Effects of mountain pine beetle on forest structure and fuel load 25 years after outbreak

Background/Question/Methods

Current mortality in Pinus contorta (Dougl. Ex. Loud.) (lodgepole pine) caused by mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) throughout much of western North America has resulted in serious concern over the outbreak's impacts on future forest structure and wildfire risk, but there is little consensus on its long-term impacts. To better understand the long-term impacts of the current epidemic, we identified and sampled areas affected by a 1980s MPB outbreak. Using aerial survey maps and 1980s Forest Service data, we identified stands with >30% lodgepole pine mortality 20–30 years ago. Stands fell into two forest type groups: lodgepole pine (>78% basal area (BA) in Pinus contorta and <11% BA in Abies lasiocarpa [subalpine fir]/Picea engelmannii [Engelmann spruce]) or mixed conifer (<78% BA in Pinus contorta and >17% BA in Abies lasiocarpa/Picea engelmannii). In summer 2010, were sampled stands to measure forest species and size structure, and down woody fuel accumulations. By comparing 1980s stand exam data with 2010 data, we found differences between the forest type groups in their post-outbreak forest changes. 

Results/Conclusions

From the 1980s to 2010, in lodgepole pine stands there was a significant increase in Pinus contorta (from 214 to 284, P=.0120), Populous tremuloides (from 252 to 1685, p<.001), Abies lasiocarpa (from 188 to 625, P=.0480), and Picea engelmannii (from 32 to 47, P=.00935) seedlings/saplings (trees 0.6 m tall to 3.8 cm diameter) per hectare (ha^-1). In mixed conifer stands, shade-tolerant species (Abies lasiocarpa and Picea engelmannii) showed the greatest increase in seedling/sapling density (from 6229 to 2784, P=.02 and from 227 to 622, P=.025), while Populus tremuloides moderately increased from 38 to 212 trees ha^-1 (P=.025). Lodgepole pine stands recovered to their pre-outbreak Pinus contorta lodgepole BA of 31 m² ha^-1 by 2010 (P=.8227), while mixed conifer stands BA was reduced from 25 to 16  m² ha^-1  in 2010 (P=.0104). Total down woody fuel loads 25 year after MPB mortality averaged 71 Mg ha^-1, 87% of which was in coarse fuels (>7.6 cm diameter), and did not differ between initial forest species composition types. Overall, the changes in species’ dominances 20-30 years after outbreak represent a significant shift in forest structure, and likely changes in forest health, fire behavior and ecosystem processes.