Interactions between woodpeckers, elk, and drought lead to the collapse of riparian willow communities

Background/Question/Methods

Willows are critical components of Rocky Mountain riparian ecosystems, particularly within Rocky Mountain National Park, where they form the dominant riparian shrub community.  However, willow decline over the past 17 years has led to degradation of riparian ecosystems and the conversion of a tall willow community to one dominated by short willows. Research on willow decline has focused primarily on the effects of ungulate browsing and altered hydrologic regimes controlled by beaver populations.  However, sapsuckers [woodpeckers] and Cytospora chrysosperma fungal infection are synergistically interacting with these known stressors.   Sapsuckers obtain highly nutritious willow sap by creating wells that have detrimental effects on willows, including initiation of fungal infection and stem dieback.  We are examining the timing and causes of willow decline by investigating the roles of climatic and biological factors using aerial photo analysis and epicormic shoot initiation dates. 

Results/Conclusions

Aerial photo analysis demonstrated that willow cover was relatively constant from 1969 through 2001, however, a 40% decline in willow cover occurred between 2001 and 2005.  Using generalized linear regression, peak and average streamflow best explained this decline.  More than two-thirds of epicormic shoots (n = 77) initiated between 2002 and 2005, however, climate variables were not strong predictors of shoot counts per year.  The synergistic effects of sapsucker wounding and Cytospora infection triggered stem dieback. Sapsuckers reduce tall willow heights and ungulate browsing removed epicormic shoots maintaining short stature willows.  We conclude that drought events in the early 2000’s set the stage for other compounding stressors to induce widespread willow decline, with severe implications for riparian ecosystems in the Rocky Mountains.