Effects of a large infrequent disturbance on a regulated river: Impacts of the 2011 flood on riparian forests on the Missouri River, USA

Background/Question/Methods

Long-term effects of altered flow regimes by dams can lead to changes in floodplain forest landscapes, including reduced regeneration of riparian pioneer tree species (e.g., Populus and Salix), habitat declines for early successional wildlife species, and colonization of former floodplain surfaces by non-native (e.g., Russian olive, Elaeagnus angustifolia) or non-riparian tree species (e.g., eastern red cedar, Juniperus virginiana).  On the Missouri River, where six mainstem dams have regulated flows for more than 60 years, a large flood occurred in the summer of 2011, inundating portions of the floodplain for more than two months.  The unprecedented duration of the flood, coupled with ecological changes from decades of flow regulation, leads to potential for dramatic changes in the ecosystem, but with high uncertainty.  We investigated initial effects of the 2011 flood on riparian forests along six segments of the Missouri River between Montana and Missouri by examining (1) pre-flood (2007-2009) to post-flood (2012) changes in woody vegetation structure and composition in young (<55 years) and older (>55 years) forest stands, (2) changes in floodplain land cover, (3) colonization of post-flood surfaces by cottonwood seedlings, and (4) pre- to post-flood changes in riparian land bird densities across different successional stages of forest.

Results/Conclusions

The flood strongly impacted vegetation structure across study segments, particularly in younger (i.e., <55 years old) stands.  Mean shrub cover and stem density declined by more than half (from 29% to 13% cover, 0.63 to 0.28 stems/m²) within younger stands, while changes in older stands (often unaffected by the flood) were much lower.  Across study segments in stands <55 years old, the mean proportions of standing trees that were recently dead were highest for the introduced Russian olive (27-64%), intermediate for eastern red cedar (24-33%), and lowest for cottonwood (0-18%).  Proportions of standing dead trees were lower in older forests, except on study segments where more extensive flooding of higher surfaces occurred.  Although area of early successional woody habitats declined, recruitment of new cottonwood seedlings was widespread on newly formed sandbars.  Many riparian bird species declined in abundance from pre-flood to post-flood surveys, particularly species (e.g., Bell's Vireo) specializing in early successional habitats, although some mature forest species also declined.  Long-term effects of the 2011 flood on riparian forest habitats along the Missouri River will depend on how vegetation recovers from flood damage and the extent to which new cottonwood regeneration compensates for losses in early successional stands.