Influence of thinning on the shelter effect of windbreaks as clarified by a wind tunnel experiment

We measured the shelter effect of model windbreaks in a wind tunnel before and after adjustment thinning based on methods for managing overcrowded windbreaks. One of the aims of this experiment was to determine whether or not the actual thinning done on an actual windbreak would sustain the shelter effect. The shelter effects were evaluated based on the critical wind speed and the shelter distance. Six model windbreaks were set to model a windbreak after the thinning (AT1–6) and one was set to model it before the thinning (BT). In models AT1–3, the trees in the middle part of the windbreak were cut and the trees in both the windward and leeward parts were left. AT1 and AT2 were models that reproduced the actual windbreak after the thinning. In the other three models (AT4–6), which were set as additional thinning models based on AT3, trees in both the windward and leeward parts were cut. The thinning ratios of these models ranged from 21% (AT1) to 84% (AT6). The shelter effects of AT1–3 were sustained or only slightly decreased compared with that of BT. The shelter effects of AT4 (thinning ratio of 63%) and AT5 (79%) were the highest among all of the models. From these results, it was confirmed that the thinning done on an actual windbreak was able to sustain the windbreak’s shelter effect. If the trees at the edges are not felled, the shelter effect of the windbreak can be sustained or improved, even if the thinning is heavier than AT2.     

The relationship between open windspeed and windspeed reduction in shelter

Windspeed reduction in shelter is generally expressed relatively and is usually assumed to be independent of open windspeed. The purpose of this study was to re-examine the relationship between open windspeed and windspeed reduction in shelter using windspeeds and wind directions measured for two windbreak systems at various distances from the windbreaks and with three windbreak porosities. Optical windbreak porosities were estimated from digitized color photographs using an intelligent (trainable) image processing program. Relative windspeed reduction in shelter was found to be related to the open windspeed. There existed a threshold windspeed at approximately 5 m s^–1, below which relative windspeed reduction varied but generally decreased as the open windspeed increased. Above the threshold windspeed, open windspeed did not have an effect on relative windspeed reduction in the sheltered zone. Using the threshold windspeed reduction, the authors propose an index to evaluate the effectiveness of a windbreak for the overal windspeed protection.Published as Journal Series No. 10815 of the Agricultural Research Division, University of Nebraska.     

Effect of shelter on temperate crops: a review to define research for Australian conditions

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. This revised version was published online in June 2006 with corrections to the Cover Date.     

Temperature and water-use efficiency by lucerne (Medicago sativa) sheltered by a tree windbreak in Tunisia

Based on the assumption that with unrestricted water availability, temperature will determine the response of sheltered crops, the effects of a Casuarina glauca Sieb. windbreak on the microclimate, water use and biological production of a lucerne crop were investigated. Degree-days (dd) were used to compute an index for the efficiency of the thermal effects of the shelter on a well- watered lucerne, under favourable (10 to 30 °C) and unfavourable (above 30 °C) temperature conditions. Water use efficiency was considered for two contrasting water regimes, with no water stress or with a large water deficit. In June, under favourable temperature and water availability conditions, temperature efficiency of the windbreak was 39 g m−2 dd−1 at a distance of three times the height of the trees compared to an exposed situation, and water use efficiency increased by 7.1 g mm−1. However, the sheltered conditions induced a decrease in above ground biomass production and water use efficiency when a highly restrictive water regime was applied. In July, daytime temperatures were disadvantageous to the lucerne, and even with a high degree of water availability, there were no significant differences between sheltered and unsheltered conditions. This research may help decision makers to manage windbreak systems by designing an appropriate irrigated area according to the shelter and choosing crops that will best benefit from sheltered conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

