Fluvial disturbance patches and cottonwood recruitment along the upper Missouri River, Montana

The disturbance patches most suitable for seedling establishment of pioneer riparian trees are also subject to future disturbances that produce high s eedling mortality. We are monitoring plains cottonwood seedling establishme nt and mortality along the Wild and Scenic reach of the Missouri River upst ream of Fort Peck Reservoir. Montana at four sites subject to livestock gra zing and four paired, ungrazed exclosures. New seedlings at these sites wer e largely restricted to surfaces inundated by spring and summer flows. Wint er ice drives and livestock grazing are important mortality factors along t he study reach. Livestock grazing reduced seedling densities, although the position of these seedlings in normal flow years means it is unlikely that they will survive future disturbance. Average values of the maximum density parameter of a Gaussian curve of seedling distribution along a hydraulic g radient of inundating discharge were 30 and 114 seedlings/m(2) on ungrazed sites in 1996 and 1997, compared to 19 and 18 seedlings/m(2) for grazed sit es. Water-surface elevations produced by ice drives and damming in the seve re winter of 1995-1996 corresponded to inundating discharges of 1,670 to 4, 580 m(3)/s. No existing trees at the study sites occurred at inundating dis charges below 1.625 m(3)/s. Seedlings established as a result of maximum su mmer flows of 827 and 1,201 m(3)/s in 1996 and 1997 were all below the elev ation of the 10-year return flow of 1,495 m(3)/s. Recruitment of plains cot tonwood trees along this reach of the Missouri River is strongly dependent on infrequent high flows that position moist, bare disturbed patches high e nough for seedlings to establish and survive subsequent flooding and ice sc our, in contrast to other reaches and streams where hydrogeomophic processe s of channel meandering and narrowing produce different patterns of disturb ance patches.