Low-intensity wildfire in old-growth Douglas-fir and western hemlock forest consumed deep duff, thinned from below, and challenged fire management

Abstract Background While wind-driven wildfires on the west slopes of the Cascades create extensive areas of stand-replacing (SR) effects, providing opportunities for Douglas-fir regeneration over multi-century time scales, recent studies have shown that forest development is strongly shaped by more frequent fires that cause predominantly non-stand replacing (NSR) effects. These NSR fires generate a complex array of stand structures and forest development pathways both within and outside of SR fire footprints. We build on these studies by describing mechanisms of tree mortality in old-growth Douglas-fir and western hemlock forest burned at low intensity on the 2023 Lookout Fire. Our data were collected in coordination with incident management staff and include pre-fire fuels and vegetation, fire behavior, and immediate post-fire and over-winter effects. We also consider implications for remote sensing and fire management. Results Average rate of spread (< 2 m h −1 ) and fireline intensity (22 kW m −1 ) were exceedingly low. High duff loadings (271 Mg ha −1 ), often derived from downed logs, were 92% consumed while high loadings of coarse-woody debris (131 Mg ha −1 ) were 36% consumed. Of trees alive before the fire, 20%, including 16% of poles (< 15.2 cm diameter at breast height, DBH), 3% of midstory trees (> 15.2 cm and < 60 cm DBH), and 0.7% of overstory trees (≥ 60 cm DBH), fell during or immediately after the fire, mostly due to combustion impacts on primary roots. Thirty-two percent of snags also fell during this period. Western hemlocks, the most common tree, were more vulnerable to root impacts than other species. Among trees of all species alive before the fire, 40%, mostly pole-sized individuals, either fell or died standing during the first post-fire winter. Trees that died standing had experienced the most crown injury. Of trees that were alive before the fire, 27% were standing alive after winter but about half of those (14% of all pre-fire live trees) experienced significant heating and structural impacts (particularly to roots) that we expect will cause delayed mortality. All told, 74% of trees alive before the fire had fallen or died standing over the winter or are expected to die in the coming years. Conclusions Low-intensity, NSR fire in old growth consumed deep duff and, between trees that have already died or are expected to die, will cause high mortality (> 70%) below the Douglas-fir dominated overstory. Western hemlocks across the size range were particularly vulnerable to root heating and, we expect, delayed mortality. Treefall during and immediately after fire along with high loadings of duff and downed logs challenged fire suppression operations on the 2023 Lookout Fire but an understanding of why trees fall could help mitigate future treefall hazards for firefighters along planned control lines.