Forest structure and regeneration in the Tsuga heterophylla-Abies amabilis transition zone, central western Cascades, Oregon

The dynamics of stands in the mid-elevation old-growthDouglas-fir (Pseudotsuga menziesii) forests of the central westernOregon Cascade Range were investigated using stand structureanalysis. Trees with different growth rates were commonly presentin the same stand, which resulted often in a weak correlationbetween tree diameter and age. Thus, age data and information onspatial dispersion and disturbance history were collected to aid ininterpreting stand development.
The population structures described were influenced by periodicfires of variable intensity and extent that resulted in a mosaic ofrelatively even-aged patches of different conifer species. Theoverall pattern depends on stochastic factors, species' life historyattributes, and the nature of the fire events. The species thatfirst dominates a site following a disturbance profoundly affectssubsequent stand composition and development. If Pseudotsuga menziesii or noble fir (Abies procera) dominate early, then treeregeneration of western hemlock (Tsuga heterophylla) and oftenPacific silver fir (Abies amabilis) will rapidly occur. Conversely,if Tsuga heterophylla establishes first, regeneration will be absent
or minimal until canopy openings are formed. Several factors thatinfluence regeneration depend on the main canopy composition.Canopy density determines how much light reaches the understorywhich in turn influences the abundance of tree seedlings andherbaceous species. Secondly, windfalls of old trees, branches, andsnags often create canopy openings that are colonized by newseedlings or filled by previously established individuals. Lastly,decaying logs and stumps provide important substrates for treeseedling establishment, especially for Tsua heterophylla. Instands without fire for at least several centuries, canopy openingsare critical for the successful regeneration of the shade tolerantspecies Tsuga and Abies amabilis. The amount of direct beamradiation during the growing season is correlated with numbers andheight growth of seedlings and saplings in larger openings.
These interactions demonstrate the need for successional schemesto include the effects of different canopy species, in addition tovarious disturbances, as factors controlling the rate and nature offorest succession.