Forest Inventory and Analysis - Methods

The forests in the Midwestern states were originally inventoried one state at a time on a rotating basis. In 2000, the Forest Inventory and Analysis (FIA) program switched from these periodic surveys to annual surveys. Now, 20 percent of the FIA field plots in each state are measured every year, with a full inventory taking five years to complete. When all plots have been measured once, they are revisited every five years. For example, plots measured in Wisconsin in 2000 were inventoried again in 2005.

Sampling

FIA sampling is carried out in Wisconsin and across the country in three phases.

Phase 1

Phase 1 is a remote sensing phase that classifies land as forest or non-forest. These spatial measurements can also be used to evaluate large-scale changes in the landscape such as forest fragmentation and urbanization. Traditionally, phase 1 relied on aerial photography, but more recently has been conducted using satellite imagery.

Phase 2

This phase involves taking forest and tree measurements in FIA ground plots that occur in Wisconsin at a density of one plot every 3,000 acres. The U.S. Forest Service inventories 650 FIA plots in Wisconsin every year, while the Department of Natural Resources forestry division contracts with a nonprofit organization, the Lumberjack RC&D Council (exit DNR), to survey 650 more. Plots are circular and four-tenths of an acre in size.

Inventory work in this phase occurs year-round and focuses mainly on tree information as it relates to timber. A variety of data are collected on each plot, including:

• Tree species
• Tree diameters and heights
• Tree locations within the plot
• Seedlings present
• Forest type
• Tree and stand age

Estimates of tree growth and mortality, and rates of tree removal due to harvesting can also be made by visiting the same plots over time.

Phase 3

Phase 3, or forest health monitoring, plots are a subset of the FIA plots. Information collected on these plots includes vegetation diversity and structure, crown condition, soil data, lichen diversity, down woody debris and ozone damage.