Across western North America, flow regulation by dams has reduced cottonwood (Populus spp.) recruitment along meandering rivers. The purpose of our work was to assess historic changes and the current status of cottonwood forests along the regulated Missouri River. We assessed historic land cover change, cottonwood (Populus deltoides) forest age distribution, and plant species composition within riparian forest stands on eight reaches of the Missouri River between Kansas City, Missouri and Fort Benton, Montana. We interpreted and digitized land cover within a GIS from historic maps (1892) and aerial photography (mid-1950s and 2006) and classified forest stands into approximate age classes (<10, 10-25, 25-50, 50-114, >114 years) based on overlays of historic imagery and field reconnaissance. We assessed plant community composition and cover in the herbaceous, shrub, and tree strata across a range of stand ages in 304 riparian forest stands. Differences in mean values of Coefficient of Conservatism, wetland score (1-5), relative cover by exotic and native species, and species richness were analyzed across stand age classes, forest type, and reach. In the Gavins Point reach in South Dakota, we reconstructed pre-settlement (1857-69) tree species abundance and compared it with present day patterns from our sampling (2007 and 2008).

Results/Conclusions

Analyses of the historic imagery suggested significant variation in cottonwood age distribution and landscape change among the eight study reaches, with a higher proportion of cottonwood recruitment over the last 50 years in the downstream three reaches and very low recent recruitment on upstream reaches. Stand-level tree species richness increased from upstream to downstream reaches, while total plant species richness per stand was highest in two reaches in the Dakotas. Indices of floristic quality (Coefficients of Conservatism) and wetland affinity of the flora varied significantly among reaches, with the lowest floristic quality and species richness in the reach below Oahe Dam in South Dakota and the highest average wetland score in the reach between Plattsmouth, Nebraska and Kansas City, Missouri. Comparisons of pre-settlement to present-day tree species composition on the Gavins Point reach suggested significant historic changes, with declines in relative abundance of elm (Ulmus americana) and willow (Salix sp.) and with exotic or invasive species that were absent or unrecorded in the 1860s (Juniperus virginiana, Elaeagnus angustifolia, Morus alba, Rhamnus cathartica) now common understory components of remnant cottonwood forests.