Graduate Thesis Or Dissertation

Spatial relationships of vesicular-arbuscular mycorrhizae, soil fauna and soil nutrients in the juniper-sagebrush-grass communities of central Oregon

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  • A Study conducted at The Island, Lake Billy Chinook, in Central Oregon, examined differences in the pattern of soil properties between a sagebrush-grass and a juniper-sagebrush-grass community. Juniper invasion is linked with the desertification process in which the sagebrush shrubs and perennial grasses decline. Patterns in soil nutrients and other properties can influence the distribution of vegetation and vice-versa. With an increase in heterogeneity of a soil resource, plants may fail to regenerate if the patch size of the resource is smaller than required. The study tested the hypothesis that juniper invasion in a sagebrush-grass community changed the heterogeneity of soil properties at scales relating to the vegetation. Soil nutrients, moisture, pH, vesicular-arbuscular mycorrhizal (YAM) infection and soil micro-arthropods were examined. Soil properties are frequently very variable such that the difference between the means of two communities may not be detectable by parametric statistical analyses alone, Spatial statistical methods adapted from geostatistics, including the semivariogram and correlogram, differentiated between the patterns of soil properties of the two communities. The semivariograms provided information on the range of autocorrelation and relative variance at increasing lags. Moran's I and Geary's C correlograms provided estimates of the average of patch and interpatch size and distribution. Communities with junipers had lower summer soil moisture, fewer soil micro-arthropods and a higher rate of vesicular-arbuscular mycorrhizal infection. Soils under sagebrush contained more moisture and soil arthropods than bare ground. However, there was more bare ground and less sagebrush in the community with junipers than in the sagebrush-grass community. Long range variation at lag distances greater than 18 m was found in both communities, indicating that pattern forming processes other than juniper were operating at these scales. Variation at scales related to sagebrush distribution occurred in soil moisture, the nitrogen fractions and soil fauna. Soil fauna also showed variation at scales relating to the grasses. YAM infection data showed no pattern structure at all scales measured, and a high nugget variance, indicating variation below scales of 0.5 m. Although the scale of the pattern (range and average patch size) did not differ between sagebrush-grass and juniper-sagebrush-grass, short range variation (<10 in) was higher than long range (>10 m) more frequently in the plots with junipers. This implied more contrast in the data and a more abrupt edge between patch and interpatch areas in plots with juniper.
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