Armillaria root disease-caused tree mortality following silvicultural treatments (shelterwood or group selection) in an Oregon mixed-conifer forest: Insights from a 10-year case study

Abstract

In 2005, the 10-year effects of two silvicultural treatments (group-selection and shelterwood) on tree-growth loss and mortality caused by Armillaria ostoyae were compared with no treatment in a mixed-conifer forest in south-central Oregon. Ten years after treatment, Armillaria-caused mortality varied by species and was greatest in Shasta red fir (38% of trees per acre) and white fir (31%) and much less in Douglas-fir (3%) and ponderosa pine (0%). Ten years after harvesting, leave-tree mortality caused by Armillaria root disease was not significantly different in treated than in the unharvested units, nor was there significant diameter-growth response to the harvesting even in large ponderosa pine and Douglas-fir. The silvicultural treatments did have some benefits: (1) leave-tree mortality appeared, at least, not to be exacerbated by harvesting; (2) more disease-resistant pine, cedar, and larch seedlings and saplings survived in the shelterwood-harvest stands and group-selection openings than in comparable areas that were not harvested; and (3) living wood fiber was recovered from the treated stands, as well as dying and dead fuels that could exacerbate wildfire losses. Insights into host-pathogen interactions and recommendations for silvicultural options are presented. This is a case study from a single site and should be interpreted as such.