5. Aesthetic appearance, a visual barrier to livestock barns.At the urban rural interface aesthetic appearance of windbreaks combined with their ability to reduce odors significantly decreases the complaints of nearby residents. For example, In Southern Missouri CAFO poultry operations in forested areas receive less complaint than due swine operations in Northern Missouri in less forested regions. Few urbanites have seen or can recognize CAFO operations due to the implementation of trees as odor reduction measures and aesthetic barriers. Within the city similar applications can be applied to the landscapes surrounding odorous factories. Complicated wind flow patterns in the urban forest landscape can be modeled using the incorporation of localized, high resolution wind and temperature fields with detailed digital imagery and infrastructure information to create best management plans for odor reduction. The Physics of Tree Health Particulate Matter & Road Improvements Particulate matter can produce a wide variety of effects on the physiology of trees. Primarily, these effects relate to the phytotoxic effect of these particles. However, Kulshreshtha et al. claim that turbulent deposition of particles can damage trees to abrasive action. Leaf surfaces under stress from particulate matter deposition show increased callus tissue formation. Fortunately for both trees and humans, trees experience less ill-health from the same ambient concentration of toxic particulate matter than do humans. Unfortunately, the increasing expansions of roads into peri-urban areas increase the number of unpaved roads, which result in the formation of a particulate barrier on the leaf and bark surface which decreases the efficiency of gaseous exchange and light absorption/reflectance and decrease the ability of trees to pollinate and form new leaves. Urban managers should also be concerned about particulate matter when it comes to the management of disease. Exposure to particulate matter indirectly increases the predisposition of plants to infection by pathogens and may alter the trees genetic structure. Urban Forest Managers need to increase partnerships with road crews, city planners, and factory managers to decrease particulate matter exposure to trees. Setting trees back from dusty roads may decrease tree benefits, yet satisfy city planners afraid of tree liabilities in vehicular crashes and protect trees from overexposure to particulate matter. Managers goals need to reflect the ability of tree death as indicators of pollution extremes. Pictures of dying trees along with survey specific recommendation for road improvements may add to a campaign for gray infrastructure improvements which reflect green infrastructure health. Canopy Cover and Soil Improvements In order to achieve the standards set by American Forests and achieve UV-radiation benefits urban foresters will have to find more open tree spaces, larger tree spaces, and healthier urban soils. Urban soils, unlike forest soils, are compacted and unfriable. Gray infrastructure engineers want soil with a high level of strength, the capacity of a soil mass to withstand stresses without giving in or rupturing. Urban forest managers should test possible tree spaces for both soil strength or, conversely, root penetrability and moisture content. A reading of low soil strength and high moisture content indicate a low bulk density of soil. Roots need open loose soil to grow and absorb water and nutrients. In too small of a tree space, or in a tree space filled with top soil but surrounded by compacted soil tree roots will wrap around the tree girdling the tree. Therefore, in areas where only high bulk soil densities exist, such as the central business district of cities, urban forest managers might need to consider other 24498302308860options than importing topsoil to small tree spaces. James Urban and Nina Bassuk suggest a soil designed to provide poor spaces while at the same Figure 4time providing the same support of compacted soils. Structural soil made of crushed loam, clay loam, organic matter and a hydrogel (potassium propenoate-propenamide copolymer) stabilizing agent meets pavement design requirements while providing open pore spaces for water and air to reach roots as well as providing a penetrable medium for roots to grow CITATION Bas97 1033 (Bassuk, Grabosky, Trowbridge, & Urban, 1997). Since structured soil can be paved over it increases the space available for root growth from a four by four foot tree box to a four by twenty foot trench. Unfortunately, the same girdling affects may happen as roots move out of the loose topsoil to the structured soil. Secondly canopy cover is dependent not upon trench volume, rather, the amount of soil in the trench. Despite the costs of structured soil it provides the opportunity to increase canopy cover in areas where trees could not grow prior to the use of structural soils. Further studies in ways to increase soil content in structural soils, increase soil moisture recharge with the use of pervious surfaces, and provide long-term results are needed to give urban forestry managers enough information to pursue canopy cover improvement projects in the central business district. Tree Liability and Acoustic Benefits Noise and odor pollution reduction by trees are dependent of the use of trees as windbreaks. Windbreaks, however pose their own risks to the safety of urbanites. Dead and dying trees within the proximity of people and property creates risk of damage to both human health and property. Improvements in the science of tree stability analysis take advantage of acoustic tools to provide information about the invisible portion of trees. Trained foresters possess the knowledge to visually assess the stability of trees, but this may not always be enough. Secondly, trained foresters are in short supply in an ever increasing urban environment. Tools accessible to those without biological backgrounds will support more rapid and precise detection of decay and structural defects in trees. Figure 5Example of Ultrasonic TomographySource: CITATION Wan 4 1033 (Wang, Allison, Wang, & Ross, 2007, p. 4)Since 1993, researchers have known that, “stress-wave propagation is sensitive to the presence of degradation in wood” (Wang et. Al., 2007, p. 3) In decayed and deteriorated wood, sound waves travel more slowly than in healthy wood. Initial detection devises used two probe systems to measure the wave transmission of a single path. This method proved limiting detecting sound trees as unsound. The lack of a standard reference velocity for use on different tree stems created the issue. Today, tomography techniques have increased. Methods include electric, ultrasonic, and geo-radar; of the three methods, ultrasonic tomography has proven to be the most effective. Ultrasonic tomagraphy can detect internal decal, locate the position of anomalies, estimate their size and shapes, and describe the characteristes in terms of mechanical properties. A 2004 study of decay in white oak (Quercus alba) and hickory (Carya spp.) by Gilbert and Smiley showed a 89% accuracy of samples which showed no cracks. Wang et. Al. in 2007 set up 12 sensors around the trunk of a tree; each sensor was magnetically attacted to a pin tapped into the bark of the tree. The acoustic wave transmission data were collected by tapping each of the twelve pins. By performing the test on subsequential horizontal planes the entire trunk is mapped without intrusive measures. The results of Wang et. Al. study show that other approaches such as visual inspection and microdrilling are needed in corrolation with with acoustic tomography. Although their approach over indicated decay and could not distinguish between large internal cracks and heartwood decay, it did confirm the initial visual indication of decay. Conclusion Particulate matter deposition, UV-radiation, and acoustics and wind speed modeling are but a few examples of where physicists and other physical scientists can get involved in the study of urban forestry. From the examples mentioned the physical benefits of trees have proven to provide benefit to city infrastructure while also providing a liability. Researchers need to fill in the gaps of physical benefits. Economists can make estimates regarding the social values of protection from UV radiation and noise and odor reduction. Secondly more physicists should consider entering into the field of environmental physics to further study methods to decrease urban forest liability and to discover more physical benefits of green infrastructure. One such goal would be to identify the magnitude of particulate matter deposition on buildings with and without trees. Forestry programs will value from increasing the literature on the physical benefits of urban trees by increasing the interest of physical scientists, engineers, and city planners in the field of urban forest management. In essence, highlight the fact that trees provide a benefit to the senses which foresters can better understand through physical evaluation will increase community’s willingness to pay for trees and the political value of trees. 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