The fire frequency-severity relationship and the legacy of fire suppression in California forests

Fire is one of the most important natural disturbance processes in the western United States and ecosystems differ markedly with respect to their ecological and evolutionary relationships with fire. Reference fire regimes in forested ecosystems can be categorized along a gradient ranging from ‘‘fuellimited’’ to ‘‘climate-limited’’ where the former types are often characterized by frequent, lower-severity wildfires and the latter by infrequent, more severe wildfires. Using spatial data on fire severity from 1984– 2011 and metrics related to fire frequency, we tested how divergence from historic (pre-Euroamerican settlement) fire frequencies due to a century of fire suppression influences rates of high-severity fire in five forest types in California. With some variation among bioregions, our results suggest that fires in forest types characterized by fuel-limited fire regimes (e.g., yellow pine and mixed conifer forest) tend to burn with greater proportions of high-severity fire as either time since last fire or the mean modern fire return interval (FRI) increases. Two intermediate fire regime types (mixed evergreen and bigcone Douglas-fir) showed a similar relationship between fire frequency and fire severity. However, red fir and redwood forests, which are characterized by more climate-limited fire regimes, did not show significant positive relationships between FRI and fire severity. This analysis provides strong evidence that for fuel-limited fire regimes, lack of fire leads to increasing rates of high-severity burning. Our study also substantiates the general validity of ‘‘fuel-limited’’ vs. ‘‘climate-limited’’ explanations of differing patterns of fire effects and response in forest types of the western US.