Volcanic Ash Mantles in Northern Idaho: What Do They Tell Us about Forest Disturbance?.

Volcanic ash from the cataclysmic eruption of Mt. Mazama blanketed much of Pacific Northwest region ~7600 y BP and has subsequently been redistributed by post-depositional processes. The close link between volcanic ash and forest productivity has spurred efforts to model the distribution of volcanic ash mantles across the region. Such efforts in northern Idaho have identified elevation as the terrain attribute that is most strongly associated with ash mantle presence and thickness. However, these models do a relatively poor job of predicting mantle distributions at lower elevations. Data from ongoing soil survey updates and research projects in northern Idaho indicate that the appearance of volcanic ash mantles does not occur gradationally in landscapes of the region. Instead, mantles are either absent or have a minimum thickness of approximately 20 cm. Such an abrupt transition is likely associated with an environmental threshold related to forest community stability. It is further suggested that current-day distributions of volcanic ash mantles may provide insights into patterns of forest disturbance over the past several millennia.