Controls of reburn severity vary with fire interval in the Klamath Mountains, California, USA

Over the past few decades, the frequency and number of large fires has increased in the western United States following a paucity of fire activity in many forests during most of the 20th century. Areas of repeated burning are of particular interest for understanding where there may be qualitative shifts in fire severity and the ecological consequences related to ecosystem resilience and climate change. In the context of recent increases in fire activity, we evaluated the relative importance of potential factors contributing to reburn severity (fire severity in areas that have burned twice) from 1985 to 2012 in the Klamath Mountains ecoregion of California. A unique component of our study was the evaluation of different scales of analysis within the ecoregion. The variables prior burn severity (continuous relative difference-normalized burn ratio values from 1 to 1300), time since fire, biophysical setting, and average monthly weather were evaluated using generalized additive models to identify the relative importance of factors related to reburn severity patterns for 28 individual repeat fires (>375 ha), shorter (<15 yr) and longer (>= 19 yr) fire interval categories, and across all repeat fires combined (79,112 ha). We found that evaluation of all repeat fires combined had masked important trends occurring across individual reburn areas of study particularly when dominated by one or more large fires. Averaging the variance of reburn severity explained across individual reburn areas (equal representation) yielded more explanatory power of predictor variables compared to the composite of all reburn areas combined. Over the period of analysis, topographic controls were relatively consistent regardless of fire interval, but vegetation characteristics had a stronger relationship than weather on reburn severity at longer fire intervals compared to shorter intervals. Likewise, we found greater similarity between prior and reburn severity at longer fire intervals than at shorter fire intervals. Our results underscore the presence of a self-reinforcing pattern of fire severity related to the biophysical setting in the Klamath Mountains ecoregion, but also highlight the importance of spatial and temporal scale of evaluation and the interactions of vegetation characteristics, time since fire, and fire weather on reburn fire severity.