Climatic controls on fire-induced sediment pulses in Yellowstone National Park and central Idaho: a long-term perspective
Introduction
In mountain landscapes, the connection between fire and major debris-flow and flash-flood events has been well established (e.g., Swanson, 1981, Wells, 1987, Wohl and Pearthree, 1991, Meyer and Wells, 1997). These events initiate in small, steep tributary basins. Their physical effects propagate down stream systems and can range from mild to catastrophic, and from transient to persistent. Here we use catastrophic in the geological sense to mean major change, without the implication of negative (or positive) ecological impacts. Effects of flash-floods and debris flows vary with location in the drainage network and the timescale of consideration. Immediate impacts in small streams may include extreme turbulence and sediment concentrations that directly cause fish mortality, with deep scouring or major infilling of channels (e.g., Bozek and Young, 1994, Meyer and Wells, 1997, Minshall et al., 2001). Over periods of several years, impacts along larger streams may range from minor and transient loading with fine sediment to major aggradation of gravel (e.g., Benda et al., 1998). Persistent alteration of channel structure may occur through addition of large woody debris and boulders that remain immobile for centuries or more (e.g., Minshall et al., 1997, Meyer et al., 2001).
The importance of fire in sediment yield and aquatic habitat disturbance is a function of fire frequency and severity, as well as the geomorphic sensitivity of the landscape (i.e., susceptibility to postfire erosion) (Swanson, 1981, Meyer, in press). In steep mountain ranges that have both erodible soils and a regime of high-severity fires, extreme postfire runoff and mass failures produce a large proportion of the overall sediment flux. In contrast, fire is much less of a factor in low-relief landscapes with only light surface fires.
Clearly, weather and climate are major factors in both fire activity and postfire sedimentation over a wide range of timescales. In xeric conifer forests of the western USA, a few wet years may allow grasses and other fine fuels to build up, so that a subsequent drought year produces widespread fires (Swetnam and Betancourt, 1998, Veblen et al., 2000). Prior to fire suppression, light surface fires recurred at intervals of a few years to a few decades in generally open forest stands. In wetter high-elevation conifer forests as in Yellowstone, more rare and severe drought may be necessary for significant areas to burn (Balling et al., 1992a, Balling et al., 1992b). Forest structure there is typically dense. Intervals of many decades to several hundred years may elapse between high-severity stand-replacing fires, and climate change over centuries and longer periods is important (Romme and Despain, 1989, Meyer et al., 1995, Millspaugh et al., 2000).
In this paper, we review postfire erosion and sedimentation processes, and explore changing landscape responses to fire in contrasting xeric and mesic conifer forests over periods of centuries to millennia. Although planning for the future over such timescales is beyond our capabilities, knowledge of linkages between climate, fire, and landscape response in the Holocene epoch (the postglacial period of the last ∼10 000 years) is critical to understand the modern environment and the potential effects of climatic trends over the last few centuries. For example, tree-rings yield detailed, spatially explicit fire histories that are limited to the age of living trees, mostly less than 500 years. Many tree-ring records reveal fire regimes prior to European settlement in the western USA, but they do not extend back before the Little Ice Age, a time of minor glacial advances and generally cooler climates in the northern hemisphere beginning ca. 1200 a.d. (Grove, 1988, Grove, 2001, Luckman, 2000). Marked warming began in the late 1800s to early 1900s and continues in the late 20th century (Pollack et al., 1998, Esper et al., 2002). Given such climatic change, fire regimes may change markedly, even without major changes in forest composition (e.g., Meyer et al., 1995, Millspaugh et al., 2000). Therefore, geomorphic responses to fire that affect aquatic ecosystems may also vary from what historical observations and shorter fire histories suggest are characteristic of a given landscape. To provide a long-term perspective on such disturbances and to better understand the influence of climate, we compare a Holocene record of fire-related sedimentation in cool, high-elevation forests of northern Yellowstone National Park with one under development for warmer, low-elevation forests of the South Fork Payette River region in west-central Idaho (Fig. 1). In both areas, fire-related sediments are well-preserved in small alluvial fans. These fans are gently sloping, conical landforms formed where small tributary drainage become unconfined and deposit sediment as they enter larger stream valleys.
