The status of forest plant communities (or "forest associations" as defined in the figure caption), because such communities make unique contributions to biological diversity. (Fig. 1)

Tree size, because forests with trees of different sizes serve as habitat for different species and have different commercial values. (Fig. 2)

Acreage of old-growth forest, because old-growth is a unique ecological community and attracts strong public interest. (Fig. 3)

Tree mortality, as a measure of significant changes in forest conditions. (Fig. 4)

Changes in forest fire acreage, because fire is a key ecological element of many forests and a significant public concern. (Fig. 5)

About one-fourth of the more than 1,400 U.S. forest associations are "critically imperiled" or "imperiled," and about 30 percent are "secure" or "apparently secure." Imperilment means that an association occurs in 20 or fewer places, covers few remaining acres, or is very vulnerable for other reasons.

The volume of wood in trees of all sizes, including very large trees, has grown in the past 50 years, although the volume in small trees (5 to 10 inches in diameter) has leveled off.

Overall forest mortality, including that from fire but not from harvest, has remained relatively stable over the past 50 years, never exceeding 1 percent per year.

From 1950 to 1997, annual acreage burned has ranged from 600,000 acres to about 15 million acres, or an average of about 3.8 million acres annually. This is about one-tenth of what was burned annually from 1924 to 1950. Acreage burned in National Forests, which are mostly in the West, increased significantly during the 1990s.

Status Of Forest Plant Communities (1) Technical Note
	"Forest associations" are ecological communities defined by similar groups of species growing under similar environmental conditions. Some occurrences of associations cover large areas; others are restricted to highly specific conditions. Many imperiled associations have been greatly reduced in extent by human activities; some are naturally rare.
Source: The Nature Conservancy & The Association for Biodiversity Information

Volume Of Wood In Trees Of Different Sizes (2) Technical Note	Old Growth Forest In California, Oregon & Washington (3) Technical Note
	Percent of total forest area 1933-1945 1992 California 56 19 Oregon 53 21 Washington 40 14 Total (CA, WA, OR) 49 18
Source: USDA Forest Service, Forest Inventory & Monitoring	Source: USDA Forest Service
Although precise estimates are not available, very large trees and old-growth forest are thought to be substantially less common today than they were in pre-European times.

Mortality As A Percent Of Inventory (4) Technical Note
	Mortality is defined as the amount of wood in trees that die during a year, expressed as a percentage of the total amount of wood in U.S. forests. It does not include trees that are cut.
Source: USDA Forest Service, Forest Inventory & Monitoring

Acres Burned In The United States, 1924-1997 (5) Technical Note	Acres Burned In National Forests, 1920-1997, 10 Year Rolling Average (6) Technical Note
Source: USDA Forest Service	Source: General Accounting Office
Fire is a natural and important component of forests. During the 1930s and 1940s, prevention and suppression efforts significantly reduced the acreage burned. In recent decades, fire has been reintroduced intentionally into some forest types, although overall acreage burned is still far less than it was in previous decades.