SAGEBRUSH STEPPE LANDSCAPES IN THE NORTHERN GREAT PLAINS: GEOMORPHIC LEGACY AND PLANT-SOIL INTERACTIONS AT THE AMERICAN PRAIRIE RESERVE

Biophysical interactions at the western end of the Northern Great Plains reflect a legacy of landscape evolution and land use in the context of semi-arid climate. For land managers working to support biotic resiliency in these systems, an understanding of these interactions is critical for effective stewardship. Here we report on plant-soil interactions at the American Prairie Reserve in north-central Montana, where the transition from cattle to bison management raises questions about controls on plant community distribution. In this region, as aridity and salinity become limiting, grasslands give way to sagebrush steppe communities, reflecting a transition to strong control of plant community distribution by the interaction of geomorphic drivers with soil development. This threshold in plant-soil interaction marks both decreased rainfall and the exposure of clay-rich, saline marine sedimentary rocks following advance and retreat of the continental ice sheet 12-20 ka. Soils in this area are primarily characterized by limited redistribution of salts, both downward within profiles and as a function of microtopographic variation and surface stability. Five primary vegetation associations with distinct plant species composition in the core of the APR are closely tied to geomorphic position and soil character. Our results indicate that the development of sagebrush steppe and reduction of soil salinity is an outcome that reflects long-term surface stability over the last ~9,000 years. A positive feedback results from establishment of prairie clubmoss and diverse herbaceous plant species in sagebrush interspaces, enhancing surface soil stability and increasing surface soil organic carbon content. Geochemical comparison of parent shales with soils reveals how salinity patterns and plant communities emerge as a function of time and surface disturbance, accounting for both natural processes of erosion and human activity. These interactions in turn drive water redistribution and grazer response, as well as human choices about land management.