农田防护林主要间距离的确定——基于林带结构与风速降低的关系

Relative windspeed reduction was measured behind nine relatively narrow, homogenous tree windbreaks with porosities between 0.13–0.33, and behind 28 combinations of model stubble barriers representing 25 different optical porosities (0.00–0.80). The optimum porosities observed were 0.25 and 0.13 for tree windbreaks and stubble barriers respectively. Based on the relationship between windbreak structure (optical porosity) and wind reduction, the chief indices for determining spacing interval, i.e., the windbreak structure index (δ) and the parameter of microclimate, represented by the problem wind (L _rp ), were determined. Additionally, investigations on shelterbelt trees were carried out, and stem-analysis techniques were used, to develop a method for determining the mature height of tree windbreaks (H ₀). Optimal spacing intervals between windbreaks could be predicted from the indices of a given windbreak structure, percentage of reduction of windspeed desired and tree growth model. A hypothetical example for determining the spacing interval of principal poplar windbreaks is given at the end of this paper. The results can be applied not only to tree windbreak design but also to other plant materials and artificial barriers for wind protection. Foundation item: This study was supported by Innovation Research Project of Chinese Academy of Sciences Biography: ZHU Jiao-jun (1965-), male, Ph. Doctor, PhD advisor. Professor of Institute of Applied Ecology, the Chinese Academy of Sciences, China, Scholar researcher of Faculty of Agriculture, Niigata University, Japan. Responsible editor: Song Funan     

Optical porosity and windspeed reduction by coniferous windbreaks in Southern Ontario

Relative windspeed reduction was measured behind nine relatively narrow, homogeneous windbreaks in southern Ontario, Canada to assess whether any characteristics of the windspeed reduction curve could be predicted from optical porosity. The latter was determined for each windbreak using high contrast black and white photographic silhouettes on a computer digitizing system. Minimum windspeeds behind the windbreaks ranged from 29 to 71% of open windspeed; these minima were located 2 to 6 multiples of windbreak height away from the windbreak. Optical porosities of the bottom half of the windbreak ranged from 0 to 31%. Multiple regression of the shelter parameters (location and value of minimum relative windspeed) on the independent variables (optical porosity, open windspeed, surface roughness, approaching wind direction relative to the windbreak, average tree diameter and average tree spacing) showed that the minimum relative windspeed could be predicted from the optical porosity of the bottom half of the windbreak. The results suggest that optical porosity can be used to predict minimum relative windspeeds and may therefore be useful as a guide in the field evaluation of windbreaks.     

农田防护林主带间距离的确定——基于林带结构与风速降低的关系(英文)

本文通过对9条分布均匀、相对较窄的不同疏透度(透光疏透度,下同)(0.13～0.33)的树木林带和不同疏透度(0.00～0.80)风障组合的野外风速观测,确定了树木林带和风障的最适疏透度分别为0.25和0.13。基于林带结构(疏透度)与风速降低的关系,确定了林带主带间距离的主要参数,即,林带结构系数(δ)和以主害风为代表的小气候参数(Lrp)。另外,通过对林带树木的野外调查,应用树木解析技术确定林带成林高(H0)。因此,树木林带的主带间距可以通过林带结构系数、希望降低风速的比例和树木生长模型来确定。本文以杨树林带为例,具体确定了杨树林带的主带间距。该研究结果不仅适于树木林带的设计,同时适于其它生物材料或人工风障的设计。图4表5参40。     

Spacing interval between principal tree windbreaks--Based on the relationship between windbreak structure and wind reduction

Relative windspeed reduction was measured behind nine relatively narrow, homogenous tree windbreaks with porosities between 0.13-0.33, and behind 28 combinations of model stubble barriers representing 25 different optical porosities (0.00-0.80). The optimum porosities observed were 0.25 and 0.13 for tree windbreaks and stubble barriers respectively. Based on the relationship between windbreak structure (optical porosity) and wind reduction, the chief indices for determining spacing interval, i.e., the windbreak structure index (δ) and the parameter of microclimate, represented by the problem wind (Lrp), were determined. Additionally, investigations on shelterbelt trees were carried out, and stem-analysis techniques were used, to develop a method for determining the mature height of tree windbreaks (H0). Optimal spacing intervals between windbreaks could be predicted from the indices of a given windbreak structure, percentage of reduction of windspeed desired and tree growth model. A hypothetical example for determining the spacing interval of principal poplar windbreaks is given at the end of this paper. The results can be applied not only to tree windbreak design but also to other plant materials and artificial barriers for wind protection.     

