CONSTRAINTS ON ESTABLISHMENT OF PLAINS COTTONWOOD IN AN URBAN RIPARIAN PRESERVE

Plot sampling and hydraulic modeling were combined to investigate esta blishment and survival of plains cottonwood along Boulder Creek, an ur ban stream on the Colorado Plains. We tested the hypothesis that estab lishment is limited to bare, moist surfaces produced by spring floodin g in the current year. No cottonwood germination was observed in 1989 when peak flow was low. A moderate peak of 11.9 m(3)/s in 1990 resulte d in cottonwood establishment on surfaces inundated by discharges less than 15 m(3)/s. Surviving sapling cottonwoods established before 1989 occurred on surfaces inundated by discharges of 15-31 m(3)/s and date d to years with peak flows over 15 m(3)/s. Since 1969, establishment o f cottonwood has been limited to a floodplain 21 m wide. Flow regulati on and channel stabilization upstream and downstream of the study area have reduced the channel movement and overbank flooding that otherwis e would have created bare, moist sites suitable for cottonwood establi shment across a wider area. GIS analysis of aerial photographs shows t hat between 1937 and 1992 the channel has not moved and forest has enc roached upon formerly open areas near the channel. A terrace, 317 m wi de, is now dominated by trees that can reproduce in the absence of dis turbance, especially the exotic crack willow.