The biological sustainability of a forest can accurately be measured via two primary methods. The first method seeks to measure the volume of forest inventory; analyzing inventory changes over time is an effective way to gauge the balance of growth and removals in a specific geographic area. In this scenario, inventory increases indicate that biological tree growth outpaces removals, and inventory decreases indicate that removals outpace tree growth.
The Forest Inventory Analysis, overseen by the US Forest Service, is the standard measure of forest inventory across the country. Every five years, timber tracts throughout all forested regions in the United States are randomly selected and inventoried. Forest Service employees survey each plot to determine the amount of forest growth and removals, and record any other changes that may have occurred. Forest service personnel identify each tree by species and measure both diameter and height. Using this very exact data, FIA can assess the condition of timberland resources at a local level.