Long-term relations among fire, fuel, and climate in the north-western US based on lake-sediment studies

Abstract

Pollen and high-resolution charcoal records from the north-western USA provide an opportunity to examine the linkages among fire, climate, and fuels on multiple temporal and spatial scales. The data suggest that general charcoal levels were low in the late-glacial period and increased steadily through the last 11 000 years with increasing fuel biomass. At local scales, fire occurrence is governed by the interaction of site controls, including vegetation, local climate and fire weather, and topography. At subregional scales, patterns in the long term fire-episode frequency data are apparent: The Coast Range had relatively few fires in the Holocene, whereas the Klamath–Siskiyou region experienced frequent fire episodes. Fire regimes in the northern Rocky Mountains have been strongly governed by millennial- and centennial-scale climate variability and regional differences in summer moisture. At regional scales, sites in present-day summer-dry areas show a period of protracted high fire activity within the early Holocene that is attributed to intensified summer drought in the summer-dry region. Sites in summer-wet areas show the opposite pattern, that fire was lower in frequency than present in the early Holocene as result of strengthened monsoonal circulation then. Higher fire-episode frequency at many sites in the last 2000 years is attributed to greater drought during the Medieval Climate Anomaly and possibly anthropogenic burning. The association between drought, increased fire occurrence, and available fuels evident on several time scales suggests that long-term fire history patterns should be considered in current assessments of historical fire regimes and fuel conditions.

Fire Climatology in the western United States: introduction to special issue

“…This hiatus, which is unprecedented in more than 7000 years of the charcoal records, coincides with the 20th century absence of widespread fire-scar dates in the tree rings, and the advent of livestock grazing and fire suppression. This South-west finding provides an interesting contrast to the conclusions of Whitlock et al (2008) and Pierce and Meyer (2008) that some Northern Rockies fire regimes have not changed appreciably relative to the modern era.…”

“…These charcoal maxima approximately coincide with exceptional 'Medieval' drought periods that are evident in extensive and independent tree-ring width based climate reconstructions (Cook et al 2004). Whitlock et al (2008) and Pierce and Meyer (2008) both emphasise the non-equilibrial nature of fire regimes, with fire episode frequencies changing more-or-less continuously in concert with climatic variations. Although the spatial extent of past high severity fires is not explicitly reconstructed with sedimentary charcoal records, they argue that modern high severity fires are not outside historical ranges of variability.…”

“…Allen et al 2002). Whitlock et al (2008) and Pierce and Meyer (2008) also highlight the temporal limitations of most tree-ring based studies to the past 500 years, which they assert are unrepresentative of either the Holocene or projected futures changes due to greenhouse gas-induced warming.…”

“…Four papers in this special issue summarise results of network-based charcoal studies in the Northern Rockies and Cascades, Klamath-Siskiyou region (15 sites, Whitlock et al 2008; and two study areas in the Northern Rockies, Pierce and Meyer 2008), and in the Southern Rockies of southern Colorado and northern New Mexico (6 sites, Anderson et al 2008a;2 sites, Allen et al 2008). Whitlock et al (2008) and Anderson et al (2008a) provide insights on millennial-scale influences of climate on fire regimes. A distinct advantage of charcoal/pollen studies are well illustrated here: not only can fire occurrence patterns be tracked through charcoal abundance indices during the past 11 000 years (the Holocene) and longer, but also the corresponding vegetation changes can be evaluated through analyses of pollen abundance indices (e.g.…”

“…For example, Whitlock et al (2008) show a multi-millennial secular increase in burning throughout the Holocene, and especially during the past 2000 years. The alluvial sediment-based studies of Pierce and Meyer (2008) show variable surges in fire activity during the centuries between ∼950 and 1150 AD (∼1050 and 850 years before present).…”