Forest Extent and Type

The forests of the U.S. cover extensive lands in both the eastern and western thirds of the country. While the amount of forest land has remained nearly unchanged since the beginning of the 20^th century, regional changes both in amount and types of forest cover have occurred as a result of changing patterns of agriculture and development. The distribution of various forest cover types is a critical determinant of the condition of forest ecosystems.

This indicator is based on data from the U.S. Department of Agriculture (USDA) Forest Service Forest Inventory and Analysis (FIA) program. The FIA program, using a statistical survey design and comparable methods across the U.S., collects various data that help assess the extent, type, age, and health of the nation’s forest land. Because the surveys are repeated over time, the FIA data provide an indication of trends in both the extent and composition of forest land. The extent data are collected for all forest lands across the nation, but species composition data over time are only available for timberland as defined by FIA data collection procedures (that is, forests capable of producing at least 20 cubic feet per acre per year of industrial wood and not withdrawn from timber utilization by statute or regulation). Timberland makes up approximately 90 percent of the forest land area in the eastern U.S. and 42 percent of forest land in the western U.S. as of 2012 (USDA Forest Service, 2014a). Extent data are collected for individual states, but have been summarized by EPA Region for this indicator. 

From 1907 to 2012, forest land nationwide increased by 25 million acres (Exhibit 1). The smallest extent, 721 million acres, occurred in 1920. By 1963, forest land rose to 753 million acres, but then fell to 733 million acres in 1987. Forest extent steadily increased to 766 million acres by 2012. There are variations in trends in forest cover among the different EPA Regions. For example, between 1907 and 2012, forest land declined by roughly 11 million acres in Region 9. Over the same period, forest land increased by 14 million acres in Region 3.

In addition to changes in the extent of forest, there have been changes in the types of forests over time (Exhibits 2 and 3).

• The largest changes in the eastern U.S. over the 1953–2012 period occurred in the maple-beech-birch forest type and the oak-hickory forest type, which gained 27.1 million acres and 26.5 million acres, respectively, since 1953. The largest decreases in the East from 1953 to 2012 were seen among the longleaf-slash pine and oak-gum cypress forest types, which decreased by 14.1 million and 12.2 million acres, respectively.
• In the West,  fir-spruce and Western hardwood forests have increased by 12.5 million acres and 12.7 million acres, respectively, since 1953. The hemlock-Sitka spruce and ponderosa-Jeffrey pine forest types have decreased by about 14.2 million and 13.5 million acres, respectively. The Western white pine forest type has decreased by 5.3 million acres, or about 96 percent of its 1953 acreage.

• Data on extent of forest land have an uncertainty of 3 to 10 percent per million acres for data reported since 1953. In 1998, Congress mandated that the FIA move to annual inventories. While data now are collected more often, fewer data are collected in any given year. Because area estimates now are based on a smaller sample size, the precision of the national estimates may be reduced relative to pre-1998 dates.
   
• Most of the specific data related to species and age classes are only collected on lands classified as timberland and not forest land in general.
   
• In addition to extent and species class, age class also influences the use of forest land as habitat by different species. For example, Smith et al. (2001, 2004) noted that younger and older stands of forest had increased over the past half-decade, while middle-aged stands of more merchantable timber had decreased.

This indicator is primarily based on data from the USDA Forest Service Resources Planning Act (RPA) resource tables, which are publicly available at http://www.fia.fs.fed.us (USDA Forest Service, 2014a). The RPA resource tables provide current and historical data on forest extent and type by state. The data are also published in a USDA Forest Service report (Oswalt et al., 2014). The 1963 data were obtained from an older USDA Forest Service report (Smith et al., 2001) because these data were excluded from the newer resource tables. Data were originally collected by the USDA Forest Service’s FIA program; original survey data are available from the FIA database (USDA Forest Service, 2014b) (http://www.fia.fs.fed.us/tools-data/).