Effects of mountain pine beetle on forest structure and fuel load 25-30 years after an outbreak in Western Colorado.

摘要

Mortality of lodgepole pine (Pinus contorta (Dougl. Ex. Loud.) caused by mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) has caused concern about long-term forest structure and wildfire hazard changes. In 2010, I identified and sampled areas affected by a 1980s MPB outbreak to quantify forest changes in the 25-30 years following mortality. Stands in Eagle County, Colorado with >30% 1980s lodgepole pine mortality were identified using USDA Forest Service aerial survey maps and inventory data. Stands fell into two forest type groups: lodgepole pine and mixed conifer. I sampled 20 stands to measure forest species and size structure and down woody fuel accumulations. I compared 1980s inventory data to 2010 data to find differences between forest type groups in their post-outbreak changes.;Lodgepole pine stands recovered to pre-outbreak overstory total basal area, density, and species composition by 2010, while in mixed conifer stands basal area and density were significantly less in 2010 than before the outbreak. In mixed conifer stands, lodgepole pine overstory basal area was reduced from 66% of total in the 1980s to 51% in 2010.Understory tree density increased roughly six-fold in both forest types between the 1980s and 2010. The overall increase in understory density was due to a ten-fold increase in seedling/sapling (trees 0.6 m tall to 3.8 cm dbh) numbers. In lodgepole pine stands, the most abundant species in the 1980s understory was subalpine fir, followed by lodgepole pine. By 2010, lodgepole pine and subalpine fir were the majority of larger understory trees; aspen and subalpine fir were most abundant among smaller understory trees. In mixed conifer stands, subalpine fir and Engelmann spruce consistently dominated all understory size classes in the 1980s and 2010. Total down woody fuel load averaged 71 Mg ha-1 and did not differ between forest type groups.;Overall, my results suggest that long-term forest recovery trajectories are dependent on pre-outbreak species composition, though understory densities are likely to increase regardless of non-host species abundances. These shifts in species and size composition by 25-30 years after outbreak likely have substantial impacts on forest health, potential fire behavior and ecosystem processes. We speculate that forest recovery following the current MPB outbreak will be similar to observed changes following the 1980s outbreak in these areas.