Tree crown dimensions are a major determinant of net primary production. Trees with large, densely foliated crowns have increased potential for carbon fixation, nutrient storage, growth, and survival. Small, sparsely foliated crowns may represent trees in decline. When biotic or abiotic stresses impact a forest, the first signs of deterioration can often be observed in the tree crowns. Because tree crowns form most of the structural architecture of forest ecosystems, they also influence the composition and vigor of understory flora and fauna, as well as the physical processes that affect soil and water quality.
The Forest Health Monitoring (FHM) Program of the U.S. Department of Agriculture (USDA) began collecting tree crown data as an indicator of forest health in 1990. This function was transferred to the Forest Inventory and Analysis (FIA) Program when the two Programs integrated in 2000. The FIA and FHM Programs operate collaboratively, where specialized indicators developed by the FHM Program are transferred to FIA when ready for implementation.
As the Nation's forest census, the FIA Program is responsible for providing the information required to assess America's forests. Most of this information is obtained from a series of permanent ground plots systematically distributed across the landscape. Conventional forest inventory data are gathered at all sample locations; additional data associated with specialized forest health indicators are collected on a 1/16 subset of the plot network. FIA refers to the conventional inventory portion of its sample as “Phase 2”, and the subset of plots and data reserved for forest health applications as “Phase 3”.
A Fact Sheet provides additional background information about the crown indicator. A more comprehensive publication discusses the rationale for the indicator in great detail.