Measuring the Benefits of Urban Forests
At a Casey Trees' conference on urban forestry, David Nowak, Ph.D, research forester at the U.S. Forest service, one of the world's foremost experts on urban forests, and a member of the team that won the Nobel Prize at the Intergovernmental Panel on Climate Change (IPCC), said out of the 20 biggest cities in the U.S., 17 have declining urban forests. "Tree cover is going down." For example, researchers have found that D.C.'s urban forestry cover decreased by 1 percent in the last 20 years, while impervious cover (hard concrete) grew by 20 percent. Now and in the future, the key to boosting urban forests may be to make better use of innovative Web applications like iTree, which "estimate value and benefits" of the tree canopy.
Nowak said to really understand urban forests you have to look at their extent or structure. You have to know "how many trees you have and where they are." The structure of an urban forest also impacts the benefits. For example, where trees are placed impacts who receives the environmental, psychological, and social benefits.
Forests can be measured in either a "bottom-up" or "top-down" manner. Bottom-up approaches involve counting species on the ground and looking at species, tree health, and the various health risks. Nowak and his team at the Forest Service participated in developing iTree, a bottom-up tool that helps manage forests. Top-down efforts are usually satellite-driven and involve high-resolution imagery and photo interpretation.
In an examination of urban forests, Nowak found that some 30 percent of vegetation is planted, while the other two-thirds is "naturally regenerating." There are also varying levels of natural vegetation within key spaces in cities. In residential areas, the share of naturally-regenerating nature is relatively low because people plant or mow, while in parks and open spaces, it's higher.
Invasive plants are also on the rise across the country. In D.C., invasive plants may even be shifting the composition of the forests. "Frontier plants are changing things."
To track all this change, Nowak said it's important to use tools like iTree, which can help local urban policymakers, planners, and landscape architects "better understand the canopy and the true value of ecosystem services." Nowak said anytime you've heard a number about the dollar value of an urban forest, it was probably based in an iTree estimate. Using "local variables such as energy, air, water quality, and climate," iTree can put a value on an area's trees and help local policymakers optimize the performance of the forest.
While landscape architects and others understand the inherent value of trees, local programs to protect trees from pests and fungus are expensive and budgets are tight, so "we need to build the financial case." Without "data and tools, it's hard."
With 20 years of data available, there are a number of applications where you can run and test models. iTree Canopy uses Google Maps to create statistically-valid estimate of tree cover, while iTree Species helps users identify the specific ecosystem service benefits of one tree over another. The system has about 5,000 trees in its database. iTree Hydro looks at tree canopy and stormwater, while iTree Design, which Nowak called the Sim City of landscape design, helps landscape architects and designers figure out the benefits of certain tree sizes and types in a landscape design. In the same way, the tool could be used to figure out the amount of financial benefits that are lost when a tree dies.
iTree 5.0 will include some new features like Google Maps, web-based data collection using mobile devices, the inclusion of data on the volatile organic compound (VOC) output of trees, and "benefit forecasting." There will also be more data on "the risks each tree type faces from insects and diseases" as well as risks from a given forest structure. For example, too many species in one place means that part of the forest could be simply wiped out with an infestation, creating a vulnerability in the overall structure.
On the value of having a tool like iTree itself, Nowak said: "This is really about urban forestry technology transfer" through a "credible, USDA-approved, public domain software."
For more on the benefits of urban forests, see ASLA's animation: Urban Forests = Cleaner, Cooler Air. Nowak was an expert advisor on the animation.
Image credit: Aerial View of Logan Circle, Washington, D.C. / Wikipedia