Fire Ecology – Case Study Low-intensity wildfire in old-growth Douglas fir and western hemlock forest consumed deep duff, thinned from below, and created challenges for fire management – a case study from the west slopes of the Cascades

Background Wind-driven wildfires on the west slopes of the Cascade Range create large areas of stand-replacing (SR) effects at multi-century timescales but less intense and more frequent non-stand-replacing (NSR) fires are now known to generate a complex array of stand structures and forest dynamics pathways in old growth, independent of, or in interaction with, SR fire. We report on the effects of low-intensity fire based on plot-based measurements of fuels and vegetation, fire behavior, and immediate and overwinter impacts in old-growth Douglas-fir and western hemlock forest ( Pseudotsuga menziesii var menziesii and Tsuga heterophylla (Raf.) Sarg.) on the 2023 Lookout Fire. Results In low intensity wildfire, high duff loadings, often in linear piles derived from downed logs, were largely consumed (89%, 247 Mg ha ^− 1 ). While towering, overstory Douglas-fir were minimally affected, 32% of poles, 7% of canopy trees (≥ 15.2 cm diameter), and 32% of snags of all sizes fell during or shortly after the fire. Most live trees fell from combustion impacts on primary roots which, for western hemlock, the most common tree species, was associated with how much duff was consumed. Further, 15% of live poles and 32% of live canopy trees, mostly western hemlock, experienced significant primary root heating. Western hemlock was particularly vulnerable to root impacts because its primary roots tended to grow exposed at the top of the mineral soil, especially where they were “perched” from having regenerated on nurse logs. Monitoring in the early growing season 7–8 months after the fire showed that, of trees alive before and standing immediately after the fire, 28% of poles and 6% of canopy trees fell and 31% of poles and 14% of canopy trees died standing with mortality strongly related to the extent of canopy injury from the fire. All told, 60% of poles and 13% of canopy trees had died by the early growing season after the fire. Primary root and root-collar heating was not significantly related to mortality of standing trees 7–8 months after the fire, but we expect that many trees will die from that cause in the coming years. Though high duff loadings were consumed by the fire and there was substantial thinning from below, the overstory of large Douglas-fir and other old-growth characteristics persisted, including most downed woody debris in large-diameter logs which will be augmented by fire-killed trees that have already fallen or will fall in the future. Conclusions NSR fire can cause substantial effects below the Douglas-fir overstory in old-growth on the west slopes of the Cascades, contributing to pyrodiversity in these ecosystems. Our coordinated pre-, active-, and post-wildfire measurements led to mechanistic insights about how these effects occur. Treefall during and immediately after fire and high loadings of duff and downed logs challenged fire suppression operations but treefall hazards for firefighters might be mitigated along shaded fuel breaks before fires ignite, guided by an understanding of why trees fall.