Mixed severity fire: Conceptual and empirical overview of ecology and case studies of restoration challenges

Background/Question/Methods

Understanding the role fire historically played in maintaining local and landscape composition and biodiversity remains an active research front. In many forest types, fire suppression and land use have altered the structure and composition of forests. In response to altered conditions, various management strategies have been used to restore forest structure, biological diversity, and the ecological role of fire. Fire regimes of forests that historically burned with frequent, low-severity fires are widely studied and provide a useful guide for ecological restoration. In contrast, setting goals for ecosystem management and restoration targets in mixed-severity fire regimes, where the frequency, severity, and effects of fires historically varied in time and space has proven more difficult. Variability in forest conditions and fire severity in space and time make active treatments in forests of mixed severity fire regimes controversial. There, target conditions for forest restoration may be based on limited historical evidence, and relationships between spatially-complicated patterns and processes are not fully understood. We surveyed the literature, burn severity data at local and landscape scales, and our own experiences in collaborative forest restoration to provide conceptual and empirical overview of mixed severity fire regimes.

Results/Conclusions

Drivers of fire severity vary by scale of observation, with fuel loading, weather, and topography predicting fire severity at small scales, and vegetation, climate and landform influencing fire severity at larger scales. Variability in spatial composition of fire severity among forest fires in the western U.S. appears to be continuous without clear classification thresholds. Thus, classifying fire regimes using a categorical approach, while convenient, may be less helpful than understanding the mechanisms driving fire severity across forest types. Variability in temporal patterns of fire severity (variation in fire return intervals) may also reflect interactions between topographic and climatic drivers and create challenges to understanding and applying historical range of variability. This variability in space and time leads to controversy in prioritizing restoration using mechanical or prescribed fire treatments. Predictions of future impacts of climate change and increasing residential development in fire-prone forests have led collaborations of diverse stakeholders to begin supporting treatments in complex forest types where landscape restoration objectives are not clearly defined. We suggest restoration be conducted with mechanistic understanding of fire’s role in maintaining species diversity and landscape resilience. Where socially and economically feasible, treatments should be implemented using experimental approaches with collaborative input to facilitate collective learning within an adaptive management framework.