Graduate Thesis Or Dissertation
 

The influence of tree cover and shade pattern upon the distribution of understory plants in ponderosa pine stands of central Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8w32r8844

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  • Seven sites of uniform topography and soil were selected within a self perpetuating ponderosa pine forest found on the eastern flank of the central Oregon Cascades. These plots were located along a vegetational gradient caused mainly by an orographic rain shadow. Data gathered included density, frequency, and cover for all understory species, amount of direct overhead tree cover, and amount of shade cast by the tree canopy. An attempt was made to relate the distributional pattern of understory species to influences caused by shade, overhead tree cover, and shrub cover within each pIot. The Dfd index and Cole's index were statistical methods used to correlate the herbaceous species pattern to overhead tree cover, shrub cover, and shade. Of the two, Cole's index appeared to be a more sensitive tool for this study. All the shrub species with the exception of Purshia tridentata showed a consistent distributional pattern to insolation. The position along the vegetational gradient appeared to be related with the shrub's affinity for an insolation class. Those shrubs restricted to the mesic end of the gradient showed an affinity for Iow insolation or deep shade. Conversely, those shrubs occurring on the xeric end showed an affinity for higher insolation. Those shrubs found in the center of the gradient showed an affinity for moderate insolation. Ten herbaceous species out of a total of 16 observed showed apparent affinities for a certain microhabitat within the stand. Plot location, shade, overhead vertical crown cover, and shrub cover influenced the distribution of herbaceous species within the plot. Species found within mesic plots showed a weak affinity for cover, but those on xeric plots showed a strong affinity for cover. A weak indication of the spatial ecological niche was gained for 15 herbaceous species, but a strong indication was observed for Trientalis latifolia. Similarly, some insight into the ecological performance of the species was gained in regard to insolation. Usually the plot location coincided with the species' affinity for an insolation class. If the species occurred mainly on the mesic end of the gradient, it showed an affinity, for deep shade (Lupinus caudatus, Fragaria virginiana, Trientalis latifolia, and Pteridium aquilinum). If the species occurred mainly on the xeric end of the gradient, it showed an affinity for high insolation (Viola purpurea, Carex inops, and Madia minirna). Five species showed some affinity for moderate insolation (Clarkia rhornboidea, Eriogonurn urnbellaturn, Kelloggia galioides , Mimulus nanus, and Stipa occidentalis), but the correlation with plot location was less clear. Four species (Festuca idahoensis, Sitanion hvstrix, Achillea rnillefoliurn, and Lathvrus lanswertii) were not restricted to any part of the gradient. These species showed preference to insolation with the plot, but this preference was not usually the same throughout the gradient. Indications of the limits of ecological potential were found for some of these species, in particular Achillea millefolium which did not occur on any plot where insolation was less than 10%.
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