Graduate Thesis Or Dissertation
 

Distribution of understory species in relation to maximum and minimum tree influence in the montane forest of the Central Oregon Cascades

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  • Twenty sites of uniform topography and soil were select ed in the montane forest found on the East flank of the Central Oregon Cascades. These sites were located along a vegetational gradient composed of five plant communities: Abies/Pachistima, Pinus/Ceanothus, Pinus Arctostaphylos-Purshia, Pinus/Purshia/Festuca, and Juniperus Festuca. An attempt was made to relate the distributional pattern of understory species to six aspects of tree influence (overhead cover, amount of shade, daily differences in shading, seasonal differences in shading, amount of litter deposit, and species of tree depositing the litter) understory species' competition, and plot location. Cole's index and hierarchical classification analysis were statistical methods used to correlate the understory species' pattern to tree influence, understory species' competition, and plot location. From frequency data of the 63 species analyzed by Cole's index, only two showed an inconsistent distributional pattern in relation to tree influence. Similarly, three distributional patterns were noted. (1) Species were distributed at random in the area of maximum tree influence regardless where the species occurred along the gradient. (2) Species were distributed at random in the area of minimum tree influence and were absent in the area of maximum tree influence regardless where the species occurred along the gradient. (3) Species at a point along the gradient were distributed at random in areas of maximum and minimum tree influence; but on more xeric plots the species were distributed similar to pattern 1, and on more mesic plots species were distributed similar to pattern 2. In pattern 3, the point along the gradient where the species were distributed at random to areas of maximum and minimum tree influence may suggest an optimum point along the gradient where the effect from tree layer influence is minimal. This point provides a basis for comparing the environmental tolerances of the species and ordinating the stands. When the species' density data were analyzed by hierarchial classification to determine what factors of maximum and minimum tree influence effected the density distribution of the species, the following patterns were noted. Normally, species with highest densities in areas of maximum insolation or sparse litter were prominent on the xeric end of the gradient, and those species with maximum densities in areas of low insolation or deep litter were prominent on the mesic end of the gradient. Chamaephyte species sampled had highest densities in areas underneath the trees and usually near the me sic end of the gradient. Therophyte species sampled had highest densities in open areas usually near the xeric end of the gradient. Thus, a theoretical model was constructed using data obtained from this structural analysis of internal distributional patterns of understory species. The distribution of the species is much wider according to the theoretical model than was actually found by constancy data, but the differences when statistically analyzed are not great enough to reject the model at the 1% significance level. The model suggests predictable patterns of species' distribution within the five community types and may reflect the relative stability of these species within the community types.
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