COS 175-4 - Drivers of reburn severity in the American West: Self-limiting severity effects of wildfire vary by region and initial fire severity

Abstract: Warmer and drier climate conditions and increased fuel loads have led to an unprecedented increase in large wildfires. With more area burned, forested areas are increasingly experiencing repeated short-interval burning (reburning) across the American West. Reburning is often associated with large high severity patches and loss of ecosystem function. However, low and moderate severity wildfires can restore forest resilience by reducing fuel loads, especially in historically fire-prone forests experiencing a present-day fire deficit. The possibility of both positive and negative wildfire feedbacks on subsequent fire severity raises important empirical questions: How strong are the moderating effects of low to moderate severity fire (i.e. self-limiting severity effects)? And how do these effects vary across large landscapes, which is critical for understanding where and when wildfires may help meet management goals. We examined reburned areas within four large western U.S ecoregions and asked (1) how strong are the self-limiting severity effects of low to moderate severity wildfire, (2) how long do moderating effects last, and (3) how are these effects influenced by initial fire severity, fire weather, climate, and ecoregion. We extracted reburn severity along with climate, topography, fire weather, and fire history data from forested areas that burned at least twice between 1985 and 2021 (n = 733 unique reburns). Using generalized additive models, we tested hypotheses about how the time between fires interacted with fire weather, climate, and initial fire severity to influence reburn severity.

The self-limiting effects of low/moderate severity fire varied substantially by ecoregion. Previous low/moderate severity fire had the strongest self-limiting effect in the Western Mountains and California Coast, where mean reburn fire severity was 25% lower in forests that re-burned 5 years after the preceding fire than in areas that had not burned for 30 years. However, self-limiting severity effects lasted longest in the Northern Mountains, where reburn severity reached its asymptote 20 years following the previous fire, versus only 10 years in the Western Mountains and 15 years in the California Coast. We found little evidence of self-limiting severity effects in the Southwestern Mountains. Overall, fires that initially burned at relatively high severity reburned at higher severity, and these areas tended to display slightly longer lasting self-limiting effects. Our findings demonstrate that self-limiting severity effects are context dependent, and that managers should consider multiple factors when determining the extent to which burning may help moderate future fire severity and contribute to management goals.