Fire Regimes Over a 1070-m Elevational Gradient, San Francisco Peaks/Dook’o’oosłííd, Arizona USA

Abstract

Abstract Background Steep elevational gradients bring multiple forest types and fire regimes together in close proximity. The San Francisco Peaks/Dook’o’oosłííd in northern Arizona rise to 3,851 m elevation with slopes that span many of the major forest types of the southwestern U.S. mountains. To reconstruct past fire regimes across this broad elevational gradient, we sampled fire-scarred trees across the south face of the Peaks, complementing previous research on forest structure, composition, and origin of aspen stands. Results At the highest elevations, bristlecone pine forests had a mean fire interval (MFI) of 19.7 years prior to a modern fire exclusion period beginning after 1879. Other high-elevation (> 2,800 m) mixed conifer forests had MFI = 5.7 years and low-elevation (< 2,800 m) pine forests had MFI = 4.0 years. After 1879 there were no large fires through the end of the 20th century. Before 1879, fires occurred in the early to middle growing season. Fire regimes were linked to climate across all elevations, with a stronger association to drought (i.e., the Palmer Drought Severity Index) than to El Niño-Southern Oscillation phase. Pulses of forest regeneration were linked to the fire regime, with the largest pulse occurring shortly after fire exclusion. In addition to fire exclusion, other factors such as post-fire sprouting and regeneration after tree harvesting, likely contributed to the current dense forest structure on the Peaks. Conclusions: Following over a century of fire exclusion, fire activity has increased on the Peaks over the past two decades, with large recent fires of uncharacteristic size and severity raising concerns about tree mortality, erosion, flooding, and infrastructure damage in surrounding human communities. Past fire regimes provide useful insight into fire-climate-forest interactions and the conditions under which existing forest communities were well adapted, but adaption to future conditions is likely to be challenging due to the rapid pace of projected environmental changes.