Changing fire regimes in the southwestern U.S. – implications for forest management and endangered species
Although fire historically played a significant role in structuring ecosystems throughout North America, recent “megafires” of the last decade have severely burned large tracts of land, with catastrophic impacts on housing, infrastructure, water supplies, forests, and wildlife. Forest managers have the paired goals of reducing fire hazards and restoring forests to provide a more natural fire regime and wildlife habitat. However, there is considerable debate on the historic role of high-severity fires in mixed-conifer forests, and restoration priorities and fire hazard reduction treatments have become increasingly contentious. USGS researchers are conducting research to determine historic patterns of fire frequency and severity in mixed-conifer forests of the southwestern U.S. to aid development of successful plans for forest management, restoration, and recovery of rare species.
In the southwestern U.S., topographic variability results in a wide range of ecosystems and fire regimes. Low-severity fires that rarely kill canopy trees were historically common in the warm and dry ponderosa pine forests. In contrast, high-severity fires that kill large patches of canopy trees were common in the wetter and cooler spruce-fir and aspen forests. In between, the mixed-conifer forests likely had a mix of low- and high-severity fire. Research suggests that dry mixed-conifer forests that contain ponderosa pine were dominated by low-severity fire regimes. In contrast, wet mixed-conifer forests are thought to have burned with relatively large high-severity patches, but data are limited.
Beginning in the late 19th century, industrial-scale livestock grazing in the southwestern U.S. removed grasses and other herbaceous plants that historically carried surface fires. The grazing and subsequent active fire suppression allowed fuels to accumulate, which led to increases in both the severity of fires and the area burned. The 2011 Las Conchas megafire in the Jemez Mountains near Los Alamos, New Mexico burned more than 150,000 acres, including an incredible crown fire run that burned 44,000 acres in the first 13 hours. The large high-severity patches create by the Las Conchas fire had substantial impacts on wildlife, tourism, and flood potential, and has converted large areas to shrublands.
Threatened and endangered species are vital parts of many ecosystems but may pose additional challenges for forest management. The endemic Jemez Mountains salamander (Plethodon neomexicanus) was listed as a federally endangered species in 2013. The recent high-severity fires in its critical habitat (mixed-conifer forests) prompted its listing and increased the urgency to reduce fire hazard and restore forest structure. However, uncertainty about the natural role of high severity fire in mixed-conifer forests has led to public objections to restoration actions in these forest types. Scientists from the USGS and University of Arizona set out to document and compare current and historic forest structure and fire regimes in the Jemez Mountains salamander habitat (Figure 1). By improving understanding of mixed-conifer forest and fire ecology, the research aims to support forest management and salamander recovery efforts.
The researchers quantified the current mixed-conifer forest habitat (e.g., stand structure and composition) and compared it to tree-ring reconstructions of historical habitat, fire frequency, fire severity, and fire-climate relationships. The analyses of tree-ring records (Figure 2) show that historical fire regimes in Jemez Mountains salamander habitat were dominated by low-severity fire during the last four centuries, both in dry and wet conifer forests. Human land use - intensive grazing followed by fire suppression - has caused the current fire-free interval (> 116 years at all sites) to lengthen to more than twice the historical maximum intervals. After a century without fire, the forest understory has become denser and dominated by mesic, fire-sensitive tree species; in some cases, dry-conifer, fire-loving species, such as ponderosa pine have been pushed out. This research indicates that more than 40% of the sites converted from dry conifer to wet conifer forest in the absence of fire over the last century.
High-severity, tree-killing fire was a component of historical fire regimes in the wetter mixed-conifer forests; however it was patchy, and not as extensive as we have seen in recent megafires (for example, during the 2011 Las Conchas Fire, there were > 2,000 acre burn patches with no surviving trees). Historically, some sites switched from mixed- or high-severity fire to low-severity fire regimes during the last few centuries, likely in response to climate variability. The research showed that most severe fires occurred during the most severe droughts and the low-severity fires occurred during moderate drought.
This research highlights how both fire suppression and droughts have affected fire regimes, forest composition and forest structure in the western U.S. This suggests that the combination of increased fuel loads and projected increases in moisture stress due to warming temperatures could lead to increased fire severity. Such possibilities need to be considered as resource managers develop sustainable management strategies for endangered species that live in these forests.
The paper, “Historical dominance of low-severity fire in dry and wet mixed-conifer forest habitats of the endangered terrestrial Jemez Mountains salamander (Plethodon neomexicanus)”, was published in Forest Ecology and Management.
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