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The mechanisms by which a porous windbreak modifies airflow, microclimates and hence crop yields are addressed, based upon recent wind tunnel experiments, field observations and numerical modelling. This paper is thus an update to the excellent reviews in Brandle (1988). It shows how a turbulent mixing layer initiated at the top of the windbreak dominates the airflow behind a windbreak. This mixing layer spreads vertically as it moves downwind, growing at a rate determined by the turbulence in the approach flow and the windbreak's 'permeability'. The roughness of the terrain and land-cover upwind, windbreak height and porosity are thus the main controls on the amount and extent of shelter provided by a windbreak. The changes in temperature, humidity, heat and evaporation fluxes given these changes in turbulence are then described. Based on the turbulent mixing layer model, the highly sheltered 'quiet zone' will be typically warmer and more humid while further downwind in the 'wake zone', cooler and drier conditions would be expected. The careful experimental studies needed to verify these theoretical predictions have not yet been published. Shade is also shown to modify the heating in the quiet zone and, depending on the orientation of the windbreak, can offset the warming in the quiet zone. Lastly, the mechanisms affecting plant productivity are described in light of these airflow and microclimate changes. A major effect of a windbreak is to reduce the incidence of low frequency, high magnitude damage events such as sandblasting or lodging. Microclimate effects, however, do not always improve productivity. For example, while shelter may improve water-use efficiency in irrigated crops by increasing yields and reducing water-use, this may not be the case in dryland agriculture.[PUBLICATION ABSTRACT]

Dispersal behavior has important effects on the persistence and recolonization of populations, but is one of the least understood traits of most organisms. Knowledge of patterns of fledging, natal, and breeding dispersal of birds in a patchy environment will assist in decisions regarding reserve design and protection or construction of corridors. I present data on movement patterns of three migratory bird species, American Robin (Turdus migratorius), Brown Thrasher (Toxostoma rufum), and Loggerhead Shrike (Lanius ludovicianus). These birds are relatively common breeders in south-central North Dakota (U.S.) in riparian woodlands and in shelterbelts (woodlots planted as windbreaks in the open agricultural environment). Field assistants and I individually marked and monitored the movements of more than 500 adults breeding in a network of shelterbelts across an 8 X 11 km study area. Most movement occurred at relatively short distances within a shelterbelt. Movements by adults between shelterbelt sites, although rare, occurred significantly more frequently between sites connected by a wooded corridor than between unconnected sites. For robins, there were on average 2.5 dispersal events between each pair of connected sites, but only 0.17 dispersal events between each pair of unconnected sites (Mann-Whitney test, significant at p < 0.009). Because unconnected and connected sites were similar in average area (1.7 to 1.9 ha), distance to next wooded habitat, and tree-species composition, this result provides a test of the hypothesis that organisms disperse preferentially along connecting corridors.

This review describes those mechanisms by which wind directly affects crop growth rates and hence yields. Wind-induced plant movement is capable of altering growth rates and leaf morphology, although this is unlikely to be a major cause of growth differences between sheltered and unsheltered crops grown outdoors. The wind's force can tear leaves or strip them from the plant. Dense plant canopies may suffer abrasion through intermittent or constant rubbing. Soil particles lifted into suspension by the wind have the potential to abrade and damage plant tissue. The wind's force can physically knock plants over, making crops difficult to harvest. Each of these mechanisms operates at a particular time of the growing season. Recovery, and hence final yield, depends on the growth stage and soil/plant moisture status when the damage occurred, the particular species and variety as well as the preceding and subsequent weather. The fact that damage effects are so dependent on the crop and the past weather makes modelling and any simple synthesis of direct wind effects difficult. The most common forms of damage likely in Australia's agricultural regions are from sandblasting and lodging. These damage events will be intermittent - their frequency depending on the local climate. Leaf tearing is likely in broad-leafed horticultural crops, and growth effects are also likely in any windy location. It is not possible to predict what the impact of this damage, and other direct effects, will be on final yields, Based on the results in the literature, protection from damage offered by windbreaks may have as large an effect on yields as incremental microclimate benefits.[PUBLICATION ABSTRACT]

