Aspen Decline on the Coconino National Forest

An accelerated decline of aspen occurred across the Coconino National Forest, in northern Arizona, following a frost event in June 1999, and a long-term drought that included an extremely dry and warm period from 2001 through 2002, and bouts of defoliation by the western tent caterpillar in 2004, 2005, and 2007. From 2003 to 2007, we monitored aspen mortality and regeneration, and measured associated stand and site variables on randomly-selected sites of the Coconino National Forest where aerial survey had detected dieback or decline. Year of death was observed or estimated since 2000. Xerophytic forests sustained greater mortality than mesophytic forests. Aspen on low-elevation xeric sites (\u3c7500 \u3eft) sustained 95% mortality since 2000. Mid-elevation sites (7500–8500 ft) lost 61% of aspen stems during the same time period; mortality is expected to continue in these sites because some remaining trees have 70 to 90% crown dieback. Less aspen mortality (16%) was observed on more mesic high-elevation sites (\u3e8500 ft). Low-elevation sites are located on northerly aspects while mid- and high-elevation sites are located on various aspects. Overall, diameter distributions showed mortality was not skewed to any particular size class, however, trees with diameters \u3e9 inches generally took longer to die than smaller size classes. Several insects and pathogens were associated with aspen mortality but appeared to be acting as secondary agents on stressed trees. Although aspen ramet production occurred to some degree on all sites with the death of mature trees, aspen sprouts were nearly nonexistent by the summer of 2007 due to browsing by elk and deer. None of the sites studied are grazed currently by domestic cattle. Widespread mortality of mature aspen trees, chronic browsing by ungulates, and advanced conifer reproduction is expected to result in rapid vegetation change of many ecologically unique and important sites