PS 24-77 - Wicked problems and sticky wickets: defining and inventorying mature and old-growth forests

Abstract: Mature and old-growth forests are valued for biodiversity, carbon sequestration, habitat, hydrologic function, aesthetics, cultural value, and spirituality. Globally, land use change and industrialization have resulted in dramatic declines in the area of old forests. We developed alternative conceptual models for defining mature and old-growth forests across the vegetation types of the United States in order to quantify their abundance and distribution. Defining old growth has been described as a “wicked problem” that involves values, science, and management, requires multiple disciplines, and can be expressed from many contradictory approaches.



Most ecological research of old growth involves characterizing composition, structure, and function of stands pre-selected as being old growth, but rarely quantifying attributes that make one stand old growth and another not. Forests are commonly placed in a successional continuum of increases in tree size, biodiversity, dead wood, wildlife habitat niches, and structural diversity, but this assumes a stand-replacing disturbance regime. Stand age is often used as a short-hand metric, but is poorly defined in low-productivity, frequently-disturbed, or gap-phase dominated forests. Time since severe disturbance or human impact is often a consideration, but ignores the impact of decades of fire suppression or the impact of Native Americans on historic forest conditions. We propose that different vegetation types can be arrayed along gradients of site productivity and fire frequency, with predictable processes and structural attributes for older forests across the gradients, that can provide a consistent framework for definitions.



Once defined, estimating the area of mature and old-growth forest presents challenges. The Forest Inventory and Analysis (FIA) program in the U.S. provides comprehensive, consistent field data across all forests, although the small footprint (0.067 ha) limits the number of potential metrics and the spatial grain of the plot classification. We applied structural criteria to FIA plots across the 72.2 million ha of inventoried National Forest and Bureau of Land Management forest land and estimated that 18% was in old growth and another 45% was mature. Protected land designations contained 49% and 35% of old growth and mature forests, respectively, While spatial models (maps) of forest structure are desirable, they often depend on mismatched scales data and model assumptions, and tend to be inaccurate at high resolutions. Therefore, identifying specific stands and landscapes of mature and old growth forests will likely require additional ground measurements and the application of local criteria and social and traditional knowledge within a consistent definition framework.