Root disease and canopy gaps in developed areas of Yosemite Valley, California

Root disease in conifers is among the most important vegetation management considerations in the Yosemite Valley of Yosemite National Park, California. Large trees with root decay have fallen in the valley causing human fatalities and property damage. Many of the problems associated with root disease can be traced back to the history of vegetation management in Yosemite Valley. Wildfire suppression and meadow draining have led to increased densities of conifers at the expense of meadows and oak woodlands. Root diseases, primarily caused by Heterobasidion annosum and Armillaria mellea, associated with bark beetles have led to conifer mortality and the formation of gaps in the forest canopy. We mapped 279 canopy gaps (mean area 1788 m 2, range 29 m 2 to 5.4 ha) that occupied about 30% of 169 ha of mixed-conifer forest in the developed eastern end of Yosemite Valley. In 1971, canopy gaps associated with root disease occupied only 8.7% of gap area detected in 1999. Gaps associated with H. annosum occupied 41 ha (82% of all gap area). Seven of these gaps were larger than 1 ha and resulted from coalescence of multiple smaller gaps. Armillaria mellea was identified as the causal agent in gaps with a total area of 4.1 ha (mean area 511 m 2, range 29–5343 m 2). Hazardous trees in canopy gaps have been felled by the park service since the 1970s, but the option of tree removal in developed areas must be continually evaluated in relation to the often competing requirements for public safety, aesthetics, and ecosystem function.