Windbreaks in southern Patagonia,Argentina: A review of research on growth models, windspeed reduction, and effects oncrops

In Patagonia, where strong winds are a constraint to agricultural production, live windbreaks are often planted in agricultural fields to protect crops, livestock, and soils from wind hazards. The major factors that determine the efficacy of windbreaks are height, porosity, orientation, length, and location in the landscape. A review of the research on the effect of live windbreaks during 1993 through 2000 is presented in the paper. Porosity and distance from windbreak were found to have major effects on relative windspeed reduction. The greatest degree of protection was for dense windbreaks (windspeed reduction of 85%) at 1H (1H = a distance of one tree height, leeward of the windbreak). Different crops showed a differential yield response to wind stress. The production of garlic (Allium sativum) was not significantly affected by wind. Tulip (Tulipa sp.) bulb yield decreased on average by 25% between 2H and 17H. The production of lucerne (Medicago sativa) at 1H was 40% higher than lucerne grown in open conditions. In contrast, strawberry (Fragaria sp.)and cherry (Prunus avium) were more sensitive to the effect of the wind. Dendrometric models (diameter, site index, volume and crown dynamics) were developed to define the wood potential productivity that could be expected from different site qualities. The prediction of height growth as a function of age, allows the estimation of the area protected by the windbreak. An economic assessment based on realistic estimates of shelter benefits is required to encourage the development of windbreak systems that could also offer other benefits, including erosion control and timber production.This revised version was published online in November 2005 with corrections to the Cover Date.     

Microclimate patterns on the leeside of single-row tree windbreaks during different weather conditions in Florida farms: implications for improved crop production

Florida citrus and vegetable crops generate billions of dollars in revenue every year. However, wind, freezing temperatures, hurricanes, and diseases negatively impact production. Windbreaks located perpendicular to the prevailing wind can increase farm production simply by reducing wind and modifying microclimate. Windbreaks can also help in managing pathogens such as citrus canker (Xanthomonas campestris pv. citri). To study the modification of wind speed, temperature, and relative humidity on the leeside of single-row tree windbreaks in southern Florida, automated weather stations were installed in 2007/2008 at 2 m above the ground along transects perpendicular to a eastern redcedar (Juniperus virginiana) and three cadaghi (Corymbia torelliana) (WB1–WB3) windbreaks. All windbreaks reduced wind speed, with minimum wind speed (~5% of the open wind speed) at two times the distance of windbreak height (2H, where H = windbreak height in m) on the leeside of a E. redcedar (~17% porosity) and at 4H (~3–30% of the open wind speed) and 6H (<50% of the open wind speed) on the leeside of cadaghi windbreaks WB1 (~22% porosity) and WB2 (~36% porosity), respectively, when the wind direction was nearly perpendicular to the windbreaks. Wind speed reduction was observed up to 31 times the windbreak height (31H). Cadaghi windbreaks reduced wind speed on the leeside even during a tropical storm event. Temperatures on the leeside of the windbreaks were warmer during the day and cooler near the windbreaks at night compared to temperature in the open fields. This study demonstrates that single-row tree windbreaks can reduce wind and modify the microclimate to enhance crop production for Florida growers.     

Effects of windbreaks on airflow, microclimates and crop yields

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. This revised version was published online in June 2006 with corrections to the Cover Date.     

Protection efficiency assessment and quality of coastal shelterbelt for Dongshan Island at the coastal section scale

The coastal shelter forest in China is under threat of destruction and degradation because of the impact of human activities. Protection efficiency assessment of the coastal shelterbelt is an important component of shelter-forest remediation planning and sustainable management. In this study, a protection efficiency index (PEI) model was established using the projection pursuit method to assess the protective quality of the coastal shelter forest at the coastal section scale of Dongshan Island, China. Three criteria were used, including forest stand structure, forest belt structure, and windbreak effect; each criterion further comprised multiple factors. Based on survey data of 31 plots in the coastal shelter forest of Dongshan Island, we calculated PEI values using a projection of a pursuit model. The result showed 64.5 % of the PEIs fell at or below the middle level, which can indicate the status of the coastal shelterbelt is unsatisfactory. To further explore whether the different bays and land use types create significant differences in PEIs and evaluation indices, we used an ANOVA to test the influence of various bays and forms of land use on coastal shelterbelts. The results showed that PEI and most of the indices differed significantly by bay; mean tree height, mean DBH, mean crown width, stand density, vegetation coverage, and wind velocity reduction differed significantly by land use. Therefore, relevant measures for different locations, bays and surrounding land use can be proposed to improve the existing conditions of the coastal shelterbelt. The results of this study provide a theoretical and technical framework for future changes and sustainable management of coastal shelterbelt on Dongshan Island.     