Section snippets
Postfire erosion and deposition: processes and patterns
Fires help to initiate sediment transport events in small, steep mountain basins via two primary mechanisms (see also Wondzell and King, this volume). The first mechanism operates when intense precipitation, often in brief summer thunderstorms, produces widespread surface runoff on severely burned slopes. Water-repellent soils and (or) surface-sealing effects reduce infiltration rates dramatically, and smooth flow paths allow rapid runoff (Swanson, 1981, Meyer and Wells, 1997, Robichaud, 2000).
Northern Yellowstone National Park study area
Most of Yellowstone National Park lies at over 2000 m elevation and is covered by dense conifer forests. We focused on geomorphic responses to fire in the rugged Absaroka Range of northeastern Yellowstone National Park and other areas of local high relief, such as Gibbon Canyon (Fig. 1). Mean annual precipitation increases markedly with elevation in northeastern portion of the park, from 360 mm at Lamar Ranger Station (2000 m elevation) to 660 mm at Cooke City (2302 m elevation), and as much as 1300
Payette River study area
The central Idaho study area is located in the South Fork Payette River canyon below Grandjean, at lower elevation (1000–2000 m) than the Yellowstone National Park study area (Fig. 1). Mean annual precipitation varies from about 600 mm in valleys to 1000 mm at higher elevations, and occurs predominantly in the colder months. Runoff is dominated by snowmelt in March to May, but thaws and large cyclonic storms sometimes generate major winter floods (Meyer et al., 2001). A pronounced summer dry
Radiocarbon dating
Charcoal samples from fire-related deposits were -dated, in part using conventional decay-counting methods but mostly by accelerator mass spectrometry (AMS) at the NSF-Arizona Laboratory. Individual charcoal fragments were selected for dating to avoid mixing of charcoal ages, and small twigs, cone fragments, needles, grass stems, and seeds were selected where possible to avoid samples with potentially large “inbuilt” ages (i.e., samples that were formed significantly before the time of fire;
Northern Yellowstone National Park high-elevation conifer forests
Fire-related deposits make up about 30% of the total thickness of the alluvial-fan sediments examined in northern Yellowstone National Park (Meyer et al., 1995). Because many postfire flood deposits lack features to identify them as fire-related, the actual percentage may be greater. The proportion of fire-induced sediment is also likely to be highly variable between small basins, given differences in amount of forest cover, bedrock erodibility, and basin morphology. For example, basins with a
Discussion
Climate, elevation, and forest type are well-known primary controls over fire severity and return intervals (e.g., Arno, 1980, Barrett, 1988, Barrett, 1994, Swetnam and Baisan, 1996, Barrett et al., 1997, Veblen et al., 2000). In general, fire return intervals are shorter for xeric low-elevation forests and longer for mesic high-elevation and subalpine forests in the Rocky Mountain region. Adaptations of conifers to fire clearly reflect these variations in regime. For example, mature ponderosa
Conclusions and implications for management
The occurrence of catastrophic fires in Yellowstone high-elevation forests is clearly dependent on climatic variations, as is the attendant disturbance in stream ecosystems. Like many river flood records (e.g., Klemes, 1989, Redmond et al., 2002), fire-related sedimentation is nonstationary because of climate change, and the probability of events changes markedly on centennial to millennial timescales. Although fire suppression and other land-use effects are clearly significant in recent
Acknowledgements
We wish to thank Ralph Mason, Chris Inoue, Danny Katzman, John Rogers, Del and Sandy Fadden, Frank Pazzaglia, Tim Lite, Lydia Rockwell, Sarah Caldwell, Wallace Pierce-Andersen, and Molly Watt for aid in field work; our colleagues Spencer Wood, Steve Wells, Tim Jull, and Charlie Luce for many helpful discussions; and John Varley (National Park Service—Yellowstone) and Kari Grover-Wier (US Forest Service) for cooperation and logistical assistance. We are grateful to the Western Regional Climate
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