Classifying mature federal forests in the United States: The forest inventory growth stage system

Abstract

President Biden’s executive order, ‘Strengthening the Nation's Forests, Communities, and Local Economies,’ (EO#14072, April 22, 2022) acknowledges the interest in mature and old-growth (MOG) forests by directing U.S. Federal agencies to define and inventory these resources on United States Forest Service (USFS) and Bureau of Land Management (BLM) lands. We propose using an effective and enduring mature forest classification system that could be adaptable to social paradigms, monitoring data streams, scientific information, and global change factors. We accommodate these design aspects by defining mature forests as a growth stage prior to the onset of old-growth attributes within a proposed Forest Inventory Growth Stage System (FIGSS). FIGSS uses the long-established USFS old-growth assessments often conducted in concert with public dialogues to identify key structural indicators of older forests. The system informs inverse modeling of the prior “mature” stage’s structural thresholds enabling initial population estimates of mature forest extent using the USFS’ nationally consistent Forest Inventory and Analysis (FIA) program data. With this approach, we estimate that approximately 45 percent of all USFS/BLM forest is mature. The FIGSS system is based on a variety of components that could be used to account for cultural values ascribed to forest conditions which could be refined across future versions such as assumptions about the relative length of growth stages, incorporating data from emerging monitoring technologies along with old-growth/mature field sampling campaigns, accommodating spectrums of site-limited and/or disturbance-driven stand development, refined variable selection processes such as machine-learning, and consideration of traditional ecological knowledge.