Effects of Beaver (Castor canadensis) Herbivory and Wildfire on Foliage Density and Woody Debris, San Pedro Riparian National Conservation Area, Arizona

Beaver (Castor canadensis) were reintroduced beginning in 1999 on the San Pedro Riparian National Conservation Area. Herbivory is the most obvious effect from beaver, but little research exists of herbivory effects after reintroduction. Fire processes may also have substantial effects to vegetation, and both beaver and fire are important ecological components for comparing effects and subsequent management decisions. The objective of this research, conducted during 2008 and 2009, was to determine any effects to foliage density caused by beaver herbivory and wildfire as compared to control sites. There were significant differences in foliage density between control, beaver, and wildfire sites, with lower foliage density and greater above-ground heights associated with wildfire sites. Although near the significance level, there were no interactions between control, beaver, or wildfire sites for changes in foliage density at different heights. Mean Fremont cottonwood, Gooddings willow, and seep willow foliage density was significantly different between control, beaver, and fire sites. Fremont cottonwood had significantly higher foliage density at control sites than at fire sites, but not between control and beaver sites or between beaver and fire sites. Gooddings willow density was significantly higher at control and beaver sites than fire sites, with no significant difference between control and beaver sites. Seep willow foliage density was significantly higher at control and beaver sites compared to fire sites, but not significantly different between control and beaver sites. Mean downed and dead wood cover was not significantly different between control, beaver, and fire-influenced sites, between beaver and control sites, between control and fire sites, or between beaver and fire sites. Management implications include more strategic wildfire planning and preparedness, achieved through integrated tamarisk control in the riparian area and use of prescribed fire in upland habitats to reduce fire size and severity that threaten the riparian gallery forest and its ecosystem services.