James R. Brandle

Windbreaks in North American agricultural systems

Windbreaks are a major component of successful agricultural systems throughout the world. The focus of this chapter is on temperate-zone, commercial, agricultural systems in North America, where windbreaks contribute to both producer profitability and environmental quality by increasing crop production while simultaneously reducing the level of off-farm inputs. They help control erosion and blowing snow, improve animal health and survival under winter conditions, reduce energy consumption of the farmstead unit, and enhance habitat diversity, providing refuges for predatory birds and insects. On a larger landscape scale windbreaks provide habitat for various types of wildlife and have the potential to contribute significant benefits to the carbon balance equation, easing the economic burdens associated with climate change. For a windbreak to function properly, it must be designed with the needs of the landowner in mind. The ability of a windbreak to meet a specific need is determined by its structure: both external structure, width, height, shape, and orientation as well as the internal structure; the amount and arrangement of the branches, leaves, and stems of the trees or shrubs in the windbreak. In response to windbreak structure, wind flow in the vicinity of a windbreak is altered and the microclimate in sheltered areas is changed; temperatures tend to be slightly higher and evaporation is reduced. These types of changes in microclimate can be utilized to enhance agricultural sustainability and profitability. While specific mechanisms of the shelter response remain unclear and are topics for further research, the two biggest challenges we face are: developing a better understanding of why producers are reluctant to adopt windbreak technology and defining the role of woody plants in the agricultural landscape.

Branching out: Agroforestry as a climate change mitigation and adaptation tool for agriculture

MEETING MULTIPLE DEMANDS UNDER CLIMATE CHANGE US and Canadian agricultural lands are being pressed to provide more environmental and economic services, while at the same time their capacity to provide these services under potential climate change (CC) is being questioned (Field et al. 2007; CAST 2011). Producers are already experiencing weather patterns outside of climate norms (e.g., the 2011 droughts in Texas, and flooding along the Missouri River in the United States and along the Red River in Canada) that have had significant impacts on production. Predictions of future climate conditions for the US Midwest include longer growing seasons that could potentially increase crop yields but also increase heat waves, floods, droughts, and insect and weed issues that may then adversely impact production (USGCRP 2009). Climate change drives many stressors and interacts with many nonclimatic stressors. This makes it difficult to forecast outcomes in any general way other than many existing threats to agricultural production, such as erosion and pests, which will most likely be exacerbated under shifting climate (Field et al. 2007; USGCRP 2009). Creating profitable and healthy operations under this unpredictable interplay of factors driven by shifting climate (and, along with it, shifting markets) will…

Influences of Trees on Abundance of Natural Enemies of Insect Pests: A Review

In this article we review the use of natural enemies in crop pest management and describe research needed to better meet information needs for practical applications. Endemic natural enemies (predators and parasites) offer a potential but understudied approach to controlling insect pests in agricultural systems. With the current high interest in environmental stewardship, such an approach has special appeal as a method to reduce the need for pesticides while maintaining agricultural profitability. Habitat for sustaining populations of natural enemies occurs primarily at field edges where crops and edge vegetation meet. Conservation and enhancement of natural enemies might include manipulation of plant species and plant arrangement, particularly at these edges; and consideration of optimum field sizes, number of edges, and management practices in and near edges. Blending the benefits of agricultural and forestry (windbreak) systems is one promising approach to field edge management that has additional benefits of wind protection and conservation of desirable wildlife species.

Three-dimensional aerodynamic structure of a tree shelterbelt: Definition, characterization and working models

In order to make recommendations to landowners with regard to the design and management of tree shelterbelts, it is necessary to understand and predict the wind flow patterns associated with shelterbelt structure. A structural description is a prerequisite for any prediction of wind flow. Optical porosity (percentage of open spaces on the side view of a shelterbelt) has been used as a structural descriptor of a shelterbelt; however, it is a 2-dimensional measure unable to fully represent the aerodynamic influence of a tree shelterbelt. Based on numerous studies observing the wind fields associated with shelterbelt structure, the overall aerodynamic structure of a tree shelterbelt in three dimensions is defined by its external structural characteristics (length, height, width, and crosssectional shape) and by its internal structural components (amounts and arrangements of vegetative surface area and volume, and geometric shape of individual vegetative elements). In order to associate the defined structure with wind speed, turbulent stress, and pressure, it is characterized using two structural descriptors the spatial functions of vegetative surface area density (vegetative surface area per unit canopy volume) and cubic density (vegetative volume per unit canopy volume). For field estimation, the two structural descriptors are expressed in three dimensions using two working models in terms of 1- or 2- dimensional sub-functions capable of being defined with field measurements. This paper discusses the rationale behind the definition, characterization, and working models for the 3-dimensional aerodynamic structure of a tree shelterbelt.

Texture as the basis for individual tree identification

Recognizing plants from imagery is a complex task due to their irregular nature. In this research, three tree species, Japanese yew (Taxus cuspidata Sieb. & Zucc.), Hicks yew (Taxus x media), and eastern white pine (Pinus strobus L.), were identified using their textural properties. First, the plants were separated from their backgrounds in digital images based on a combination of textural features. Textural feature values for energy, local homogeneity, and inertia were derived from the co-occurrence matrix and differed significantly between the trees and their backgrounds. Subsequently, these features were used to construct the feature space where the nearest-neighbor method was applied to discriminate trees from their backgrounds. The recognition rates for Japanese yew, Hicks yew, and eastern white pine were 87%, 93%, and 93%, respectively. The study demonstrates that the texture features selected and the methods employed satisfactorily separated the trees from their relatively complex backgrounds and effectively differentiated between the three species. This research can lead to potentially useful applications in forestry and related disciplines.

