Forest stand structure characteristics for the Cispus Adaptive Management Area, Cascade Range, United States of America: Implications for old growth, fire hazard, silviculture, and landscape management (Washington)

Low elevation forests between 150 and 1000 years old were studied and compared with younger forests in the Cispus Adaptive Management Area of the Gifford Pinchot National Forest, Washington, U.S.A. Species, diameter distributions, and other characteristics commonly associated with old growth forests were quite variable, and do not create a uniform pattern of “old growth.” Multiple canopy strata was the most consistent characteristic of all old forests examined. Age distributions in these old forests were categorized as: single cohort, clumped, or well distributed. Shade tolerant species began to gain dominance (in basal area and tree numbers) over Douglas-firs approximately 200–400 years after stand establishment. Some large, dominant Douglas-firs remained alive for many more centuries, but their diameter distribution became discontinuous. The shift from Douglas-firs to more shade tolerant species was probably because new Douglas-firs were not recruited to the shaded understory.; Stands with large shade tolerant species (indicating infrequent fires) are more likely to be on low sidehills or canyon bottoms than on south aspects and upper or mid sidehills, while Douglas-fir dominated stands (indicating more frequent fires) are more likely to be on steep slopes.; Diameter growth rates were low in dense stands and on poor sites. The slow diameter growth in trees in lower canopy strata indicates these trees would take very long to grow to the upper canopy without an intervening disturbance. Large western redcedars in old growth stands probably either began with stand establishment and grew rapidly at first or were released and grew to the upper canopy after a quite large disturbance.; Forests with old growth characteristics of multiple species, large trees, and multiple canopies probably grew on only certain locations—sites where the trees could grow to large sizes and physiographic locations where fires were infrequent. Using tools such as LMS and GIS, forest managers can stratify the landscape into areas of different growth and fire potential, thus manage each area appropriately. It will be difficult to provide old growth forests on more than 80% percent of the area as targeted in the forest's objectives because of the growth and fire limitations.