The role of shelter in Australia for protecting soils,plants and livestock

The purpose of this review is to examine the current knowledge of the role of trees in providing shelter for pastures, crops, and livestock, for controlling erosion of soils and improving productivity and sustainability of agricultural production in Australia — and the extent to which this knowledge has been applied.Land degradation — tree loss and associated soil salinity, water and wind erosion, soil acidification, soil structural decline and nutrient degradation — is evidence that our primary production systems are not sustainable. We have sought increased production without proper consideration of the ecological context of that system. About half of Victoria's crop and pasture lands are affected or at risk, and in Western Australia about 25% of the cleared agricultural land is wind-eroded and 60% is potentially susceptible, salinity affects 0.43 m ha and half of the divertible surface water is affected by salinity. Similar problems occur in other States. At least 43 m ha or 13% of our rangelands are seriously degraded by wind erosion caused by overgrazing, often coinciding with drought or a run of drier years.Minimum tillage and stubble management for erosion control in cropping has been a major extension and research activity in Australian agriculture. Severe weather, combined with imperfect adoption of appropriate grazing and crop management systems, shows the weakness of complete reliance on these methods of erosion control. An effective system must accommodate the impact of extreme events, which are the most damaging. However, the complementary use of windbreaks to reduce soil erosion is rare, and their establishment has not been promoted, despite the wide-spread adoption of this technology by other countries.In the cropping and higher rainfall grazing areas, the systematic planting of 10% of the land in a net of shelterbelts/timberbelts/clusters could achieve a 50% windspeed reduction; this would substantially improve livestock and pasture production in the short and long-term. Wind erosion could be dramatically reduced and crop production probably increased by the use of windbreaks. Wheat and oat yield at Rutherglen (Victoria), and lupin yield at Esperance (Western Australia), were increased in the sheltered zone by 22% and 47%, and 30%, respectively.In semi-arid and dry temperate areas, planting of 5% of the land to shelter could reduce windspeed by 30–50% and soil loss by up to 80%. This planting would also contribute substantially to achieving other objectives of sustainable agriculture. Agroforestry — particularly timberbelts applications — will be important in the long-term strategy for achieving revegetation. If some of the trees yield a marketable product then the adoption of the system will be more readily achieved.In the arid (pastoral) areas there is an urgent need to promote the ethic that preservation and improvement of the perennial grass and shrub vegetation is critical for the protection of the soil and maintenance of land capability. Control of animal grazing remains the sole means of preventing erosion in much of this zone. While satellite imagery allows us to assess the condition of leasehold lands, we have failed to achieve stocking policies that will halt the degradation of our rangelands.     

Effect of windbreak on the yield of cotton crop in semiarid regions of Haryana

The beneficial effects of shelterbelts and windbreaks have been convincingly demonstrated in the temperate regions of the world. Investigations conducted on the biological effects of windbreaks are relatively few in arid and semiarid zones. In the semiarid regions of Haryana under Social Forestry programmes large scale plantations along roads, canals, fields, etc. were undertaken. In this region, an area having a windbreak of Dalbergia sissoo (18 years old) was selected and cotton was raised in the agricultural fields. The tree belt is able to reduce the windspeed by 15 to 45% depending upon season and wind speed. Observations on morphological characters and crop yield revealed that the belt height had a significant effect on plant growth. In general growth and cotton productivity increased upto distance of four times the tree height to the leeward of belt. Depending upon the orientation of tree belt, an increase in cotton yield was found to be 4 to 10%. Plant growth (in terms of leaf area, leaf number, plant height) and yield (seed cotton production) were observed to be high in sheltered area as compared to open fields. The results indicate that windbreak establishment should be adopted and integrated to a whole farming system, in semiarid regions.     

