Historical fire regimes, reconstructed from land‐survey data, led to complexity and fluctuation in sagebrush landscapes

Abstract

Sagebrush landscapes provide habitat for Sage-Grouse and other sagebrush obligates, yet historical fire regimes and the structure of historical sagebrush landscapes are poorly known, hampering ecological restoration and management. To remedy this, General Land Office Survey (GLO) survey notes were used to reconstruct over two million hectares of historical vegetation for four sagebrush-dominated (Artemisia spp.) study areas in the western United States. Reconstructed vegetation was analyzed for fire indicators used to identify historical fires and reconstruct historical fire regimes. Historical fire-size distributions were inverse-J shaped, and one fire > 100 000 ha was identified. Historical fire rotations were estimated at 171-342 years for Wyoming big sagebrush (A. tridentata ssp. wyomingensis) and 137-217 years for mountain big sagebrush (A. tridentata ssp. vaseyana). Historical fire and patch sizes were significantly larger in Wyoming big sagebrush than mountain big sagebrush, and historical fire rotations were significantly longer in Wyoming big sagebrush than mountain big sagebrush. Historical fire rotations in Wyoming were longer than those in other study areas. Fine-scale mosaics of burned and unburned area and larger unburned inclusions within fire perimeters were less common than in modern fires. Historical sagebrush landscapes were dominated by large, contiguous areas of sagebrush, though large grass-dominated areas and finer-scale mosaics of grass and sagebrush were also present in smaller amounts. Variation in sagebrush density was a common source of patchiness, and areas classified as "dense" made up 24.5% of total sagebrush area, compared to 16.3% for "scattered" sagebrush. Results suggest significant differences in historical and modern fire regimes. Modern fire rotations in Wyoming big sagebrush are shorter than historical fire rotations. Results also suggest that historical sagebrush landscapes would have fluctuated, because of infrequent episodes of large fires and long periods of recovery and maturity. Due to fragmentation of sagebrush landscapes, the large, contiguous expanses of sagebrush that dominated historically are most at risk and in need of conservation, including both dense and scattered sagebrush. Fire suppression in Wyoming big sagebrush may also be advisable, as modern fire rotations are shorter than their historical counterparts.

Fire patterns in the range of the greater sage-grouse, 1984-2013 - Implications for conservation and management

“…To make this assessment, we generally were hindered by a lack of historical fire rotation estimates for the different vegetation types among different regions. However, we can draw a few tentative conclusions based on estimates we do have of historical fire rotations for big sagebrush communities in different regions as presented in Baker (2013) and Bukowski and Baker (2013). For Big Sagebrush in the western region, our calculated modern fire rotations for all management zones (61-150 years across the entire region) likely represent a substantial reduction compared to historical fire rotations, which were estimated to be 171-342 (Bukowski and Baker, 2013).…”

“…Historical fire sizes in sagebrush ecosystems are poorly understood. Some suggest that infrequent large fires were part of historical sagebrush fire regimes (Baker, 2011;Bukowski and Baker, 2013), although others suggest that the sizes of sagebrush fires during recent decades may be unprecedented (Keane and others, 2008). In terms of conservation efforts for the greater sage-grouse, it is arguably less important to know how current fire sizes compare to historical, than it is to understand how current fire sizes affect the species and its current remaining habitat.…”

“…Other studies have documented that at least a few to several decades are required for sagebrush ecosystems to recover after fire (for example, Welch and Criddle, 2003;Lesica and others, 2007), which would suggest even longer mean fire rotations are required to maintain a dominance of mature sagebrush communities on the broader landscape (Baker, 2006;Lesica and others, 2007). Supporting this notion, Bukowski and Baker (2013) used historical General Land Office Survey data from the late 1800s to early 1900s that documented fire evidence and vegetation types for large areas of the Western United States, and estimated that historical fire rotations were 171-342 and 137-217 years for Wyoming Big Sagebrush and Mountain Big Sagebrush, respectively.…”

“…Mean fire return intervals may have ranged from decades in colder-moister sagebrush ecosystems (Miller and Heyerdahl, 2008) to hundreds of years in hotter-drier sagebrush ecosystems (Bukowski and Baker, 2013), allowing for recovery and persistence of sagebrush communities adapted to those conditions. However, during recent decades, fire probability and occurrence have increased across large portions of the cold desert ecoregion of Western United States (Whisenant, 1989;Knick and Rotenberrry, 1997;Chambers, 2008;Miller and others, 2011;Baker, 2013;Balch and others, 2013), hindering recovery of sagebrush and threatening sage-grouse habitat.…”

Effects of drought and fire on bird communities of the Kofa National Wildlife Refuge, Arizona

“…Fire frequency and extent have increased across western North America in recent decades (Saab and Powell, 2005;Bukowski and Baker, 2013). In recognition of this new dynamic, it is important to properly assess the effects of fire in avian communities, particularly in desert ecosystems.…”