Urban Forest Inventory and Assessment Pilot Study

The urban forest (trees in and around communities) is an integral part and significant contributor to Wisconsin’s urban landscapes. While frequently observed, a relatively small amount is known about the resource. A partnership between the USDA Forest Service Forest Health Monitoring, Wisconsin DNR Urban and Community Forestry Program and USDA Forest Service Research launched a pilot study in Wisconsin. The study is designed to provide information on the composition, condition, and benefits provided by Wisconsin’s urban forest.

Figure 1. Wisconsin urban areas.

What We Found

The pilot study established and sampled 111 field plots falling within urban areas during the summer of 2002. Urban areas were delimited using the 1990 U.S. Census definition of urban and crossed all ownership boundaries (Figure 1).

The inventory included trees on all land uses (e.g. residential, ROW, commercial, etc.). Residential was the dominant land use, covering 38% of urban areas (Table 1). It was found that Wisconsin’s urban areas contain over 26.9 million trees, averaging 36.9 trees per acre and with an estimated total structural/replacement value of $10.9 billion. Tree size averaged 5.4” DBH. The predominant species were Boxelder, White Ash, Green Ash, and White Pine, of the 56 tree species found to occur in the urban forest plots (Figure 2).

Figure 2. Species composition of Wisconsin’s urban forest.

Forest health data collected on crown conditions and occurrence of damage indicated the urban forests of Wisconsin are generally healthy and vigorous. Despite this, 19% of urban trees showed signs of damage (conks, vines, open wounds, etc.) White ash and Boxelder were the most frequently damaged trees. The average annual mortality within urban areas was estimated to be 7.3%. To sustain tree cover at its current level (14%), it was estimated that about 2.6 million trees need to be established annually. Some of these trees will be naturally established through regeneration, but many will likely need to be planted. This translates to 1 new tree per year for every 10 existing trees within urban areas.

Pests can still impact a healthy forest. In particular, the emerald ash borer (EAB) poses a risk to a significant (20%) part of Wisconsin’s urban forest.

• EAB could eliminate 5.4 million Ash in urban areas with an associated structural/replacement value of $1.5 billion.

These are conservative values and do NOT consider the cost to remove the existing trees and stumps, nor the lost environmental, social and economic services provided by the trees.

The study was able to quantify some of the environmental benefits that Wisconsin’s urban forests provide including pollution removal, carbon storage, carbon sequestration, and energy reduction.

• Trees within Wisconsin urban areas and immediately surrounding forest remove $36.4 million worth of pollution (ozone, particulate matter, nitrous oxide, etc.) annually.
• Wisconsin urban trees store $42 million worth of carbon.
• An additional $2.4 million worth of carbon is sequestered annually by the urban forest.
• Trees reduce heating and cooling expenses by $24.3 million annually, with an additional $1 million worth of carbon production avoided because of reduced energy demand (Figure 3).

These values tend to increase with increased size and numbers of healthy trees. Sustaining forest health and longevity is critical to sustaining these benefits through time.

Figure 3. Estimated energy savings and carbon emission avoidance savings by season due to urban trees in Wisconsin.

What This Means

The urban forests of Wisconsin provide significant social, economic and environmental benefits to the people of Wisconsin. The resource itself is worth billions of dollars and annually provides functional benefits of over $64 million per year. This amount is just for pollution removal, carbon sequestration, and reduced building energy consumption. Many other environmental, economic and social benefits, such as storm water reduction, increased property values, and improved neighborhoods are yet to be quantified.

Wisconsin’s urban forest is generally healthy, yet a concerted effort needs to be made to maintain this valuable resource. Urban trees impact about 80% of the state’s population daily. The benefits associated with this resource will only be realized with conscientious urban forest management over time.