Willows are critical components of Rocky Mountain riparian ecosystems, particularly within Rocky Mountain National Park (RMNP), where they dominate the riparian shrub community. However, willow decline over the past 15 years has led to a dramatic change in riparian ecosystems in the Park. Our research has shown that willow canopies have decreased by an average of 65% within the Kawuneeche Valley, at the headwaters of the Colorado River. Research on willow decline has focused primarily on the effects of elk browsing and altered hydrologic regimes controlled by beaver populations. However, another key secondary stressor, fungal infection by Cytospora spp., is interacting in a novel way with these known factors. Cytospora spp. is a native facultative wound pathogen, requiring physical damage to the stem before infection can occur. Studies on other plant species have shown that when a plant is previously exposed to stress, the fungal infection is more likely to kill the stem. We are investigating the primary interacting stressors – browsing, drought due to absence of beavers, and wounding – that lead to the death of willow stems. Using three thirteen year old ungulate exclosures along a hydrologic gradient, we wounded 60 stems during the growing season and 60 stems during the dormant season to investigate the timing of the wound in relation to fungal infection. In addition, we examined 59 stems that were wounded by sapsuckers.
Results/Conclusions
Sapsuckers selected stems significantly larger in diameter than average, both inside and outside exclosures. A high percentage of stems above sapsucker wounds were killed by fungal infection, while compensatory epicormic shoots below the wound began sprouting an average of three to four years prior to the death of the apical meristem. Using dendrochrolological techniques, we found that a high proportion of stems died in 2006, four years after the extreme 2002 drought. These results show that underlying drought stress, in addition to the presence of a wound, can lead to death of a stem by fungal infection. Therefore, changing hydrologic regimes may set the stage for other compounding stressors to induce widespread willow decline, with severe implications for riparian ecosystems in the Rocky Mountains.
