The effect of salvage logging on surface fuel loads and fuel moisture in beetle-infested lodgepole pine forests

• Harvest increased fuel loads with concomitant decreases in fuel moisture content.
• Removal of overstory leads to microclimate changes in wind and soil temperature.
• Fuel in harvested stands dry faster following precipitation and remain drier.
• Fire hazard may increase in post-harvest slash prescriptions that retain biomass.

Widespread tree mortality from mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) outbreaks has prompted forest management activities to reduce crown fire hazard in the Rocky Mountain region. However, little is known about how beetle-related salvage logging and biomass utilization options affect woody surface fuel loads and fuel moisture dynamics. We compared these attributes in salvage-logged lodgepole pine (Pinus contorta var. latifolia Engelm. Ex S. Wats.) stands harvested using either biomass removal (whole-tree harvest) or biomass retention (bole only harvest) prescriptions with untreated MPB-infested stands. Both prescriptions roughly doubled 1-h and 10-h fuel loads compared to untreated forest. Biomass retention left ten times more 1000-h fuels compared to biomass removal prescription (28 vs 3 Mg ha−1). Overall, the woody fuel load was more than twice as high with biomass retention compared to biomass removal (60 vs 25 Mg ha−1). Fuel moisture content was lower in salvage logged units compared to untreated forest plots, but it did not differ among the biomass prescriptions. Fine (10-h) and heavy (1000-h) fuels dried to a critical ignition threshold 3–8 weeks earlier in the two prescriptions, respectively, compared to the untreated forests. Salvage logging removes canopy fuels and crown fire hazard, but we found that depending on the amount of biomass retained it can both increase surface fuel load and decrease fuel moisture compared to untreated stands. In the coming years, snag fall will transfer crown to surface fuels in untreated beetle-killed stands adding coarse surface fuel loads surpassing those in treated stands.