Since the mid 1990s a dramatic decline in the condition of tall willows has occurred in Rocky Mt. National Park, especially in the Kawuneeche Valley (Colorado River). The matched photos below from 1995 and 2008 illustrate the dieback. This problem is of great concern because beavers are the main drivers of the Colorado River floodplain hydrologic regime, and without tall willows they could not build dams. But what was causing this dieback of tall willow stems?
The most obvious cause is heavy browsing by elk and moose. However many tall willow stems are dead without any obvious browsing damage. Ph.D. student Kristen Kaczynski is working on these issues and is exploring the possibility that stem death is caused by species of Cytospora fungi. The fungi do not harm intact stems, but once a stem is damaged the fungi enters the vascular tissue and kills the stem. The main cause of damage to willow stems is by red-naped sapsuckers, small woodpeckers who feed on willow sap and create geometric patterns on the stems with their sap wells. The stem develops fungal pycnidia (black dots on center photo below) and some erupt in sexual fruits (right).

2014Kaczynski&CooperWEM
kaczynski&CooperFEM2013
2014Kaczynski et al. Botany