Physics of the Urban Forest<br />Understanding and Developing the Physical Benefits of Green Infrastructure<br />by <br />Courtney Bonney <br />MSES/MPA December 2009 Candidate <br />for<br />Dr. Burnell Fischer<br />Urban Forest Management<br />Indiana University – Bloomington<br />James Urban, a Fellow of the American Society of Landscape Architects, claims the physics of a human being are actually very close to the physics of a tree. He recognizes that urban forest managers, whether they are landscape architects or arborists need to understand the science behind the trees which benefit them and the science behind the benefits themselves. When a city engineer describes the physical framework of a community, its pipes, roads, buildings, utilities and other man-made objects are the first objects which come to mind. Conversely, trees and greenery are too often the only objects of interest to arborists and horticulturalists. In the past city planners, architects, and engineers have overlooked the physical nature of trees and the urban forest’s structural component in the urban landscape. Trees appeared to be “individuals in streets, parks and gardens, or as components of woodlands as ‘relics’ surviving from the forest before urbanization,” rather than an integral part of the city infrastructure CITATION Buc06 p 1 l 1033 (Bucur, 2006, p. 1). In the mid-1990s a revolution began in the understanding of trees as urban structures with the introduction of the term green infrastructure to the developing urban forestry vernacular.<br />The city infrastructure can be divided into green infrastructure and the traditional gray infrastructure. Green infrastructure may be best represented by the urban forest and the individual trees which comprise it represent the structural benefits of green infrastructure. Like gray infrastructure, green infrastructure is constructed by man to provide benefit to the human population. For instance, Trees provide protection from ultraviolet radiation and particulate matter and reduce both noise pollution and urban odor independent of the intrinsic worth of urban trees. In order to promote green infrastructure development urban forest managers—whether landscape architects or arborists— need to understand the science used to describe the benefits provided by trees and the science used the in the management of the trees which benefit the city. <br />The Physics of Tree Benefits<br />Particulate Matter<br />Although trees provide many biological benefits, community trees also provide benefit simply due to their structural integrity: A focus on the physical properties rather than biological processes may increase involvement of engineers, city planners, and architects in urban forestry programs.<br />Particulate matter regulated as PM10 by the National Ambient Air Quality Standards is becoming a real and serious problem in the urban environment. Typically researchers in urban forestry have focused on the ability of trees to absorb other air pollutants regulated by NAAQs, NOx, CO, SO2, etc. Particulate matter although listed with the other pollutants is a fundamentally different air pollutant. SO2 and NOX are removed from the atmosphere more quickly than PM10. PM10 not only hurts air quality it also can be damaging to the city infrastructure itself. <br /> Emissions particular to urban areas motor emissions from motor vehicles and road dust comprise over fifty percent of the ambient particle load. As would be expected, monitoring devices in the UK show that concentrations of PM10 are lower than at urban locations CITATION Bec98 l 1033 (Beckett, Freer-Smith, & Taylor, 1998). Although scientists cannot prove increased exposure to air borne particulate matter causes asthma, studies show increased asthma related morbidity and mortality. Health effects of particulate matter are not limited to cardiovascular and respiratory health issues; studies have shown that infants exposed to large concentrations of particulate matter have smaller birth weights and head sizes and that new studies are needed to research the health effects of smaller particles, PM2.5, this finer fraction of the total ambient PM exacerbates illness and is "
capable of being inhaled deeper in to the lungs causing alveolar inflammation"
according the UK department of Health CITATION Bec98 l 1033 (Beckett, Freer-Smith, & Taylor, 1998). Besides health considerations, air-borne particulate matter decreases visibility. Particles ranging in size from .1 to 1 micrometers scatter light because their diameters are close to the wavelength of visible light (0.34-.72 micrometers) CITATION Bec98 l 1033 (Beckett, Freer-Smith, & Taylor, 1998).<br /> Current research on particulate matter abatement by trees is being conducted in Britain, home to the smog of the early 1950s which first raised the issue of modern day particulate matter pollution. These incidents created a need for congress to pass the Clean Air Act of 1956. Afterward, In the US and abroad particulate matter was regulated as PM10, because the majority of all particulate matter in the atmosphere is represented by PM10; PM10 refers to particulate matter having an aerodynamic diameter of less than 10 micrograms and is measured in micrograms per cubic meter of air. Particles can be removed from the atmosphere in a multitude of ways: sedimentation, diffusion, turbulence, washout, and occult deposition to name a few.