Thirty Year Change in Lodgepole and Lodgepole/Mixed Conifer Forest Structure Following 1980s Mountain Pine Beetle Outbreak in Western Colorado, USA

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Forest Ecology and Management

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Current mortality in lodgepole pine caused by mountain pine beetle (MPB) throughout much of western North America has resulted in concern about future forest structure. To better understand the long-term effects of the current mortality, and how it might differ depending on forest species composition, we measured forest vegetation and woody fuel accumulations in forest affected by a MPB outbreak in the late 1970s and early 1980s and compared conditions to 1980s USDA Forest Service data to quantify changes in the approximately 30 years following tree mortality. Stands were classified into two forest type groups based on species composition prior to 1970s/1980s MPB mortality: lodgepole pine and mixed conifer. In the 30 years after MPB mortality, lodgepole pine stands’ overstory recovered to 91% of pre-mortality total basal area and 93% of overstory trees ha−1. Mixed conifer stands’ basal area and overstory trees ha−1 remained significantly reduced. In both forest types relative basal area and trees ha−1 of non-pine species increased, and understory trees ha−1increased roughly fivefold. In lodgepole pine stands, the most abundant species in the 1980s understory was subalpine fir, followed by lodgepole pine. By the 2010s, subalpine fir and aspen were the most abundant understory tree species. In mixed conifer stands, subalpine fir and Engelmann spruce dominated all understory size classes in the 1980s and the 2010s. Total down woody fuels were greater in mixed conifer (103 Mg ha−1) than lodgepole pine stands (60 Mg ha−1) due to higher rotten fuel accumulation in mixed conifer than lodgepole pine stands. Overall, our 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-pine species abundances. These shifts in species and size composition by 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 in these areas will be similar to observed changes following the 1970s/1980s outbreak.