Riparian willow decline in Colorado: interactions of ungulate browsing, native birds and fungi

Abstract

Willows (Salix spp.) are critical components of Rocky Mountain riparian ecosystems. They provide food for ungulates and beavers; habitat for resident and migratory bird populations, and amphibians; and are integral components of the structure and function of montane riparian ecosystems. In Rocky Mountain National Park (RMNP), willows form the dominant riparian shrub community. However, willow decline over the past 17 years has led to a dramatic change in riparian ecosystems in RMNP, resulting in the conversion of a tall willow community to a community dominated by short willows, with cascading effects on habitat for beaver and migratory and resident songbirds. Research on willow decline has focused primarily on the effects of ungulate browsing and altered hydrologic regimes controlled by beaver populations. However, damage from sapsuckers [woodpeckers] and Cytospora chrysosperma fungal infection are interacting with these known stressors. My dissertation research investigates willow decline using a multifaceted approach and covers three main topics: 1. The biotic and climatic factors contributing to the willow decline; 2. The spatial and temporal dynamics of willow decline; and 3. The effect of altered water tables and increased temperatures on Cytospora fungal infection and willow production. My research provides a comprehensive new understanding of the dynamics of willow decline in RMNP that can be applied to riparian sites throughout the Rocky Mountain ecoregion. My first study explains the, previously unidentified, interaction of sapsucker wounding, Cytospora fungal infection and ungulate browsing in the decline of the riparian ecosystem. My second study demonstrated that the increase in moose populations explained the sharp decline in willows that occurred between 2001 and 2005. Past climate, such as the droughts of the early 2000s, was not the main driver in the decline. Finally, my third study found that willow stems are highly susceptible to fungal infection and my experiment demonstrated that once C. chrysosperma is present on a wound, it will form enlarging cankers under a wide range of environmental conditions. Results from my dissertation research support the conclusion that willow decline is more strongly driven by biotic, rather than climatic stressors. This new understanding of the interactions resulting in willow decline will allow land management agencies to develop more effective restoration strategies.