Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

It is important to note that the age of a tree does not necessarily convey information about the size of the tree. Fast-growing species attain sizes comparable to much older trees of another species, and trees of the same species and age growing in different locations may be very different in size. In addition, processes such as forest fires and hurricanes can act to limit the age of trees in a region (e.g., hurricanes are more prominent in the eastern United States).

The Data

This indicator presents data for a subset of all forests in the United States––those defined by the USDA Forest Service as “timberlands.” Timberlands is a designation that covers lands on which harvesting is not prohibited by law and which grow an average of 20 cubic feet of wood per acre per year. Thus, the data presented here do not include national parks and wilderness areas and other natural and semi-natural forestland not classified as timberlands and thus not included in previous inventories. As a result, these data describe nearly all eastern forests, but only about 40% of western forests. Data on slow-growing forests and those in parks and wilderness areas are being collected, but they are not yet available.

Data Source: Data for this indicator were collected by the USDA Forest Service Forest Inventory and Analysis (FIA) program, which is described in the Forest Area and Ownership technical note.

Data Collection Methodology: The age of a stand of trees is a classification based on the mean age of trees with dominant or codominant crown positions in the stand. Dominant/codominant crowns are those tree crowns dominating or sharing space in the upper layer of the tree canopy. The age of these trees is generally determined using tree cores from which annual growth increments were counted.

2003 Web Site Update: Data for 2002 were added in this update. Data were acquired from the Forest Service and are available on the Web at http://fia.fs.fed.us.