Greenhouse gases in the atmosphere, mainly carbon dioxide (CO^sub 2^), can be mitigated by the planting of trees and shrubs. Appropriate agroforestry practices in Saskatchewan include field and farmyard shelterbelts, wildlife plantations, poplar plantations and managed woodlots. A study was conducted to determine the amount of carbon held in prairie shelterbelts. The effect of the soil type and tree species on biomass and carbon content was measured in shelterbelts in the brown, dark brown and black soil zones of Saskatchewan. For some of the main shelterbelt species, the mean aboveground carbon content was 79 kg/tree (32 t/km) for green ash, 263 kg/tree (105 t/km) for poplar, 144 kg/tree (41 t/km) for white spruce and 26 t/km for caragana. In the brown and the dark brown soils, which are more arid than the black soil zone, trees had 60.6% and 65.5%, respectively, of the biomass and carbon content of trees and shrubs in the black soil zone. Improved, fast-growing poplar clones had the greatest biomass at maturity and fixed the greatest amount of carbon. Simple equations were developed to calculate the carbon contents of prairie shelterbelts, based on easily measured tree or shrub parameters. This paper will discuss the results of this particular study and the broader implications of this work.[PUBLICATION ABSTRACT]

The fact that the shelter created by windbreaks can have a significant, positive effect on crop production is supported by eight decades of research from many countries around the world. Although the concept of planting windbreaks to enhance crop production has general currency in Australia, the practice is not as wide as it could be. This review of the last decade of windbreak literature defines the research needed to encourage wider utilisation of windbreak technology. After outlining the principal mechanisms behind the effect of shelter on temperate crops, the review discusses relevant literature of the past decade especially that from Australia. The main mechanisms discussed are: the protection of crops from physical damage; soil conservation; the direct augmentation of soil moisture; and the alteration of the crop energy balance and plant water relations. Also discussed are the elusiveness of the shelter effect, competition from windbreak trees, and the modelling of windbreak systems. Suggestions for future research in Australia include: quantifying the competition of various windbreak species and the effect of root pruning on both crop and tree; a model of crop energy and water relations at the tree-crop interface; an economic model and a farmer-oriented decision support tool.[PUBLICATION ABSTRACT]

The effects of windbreaks on pastures are reviewed, with an emphasis on temperate grazing systems. Mechanisms of plant response to shelter are dealt with in brief. Few papers on measured responses of pasture species to shelter were located in a search of the global literature for the period 1972-97. Except in cold climates, where the benefits of snow-trapping on water availability can be demonstrated, there were few reports of increased production of pasture in response to shelter. A significant result was obtained in a summer rainfall environment in Australia, where a 43% increase in wool production was obtained over three years in small plots sheltered with iron sheeting on the fences. The gain was attributed to increased pasture growth. In New Zealand, one study over three years with a narrow, permeable shelterbelt in a windy, dry summer environment showed a 60% increase in pasture growth in the sheltered zone. However, another study on a high rainfall site with a dense, wide shelterbelt found no substantial shelter effect on pasture. In dry, hot and windy climates there appears to be scope for protecting spray-irrigated pasture with windbreaks. The feasibility of evaluating shelter effects on pastures or crops from old windbreaks is questioned. Variability of soil over the site can not be satisfactorily accounted for and there are problems in defining the true 'unsheltered' yield. Shelter effects on pastures could best be determined by comparing production in small completely sheltered plots and open plots. Effects in and near the competitive zone should be measured for living windbreaks. Modelling could then be used to evaluate windbreak systems. We are not yet in a position to provide unequivocal advice to farmers on windbreak outcomes for particular purposes or regions.[PUBLICATION ABSTRACT]

We use a nonhydrostatic shelterbelt boundary-layer turbulence model with Mellor-Yamada second-order closure to evaluate quantitatively the dynamic processes of surface boundary-layer flow perturbed by shelterbelts of different densities and to understand the shelter mechanism. We first analyze the drag exerted on air by shelterbelts of different densities, a root cause of any shelter function, and the resulting wind reduction. The results show that the effectiveness of a shelter is determined not only by its total drag but also by the distribution of the drag-generated momentum deficit in the sheltered area, and that medium-dense shelterbelts have the maximum shelter effect. We also analyze the horizontal momentum budget and find that the shelter mechanism is the product of several processes. The results reveal that strong vertical mean transport and the pressure gradient also play important roles in shelter efficiency. The pressure perturbation caused by the shelter extends far downstream of the shelter, and combines with advective transport to provide the larger shelter efficiency of medium-dense shelterbelts. We finally analyze the changes of perturbed pressure, turbulence, and vertical velocity with shelterbelt density to further clarify the shelter mechanism.

The paper refers to the possibilities of the evaluation of the wind erosion risks by using a model created in GIS. The model exploits the pedological information database for determining the potential risks of soils by wind erosion. The following data are the database of the agricultural land use, meteorological data and the topographic maps for determining the direction of wind and climatic conditions. Using the data transferred to the graphic form, it is possible to create the digital terrain model and to regionalise the meteorological data. Consequently, the wind barriers are localised in the landscape and it is possible to create the zone of efficiency around each barrier (protecting the land from the erosive effects of the wind) according to the characteristics of their height and density.