated into one of these flow models, Aquilon. The relative velocity between air parcels and particles is simply represented through the addition of a particle settling velocity, i.e. the particle fall velocity in still fluid. The dynamic part of this model has been previously validated in two-dimensional heterogeneous cases (roughness change, forest edge flow) and tested in a more complex three-dimensional heterogeneous case (urban forested park). In order to test the ability of this Eulerian approach to simulate accurately airborne pollen concentration and pollen deposition rate within and above heterogeneous vegetation canopies, the model is validated against two field experiments where the airborne concentration and the deposition rate of maize pollen (Zea mays) were measured downwind from source plots.We also compare the outputs of Aquilon with those of a Lagrangian model previously tested against the same dataset. Generally speaking the model performs well, with a similar accuracy to the Lagrangian model. However, both models underestimate the measured maximum in ground pollen deposition just downwind from the maize plot. It is shown that this discrepancy may be due to an underestimation of the pollen settling velocity in this region. As the Stokes number, defined as the ratio between the maize pollen response time and the characteristic time of the turbulent structures at dissipation-range scale, is about 1 in the wake region behind the source plot, it is likely that turbulence leads to an increase in the apparent settling velocity there.

International audience; The damage caused by windstorms to forest ecosystems is often very heterogeneous. In order to improve the stability of forested landscapes, it is of great importance to identify the factors responsible for this spatial variability. The structure of the landscape itself may play a role, through possible influences of canopy heterogeneities on the development of turbulence. For the purpose of investigating the role of landscape fragmentation on turbulence development, we used a numerical flow model with a k–ε turbulence scheme model, previously validated in simple cases with well-defined surface changes (roughness change and forest edge flow). A series of two- and three-dimensional simulations were performed over a heterogeneous urban forested park in Europe, which was severely damaged in various places by the Lothar windstorm in December 1999. The model shows the development of a region of strong turbulence, resulting from the generation of large wind shear at the top of the canopy. A sensitivity study shows how the location, extension and intensity of the region depend on canopy characteristics such as the leaf density, the nature of the edge or the presence of gaps and clearings. Simulations performed in conditions representative of the windstorm show that the location of the damaged areas corresponds very closely to the regions where the turbulent kinetic energy was above a certain threshold.

International audience; This paper presents a Lagrangian budgeting approach to quantify the uptake of CO2 by vegetation at horizontal scales of several tens of kilometers. For this purpose, CO2 and meteorological parameters were measured from a small aircraft during four flights in June 2001 over a flat homogeneous and productive temperate forest in the Landes region ( southwestern France). Additional CO measurements were made in order to identify and quantify the potential influence of anthropogenic emissions on the net CO2 flux derived from the measurements. For one of four flights, Lagrangian conditions were nearly perfectly fulfilled. On average, the CO2 mixing ratio in the boundary layer decreased at a rate of 0.11 ppm km(-1), yielding an average CO2 uptake by the forest of 16 +/- 2.5 mumol m(-2) s(-1) between 1230 and 1430 UT. Our result is about 15% smaller than the local net ecosystem exchange measured by eddy covariance at a tower north of the flight domain and about 12% higher than a regional estimate based on remote sensing data for the whole experimental area. The contribution of anthropogenic emissions to the regional CO2 budget was estimated from the CO measurements to be to

International audience; The physiological and physical processes controlling ozone dry deposition to vegetated surfaces are still not fully understood. In particular, the role of the understorey and the possible action of dew on ozone deposition have not received much attention so far. This paper presents the results of an experiment aimed at quantifying ozone dry deposition to a maritime pine forest in the “Les Landes” area in France. Ozone deposition fluxes were measured using the eddy-covariance technique above and within the canopy. We investigate the factors acting on ozone deposition in both dew-wetted and dry conditions. The values obtained for the ozone deposition velocity are well in the range of previously published measurements over coniferous forests. For the present forest, ozone uptake by the understorey is a significant portion of ozone deposition to the whole pine stand. The understorey contributes more to the overall ozone flux than to the other measured scalar fluxes (sensible heat and water vapour). During dry nights the surface conductance for ozone and the friction velocity are strongly correlated, showing that ozone deposition is largely controlled by dynamical processes. During the day, in dry conditions, the canopy stomatal conductance is the major parameter controlling ozone deposition. However, in winter, when the stomatal conductance is low, the influence of dynamical processes persists during day-time. It is also found that surface wetness associated with dew significantly enhances ozone deposition, during the night as well as in the morning.

