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

however, can vary by Armillaria clone, site and soil characteristics, disturbance history, and plant association (McDonald et al.1987, Curran et al. 2007). Single genets (clones) of A. ostoyae (“the humongous fungus”) have been estimated to be 2,400 ac in size and 2,200 years old in northeast Oregon (Schmitt and Tatum 2008), and wildfires may have little effect in reducing populations of subterranean root pathogens such as A. ostoyae (Ferguson et al. 2003, Fields 2003). A name change to Armillaria solidipes has been recently proposed for A. ostoyae (Burdsall and Volk 2008). Besides tree mortality, Armillaria infections may cause crown dieback, resinous-root lesions, treegrowth reductions, lower-stem deformations, stand-structural changes, and down-wood accumulations (Bloomberg and Morrison 1989, Reaves et al. 1993, Cruickshank et al. 1997, 2009, Mallett and Volney 1999, Cruickshank 2002, Fields 2003, Omdal et al. 2004). The association between tree wounding and Armillaria infection is not well known. Severe wounding of roots or stems could exacerbate existing root infections and possibly result in tree mortality. Armillaria root disease has been associated with stressed trees resulting from soil disturbance, high stand densities, drought, or other pest attack (Wargo and Shaw 1985, Hadfield et al. 1986, Williams et al. 1986, Shaw and Kile 1991). In severely infected forests in the Pacific Northwest, tree mortality caused by A. ostoyae has been estimated at 25 ft/ac per year on 1,500 ac in south-central Washington (Shaw et al. 1976), 50 ft/ac per year on 575 ac in south-central Oregon (Filip 1977), and 30 ft/ac per year on 2,500 ac in central Oregon (Filip and Goheen 1982). Dead root systems may be completely colonized by Armillaria 1 to 5 years after tree death or harvesting, depending on Armillaria genet, tree species, size of the root system, number of infected-root lesions, and extent of root colonization by insects or other fungi. Because Armillaria can persist for millennia on infected mixedconifer sites and therefore eradication is futile, we tested the hypothesis that silvicultural treatments can reduce growth loss and mortality caused by Armillaria and reestablish Armillaria-tolerant tree species. The study was conducted in an uneven-aged mixed-conifer forest (but predominantly true fir). The objectives of our study were to determine whether significant differences occur between treated and untreated areas in the amount of 10-year leave-tree dbh-growth loss and mortality with shelterwood harvesting or group-selection harvesting, and to evaluate the frequency of mortality among six A. ostoyae“tolerant” species that were regenerated in the shelterwood and group-selection stands: ponderosa pine, lodgepole pine (Pinus contorta), sugar pine (Pinus lambertiana), coastal Douglas-fir, western larch, and incense-cedar.