Prince George's County is mauling its way through its woodlands and forests with convenient zoning ordinances that allow trees to be mitigated (cut down) with a simple request for a variance. The underlying assumption is that trees contribute little while shopping malls are the key to economic development and quality of life. And without a doubt we need quality shopping malls as part of any sustainable economic development strategy, however consideration of trees should be part of the equation not a bothersome after thought. Somehow the county leadership has arrived at the idea that it is better to cut down trees than to rehabilitate and adaptively re-use already disturbed and degraded, treeless properties. Behind this proposition is the obvious profit margin gain that comes from paving over open space rather than reusing existing development within exitisting communities.
So it is easy to sell a mall project and put forth the number of service industry jobs that it will create as well as calculate the sales and property tax that might be collected. It is of course almost impossible to calculate the opportunity costs of the trees. So we don't; we present a one-sided and short-sighted option based upon a baseless premise of infinite resources available for development. Let me tell you about what we lose every time we cut down a mature tree and pave over a woodland plot.
A community's trees and its urban forest remains one of the first impression a community projects to visitors. The tree's of an urban woodland, forest or landscape is an extension of its pride and community spirit. Trees enhance community economic stability by attracting businesses and tourists; people linger and shop longer along tree-lined streets; apartments and offices in wooded areas rent more quickly and have higher occupancy rates; businesses leasing office spaces in developments with trees find their workers are more productive and absenteeism is reduced.
Urban forests, woodlands and landscapes can increase traffic safety by enhancing traffic calming measures, such as narrower streets, extended curbs, and other infrastructure. According to Tall trees give the perception of making a street feel narrower, slowing people down. Closely spaced trees give the perception of speed (they go by very quickly) slowing people down. A treeless street enhances the perception of a street being wide and free of hazard, thereby increasing speeds. Increased speed leads to more accidents. Trees can serve as a buffer between moving vehicles and pedestrians. Street trees also forewarn drivers of upcoming curves. If the driver sees tree trunks curving ahead before seeing the road curve, they will slow down and be more cautious when approaching curves.
According to a website composed and compiled by Kathleen Alexander trees are a "free" resource that absorbs carbon and pollutants from the air generated by our cars and trucks. One tree compensate for automobile fuel use equivalent to driving a car between 7,200 and 8,700 miles per year (Prince George's County just cavalierly agreed to cut 38 trees down for a parking lot). Trees according to the Maryland Department of Natural Resources, create organic matter on the soil surface from their leaf litter. Their roots increase soil permeability that results in: Reduced surface runoff of water from storms; reduced soil erosion and sedimentation in streams; increased groundwater recharge that is significantly reduced by paving; lesser amounts of chemicals transported to streams; and reduced wind erosion of soil. Without trees, Maryland DNR contends, cities and counties would need to increase sewage and storm water drainage channels and waste treatment capacities to handle increased water runoff. These are costs that our leadership prefers its citizens to discover after the mall is in place. This is the externalization of costs onto the tax payer that comes when the trees are cut down because we could not rebuild, readapt or refresh and existing sight because it would cut the profits of a few.
Prince George's County needs development and it needs retail. It also needs ecosystem services and it needs to look to its establish communities for develop0mental opportunities not always towards a field of trees. We need to find away to rebuild and refresh existing malls and the grey fields of our existing established communities not always find the quickest most expedient green field because a few think that paving over a green space is not harming anything; that green spaces exist so that a few can make money; that someone somewhere later can bear the costs of replacing the lost ecosystem services.
 Michigan State University Extension, Urban Forestry #07269501, “Benefits of Urban Trees”
 National Arbor Day Foundation pamphlet #90980005
 Kathleen Alexander. Benefits of Trees In Urban Areas. [accessed January 14, 2012] http://www.coloradotrees.org/benefits.htm#16
a) this answer depends on tree density per acre, diameter structure, species composition, and growth rates. Estimates from Chicago are 2.7 t C/ac of tree cover/yr (Nowak 1994b); the Chicago area: 2.2 t C/ac of tree cover/yr (Nowak, 1994b); and in Brooklyn, NY: 1.0 t C/ac of tree cover/yr (Nowak et al., in review). These are gross carbon sequestration estimates and do not account for carbon emitted due to decomposition. The Chicago estimates are likely liberal as they do not account for tree condition or stand structure effects on growth. Gross carbon sequestration estimates for individual trees in Brooklyn, by various diameter classes are (Nowak et al., in review):
DBH Class (in) Carbon Sequestration (lbs/yr)
b) Estimate of carbon emitted per vehicle mile is approximately 0.24 lb C/mi (see Nowak, 1993 for calculation and references) but is as high as 0.29 lb C/mi if carbon produced from transportation and fuel processing is included. Thus, a car driven 26,000 miles will emit 6,240 lbs C (22,880 lbs CO2) or 7,540 lbs C (27,647 lbs CO2) if the whole fuel process is included. Thus, one acre of tree cover in Brooklyn can compensate for automobile fuel use equivalent to driving a car between 7,200 and 8,700 miles, depending on which estimate you choose to use. However, when the tree dies, most, if not all, of the carbon stored will eventually be released back to the atmosphere and form CO2. Thus, the CO2 gains made by trees are sustained as long as the forest structure is sustained. Also, the gains made are only good for the first generation of trees, unless the carbon is prevented from decomposing. If first generation decomposes, the second generation of trees will only compensate for the loss of the first generation (Nowak et al., in preparation).' Trees remove several tons/day of O3, CO2, SO2, NO2, PM10. How many trees does it take to remove so many tons of one element?
We are currently completing a comparison of pollution removal by trees in 50 cities across the United States. Pollution removal varies based on meteorology, amount of tree and shrub cover (acres), pollution concentration, and length of growing season. Pollution removal (ozone, particulate matter, sulfur dioxide, nitrogen dioxide, and carbon monoxide) by trees and shrubs in Chicago in 1994 was estimated at 651 tons (rates varied for each pollutant) (Nowak, 1994c). In Brooklyn,1994 pollution removal (same 5 pollutants) by trees and shrubs was estimated at 287 tons (Nowak et al., in review). Average individual tree pollution removal estimates for Brooklyn by various diameter classes are:
DBH Class (in) Pollution Removal (lbs/yr)
Differences in removal rates per tree by diameter classes are due to differences in the average amount of healthy leaf area per tree among the diameter classes.
 The Maryland Department of Natural Resources Forest Service. [accessed January 14, 2012] http://www.dnr.state.md.us/forests/publications/urban4.html