High-frequency pressure variations in the vicinity of a surface CO2 flux chamber

We report measurements of 2 Hz pressure fluctuations at and below the soil surface in the vicinity of a surface-based CO 2 flux chamber. These measurements were part of a field experiment to examine the possible role of pressure pumping due to atmospheric pressure fluctuations on measurements of surface fluxes of CO 2. Under the moderate wind speeds, warm temperatures, and dry soil conditions present at the time of our observations, the chamber had no effect on the pressure field in its near vicinity that could be detected above the level of natural pressure fluctuations in the vicinity. At frequencies at or <2 Hz, pressure fluctuations easily penetrated the soil to depths of several cm with little attenuation. We conclude that the presence of the chamber does not introduce pressure perturbations that lead to biases in measurements of surface fluxes of CO2.

Modelling the effect of shelterbelts on maize productivity under climate change: An application of the EPIC model

Abstract The potential of shelterbelts to ameliorate climate change induced crop stress, particularly in semi-arid regions such as the North American Great Plains, is examined. Specifically, the microlimate effects of shelterbelts, synthesized from empirical studies in the literature, are inserted into the Erosion-Productivity Impact Calculator (EPIC) crop model to simulate the response of dryland maize to shelter at The University of Nebraska Agricultural Research and Development Center (ARDC) near Mead, Nebraska. Though lack of extensive observed maize yield data precluded rigorous validation, the shelterbelt version of EPIC and a version simulating an unsheltered control were tested with 2 years of observed maize yield data from ARDC. EPIC underpredicted the observed ratio of shelter to open field maize yields as expected because not all benefits of shelter to crops can be incorporated into EPIC. However, the two versions correctly simulated the magnitude of difference in the shelterbelt to open field ratios between the 2 years. The two EPIC versions were then subjected to prescribed increments to temperature, and increments/decrements to precipitation and windspeed to examine differences in crop productivity between the two EPIC versions. The results show that simulated shelter increases dryland maize yields above corresponding unsheltered yields for almost all levels of climate change. The model results suggest that shelterbelts provide a night-time cooling that partially compensates the tendency of warming to shorten the growing season. They also suggest that evapotranspiration is reduced in shelter, thus reducing crop moisture stress. The positive effect of shelter on dryland maize at all levels of temperature increase is greatest for the most severe changes: maximum precipitation deficiency and greatest increase in windspeed. Despite methodological limitations, the findings suggest that shelterbelts may afford important protection from climate warming.

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.

Ecological Development and function of Shelterbelts in Temperate North America

As the world’s population continues to expand, the pressure on farmland, both from expansion of urban areas (United Nations, 2002) and from a need to produce more food and fiber (Hewitt and Smith, 1995; Gardner, 1996), will increase. In direct competition with the increasing demand for more food and fiber is a growing public desire for conservation of natural systems and a focus on quality of life issues (Matson et al., 1997; Jackson and Jackson, 2002; Pimentel et al., 2004). These two societal needs are clearly linked. Unfortunately, they are antagonistic, not complementary. The impacts of intensive agriculture, needed to increase food and fiber production, extend well beyond the field border (CAST, 1999). Similarly, many species found in natural systems, both flora and fauna, do not remain within protected reserves provided for their benefit and are impacted by land-use decisions in surrounding areas. A challenge to resource managers is to develop management strategies that support both sets of needs and lead to the “right compromise” between production agriculture, sustainability, and conservation of native floral and fauna (Mineau and McLaughlin, 1996; Swift et al., 2004). Shelterbelts and other types of linear forest systems, such as riparian buffer strips (Benton et al., 2003), can support both sets of needs and be a link between production agriculture and protection of biodiversity. These systems, both planted and naturally occurring, provide various ecosystem services (Guertin et al., 1997). While this review focuses on shelterbelts, many of the principles discussed apply to other linear forest systems.

Estimation of the three-dimensional aerodynamic structure of a green ash shelterbelt

The three-dimensional aerodynamic structure of a tree shelterbelt is described by two structural descriptors: vegetative surface area density (vegetative surface area per unit canopy volume) and cubic density (vegetative volume per unit canopy volume). Based on destructive measurements, estimates of both descriptors for a two-row 31-year-old green ash (Fraxinus pennsylvanica Marsh.) shelterbelt are developed. In order to estimate the structural descriptors in three dimensions based on data measured in two dimensions, equations to predict vegetative surface area and volume, their marginal distribution with height, and their marginal distribution across width at a given height are derived for each tree component: trunk, branches, leaves and seeds. The procedure to use these equations to estimate the structure of the green ash shelterbelt either in three dimensions or in the width and height dimensions is demonstrated. The estimated structure can be used to test current models of turbulence through a tree shelterbelt under a field conditions, and to simulate the wind fields as influenced by the actual structure of a tree shelterbelt. The developed equation can be used to generate the three-dimensional structure of a shelterbelt with a design similar to the sampled shelterbelts or a shelterbelt of green ash mixed with other species similar to those in the sampled shelterbelts. Thus, the boundary-layer flows as influenced by the overall structure of tree shelterbelts with different designs can be numerically simulated. These simulated results can provide guidance for shelterbelt design.