OOS 17-2 - Scaling burn severity patterns across regions and fire regimes yields insights into historically climate-limited fire regimes

In many temperate and boreal forests worldwide characterized by high-severity fire regimes, large wildfires have historically been limited by cool, wet climate and have thus been very infrequent, with forests burning severely every one to several centuries. As climate warms, large increases in future area burned are expected to occur in such systems, where fire activity has been strongly limited by climate and ignitions rather than fuel. Developing pre-and post-fire response plans in this context requires an understanding of the high-severity patch structure within fire events (i.e., the size and shape of patches), as areas burned at high severity are where management intervention is often of greatest priority. However, the sparseness of empirical data for infrequent fire regimes limits our understanding of fire and fire effects, and presents a challenge for quantifying the expected range of burn severity patterns using data from these focal regions. One key area exemplifying these challenges is northwestern Cascadia, the region west of the Cascade Crest in Washington and northern Oregon, which has seen recent years of large fires following nearly a century of little fire activity. Leveraging a satellite fire severity dataset of 1,615 fire events from the Northwest US between 1985 – 2020, we present an approach for characterizing expected patch-level burn severity patterns in data-sparse and infrequent-fire regions such as northwestern Cascadia. We demonstrate how spatial scaling relationships relating fire event size to high-severity patch size and shape, when combined with regional area burned and fire size distribution projections, can be harnessed to help anticipate future fire effects. Using a simulation approach, we quantify how for a given total burned area, the range of burn severity patterns expected might vary, depending on the size of future fire events. Our results illustrate how near-term shifts in fire size distributions towards larger fire events will lead to increasingly large high-severity burn patches with interior areas that are increasingly far from live-tree seed sources following fire. These results have important post-fire management implications in forested regions characterized by infrequent but severe fire events. Our approach can be harnessed to characterize emergent properties within and among fire regimes and detect shifts in those properties under a changing climate.