Forest Health Decline in a Central Oregon Mixed-Conifer Forest Revisited After Wildfire: A 25-Year Case Study

Abstract

Abstract A 500-ac mixed-conifer forest near Cache Mountain in central Oregon was examined in 1979, 1992, 2002, 2004, and 2005 to document causes of forest health decline and subsequent wildfire damage. The site is dominated by grand fir (Abies grandis) and ponderosa pine (Pinus ponderosa), with some lodgepole pine (Pinus contorta) and subalpine fir (Abies lasiocarpa). Part of the area was clearcut or shelterwood harvested from 1983 to 1985. Between 1979 and 1992, grand fir increased substantially, whereas ponderosa pine decreased in stems and basal area/ac in the unharvested areas. From 1979 to 2002, grand fir experienced severe mortality that was caused primarily by the root pathogen, Armillaria ostoyae, and the fir engraver (Scolytus ventralis). In 2003, a wildfire burned all of the study area, and by 2004, most of the grand fir, subalpine fir, and lodgepole pine was killed. The least amount of mortality from fire occurred in the larger-diameter ponderosa pine. Two years after the 2003 fire, some of the grand firs with bole or crown scorch that were alive in 2004 were killed by fir engravers by 2005. For ponderosa pines, only a few trees with bole or crown scorch that were alive in 2004 were killed by bark beetles, mostly mountain pine beetle (Dendroctonus ponderosae) and red turpentine beetle (Dendroctonus valens), by 2005. This case study has relevance to current interpretations of forest health in similar mixed-conifer forests, the major causes of forest health decline, and the role of fire in forest health.