The benefits of trees in the urban environment are both qualitative and quantitative. Trees offer an aesthetic enhancement to the urban environment. There is something magical about driving, biking, or walking down a street lined with mature trees. There is a connection to the past – with the people who planted those trees and all the residents that have consciously or unconsciously enjoyed their presence. But in today’s society money talks. Lucky for trees – it turns out they have an economic voice – and its pretty loud.
Researchers at the USDA Forest Service, Northern Research Station have developed a tool for discovering, among other things, the economic impact that trees have in the urban environment. Its called the Urban Forest Effects (UFORE) model. This model is able to estimate the economic value of various ecosystem services provided by urban trees. For example, would you have guessed that the urban forest of Minneapolis removes around 384/tons of pollution each year? And the value of this service? $1.9 million/year. How about the current ‘hot-topic’ – carbon. Well, Minneapolis trees sequester around 8,900 tons/year – worth around $164,000/year according to the UFORE model. Urban trees also happily store this sequestered carbon for us. The Minneapolis urban forest is estimated to store 250,000 tons of carbon – a value of $4.9 million. (For more information visit http://www.nrs.fs.fed.us/tools/ufore/).
rban citizens live in highly developed and altered landscapes. We have houses connected to gas lines, sewer lines, electric lines, cable lines, phone lines, paved roads, sidewalks, and parking lots. In many cases our urban trees must coexist with these structures in close proximity. Furthermore, trees need to deal with all the construction and renovations that must be done to keep these urban systems going. Now, no one is suggesting we get rid of our modern conveniences to make room for growing bigger and better trees…well, some probably do! So we need to understand how our construction activities effect the nearby trees.
This is the goal of a 20 year study undertaken by University of Minnesota Urban and Community Forestry Professor, Gary R. Johnson. In 1993-1994 the Kenwood neighborhood of Minneapolis underwent a complete utility and infrastructure renovation. New utility lines (sewer, electric, water and gas) were run, streets were widened, curbs were lowered, new sidewalks installed, and the boulevard was regraded. The goal of the study was to see how trees planted in the area between the sidewalk and street – also known as the boulevard – performed when subjected to intensive construction activities. For comparison, a control group of boulevard trees that did not experience any construction activity was also established nearby. Five different genera of trees were looked at including maple (Acer spp.), American elm (Ulmus americana), ash (Fraxinus spp.), hackberry (Celtis occidentalis), and linden (Tilia spp.). Stem diameter (DBH) and condition ratings were taken for each tree in the study three times over the course of 20 years. Survival was also noted.
A complete statistical analysis is still forth coming but several trends are appearing from the data. For example – elm and maple trees experienced a greater percentage of loss in the construction group compared to the control group. Ash and linden on the other hand had a smaller percentage of loss in the construction group compared to the control group. Causes of tree loss are unknown. Elms by far had the greatest percentage of loss over all genera with 57% loss in the construction group and 45% loss in the control group. Average increase in DBH was not significantly different between construction and control groups among the various genera.
This represents a case study of boulevard trees in one neighborhood of Minneapolis. More studies are needed to form any definitive answer on the effect of construction on our boulevard trees. A complete report of this study is forthcoming. Stay tuned.