Pedogenic Pathways in Volcanic Ash Mantles of the Northern Rocky Mountains.

The eruption of Mount Mazama deposited a mantle of volcanic ash across the Pacific Northwest region. Two decades of collaborative research between the University of Idaho, USDA-Natural Resources Conservation Service, and the US Forest Service has documented pathways of soil formation in forested regions of the northern Rocky Mountains influenced by this ash. Under the dominant climatic conditions of the region – xeric/udic soil moisture regimes; frigid/cryic soil temperature regimes – weathering of tephra typically gives rise to a colloidal fraction dominated by allophane and ferrihydrite. This pedogenic pathway results in allophanic Andisols (Udivitrands, Hapludands) and Andic subgroups dominating the landscape. At higher-elevation sites with greater effective precipitation, podzolization processes are favored, resulting in formation of Spodosols and Spodic intergrades. These soils are extremely acid and have high aluminum activity as a result of podzolization. In areas where stand-replacing disturbance has occurred, soil genesis may proceed along two pathways. Where bracken fern communities become established following disturbance, increased belowground additions of organic matter give rise to non-allophanic Andisols. Through this pathway, soils develop relatively high levels of exchangeable aluminum and do not appear to be conducive to conifer regeneration, resulting in an arrested stage of secondary plant community succession. In other areas, erosional processes following stand-replacing disturbance such as severe fire can result in almost complete loss of the ash mantle. This promotes an irreversible transition to another ecological state in which soils are no longer able to support the forest communities that existed prior to disturbance. This research underscores the importance of environmental factors and disturbance in determining the direction and extent of pedogenesis.