<br />Trees capture particles by impaction processes. Impaction increases if the particles in the airstream come in contact with moist, rough, or electrically charged surfaces. Although these particles may remain on the tree surface for some time, McPherson estimates that up to fifty percent of deposited particles are re-suspended. Secondarily, although trees provide a rough, moist environment they do not significantly contribute to impaction due to electrostatic and radiometric forces.<br />Trees produce their own particulate matter in the form of pollen which may also have severe health effects, but even under high pollen counts the particle mass remains one fifth of the daily average ambient quality standards of 50 micrograms per cubic meter CITATION Bec98 l 1033 (Beckett, Freer-Smith, & Taylor, 1998).<br />Overall trees are a sink, not a source of particulate matter pollution. According the Manning and Feder (1980) forest canopies are the most effective vegetation type for capturing particles. The greater surface roughness of tree canopies provide opportunities for increased turbulent air flow resulting in "
turbulent atmospheric mixing, the removal of a significant surface boundary layer resistance and thus the efficient deposition of pollutants.<br />Ultraviolet Radiation<br />UV radiation in the urban environment is a topic often left unaddressed. UV radiation is both beneficial and detrimental depending upon the amount and duration of exposure. UV radiation causes twenty percent of cataract related blindness and has been linked to both skin cancer and suppression of the immune system response CITATION Wor02 l 1033 (World Health Organization, 2002). According to Skin Cancer Foundation Statistics one in every five Americans will develop skin cancer in their lifetime and the number of cancers caused by UV radiation is expected to rise. As ozone levels are depleted, the atmosphere is less able to filter out UV radiation, thus, increasing the amount of UV exposure. While skin cancer risk is on the rise, the risk of cataracts is decreasing. According to the World Health Organization an estimated 3.2 million people go blind each year from cataracts. However, the majority of those who have cataracts do not live in cities. Shade from city infrastructure provides protection from UV rays decreasing the incidence of cataracts. City trees add to UV-protecting city infrastructure; green infrastructure, like gray infrastructure, will decrease additional cases of cataracts while creating relatively safer recreational Figure 1: American Forest Goalsareas for those predisposed to skin cancer. <br />Urban Forestry managers look to Urban Forest Canopy Cover as a surveying tool to American Forest Urban Forest Canopy Cover GoalsFor metropolitan areas east of the Mississippi and in the Pacific Northwest:Average tree cover counting all zones40%Suburban residential zones50%Urban residential zones25%Central business districts15%For metropolitan areas in the Southwest and dry West:Average tree cover counting all zones25%Suburban residential zones35%Urban residential zones18%Central business districts9%Source:http://www.americanforests.org/resources/urbanforestsdescribe the quality and quantity of the urban forest. American Forests after doing an Urban Ecosystem Analysis of 40 metropolitan areas for 10 years suggested setting an average urban canopy goal of forty percent east of the Mississippi and in the Pacific Northwest and a twenty-five percent goal in the drier regions of the Southwest and West (Table #). American Forests sets its goals based on the observed canopy cover deficit over those ten years as well as the calculated benefits from trees such as improved air quality. Although, Canopy cover, like other shade producing infrastructure, decreases exposure to solar radiation, American Forests does not include benefits from UV-B protection. These goals, however, still reflect an optimal level for the benefits of UV-B protection. <br />Figure 2: The Decrease in Canopy Cover EffectivenessProtection from UV-radiation by trees as measured by the Ultraviolet Protection Factor (UPF), synonymous with SPF, ranges from 0 to 10 UPF. Purdue University and USDA researchers claim, based on models of the relative irradiance above and below canopy cover, that the typical urban forest provides a protection factor of 2 to pedestrians. To obtain a protection of 10 or greater SPF, pedestrians can stand either directly under a tree’s crown or find tree cover like that in a forest (>90% tree cover).