Measurements of turbulent exchange in the understorey of a forest canopy are necessary to understand and model both the functioning of the lower stratum and its contribution to turbulent exchange at the whole canopy scale. Eddy covariance measurements of eddy fluxes just above the floor of a forest canopy must be thoroughly validated, given the particular conditions prevailing there. Our objective is two-fold: (i) check the overall quality of such eddy flux measurements through the analysis of the understorey energy balance closure and (ii) define quality criteria for each half-hourly sample, based on the residual term of the energy balance. A subset of the EUROFLUX data base, collected within a pine forest canopy in south-west France, was used for this purpose. During this experiment, all heat storage terms were carefully measured, which allowed the closure of the understorey energy balance to be rigorously tested. As in most experiments storage term measurements are not available, we also developed a method to estimate them, in order to apply the above-mentioned data selection method. The energy balance closure was found to be quite satisfactory (the slope of the sum of eddy fluxes and storage terms versus transmitted net radiation is 0.99, the intercept is less than 1 W m 2 , r 2 is 0.94, there is no deviation from a linear trend). The data selection procedure allows a fair description of the daily and day-to-day variation of turbulent fluxes while rejecting the most dubious data, whether experimental or estimated storage terms are used. This analysis proves the validity of eddy flux measurements in the lower part of the forest and offers tools for flux data selection, depending on the type of studies such data are intended for. © 2001 Elsevier Science B.V. All rights reserved.

International audience; An analogy has been established between a plane mixing layer and the atmospheric flow near the top of a vegetation canopy. It is based on a common feature, a strong inflection in the mean velocity profile, responsible for hydrodynamical instabilities that set the pattern for the coherent eddies and determine the turbulence length scales. In an earlier study, this analogy was tested using a small data set from thirteen experiments, all in near-neutral conditions. It provided a good prediction of the streamwise spacing Λw of the dominant canopy eddies (evaluated from time series of vertical velocity) that appears to depend on a shear length scale Ls = U(h)/U'(h), where h is canopy height, U is mean velocity and U' the vertical gradient dU/dz. The present analysis utilizes an extensive data set of approximately 700 thirty-minute runs, from six experiments on two forest sites and a maize crop, with a large range of stability conditions. Λw was estimated for each run using the wavelet transform as an objective, automated detection method. First, the variations of Λw and Ls with atmospheric stability are discussed. Neutral and unstable values exhibit a large scatter whereas in stable conditions both variables decrease with increasing stability. It is subsequently found that Λw is directly related to Ls, in a way close to the neutral prediction Λw /h = 8.1Ls/h.The Strouhal number Str = Ls /Λw is then shown to vary with atmospheric stability, weakly in unstable conditions, more significantly in stable conditions. Altogether these results suggest that, to some extent, the plane mixing-layer analogy can be extended to non-neutral conditions. It is argued that the primary effect of atmospheric stability, at least in stable conditions, is to modify the shear length scale Ls through changes in U(h) and U'(h), which in turn determines the streamwise spacing of the active, coherent motions.

International audience; Turbulence data collected within the roughness sublayer, from a number of sites with varying canopy structure and atmospheric conditions, has been used to investigate coherent structures. An extensive data set of approximately 700 thirty minute runs has been analysed using the wavelet transform as an objective, automated detection method. Spatial separation of coherent structures was evaluated from vertical velocity time series. The variations of the resulting values with atmospheric stability were investigated. Mean eddy separation was shown to be primarily controlled by a canopy shear scale based on the recognition that canopy flows share common features with a plane mixing layer flow.

