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

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3b591c239

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  • The dynamics of stands in the mid-elevation old-growth Douglas-fir (Pseudotsuga menziesii) forests of the central western Oregon Cascade Range were investigated using stand structure analysis. Trees with different growth rates were commonly present in the same stand, which resulted often in a weak correlation between tree diameter and age. Thus, age data and information on spatial dispersion and disturbance history were collected to aid in interpreting stand development. The population structures described were influenced by periodic fires of variable intensity and extent that resulted in a mosaic of relatively even-aged patches of different conifer species. The overall pattern depends on stochastic factors, species' life history attributes, and the nature of the fire events. The species that first dominates a site following a disturbance profoundly affects subsequent stand composition and development. If Pseudotsuga menziesii or noble fir (Abies procera) dominate early, then tree regeneration of western hemlock (Tsuga heterophylla) and often Pacific 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 that influence regeneration depend on the main canopy composition. Canopy density determines how much light reaches the understory which in turn influences the abundance of tree seedlings and herbaceous species. Secondly, windfalls of old trees, branches, and snags often create canopy openings that are colonized by new seedlings or filled by previously established individuals. Lastly, decaying logs and stumps provide important substrates for tree seedling establishment, especially for Tsuga heterophylla. In stands without fire for at least several centuries, canopy openings are critical for the successful regeneration of the shade tolerant species Tsuga and Abies amabilis. The amount of direct beam radiation during the growing season is correlated with numbers and height growth of seedlings and saplings in larger openings. These interactions demonstrate the need for successional schemes to include the effects of different canopy species, in addition to various disturbances, as factors controlling the rate and nature of forest succession.
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