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

Physiognomic patterns may vary significantlyduring succession despite a tendency for larger-growthforms to gradually replace smaller ones. Development ofunderstory structure was observed for 25 yr after harvestof Pseudotsuga forests on two sites in the western Cas-cade Range, Oregon. We examine the influences ofdisturbance intensity and initial vegetation structure onthe origin, direction, and rate of physiognomic change.
Broad-scale changes in vegetation structure differedbetween sites. On Watershed 1, herbs dominated for 11yr, after which shrubs became co-dominant. In contrast,Watershed 3 never exhibited a distinct, transitional shrubphase - herbs dominated for 18 yr, after which treesassumed co-dominance.
The pattern and rate of physiognomic successionalso varied among pre-disturbance plant communitiesand with disturbance intensity. Differences amongcommunities largely corresponded with initial vegeta-tion structure, reflecting the disturbance tolerance offorest herbs and shrubs. Canopy closure occurred mostrapidly in the initially depauperate, but tree-dominatedCoptis community. Along the disturbance gradient,shifts from herb to shrub dominance occurred earlier onburned than on unburned sites due to rapid developmentof invading shrubs, whose germination and establish-ment were stimulated by fire. However, subsequenttransitions to tree dominance showed no clear relationshipwith disturbance intensity.
These long-term trends suggest that pre-disturbancecommunity structure and disturbance intensity are majordeterminants of physiognomic succession, but that theireffects may be modified by historical or stochastic fac-tors such as limited seed availability or local fluctuationsin weather.
Keywords: Clearcut logging; Community structure; Forestunderstory; Growth-form; Seral origin; Slash burning;Succession.