LONG‐TERM COTTONWOOD FOREST DYNAMICS ALONG THE UPPER MISSOURI RIVER, USA

Abstract

ABSTRACTThe upper Missouri River bottomland in north‐central Montana, USA, retains much of the physical character it had when traversed by Lewis and Clark around 1805. We used geospatial data to quantify long‐term changes in the distribution of bottomland vegetation, land use patterns and channel planform for a 257‐rkm segment of the Missouri River above Fort Peck Reservoir. This segment is less ecologically altered than downstream segments, but two dams completed in the mid‐1950s have decreased the frequency and magnitude of floods. The area of forest is sparse because of geomorphic setting but, contrary to public perception, has remained relatively constant during the past century. However, the stability of forest area obscures its spatial and temporal dynamics. We used state and transition models to quantify fates and sources of forest during two periods: 1890s–1950s and 1950s–2006. Total forest area was 6% greater in 2006 than it was in the 1890s, largely due to reduced forest loss to erosional processes and gains related to progressive channel narrowing. Channel narrowing resulted in part from human‐caused peak flow attenuation. A modified transition matrix, used to examine future steady‐state conditions, projected little change in forest area; however, these projections are likely an overestimate. The extent to which 2006 forest area represents a transient adjustment to a new flow regime versus a dynamic, quasi–steady state will be determined by the long‐term interplay among hydrologic factors, channel processes, water management and land use practices. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.