Extensive Die Back and Mortality of Riparian Salicaceae along the Bill Williams River, Arizona
 
Patrick Shafroth1*, April Johnson2, Richard Thaxton3, Steven Sesnie4, Jonathan M. Friedman1
 
1 U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO, USA; shafrothp@usgs.gov
2 Pennsylvania State University, University Park, PA, USA; ajbaconjohnson@gmail.com
3 Colorado State University, Fort Collins, CO, USA; rthaxton@usgs.gov
4 U.S. Fish & Wildlife Service, Albuquerque, NM, USA; steven_sesnie@fws.gov
 
Low water availability is the most commonly reported cause of stress, die back and mortality in riparian forests dominated by trees in the Salicaceae family (cottonwood and willow) in semiarid western North America. Reduced water availability can be the result of low streamflow conditions that result from weather or climatic factors, but often water management associated with dam operations and/or groundwater pumping contribute to low water stress in riparian forests. Our study aims to quantify how changes in flow, groundwater levels, precipitation, and temperature affect the survival and growth of Salicaceae trees on the Bill Williams River (BWR) in western Arizona.
 
The Salicaceae forests of the lower BWR are unique and of high conservation value. Extensive crown die back and mortality of riparian trees along the lower Bill Williams River occurred between 2014-2017. We used remotely sensed (Enhanced Vegetation Index--EVI) and field (tree cover, mortality, growth) data to quantify this forest decline. In addition, we analyzed surface and groundwater data to examine the working hypothesis that riparian forest die back is due to low water availability. At the landscape scale, departures from long-term average EVI values indicate relatively stable conditions in the upper 36 km of the BWR, but dramatic declines in the lower 14km of the BWR between 2014-2017. Field data from 26 plots sampled in spring 2017 reveal that the dead cottonwood and willow trees in the lower BWR plots ranged from 30-100%, and living trees in those plots had crown volumes only 5–37% of potential maximum. A several year period of reduced surface water flow volumes released from Alamo Dam at the upstream end of the BWR, combined with groundwater pumping upstream of the declining forest suggest a connection between reduced water availability and Salicaceae die back and mortality.