20 must plant replacement trees of the same total girth For example, removal of a 55-inch tree would require planting seven young trees of 2 5-inch diameter or caliper The law is administered by the Urban Forestry Administration (UFA), which also is responsible for the city’s street trees 41 • In 2005, Mayor Anthony Williams signed the Conference of Mayors’ Climate Protection Agreement, adding Washington to 140 other cities that signed the accord Mayor Williams recognized that Washington could abate rising carbon dioxide in its air by adding many more acres of trees • In 2007, the city settled two crucial negotiations with the US EPA to receive permits under federal air and water laws; both include forest expansion as one step towards compliance One is the State Implementation Plan for 8-hour ozone, key to lowering regional asthma rates The other concerns the MS4 sewer-water system to reduce sewage-laden water spilling into Rock Creek and the Potomac and Anacostia rivers in storms Today there are signs that Washington’s tree loss has stabilized But the area canopied by trees could shrink again due to land development and population growth In 2012, estimates of city land covered by trees were 37 2 to 35 1 percent 42 The bottom line is that Washington must expand its net area of tree canopy continuously to abate the region’s “heat island” in the 21st century Fig. 32 Rush hour on Mass Ave near 18th Street. Visible are some of the denser commercial buildings, which replaced earlier mansions and townhouses.
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