International audience; Two experiments were performed in a confierous forest (maritime pine) in the southwest of France, one in 1994 and the other in 1995. Two sites were chosen, differing by age, height and structure of the trees, as well as the nature of the understorey. In both cases measurements of turbulent fluxes were made at two levels above and within the forest canopy, using sonic anemometers and open-path infrared CO2-H2O analysers. The flux differences derived from the two measurement levels allowed the Radiation and Water Use Efficiencies (RUE and WUE, respectively) to be evaluated for both canopy crowns. The results are based on the analysis of about ten days from each experiment. For both campaigns RUE is significantly larger during cloudy conditions when the fraction of diffuse radiation () increases. An empirical linear relation between RUE and is established for each site, with a smaller intercept and a larger slope for the older forest. In clear conditions (), RUE is about 30 % lower for this forest. Tree photosynthesis, estimated as the net CO2 flux of the foliated layer Fc, appears poorly correlated (r2 < 0.4) with transpiration (net water vapour flux E). This is shown to result from strong variations in the atmospheric saturation deficit D during both campaigns. At both sites WUE turns out to be a hyperbolic function of D (). The coefficient k is 50 % larger for the younger forest. This is in agreement with the values obtained for RUE, and indicates that photosynthetic rates decrease with the age of the trees.

International audience; Previous measurements of water loss from small-dish evaporimeters mounted at the height of irrigated crops grown under conditions of extreme local advection in the Sudan are reexamined. From these evaporimeter measurements, it is possible to calculate fractional changes in the saturation deficit. Relationships between canopy conductance and saturation deficit are briefly reviewed and introduced into the Penman-Monteith equation to calculate transpiration rates as a function of distance downwind of the boundary between the upwind desert and the irrigated crop. In contradiction to most theoretical predictions, these new calculations show rates of transpiration to undergo only modest changes with increasing fetch. This occurs because of the feedback interaction between saturation deficit and stomatal conductance. This result is in good accord with a recent study suggesting that a dry-moist boundary transition may be best modelled as a simple step change in surface fluxes and further that the advective enhancement of evaporation may have been overestimated by many advection models. Larger effects are expected on dry matter yields because of the direct influence of saturation deficit on the yield-transpiration ratio.

A set of daytime and nighttime turbulence data acquired above a maize crop is analyzed, using various properties of the wavelet transform. Wavelet variances and covariances provide characteristic duration scales for the turbulent motions contributing most to the signal energy. Subsequent detection of their signatures in the raw time series allows a characteristic gust frequency to be determined. Once normalized with friction velocity and canopy height, this frequency appears to be similar to that observed above a pine forest, which supports the postulate that transfer processes over plant canopies are dominated by populations of canopy-scale eddies, with universal characteristics. Conditional averaging of the corresponding patterns provides a clear picture of the ejection-sweep processes. Around the inversion time of the mean vertical temperature gradient, two scales of motions are inferred from the observation of wavelet variances and covariances. A technique based on the inverse wavelet transform is then developed for partitioning the original signals into small- and large-scale components, using a cut-off scale deduced from wavelet variances and covariances. Despite their wavelike aspect, the extracted large-scale components are not gravity waves. Their nature is unclear but the proposed methodology has potential applications for studying wave-turbulence interactions.

International audience; The existence of ramp structures in scalar fields such as air temperature has been reported in laboratory flows over smooth and rough walls, in the atmospheric boundary layer and in flows in and above forests. They have been recognized as the signature of coherent turbulent structures. The aim of this paper is to present some observations and analyses of these features in the agricultural environment. Evidence is given from samples of time traces recorded during experiments conducted in maize crops and orchards. Ramps of air temperature, surface temperature, humidity and CO2 concentrations are shown to occur under stable, neutral and unstable conditions. Ramp structures are more apparent above short canopies than within them, in contrast to taller tree canopies where ramps are seen most clearly near the canopy top. Under stable conditions, they are sometimes found in association with trapped gravity waves. It is demonstrated that the frequency of occurrence of the coherent structures is related to a wind shear scale characteristic of the canopy flow.

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.