<br /> Neither advocating for full tree cover in urban areas nor directing pedestrians to spend their day standing under a tree supports the message of urban forest managers. Fortunately, supporting the American Forests goal of forty percent supports a cost efficient level of UV-Radiation protection. Unsurprisingly, UV-B radiation decreases as canopy cover increases, but the amount of protection increases with a diminishing rate of return. Thus, it makes more sense to focus tree planting efforts to increase tree canopy cover from fifteen to thirty percent in many urban areas rather than to increase canopy cover past the forty percent recommendation set by American Forests in a few urban areas. <br />Forest managers should target areas of high risk to exposure. The accumulative effect of a lifetime of UV radiation exposure reflects a large number of areas and exposure scenarios. Studies which look at exposure to school children show that students in primary school are more exposed to UV radiation than are secondary school students. As would be expected, elementary and middle school students spend a larger amounts of time outdoors than do high school students. Goals set for elementary school playgrounds should be higher than for secondary schools to reflect the higher need for shade protection. Similarly, future studies are needed to better formulate UV-radiation target areas based on exposure risk. Foresters can increase protection by through species selection and management practices. Australian researchers using UVR sensitive polysulphone (PS) badges rather than UV meters have shown significant differences in UV-radiation by tree species. Management practices such as increased planting density and planting/pruning for optimal height, width, and shape of canopy have proven to have a more significant effect than individual tree species performance. Further studies reflecting more species, latitudes, and management practices are needed to model the benefits of UV-radiation protection in urban areas. UV meters may, if proven reliable, be a simple, inexpensive way for local entities to measure species UV-radiation protection differences verses management practice differences. <br />Noise Pollution<br /> Noise or unwanted sound that causes either physiological or psychological effects on people is an increasing issue in urban areas. Noise caused by factories, construction, and traffic congestion increases as the number of residents increases. School children exposed to noise pollution have more difficulty learning. Noise is both a distraction and can also cause language problems. A 1984 study performed by Moch-Sibony indicated that children in noisy school settings, specifically a school near a Paris Airport, lose the ability to discrimination between similar sounding words than children in sound-attenuated schools. Hearing loss, noise induced threshold shifts (inability to hear below a certain sound pressure level), and tinnitus are all possible physical effects of exposure to high intensity sounds over long duration, but are not the significant concern of noise pollution. Citizens expect their ability to hear to decrease over time, what they do not expect are the psychological effects of noise pollution. Perceived noisiness rather than loudness may be a better predictor of adverse reactions to sound. Yet, “noisiness” does not a unique meaning, therefore, an understanding of noise pollution ought to begin with a study of the physical principles of sound and move from physical principles to perceived notions of noisiness.<br />Characteristics of noise include sound pressure level (measured in decibels), sound frequency (measured in Hertz), type of sound, and variation in time. Sound pressure levels range from 20 dB in quiet rural settings to 120 dB near jet-aircraft at takeoff. In cities, 50 to 90 dB is a typical range of sound pressure levels. High and low frequencies affect people differently. Humans are not sensitive to frequencies 1,000 to 4,000 Hz and can hear a range of 20 to 20,000 Hz. Community annoyance is often determine most by type of sound, recognition of sounds, and the impulsiveness of sound fluctuations rather than the sheer force of the sound.<br />For every 10 dB increase in physical intensity the perceived loudness is twice<br />Woodlands reduce noise pollution by scattering and absorbing sound. The influence of distance on sound levels in woodlands is expressed by the relationship:<br />Sd=S0-20logdd0<br />Where Sd is the sound level at distance d, S0 is the sound level at the source, and d0 is the distance where the sound level is known. A 90 dB source moving through a 30 meter shelter belt with 2 m in row spacing of 25m tall deciduous trees will decrease by about 10 decibels for the first 50 meters, 10 decibels for the next 25 meters and 5 decibels for the next 25 meters. Attenuation of sound is greatest when shelterbelts are located halfway between the source of the noise and the receiver. Visibility can also indicate the relative attenuation of noise levels. As visibility decreases and width of tree belt increases noise level attenuation increases. A shelterbelt with a width of 25 meters and a visibility of 12 meters the range of attenuation is between 6 and 10 dB. For a medium width of 10 meters and a 3 meter visibility the range of attenuation is also between 6 and 10 dB CITATION Buc06 p 23 l 1033 (Bucur, 2006, p. 23). Therefore, forest managers can receive similar results of large plantings spaces in smaller spaces by increasing vegetative density.