Milwaukee has begun to quantify the costs and benefits of trees in its urban ecosystem. Reductions in storm-water flow, conservation of energy, and improvements in air quality were studied to determine the financial contribution of the tree canopy to the city.

An analysis using a Geographic Information System (GIS) indicated that only about 16 percent of Milwaukee has tree canopy cover and of this 80 percent is on private property. This relatively low tree cover, which varies from 1 to 42 percent per ownership unit, can be attributed to human development and Dutch Elm Disease.

The existing tree canopy cover reduces storm-water flow by up to 22 percent and provides an estimated $15.4 million in savings. If all the trees in Milwaukee were removed, the additional storm-water would require the construction of an estimated 357,083 cubic feet of water retention capacity.

The city's trees also sequester an estimated 1,677 tons of carbon annually, a benefit valued at $1.5 million. By maximizing urban tree canopy cover to match existing well-canopied sites, 4,793 tons of carbon could be sequestered annually. The resulting summer energy savings are estimated to be $650,000.

Currently, benefits from trees in urban areas are not derived primarily from marketed products or raw materials but from improvements in temperature and other environmental measures, such as storm-water flow, water quality, energy use, real estate values, pollution control, and health and psychological benefits. Whether they are on publicly or privately owned land, trees in the urban ecosystem provide unique benefits.