Urban Forest Age Structure 

In 2009, the City completed a tree inventory to determine the species, size, location, and condition of all city-owned street and park trees. The tree population includes a healthy mixture of young, medium, and older individuals. The tree inventory does not include trees on private property or those in City-owned natural areas. Source: Ann Arbor Urban Forest Inventory

 What is age structure?

Age structure is the relative abundance of individuals of different ages making up a population. Tree size, represented as the diameter at breast height (DBH), or the tree diameter at 4.5 feet above the forest floor, is a good predictor of tree age. Young trees have small DBH measurements. As trees mature, DBH increases.

What does age structure tell us about the health of the urban forest?

In the same way that high species diversity reduces a forest's vulnerability to species-specific pests and diseases, a diverse age structure reduces the possibility that all the trees in the forest will begin to die at the same time. A healthy mixture of young, medium, and old trees provides a nearly constant turnover of generations over time as new trees replace the old. In addition, trees of different sizes provide more complex habitat for wildlife and can support a greater number of species.

For more information

• Ann Arbor Forestry
• Ann Arbor Tree Inventory

Last Updated: July 2009