Native riparian tree establishment along the regulated Dolores River, Colorado

This study investigated influences of flow regulation of the Dolores River, Colorado, by McPhee Dam on establishment of 3 native riparian tree species (Populus angustifolia, Populus deltoides subsp. wislizenii, and Acer negundo). Tree establishment was documented with dendrochronology at 2 reference sites, the unregulated Upper Dolores and San Miguel rivers, and at multiple reaches along the Lower Dolores River for a pre-dam, canal-diverted period (1961-1984) and a post-dam period (1985-2008). Tree establishment along the Lower Dolores River relative to unregulated reference rivers was negatively affected by river regulation for A. negundo, but not for P. angustifolia and P. deltoides. For P. angustifolia, similar establishment occurred at the Upper and Lower Dolores rivers; establishment was low from 1969-1988, with greater establishment occurring prior to and following this period. For P. deltoides, pulses of tree recruitment occurred along the Lower Dolores River after river damming during 1989-1993 and 2005-2008, and overall recruitment after river damming was significantly greater than that observed for unregulated rivers. Establishment of A. negundo was low at the Lower Dolores River during the last 8 years (2001-2008) of the study period. Our results suggest that flow releases from McPhee Dam into the Lower Dolores River between 1985 and 2008 provided appropriate conditions for Populus establishment, particularly at low topographic positions within the active channel in recent years, whereas A. negundo may require greater flows to bolster establishment at the higher topographic positions where it often occurs.