A comparison and integration of tree-ring and alluvial records of fire history at the Missionary Ridge Fire, Durango, Colorado, USA

We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised the valley-filling alluvial sediments in the lower basin, and created exposures of fire-related of deposits of late-Holocene age. Tree-ring and alluvial sediment fire history records were created separately, and then compared and integrated to create a ~ 3000 year record of past fire activity. The tree-ring record showed that from ad 1679 to 1879, there were frequent surface fires, while patches of high-severity fire occurred during widespread fire years. The alluvial record showed that a lowto moderateand mixed-severity fire regime has likely been dominant over the past ~ 2600 calibrated calendar years before present, as shown by locally episodic deposition of charcoal-rich, fine-grained sediments. Radiocarbon dating suggested that in two stratigraphic sections, there was rapid deposition of several fine-grained sediment layers. One of these episodes occurred during the Medieval Climatic Anomaly (ad 900–1300). A charcoal-rich debris flow deposit in the oldest exposed part of the stratigraphic record dated to ~ 2600 calibrated calendar years before present. This event was potentially equivalent in magnitude to the debris-flow events following the recent wildfire in the study area, and is evidence of a high-severity fire that burned a large proportion of the study basin. The timing of this event coincides with a period of less frequent, yet more severe wildfires in a nearby lake sediment record, and is associated with the end of a Neoglacial period of cooler and wetter temperatures. The Holocene 20(7) 1047–1061 © The Author(s) 2010 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959683610369502 http://hol.sagepub.com Received 28 August 2009; revised manuscript accepted 9 March 2010 Corresponding author: Erica Bigio, Laboratory of Tree-Ring Research, University of Arizona, 105 W. Stadium, Tucson AZ 85721, USA Email: [email protected] Research paper 1048 The Holocene 20(7) by most tree-ring records (~ 500 years or less) is too brief to resolve centennialor millennial-scale fire regime changes that are climatic in origin, such as effects of the ‘Medieval Climatic Anomaly’ (MCA, ad 900–1300) or the ‘Little Ice Age’ (LIA, ad 1300–1850) (Pierce and Meyer, 2008; Whitlock et al., 2008). The MCA was a period of increased climatic variability and several extended droughts across the western US (Cook et al., 2004; Meko et al., 2007; Stine, 1994), while the LIA was a period of inferred cooler and wetter climate throughout the Northern Hemisphere (Armour et al., 2002; Cook et al., 2004; Grove, 1988; Petersen, 1994). One consequence of the temporal limitation of tree-ring records is that inferences about anthropogenic causes of recent fire regime changes (e.g. livestock grazing or fire suppression) may be complicated by longer-term climatic changes. Recent high-severity ‘megafires’ may not be unprecedented, and the events and trends of the past two decades may be a largely climateinduced response of ecosystems, similar to prior Holocene episodes of severe fires, as shown by alluvial sediment records in the northern Rockies and southern New Mexico (Frechette and Meyer, 2009; Pierce and Meyer, 2008; Pierce et al., 2004). In order to discuss fire events and fire regimes throughout this paper, we begin with the definitions of fire severity and fire regimes in western ecosystems. At the stand level, low-severity fires char litter and surface fuels, leaving no exposed soil, and induce little heat damage to the underlying soil. Tree mortality is limited to scattered individuals. Moderate-severity fires char litter and portions of the underlying duff, with some heat damage and partial exposure of mineral soil. Moderate-severity fires result in some tree mortality, though limited to individual trees or small groups. High-severity fires completely consume the litter and duff, while exposing and damaging the underlying soil and causing at least 80% tree mortality (DeBano et al., 1998). Mixedseverity fire events contain both lowand high-severity fire in adjacent forest patches of approximately 10–20 ha (Fulé et al., 2003; Iniguez et al., 2009), and therefore, create partial exposure of mineral soil at the scale of tributary watersheds. A fire regime describes the role of fire in an ecosystem, by defining the frequency, severity and seasonality of fire events (Agee, 1993). Questions about the past versus present role