Graduate Thesis Or Dissertation
 

Structural development of late successional forests in the central Oregon Coast Range : abundance, dispersal, and growth of western hemlock (Tsuga heterophylla) regeneration

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  • Patterns of western hemlock regeneration were studied in relation to forest structural development and environment in the Oregon Coast Range. Density of western hemlock seedlings was examined across the climatic gradient from cool, moist coastal areas to the seasonally hot and dry Willamette Valley Margin. Seedling densities were most strongly related to stand-level variables including stand age, frequency and abundance of overstory western hemlock trees, abundance of coarse woody debris, decreasing shrub cover, and increasing overstory conifer cover. Significant regional climate and topographic variables included mean annual temperature, precipitation and elevation. Dispersal distances between western hemlock seedlings and potential parents were characterized by measuring distances between seedlings and potential parents. Cone production in western hemlock trees by size class was estimated in four late successional Coast Range stands. Dispersal distances were most commonly short - generally within 10 meters of the nearest potential parent tree. Cone production was related to diameter and crown position, with larger codominant and dominant trees producing more cones than smaller trees. Dynamics and age structure of western hemlock in late successional forests was described in terms of age and size distributions, rooting patterns, and growth rates of western hemlock seedlings, saplings, and trees across the climatic gradient and related to stand level factors in six late successional forests. Time between initial and subsequent western hemlock regeneration pulses varied between 40-70 years. These patterns were observed in all stands. Striking differences in substrate colonization patterns were observed between seedlings and trees and at climatic extremes. Seedlings were predominantly observed on decaying wood while trees were generally observed on forest floor substrates. An exception to this pattern occurred in near-coastal sites where western hemlock of all size classes were found almost entirely on decaying wood substrates. Growth rates across size classes were also calculated and related to regional and stand-level variables. Highest growth rates occurred in low elevation coastal sites, in stands containing the highest levels of coarse woody debris, and the highest level of canopy gaps. Seedling growth rates averaged 4 cm/year for seedlings, and between .2-.6 cm/year diameter growth for overstory saplings and trees.
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