<br />Figure 3Percentage of Annoyed Citizens by Sound Pressure LevelSource: http://www.noisepollution.org/library/whonoise/whonoise.htmUnder the concept of perceived noisiness, two sounds equally loud may not be equally noisy. The difference may be between the time and frequency structure of the sound. Presently practitioners of noise attenuation use continuous energy equivalent noise levels to describe community noise. This index although physically correct is not applicable to comparing noise situations of unequal character. As discussed above impulsive noise provides more annoyance than continuous noise. Rail noise, which is often louder than traffic noise, can be more amenable, because it lacks the intermittent nature of road traffic.<br />Odor Pollution<br />Increases in noise and odor pollution are symptoms of an ever expanding urban population. As development begins to focus on marginal spaces and the expansion into peri-urban areas new problems arise such as noise pollution and urban pollution. Between 1990 and 2000 urban expansion occurred largely in both forested (33.4%) and agricultural (32.7%) land (Nowak et. Al., 2005, p. 379). Areas of the largest urban expansion into agricultural lands such as Nebraska, Indiana, Illinois, and Wisconsin have large numbers of (confined animal feeding operations). In North Carolina, residents living within two miles of a CAFO experienced increased frequencies of headaches and burning eyes than residents living two miles away from CAFO. Residents complain they cannot open windows of enjoy the outdoors causing mood disturbance, tension, depression, anger, fatigue, and confusion CITATION The08 l 1033 (Starmer). Finally, property values around CAFOs are significantly decreased due to diminished marketability. Studies on odor pollution in the urban environment are limited; as urban areas expand, citizen complaints will increase studies into the effects of odor pollution. Studies similar to those done by the National Resource Conservation Service on CAFOs are now being conducted on urban sources of pollution.<br />Shelterbelts used to decrease noise pollution and reduce wind speed have the added benefit of odor reduction. The Missouri NRCS suggests windbreaks with one to three rows of alternating conifer and deciduous tree species. Shrubs and understory trees should be planted to maximize odor interception and the dilution of air. Odor reduction is based primarily on the interception of odor causing particulate matter. As discussed previously, “as leaf surface roughness increases, the capture ability of particles and odor increases” CITATION Mis04 p 1 l 1033 (NRCS Missouri, 2004, p. 1). Leaves with complex shapes, like the leaves of conifers, provide the most effective form of interception. Other forms of windbreak odor reduction include CITATION Mis04 p 5 l 1033 (NRCS Missouri, 2004, p. 5):<br /><ul><li>Dilution and dispersion of gas concentration of odor b a mixing effect
Deposition of odorous dusts and other aerosols around windbreaks
Collection and storage within tree wood of the chemical constituents of odor pollution
Containment of odor by plantings close to the odor source.
Aesthetic appearance, a visual barrier to livestock barns.</li></ul>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. <br />The Physics of Tree Health<br />Particulate Matter & Road Improvements<br />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. <br />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.<br />Canopy Cover and Soil Improvements<br />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 l 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. <br />Tree Liability and Acoustic Benefits<br />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.<br />Figure 5Example of Ultrasonic TomographySource: CITATION Wan p 4 l 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.<br />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. <br />Conclusion<br />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. The goal is, then, to expand the dimensions of tree benefits through new technical information which may inspire policymakers to revisit information which they may have put aside, while cutting the costs by using physical principles to increase the feasibility of implementation.<br />Bibliography BIBLIOGRAPHY Bassuk, N., Grabosky, J., Trowbridge, P., & Urban, J. (1997, Spring). Structured Soil: An Innovative Medium Under Pavement that Improves Street Tree Vigor. Retrieved February 28, 2008, from University of Michigan: http://www-personal.umich.edu/~sarhaus/courses/NRE501_F2000/lloyd/strctsoil.htmBeckett, K. P., Freer-Smith, P. H., & Taylor, G. (1998). Urban Woodlands: Their Role in Reducing the Effects of Particulate Pollution. 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Ultraviolet Radiation: Global Solar UV Index. Retrieved February 28, 2008, from World Health Organization Factsheets: http://www.who.int/mediacentre/factsheets/fs271/en/World Health Organization. (2006). WHO-Facts Sheet: Health Consequences of Excessive Solar UV Radiation, Global Disease Burden from Solar Ultraviolet Radiation. (B. K. Chandy, Ed.) Kuwait Medical Journal , 38 (3), 254-258.<br />