Status and Trend of Cottonwood Forests Along the Missouri River
GEOLOGICAL SURVEY FORT COLLINS CO BIOLOGICAL RESOURCES DIV
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From 2007-2009, we studied the current 2006 and historic 1892, mid-1950s extent, current age distribution, and plant species composition of plains cottonwood Populus deltoides and non-cottonwood riparian stands along eight study segments of the Missouri River between Fort Benton, Montana and Kansas City Missouri, covering 930 river miles 1500 km or over 13 of the rivers length. These segments included all of the unchannelized and unimpounded segments below Fort Benton, as well as portions of two impounded and one channelized segment. Based on GIS analyses of historic maps and aerial photography, the combined area of forests, woodlands, and shrublands in the historic floodplain declined 47 across all study segments from 1892 to 2006, with losses linked to clearing for human land uses primarily agricultural cropland or inundation by reservoirs. Most forest loss occurred between 1892 and the 1950s, while most shrubland loss has occurred from the 1950s to 2006. As of 2006, we estimate that a total of 75,600 hectares 186,900 acres of shrubland, woodland, and forest occurred within the mapped area of the eight study segments, with 66,800 hectares 165,000 acres or 88 of the total, composed of patch types which contain cottonwood as a significant component. Most 62 of the cottonwood area is composed of stands 50 years old, and only 14 is from stands that have recruited in the last 25 years. These patterns, along with significant historic declines in shrubland and sandbar area on most segments, indicate that the fluvial geomorphic dynamics that drive cottonwood recruitment have been reduced, and cottonwood regeneration compromised, under the river management practices of at least the last 25-30 years. A significant proportion 24 of forest in the 25-50 year age class, however, suggested that a temporary pulse in recruitment accompanied geomorphic adjustments in the channel during the first 2-3 decades after dam closure on some segments.