of high-severity crown fires are most actively debated in the context of pure ponderosa pine (Pinus ponderosa) and mixed conifer forests (pine, Douglas-fir, true firs and other conifer species mixtures) in the western US (Allen et al., 2002; Baker et al., 2007; Brown et al., 2008; Hessburg et al., 2005). Frequent, low-severity surface fires were clearly predominant in most pure ponderosa pine landscapes of the southwestern US prior to ad 1900 (Allen et al., 2002; Swetnam and Baisan, 1996). ‘Mixed’ or ‘variable’ severity fire regimes, including both low-severity surface fire and patches of high-severity crown fire (~ 10 ha), occurred during the pre1900 era in some ponderosa pine dominant and mixed conifer landscapes in southern New Mexico and Arizona (Iniguez et al., 2009; Swetnam et al., 2001). Crown fires also played a significant role in ponderosa pine dominant forests in the Colorado Front Range (Brown et al., 1999; Ehle and Baker, 2003; Sherriff and Veblen, 2006), while the evidence of past crown fire activity in the Black Hills of South Dakota is currently being debated (Brown, 2006; Brown et al., 2008; Shinneman and Baker, 1997). Millennial-length fire history records are valuable for determining the full range of variability in historical fire regimes, and can help evaluate the relative impacts of climate variability and twentieth-century land-use practices on recent wildfire activity. Alluvial fan sediment records provide fire history information for a specific area, because each sampling site represents the contributing area of a watershed. Fire-related sedimentation events from several sites are then composited to create a regional fire history chronology extending up to several thousand years (Meyer et al., 1995; Pierce and Meyer, 2008; Pierce et al., 2004). These methods have been used to interpret both lowand highseverity fire regimes in xeric to mesic forests types throughout the Rocky Mountains (Frechette and Meyer, 2009; Jenkins, 2007; Meyer et al., 1995; New, 2007; Pierce et al., 2004). In this study, we compared tree-ring and alluvial fan records of fire history for a single low-order drainage basin with predominantly mixed conifer forest located in southwestern Colorado. Most of this drainage was burned by the Missionary Ridge Fire in 2002, and post-fire erosion exposed charcoal-rich sediment layers in alluvial stratigraphy. We used tree-ring records to better interpret fire event information from the alluvial sediments, while also improving our understanding of long-term changes in mixed-severity fire regimes in the Southern Rockies. Alluvial fan fire history records have been documented at several locations in the western US, and thus far, this proxy method has been compared with stand-age reconstructions at one study area in southern New Mexico (Frechette and Meyer, 2009; Meyer et al., 1995; Pierce et al., 2004). Tree-ring records have previously been compared with fire events reconstructed from charcoal deposition in lakes and bogs (Allen et al., 2008; Whitlock et al., 2004) in the same study areas. The most challenging aspect of the comparison of tree-ring records and charcoal-based fire history records is the different temporal resolution of the available dating methods. Radiocarbon dating is used to determine the age of charcoal pieces in lake, bog or alluvial fan deposits, which yields calendar age ranges of 200–400 years for each event. In contrast, tree-ring records represent fire events with annual to decadal resolution, and can more precisely determine the location of the fire. In previous comparisons of tree-ring records with charcoal records from lake or bog sediments, charcoal records were unable to capture all of the events observed in the tree-ring record, though general trends in the data were similar. In addition, Frechette and Meyer (2009) used stand-establishment data from the contributing watershed of an alluvial fan to constrain the age of a fire-related sedimentation event to the late 1800s, which was dated with radiocarbon methods (ad 1700–present). Questions to be addressed in this study include: (1) How does the fire history information derived from sediment deposits within the past 500 years compare with the fire history reconstructed from tree-ring records? (2) What does the combined fire history record suggest about the timing, extent and severity of past fires in the study basin? (3) Is there evidence of standreplacing fires and post-fire geomorphic responses similar to the 2002 Missionary Ridge Fire event over millennial timescales?