PARENT SESSION
Oral Session #70: Riparian Ecology. Presiding: R. Cosgriff.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas I.
Water relations and biomass adjustments of cottonwood in response to river regulation.
Williams, Christopher1,2, Cooper, David1,3, 1 2 3
ABSTRACT- The decline of riparian cottonwood forests in western North America has been attributed to river regulation by large dams that reduce peak flows resulting in lower riparian water tables. Water table depression can cause water stress in riparian vegetation resulting in accelerated plant senescence, branch sacrifice, or death. We examined the link between river regulation by Flaming Gorge Dam and Fremont cottonwood decline along the Green River. Floodplain hydrology and cottonwood water relations were monitored on the unregulated Yampa River and the regulated Green River. We measured floodplain water levels, soil water, cottonwood xylem potential, and leaf-level stomatal conductance throughout the 1998 and 1999 growing seasons. Leaf and root biomass of mature cottonwood stands were quantified. Green River regulation has lowered the annual peak flow by 65% and approximately doubled baseflows. The reduction in stage change has lowered soil water in Browns Park by as much as 70% when compared with the Yampa River floodplain. Similar daily and seasonal trends in stomatal conductance and xylem potential indicated that cottonwood along the regulated river were not more water stressed. Stand leaf area was 10% to 30% lower along the regulated river and root density and biomass were 2 to 3 times higher for cottonwoods along the unregulated river. Similar water relations but lower biomass suggest that cottonwoods along the Green have experienced dieback to a level supported by the water available under the regulated river hydrologic regime.
KEY WORDS: cottonwood water relations, river regulation, floodplain hydrology, leaf area, root biomass, xylem pressure potential, stomatal conductance