Physiognomic development of Pseudotsuga forests in relation to initial structure and disturbance intensity

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Abstract
Physiognomic patterns may vary significantly during succession despite a tendency for larger‐growth forms to gradually replace smaller ones. Development of understory structure was observed for 25 yr after harvest of Pseudotsuga forests on two sites in the western Cascade Range, Oregon. We examine the influences of disturbance intensity and initial vegetation structure on the origin, direction, and rate of physiognomic change.Broad‐scale changes in vegetation structure differed between sites. On Watershed 1, herbs dominated for 11 yr, after which shrubs became co‐dominant. In contrast, Watershed 3 never exhibited a distinct, transitional shrub phase ‐ herbs dominated for 18 yr, after which trees assumed co‐dominance.The pattern and rate of physiognomic succession also varied among pre‐disturbance plant communities and with disturbance intensity. Differences among communities largely corresponded with initial vegetation structure, reflecting the disturbance tolerance of forest herbs and shrubs. Canopy closure occurred most rapidly in the initially depauperate, but tree‐dominated Coptis community. Along the disturbance gradient, shifts from herb to shrub dominance occurred earlier on burned than on unburned sites due to rapid development of invading shrubs, whose germination and establishment were stimulated by fire. However, subsequent transitions to tree dominance showed no clear relationship with disturbance intensity.These long‐term trends suggest that pre‐disturbance community structure and disturbance intensity are major determinants of physiognomic succession, but that their effects may be modified by historical or stochastic factors such as limited seed availability or local fluctuations in weather.