Two decades of stability and change in old-growth forest at Mount Rainier National Park

We examined how composition and structure of old-growth and mature forests at Mount Rainier National Park changed between the mid-1970s and mid-1990s. We assessed whether the patterns of forest dynamics observed in lower elevation old-growth forests in the Pacific Northwest held true for the higher-elevation forests of the Park. We used measurements of tree recruitment, growth, and mortality on 18 permanent plots that spanned the range of forested environments in the Park. Similar to previous studies, there was little change in stand structure and composition, while a relatively large number of individual stems died or were recruited into the tree population. Most recruitment was of shade-tolerant tree species. Unlike some previous studies, in many stands recruitment of shade-tolerant individuals occurred without substantial mortality of shade-tolerant trees in the upper canopy. Habitat characteristics associated with old-growth forest changed little in most stands. One exception was a mature noble fir stand in which a brief episode of tree mortality, apparently due to drought and pathogens, increased similarity to old-growth structure. Plots in old-growth forest on the coldest and wettest sites in the Park had low similarity to the published definition of upper-slope old-growth forest at both the beginning and end of the study, suggesting that the existing definition may not apply at these environmental extremes.