A model to evaluate windbreak protection efficiency

The effectiveness of windbreaks in windspeed reduction is often evaluated without regard to the objects to be protected. In fact, many objects may have different sensitivities to wind and often require different degrees of wind protection. Since commonly used indexes do not consider the sensitivity to wind, a concept of specific protection efficiency is developed. A critical windspeed is used to represent the sensitivity of each object. Windspeeds greater than this value are considered damaging. A dimensionless protection index is defined to evaluate windbreak efficiency. The maximum index value is 1 for the highest protection, and the index is negative when sheltered windspeed is greater than the critical windspeed. This index can be compared, summed, and averaged across different windbreaks, objects, and leeward locations. A sample of critical windspeed values was compiled from the literature. The index was evaluated using actual wind data measured under both sheltered and open conditions. The results indicated that the index can be used for evaluating windbreak effectiveness in terms of objects protected under various conditions. This model could be used as a tool for windbreak-related research and policy making.     

Alteration of soil water content consequent to root-pruning at a windbreak/crop interface in Nebraska, USA

Root-pruning is generally recommended as an appropriate treatment to reduce competition for soil water and/or nutrients and suppression of crop yield in areas adjacent to windbreaks. Several recent studies suggest, however, that factors other than soil water might be causing yield reduction at the interface. For two consecutive years, we evaluated root-pruning effects on soil water at the windbreak/crop interface under both cropped (soybean [Glycine max (L) Merr.] variety Iroquois, 1997) and non-cropped (1998) conditions in Mead, Nebraska, USA. Volumetric soil water content near the windbreaks was systematically measured at various soil depths, distances from the windbreak, and windbreak exposures using Time Domain Reflectometry (TDR). Overall differences in soil water content between root-pruned and non-pruned plots in soybean were smaller in magnitude at all distances in both the west (windbreak on the east side) and the east (windbreak on the west side) exposures in 1997, compared with the non-cropped condition in the south exposure in 1998. With a soybean crop in 1997, volumetric soil water content in the east exposure averaged 2.3% greater in the top 30 cm of the soil profile at a distance of 0.75H (H = windbreak height) into the field from the windbreak when compared to the non-pruned treatment. In the west exposure, however, the differences were undetectable at corresponding distance and depth. The increase in soybean yield in root-pruned plots corresponded well with the observed differences in soil water content at various distances, especially in the east exposure. Under a non-cropped condition in 1998, soil water content in the root-pruned plots was significantly greater than the non-pruned plots in the top 45-cm profile, averaging 3.3% at 0.75H and 2.2% at 1.0H. Beyond 1.0H, the increase was not significant. These results agree with the previously reported range of crop yield suppression near windbreaks, indicating that soil water competition between the crop and windbreak is highly related to, and probably plays a leading role in yield suppression within the competition zone.This revised version was published online in November 2005 with corrections to the Cover Date.     

An assessment of windbreaks in Central Wisconsin

Soil erosion by wind is a persistent problem in central Wisconsin. The extent and effectiveness of windbreaks in erosion-prone areas are unknown. We investigated the potential effectiveness of windbreaks established for soil erosion control in Portage County, Wisconsin. Our objectives included quantification of their extent and condition, development of a routine method for field assessment, and compilation of a reference database of windbreak information. We used aerial photographs to identify the windbreak population, and a two-stage, stratified random sampling technique to obtain samples for field evaluation. Variables and attributes examined included species, number of gaps, height, width, porosity, spacing, live crown ratio, crown condition, and a condition rating. Methods included simple photo-interpretation techniques, field measurements, optical scanning techniques, and data manipulation in geographic information systems. We identified over 2600 windbreaks comprising a total extent of 834 km; only a small proportion of this may offer effective protection at critical periods. Collectively, field windbreaks protect a very small proportion only of the county‘s agricultural land area from wind erosion. These results imply a need for increased planting rates, explicit maintenance or renovation of existing barriers, and windbreak designs that are both sustainable and compatible with current agricultural production efforts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of windbreak orientation,shade and rainfall interception on wheat and lupin growth in the absence of below-ground competition

