Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests

Abstract

Western US ponderosa pine forests have recently suffered extensive stand-replacing fires followed by hillslope erosion and sedimentation. These fires are usually attributed to increased stand density as a result of fire suppression, grazing and other land use, and are often considered uncharacteristic or unprecedented. Tree-ring records from the past 500 years indicate that before Euro-American settlement, frequent, low-severity fires maintained open stands. However, the pre-settlement period between about ad 1500 and ad 1900 was also generally colder than present, raising the possibility that rapid twentieth-century warming promoted recent catastrophic fires. Here we date fire-related sediment deposits in alluvial fans in central Idaho to reconstruct Holocene fire history in xeric ponderosa pine forests and examine links to climate. We find that colder periods experienced frequent low-severity fires, probably fuelled by increased understory growth. Warmer periods experienced severe droughts, stand-replacing fires and large debris-flow events that comprise a large component of long-term erosion and coincide with similar events in sub-alpine forests of Yellowstone National Park. Our results suggest that given the powerful influence of climate, restoration of processes typical of pre-settlement times may be difficult in a warmer future that promotes severe fires.

Over 16,000 Years of Fire Frequency Determined from AMS Radiocarbon Dating of Soil Charcoal in an Alluvial Fan at Bear Flat, Northeastern British Columbia

“…Although the youngest age appears older than other records (e.g. Pierce et al 2004;Sanborn et al 2006), late-Holocene fire frequencies at Bear Flat are similar or lower than those inferred at other locations (Long et al 1998;Hallett et al 2003a;Millspaugh et al 2000;Lertzman et al 2002;Hallett and Walker 2000). It is possible that the youngest records have been removed by 20th century agricultural ploughing.…”

“…Pierce et al 2004). The record also shows a record of forest fires beginning in Late Glacial times, consistent with the deglaciation history of the region (Catto et al 1996).…”

“…This distribution was obtained by summing the probability ranges of all the calibrated 14 C ages. We summed the probability distributions derived from Calib 5.0 (Reimer et al 2004), which is the same method employed by Meyer et al (1995) and Pierce et al (2004), except they used an older version of the calibration. This plot shows striking increases in fire at several periods (e.g.…”

“…The long-term record of forest-fire history has been studied in a number of regions (Meyer et al 1995(Meyer et al , 2001Cwynar 1987;Long et al 1998;Hallett et al 2003a,b;Millspaugh et al 2000;Horn and Sanford 1992;Turcq et al 1998;Lertzman et al 2002;Hallett and Walker 2000;Pierce et al 2004) of the world, but there is little information on fire history in more northerly regions of Canada. There are basically 2 kinds of studies: 1) direct measurement of charcoal found in alluvial and colluvial deposits (e.g.…”

“…There are basically 2 kinds of studies: 1) direct measurement of charcoal found in alluvial and colluvial deposits (e.g. Meyer et al 1995Meyer et al , 2001Lertzman et al 2002;Pierce et al 2004); and 2) studies of the charcoal record found in lake sediments (e.g. Long et al 1998;Millspaugh et al 2000).…”