Climatic and Landform Effects On the Distribution of Fine-Textured Volcanic Ash in a Temperate Forest Ecosystem.

Soils of temperate forest ecosystems of the Inland Northwest, USA are often significantly influenced by fine-textured volcanic ash.  These soils have been associated with high forest productivity and increased soil water holding capacity when compared with similar soils, but without volcanic ash. Currently, there is increasing awareness of these properties by forest land use planners and a subsequent desire to spatially identify the distribution of these soils. Past volcanic ash modeling efforts primarily focused on landform effects upon ash distribution with varying degrees of success; however, there is growing evidence that climate induced changes to vegetation communities may play as large a role in volcanic ash distribution as topography. A study area in the Clearwater Mountains of north central Idaho, USA was selected to explore the interaction between climate and topography on volcanic ash distribution. Fourteen topographic and fifteen climatic variables were selected for analysis. Multicollinearity between independent variables was addressed through correlation matrices and cluster analyses. Subsequent exploratory analyses tested independent variable parameter stationarity assumptions inherent to traditional ordinary least squares means analyses (OLS). Results indicate that degree days >5°C (DD5), the ratio of summer to total precipitation (PRATIO), elevation (ELEV), plan curvature (PLANC), and compound topographic index (CTI) are significantly related to the distribution of volcanic ash.  However, the parameter estimates associated with each of these variables were found to be non-stationary (p<0.05), indicating that traditional OLS means tests are unsuitable to capture the complex interactions responsible for volcanic ash distribution. Parameter estimates not only varied within space, but also switched signs. Overall, thick ash mantles were associated with cool/moist climates at higher elevations, divergent landscape positions and in zones of increasing deposition.  These findings suggest that volcanic ash is stabilized by lush vegetation communities associated with cool/moist environments and is less susceptible to gravitational movement on divergent topography.  Geospatial maps were created to illustrate these varying effects accompanied with a predictive surface utilizing nonstationary parameter estimates.