Document: DAV-3-69-42

Disturbance in mixed-conifer forests of Yosemite Valley, California.

RIZZO, D.M.* and G.W.SLAUGHTER

University of California, Davis, CA 95616, USA ¹

Abstract:
Fire suppression and meadow draining have led to increased densities of conifers at the expense of previously dominant meadow and oak woodland vegetation types since Euroamerican settlement of Yosemite Valley 150 years ago. Mixed-conifer forests now cover over 50 % of the area in the valley; Pinus ponderosa and Calocedrus decurrens are the dominant species with lesser numbers of Quercus kellogii, Abies concolor, and Pseudotsugae menziesii. Based on a transect survey of the undeveloped western end of the valley, we estimate that approximately 31 % of the mixed-conifer forest is in expanded gaps. Surveyed gaps had a mean area of 1007 m² (median 459 m², range of 81 to 8300 m²). Root disease was associated with over 80 % of all gaps and was the most common form of disturbance. Heterobasidion annosum (30% of gaps) and Armillaria mellea (30%) were the most common root pathogens; other pathogens were associated with 7 % of gaps. Bark beetles were associated with all root disease gaps, often as the final cause of death of infected trees, but were solely responsible for tree mortality in 14% of gaps. Gaps caused by combinations of biotic and abiotic (fire, wind) factors were associated with 19 % of gaps. Given the extent of root disease in the valley, excluding such long term factors as climate, and with fires suppressed, we believe root disease is now the predominant ecological factor driving forest succession in Yosemite Valley. Root disease gaps differ from episodic gap-causing factors, such as fire, wind, or bark beetles, in that they may enlarge over decades due to spread of the pathogen to adjacent trees. Enlarging gaps tend to have chronic tree mortality in all stages of decay, whereas episodic occur during one or a few years leading to a homogeneous cohort of trees decaying at a similar rate through time. The high prevalence of root disease in Yosemite Valley appears to be the result of a series of natural resource management actions taken many years earlier.

Keywords: canopy gaps, disturbance, pathogens, bark beetles, conifers

This abstract is being presented at: 9:15 AM in session:
Oral Session #22: Multiple Disturbance Effects, Including Fire.