Factors influencing the distribution of late-seral woody species along the upper Missouri River in central Montana

The spatial extent of riparian areas has been reduced by an estimated 80% since Euro-American settlement. This decline has occurred due to many factors, such as grazing, deforestation, urbanization, invasive plants, pollution, and altered flow regimes. While most riparian research and management has focused on early seral species such as cottonwoods and willows, other woody species, including green ash, box-elder, chokecherry, serviceberry, currant, and red-osier dogwood, also provide important structure and diversity to riparian forests. These species, called late-seral woody species, are typically less prominent than other tree and shrub species in western riparian forests. Recent studies suggest that along the Missouri River in central Montana late-seral woody species distribution is more limited than expected given their prominence in other areas, such as stream sides in eastern Montana. The objectives of this study were to determine whether late-seral woody species abundance is related to grazing intensity, and to assess differences in site conditions that could explain the abundance of late-seral species across a variety of sites. These objectives were examined through a community analysis comparing environmental factors relating to water availability and grazing use, past and present, across a gradient of species composition. These analyses revealed three dominant environmental influences on late-seral species presence and abundance: elevation above the river, geomorphic landform, and soil texture. Late-seral species were most often found at low elevations within the bottomlands and in floodplain channels. Calculated available water content was similar across vegetation types, which was unexpected. Differences in present grazing use were not detected across changing species composition, though historic records did show dramatic increases in cattle numbers in Montana through the 1970’s with only slight declines since this time. These findings, regarding both grazing and environmental variables, could be important for the restoration of late-seral species. If management and restoration of these important riparian species is to take place, understanding the current limitations to their distribution is imperative