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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A procedure to assess pesticide emission to the air and characterise possible air pollution sources was carried out using a tracer dye and 2mm PVC lines during air-assisted spraying of an artificial vineyard. Three experiments were performed to evaluate the method feasibility, quantify upward movements of sprayed droplets and investigate the influence of microclimatic variables on pesticide emission. During each experiment two test series were carried out with two droplet size distributions (very fine and fine spray, according to the BCPC classification). The amount of sprayed liquid collected at 2. 5m above ground varied between 9. 0% and 10. 7% of the total dose applied for very fine spray and between 5. 6% and 7. 3% for fine spray. In stable atmospheric conditions the spray drifted along the mean wind direction over the crop whereas in unstable conditions the sprayed liquid plume was larger, with a greater amount of material sent to higher levels. A statistical model based on a simple multiple regression featuring droplet characteristics and microclimatic variables (wind speed, temperature, stability parameter and relative humidity) provided a robust estimate of spray loss just above the crop, with an acceptable determination coefficient (R 2=0. 84). This method is therefore suitable for quantifying spray drift and provides a way to study the influence of several variables on the amount of pesticide released into the atmosphere by air-assisted spraying, with suitable accuracy. [Copyright &y& Elsevier]

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 5 h, 10 h, 15 h, 20 h, 30 h, where h is the canopy height) between forest blocks of length 8.7 h, 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Forested landscapes often exhibit large spatial variability in vertical and horizontal foliage distributions. This variability may affect canopy-atmosphere exchanges through its action on the development of turbulent structures. Here we investigate in neutral stratification the turbulent structures encountered in a maritime pine forest characterized 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 results of large-eddy simulations are used and analyzed together. In stand conditions, far from the edge, canopy-top structures appear strongly 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, autocorrelation and spectral analyses are not quite appropriate to characterize 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 region, confirming the hypothesis of spectral short-cuts in vegetation canopies. In the edge region specific turbulent structures develop just below the crown layer, in addition 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 direction 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. [ABSTRACT FROM AUTHOR]

Abstract: 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 9h from the edge (where h is the mean canopy height). The length of the adjustment region is shown to be greater than 10–15h, 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. [ABSTRACT FROM AUTHOR]

Upward spray losses over vines were assessed during a typical air-assisted application using a fluorescent tracer dye and PVC lines as samplers. Linear multiple regression and fuzzy logic inference models were used to evaluate the effects of micrometeorological conditions on pesticide application for two spray qualities (fine and very fine). For the fine spray application (volume median diameter [VMD] 134μm), the significant variables for the multiple regression were wind speed, air temperature and wet bulb temperature depression, with a coefficient of determination of 0.70. For the very fine spray application (VMD 65μm), atmospheric stability was also significant, with a coefficient of determination of 0.82. Spray losses were also predicted using fuzzy inference systems, and high coefficients of determination were obtained (R 2=0.72 for the fine spray and 0.66 for the very fine spray). Interpretable rules were established for the characterisation of micrometeorological parameters using the two sprays. Both analysis tools can be combined with mathematical modelling in order to evaluate air pollution and spray drift from simplified field tests. [Copyright &y& Elsevier]

Virot et al. [E. Virot et al., Phys. Rev. E 93, 023001 (2016)] assert that the critical wind speed at which ≥0% of all trees in a population break is ≈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. [ABSTRACT FROM AUTHOR]

Riparian corridors often act as low-land climate refugia for temperate tree species in their southern distribution range. A plausible mechanism is the buffering of regional climate extremes by local physiographic and biotic factors. We tested this idea using a 3-year-long microclimate dataset collected along the Ciron river, a refugia for European beech (Fagus sylvatica) in southwestern France. Across the whole network, canopy gap fraction was the main predictor for spatial microclimatic variations, together with two other landscape features (elevation above the river and woodland fraction within a 300m radius). However, within the riparian forest only (canopy gap fraction < 25%, distance to the river < 150m), variations of up to -4°C and + 15% in summertime daily maximum air temperature and minimum relative humidity, respectively, were still found from the plateau to the cooler, moister river banks, only ~ 5-10m below. Elevation above the river was then identified as the main predictor, and explained the marked variations from the plateau to the banks much better than canopy gap fraction. The microclimate measured near the river is as cool but moister than the macroclimate encountered at 700-1000m asl further east in F. sylvatica's main distribution range. Indeed, at all locations, we found that air relative humidity was higher than expected from a temperature-only effect, suggesting that extra moisture is brought by the river. Our results explain well why beech trees in this climate refugium are restricted to the river gorges where microtopographic variations are the strongest and canopy gaps are rare., (© 2024. The Author(s) under exclusive licence to International Society of Biometeorology.)