Temporal and spatial variation of broadleaf forest flammability in boreal Alaska

Maija Wehmas

M. S. Thesis
July 2018
University of Alaska Fairbanks


Context: Climate warming has altered the wildfire regime in boreal Alaska. One predicted outcome is a potential reduction in landscape flammability due to an increase in broadleaf forest relative to conifer forest. However, the current and future flammability of broadleaf forest in a warming climate is not well understood.

Objectives: To investigative the flammability, likelihood of burn, of upland boreal forest patches in Interior Alaska, we used pre-fire and post-fire geospatial data. Our objectives were to assess burning of broadleaf forest patches during Normal (less than 1 million hectares burned within a fire season across the state of Alaska) vs. Large Fire Years by week within a fire season, and by topographic position.

Methods: We estimated the flammability of upland broadleaf forest patches during Large and Normal Fire Years using 30-meter land cover and fire severity grids. We then looked at variation in broadleaf burning base on topographic effects using a solar radiation index to eliminate potential deviations within the vegetation. Finally, Moderate Resolution Imaging Spectroradiometer (MODIS) hotspots were used to track the spatial extent of burns during the fire season by examining the periods of fire activity and intensity.

Results:Flammability of broadleaf forest patches varied in both time and space. Normal Fire Years showed a mean patch burned of over 50% patch, while Large Fire Years showed a higher mean patch area burned with over 75%. Burning of broadleaf patches varied with topographic position as indexed by potential insolation. Finally, broadleaves forest patches burned most frequently in late June-early July with over 90% of the patches having burned by late July. Similarly, conifer forest patches burned most frequently in late June- early July; however, conifers didn’t cumulatively burn over 90% of the patches until early August.

Conclusions: Contrary to the belief that broadleaves act as fire break, broadleaf forest patches in boreal Alaska were susceptible to burning even during Normal Fire Years. With shifting fire regimes, an increase in the flammability of broadleaf forest is likely due to more extreme fire weather events. Thus, broadleaves effectiveness as fire breaks during fire-control operations will decrease in the future despite the frequency of broadleaf forest patches on the landscape is likely to increase with more frequent and severe wildfires.