Two conceptual models describe the successional and structural development of mature and transitional stands toward old-growth Pseudotsuga menziesii (Mirbel) Franco forests in the Pacific Northwest. The "catastrophic/ maturational" model is based on infrequent catastrophic disturbances that initiate the successional cycle, while the "chronic/ partial fire" model is based on low- to moderate-severity fires which, over long periods of time, can create old-growth conditions through multiple disturbances. Although both models can produce similar structural characteristics, quantitative differences in speed and pathways of structural development with respect to these models are not well understood. To elucidate these quantitative differences, diameter distributions of live trees were investigated in a chronosequence of mature, transitional, and old-growth stands subjected to these two disturbance models. Stands characterized by the "catastrophic/ maturational" model exhibited varying degrees of rotated sigmoid distributions up to 250-300 years of age because of long establishment periods and differential growth and mortality rates of the component species. They also showed a modest trend toward reverse-J distributions with increasing stand age. Stands characterized by the "chronic/partial fire" model had a stratum of distinctly older emergent trees with younger cohorts of Pseudotsuga menziesii, Tsuga heterophylla (Raf.) Sarg., and Thuja plicata Donn. and exhibited reverse-J distributions within about 150-200 years after disturbance. These results suggest that reconsideration of current definitions of what constitutes old-growth on Tsuga sites may be necessary. A conceptual model is presented, which links pathways of structural development to disturbance regimes. This model indicates that active management can hasten the development of old-growth structure, even in some older forests.
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