In the temperate cropping regions of Australia, the benefits of shelter from windbreaks are often offset by tree-crop competition. The aim of this trial was to quantify microclimate and crop growth close to shade-cloth windbreaks with various orientations, to determine the effect of shelter and above-ground competition on the growth and productivity of wheat (Triticum aestivum) and lupins (Lupinus angustifolius) in the absence of below-ground competition. The trial was conducted in southwestern Australia in 2000 and 2001. The windbreaks modified windspeed, temperature, light and rainfall both spatially and temporally. Consequently, there were spatial differences in the phenology, morphology and productivity of the crops and the incidence of fungal disease adjacent to the windbreaks. Mean grain yield within three times the height of the windbreaks (H) was 97, 102, 82, and 98% of yield beyond 3 H on the northern, eastern, southern and western aspects respectively for wheat and 99, 87, 90, and 87% respectively for lupins. Changes in windspeed and rainfall were not significantly correlated with the relative yield of either crop. However, photosynthetically active radiation was significantly correlated with yield for wheat but not for lupins. Given these responses, and the practical difficulties of reducing below-ground tree-crop competition to the point where light is more limiting for crop growth than water, it is recommended that windbreaks continue to be oriented primarily to protect against damaging winds, rather than to minimise shading in the medium and low rainfall areas of Australia.     

Estimates of additional Maize (Zea mays) yields required to offset costs of tree-windbreaks in Midwestern USA

Field windbreaks can increase crop yield within a protected zone. However, they also take land out of crop production and compete with adjacent crops. Although the beneficial aspects are generally recognized, the question arises whether the windbreak will increase crop revenue enough to offset costs over time. Achieving additional yields to offset windbreak costs might be a sufficient incentive for a producer to plant a windbreak. Additional maize (Zea mays) yields necessary to break even with costs are calculated for four typical Midwestern USA field windbreaks: poplar (Populus spp.), mixed tree/shrubs (Populus spp., Acer saccharinum L./Physocarpus spp., Viburnum spp., Cornus spp.), and two and four-row spruce (Picea spp.) windbreaks. Five lifespans, two management and two cost scenarios, and three protected zone widths to account for changing sheltering effects are evaluated. Greatest additional yields are for a 4-row spruce windbreak with intensive management at high cost and a 10-year lifespan: 15.38 Mg ha^–1 yr^–1 within 6H, 7.69 Mg ha^–1 yr^–1 within 12H and 6.15 Mg ha^–1 yr^–1 within 15H. If a 50-year lifespan is implemented, the additional yields are about 11% of those in 10-year lifespan. Smallest additional yields are for a mixed tree/shrubs windbreak with extensive management at low cost and a 50-year lifespan: 0.56 Mg ha^–1 yr^–1, 0.28 Mg ha^–1 yr^–1 and 0.22 Mg ha^–1 yr^–1, respectively. The mixed windbreak is likely to have actual maize yield increases comparable to the added maize yields required to break even as long as the lifespan is 30 years or longer with a minimum protected zone of 12H. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impact of field windbreaks on visual appearance of agricultural lands

A sample of 1,500 farmers and 2,000 non-farmers were surveyed during 2005–2006 to examine the impact of field windbreaks on visual appearance of agricultural lands in Iowa, USA. About 73% of respondents indicated that field windbreaks diversified visual appearance of agricultural landscapes, whereas 67% of these respondents thought that windbreaks made agricultural lands visually more appealing. An improved aesthetics of the area was fourth most frequently mentioned reason for planting field windbreaks reported by 21% of respondents. When compared to other windbreak benefits, however, improved aesthetics of agricultural landscapes was ranked as the least important windbreak benefit. Majority of respondents (50%) preferred groups of trees planted in straight rows followed by groups of trees nested between agricultural fields (23%), groups of trees planted in non-straight rows (20%), and single trees dispersed between fields (7%). Respondents preferred windbreaks that were tall, wide, long, and continuous. Most of them (74%) preferred field windbreaks consisting of trees and shrubs, whereas 69% preferred windbreaks in which conifers were mixed with hardwoods. Respondents who preferred groups of trees planted in non-straight rows were 5.0 times more likely to indicate that windbreaks improved visual appearance of agricultural lands than those who preferred single trees. Those who preferred groups of trees nested between agricultural fields or groups of trees planted in straight rows were 3.6 and 2.6 times more likely, respectively, to indicate that windbreaks improved visual appearance of agricultural lands in comparison to those who preferred single trees.