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The size and severity of wildfires are increasing in many parts of North America due to increased fuels from a century of fire exclusion combined with warming climate. Climate change is forecasted to continue to increase fire activity, even in wet or cold areas where fire is not currently common. Fire is an important ecosystem process, but human expansion into wildlands combined with the increasing severity of modern fires pose many ecological and societal challenges. For example:

1. wildfires produce dangerous post-fire debris flows and flooding and threaten shrinking water supplies;
2. wildfire smoke reduces air quality and threatens public health even far from the fire;
3. fires are increasingly threatening life and property; and
4. anomalously severe fires are killing forests that have existed with fire for centuries.

Understanding past, present, and future patterns and drivers of wildfire across the range of forested ecosystems in North America is necessary for science-based management and social planning and adaptation.

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Trees preserve records of many environmental conditions in their annual growth rings. This includes climate, expressed as wide and narrow rings, but some trees also record fires as scars in the tree rings. Tree-ring fire scars provide information about the year, season, severity, frequency, size, and fire-climate relationships of fires that occurred centuries to millennia prior to modern records. In order to maximize the utility of tree-ring fire scar data, for the first time USGS scientists and partners compiled all known tree-ring fire-scar records in North America and described the new network in terms of sample depth, geography, topography, vegetation, climate, and human variables. The result was the North American tree-ring fire-scar network. This rich dataset from more than 2,500 sites across North America, provides new opportunities to understand the influences of climate, humans, and land use on past, current, and future fire regimes across the continent.

The team found that tree-ring fire scars from more than 37,000 trees across North America provide evidence of historical low-severity fire, often in areas that have not burned for a century or more. Fire scars were found on 91 tree species in all but two ecoregions (Arctic Cordillera and Tropical Dry Forests) of the North American continent, ranging from sea level to > 4,000 m in elevation. Notable gaps in the network exist in southern Mexico, northwestern Canada, and the east coast of the United States, which should be prioritized for future study.

The new network illustrates that human activities, including cultural burning, can strongly influence fires regimes. This is exemplified by a stark contrast at the U.S./Mexico border where fires were stopped circa 1900 in the U.S., creating a large fire deficit that set the stage for modern megafires, but continued to burn in northern Mexico where they maintain resilient ecosystems. Fire scars were found across a broad range of climates, from deserts to marine coastal locations to seasonally cold taiga, and climate change will likely have varying influences on future fire across this range.

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Collectively, this new North American fire-scar network provides a unique, important long-term perspective on modern changes in fire regimes that can help guide societal and management responses. Future work with the network, supported by the USGS Climate R&D Program and the USGS Powell Center for Synthesis, includes analyses of centuries-long relationships between climate and fire to better forecast future fire regimes with a changing climate.

The paper, “The North American tree-ring fire-scar network,” was recently published in Ecosphere.