Drivers of Fuels, Flammability, and Fire Behavior in Young, Post-Fire Lodgepole Pine Forests

Fire and bark beetles have affected vast areas of forest over the past several decades raising concern about the risk of subsequent burning. Little is known about how fuel loads and fire behavior vary shortly after burning, nor how forest flammability might differ between stands recovering from fire and bark beetles. To address this, we investigated the variation and drivers of fuel characteristics (Chapter 2) and fire behavior (Chapter 3) in 24-year-old post-fire lodgepole pine (Pinus contorta var. latifolia) stands that regenerated after the 1988 Yellowstone Fires. To assess differences in flammability between disturbance types (Chapter 4), we intensively sampled meteorological conditions and fuel moisture content in adjacent burned and bark beetle-affected forest sites. Both sites were approximately 24 years since disturbance. Our results indicate that fuel characteristics varied tremendously across the post-1988 Yellowstone landscape and were sufficient to support fire in all stands. Total surface-fuel loads in post-disturbance forests were similar or greater than those reported in mature lodgepole pine stands; however, 88% of fuel was in the 1000-hr fuel class, and litter, 1-hr, and 10-hr surface fuel loads were lower than values reported for mature lodgepole pine forests. Pre-fire successional stage was the best predictor of 100-hr and 1000-hr fuel and strongly influenced the size and proportion of sound and rotten logs, where post-fire stand structure was the best predictor of litter, 1-hr, and 10-hr fuels. Available canopy fuel loads and canopy bulk density met or exceeded loads observed in mature lodgepole pine forests, exhibited a strong positive relationship with post-fire lodgepole pine density, and were the primary drivers of crown fire behavior. Meteorological conditions in post-fire sites exhibited symptoms of earlier snowmelt, greater evapotranspiration, and greater drought stress than post-bark beetle sites, and live fuel moisture content mimicked these differences as post-fire sites broke dormancy earlier and experienced longer, more severe drought conditions than post-bark beetle sites. Dead fuel moisture content was similar in burned and bark beetle affected sites in July, but had a greater response to heavy August precipitation that resulted in higher dead fuel moisture content on the post-burn sites. In sum, our data suggest that 76% of the young post-fire lodgepole pine forests have 1000-hr fuel loads that exceed levels associated with high-severity surface fire, and 63% exceed canopy bulk densities associated with spreading crown fire. Fire simulation modeling predicted active crown fire in 90% of stands at wind speeds >20 km hr-1, regardless of fuel moisture condition. We conclude that 24-year old lodgepole pine forests can readily support fire intervals shorter than those observed historically in Yellowstone National Park, and that dead fuel moisture content appears more dynamic while foliar fuel moisture content might be less dynamic on post-fire sites than post-bark beetle sites. Overall, the potential for crown fire is high across the post-1988 Yellowstone landscape, and post-fire sites appear to be more flammable than post-bark beetle sites during dry periods. Given a less developed canopy seed bank and a high potential for crown fire, young post-fire lodgepole pine forests are likely to have lower reproductive potential than comparable mature forests. Progressive reductions in tree recruitment after short-interval fires may lead to self-limiting dynamics where lack of fuels limit continued short-interval burning.