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Scott_et_al - Ecological Applications 7(2 1997 pp 677690...

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677 Ecological Applications, 7(2), 1997, pp. 677–690 FLOOD DEPENDENCY OF COTTONWOOD ESTABLISHMENT ALONG THE MISSOURI RIVER, MONTANA, USA M ICHAEL L. S COTT , G REGOR T. A UBLE , AND J ONATHAN M. F RIEDMAN United States Geological Survey, 4512 McMurry Avenue, Fort Collins, Colorado 80525-3400 USA Abstract. Flow variability plays a central role in structuring the physical environment of riverine ecosystems. However, natural variability in flows along many rivers has been modified by water management activities. We quantified the relationship between flow and establishment of the dominant tree (plains cottonwood, Populus deltoides subsp. monilifera ) along one of the least hydrologically altered alluvial reaches of the Missouri River: Coal Banks Landing to Landusky, Montana. Our purpose was to refine our understanding of how local fluvial geomorphic processes condition the relationship between flow regime and cottonwood recruitment. We determined date and elevation of tree establishment and related this information to historical peak stage and discharge over a 112-yr hydrologic record. Of the excavated trees, 72% were established in the year of a flow . 1400 m 3 /s (recurrence interval of 9.3 yr) or in the following 2 yr. Flows of this magnitude or greater create the necessary bare, moist establishment sites at an elevation high enough to allow cottonwoods to survive subsequent floods and ice jams. Almost all cottonwoods that have survived the most recent flood (1978) were established . 1.2 m above the lower limit of perennial vegetation (active channel shelf). Most younger individuals were established between 0 and 1.2 m, and are unlikely to survive over the long term. Protection of riparian cottonwood forest along this National Wild and Scenic section of the Missouri River depends upon maintaining the historical magnitude, frequency, and duration of floods . 1400 m 3 /s. Here, a relatively narrow valley constrains lateral channel movement that could otherwise facil- itate cottonwood recruitment at lower flows. Effective management of flows to promote or maintain cottonwood recruitment requires an understanding of locally dominant fluvial geomorphic processes. Key words: bottomland; dendrogeomorphology; disturbance; flood; Missouri River; Montana; patch dynamics; Populus deltoides; riparian ecosystems; seedling establishment. I NTRODUCTION Variation in streamflow is central in structuring the physical environment of riverine ecosystems and in determining community composition of lotic and ri- parian environments (Hupp and Osterkamp 1985, Poff and Ward 1989). Flow regime influences species abun- dance by determining the spatial and temporal occur- rence of suitable habitat patches (Bain et al. 1988, John- son 1992, Auble et al. 1994, Poff and Allen 1995). Although extreme flow variation can eliminate species (Zimmerman 1969, Bain et al. 1988), floods or droughts are necessary for the persistence of some species of fish (Meffe 1984) and plants (Nilsson et al. 1991, Fried- man et al. 1996).
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