Vignettes > Fire geomorphology: Interactions among climate, fire, and vegetation

Fire geomorphology: Interactions among climate, fire, and vegetation

Kerry Riley
Boise State University, Geoscience
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Continent: North America
Country: United States
City/Town: Frank Church-River of No Return Wilderness


Climate Setting: Northwest
Geologic Setting: Idaho Batholith and Challis Volcanics
Type: Process, Stratigraphy, Chronology

Figure 1 - The Middle Fork Salmon River (MFSR) is ~7440 km^2 and is located in the Frank Church-River of No Return Wilderness in central Idaho. Ten study sites have been classified into upper and lower basins based on the type of vegetation found within the study basin contributing area reflecting differences in temperature and precipitation. The map on the right shows fire perimeters from the US Forest Service for the five largest (years with the greatest acreage burned) fire years recorded during the last 30 years. Details

Figure 2 - Three records of climate and fire are presented. The bottom graph shows the annual total percent of the Middle Fork Salmon River basin area burned during the last 100 years. Data labeled in boxes reports the year and percentage of area burned for the four largest fire years in the record. The middle graph shows the Palmer Drought Severity Index (PDSI) for gridpoint 69 located in central Idaho (Cook, 2004). Negative numbers represent periods of aridity and positive numbers represent wet periods. The top graph shows summer (June, July, and August) mean max temperature for central Idaho (PRISM Climate Group, Oregon State University, created 4 September 2011 ). Details

Figure 3 - A recent (1997) debris flow (surface 1) initiated in the headwaters of Reservoir Creek and incised through alluvial fan surface 2 exposing a sequence of deposits from individual erosional events. Fire-related deposits, collected from an exposed vertical profile in surface 2, were radiocarbon dated to reconstruct the depositional history of alluvial fan surface 2. The fire and fire-related depositional history spans the last 6500 cal yr BP. Fan surface 3 is undated but is older than surface 2. The recent debris flow deposit contains poorly sorted angular rocks. The fine-grained matrix has been removed in the distal deposit (bottom of photograph) by high water in the main-stem river. All three fans have been truncated by the main-stem river. Details

Figure 4 - A recent debris flow incised through an alluvial fan surface exposing past layers of deposition. Fire-related deposits are radiocarbon dated and their calibrated ages are shown. The total depth of incision is ~3.5 m and the modern channel is filled with a series of sheetfloods that are not fire-related. Details

Figure 5 - Fire histories were reconstructed using charcoal found in discrete deposits within incised alluvial fans. The smoothed data represents the sum of subsets of fire probabilities. The inverted bar graphs below the smoothed data show the sample count and represent the number of charcoal fragments creating the fire reconstruction. Upper basin sites (blue data) are cool and moist with higher elevations (2750–1380 m). These basins receive an average of ~750 mm of precipitation per year. Lower basin sites (red data) encompass elevations ranging from 2650 – 1102 m and are warmer and drier than upper basin sites. Lower basin sites receive ~400 mm of precipitation per year. The combined upper and lower basin fire history is shown in black and represents all fires reconstructed in the Middle Fork Salmon River. Grey bars indicate periods of time when upper and lower basin sites where burning. This synchronicity suggests large scale climate driving the timing of fire, in contrast to ecosystem differences driving variability in the timing of fire. Details

Figure 6 - This figure illustrates the complex response associated with changes in climate, vegetation, fire regimes, and erosional response over Holocene timescales in the Middle Fork Salmon River (MFSR). Graph of 49 radiocarbon dated deposits and represents the total reconstructed fire record for the Middle Fork Salmon River. The x-axis is calibrated years BP moving forward in time from left to right with 0 cal yr BP being 1950 AD. The data in the inverted bar graph represents the sample count of the number of dated charcoal fragments creating the above smoothed curves. Red bars are lower basin samples and blue bars are upper basin samples. Curves are the cumulative sum of 14C age distributions and are not normalized. The black curve is a 100-yr running mean and represents all dated fires (n=49) collected from within all deposits types (i.e., debris flows, burn surfaces, hyper-concentrated flows, sheetfloods, and over-bank deposits). The fire record is deconstructed into data subsets highlighting spatial distribution of fires and the type of deposition process delivering sediment over time. The subset curves (not smoothed) represent dated upper basin and lower basin subsets confidently identified as debris flow deposits or sheetflood deposits. Two regional climate proxies are compared to the MFSR fire record. Details


In steep forested landscapes, fire plays a crucial role in the timing and magnitude of erosion. Fire is a natural disturbance to vegetation and surface erosion and is a primary driver of ecosystem change across the earth (e.g. Swetnam, 1993). In order to understand, predict, and manage for future effects of fire, it is important to understand historical relationships among fire, climate, vegetation, and erosional response. It is well established that severe summer drought is a primary driver of widespread fire on annual timescales (e.g. Westerling et al., 2006). However, fire regimes reflect both climate conditions and vegetation (fuel) characteristics, which change over decadal to millennial timescales. The natural range of variability of an ecosystem describes the nature of fires occurring over an extended period of time (Brown, 1995). But what timescale is relevant to assess fire variability in order to predict and manage for future changes? If climate is a primary driver of wildfire, then timescales that involve millennial global climate cycles are relevant.

The Middle Fork Salmon River (MFSR) flows from the southwest to northeast and drains 7440 km2 of the Frank Church-River of No Return Wilderness in central Idaho (Figure 1). The lack of prior management influences (e.g. fire suppression and logging) on the landscape makes the MFSR an excellent place to study the natural role of fire without the influences of human disturbance. During the last 30 yrs, large wildfires have increased in duration and frequency throughout the western United States (Westerling et al. 2006). Fires have burned over 40% of the MFSR during the last 30 years (Figure 1). Big fire years have occurred during dry and warm summers (Figure 2).

Have large wildfires occurred in the past or are recent increases in fire frequency, duration, and size an anomaly over Holocene (1000's of years) timescales?
This study investigates the impact of Holocene climate change on spatial and temporal variations in the timing, frequency, and severity of fire. We 14C-date charcoal fragments found in discrete deposits within multiple stratigraphic profiles exposed in recently incised alluvial fans (Figure 3). One stratigraphic profile is a snapshot of a sequence of depositional events that have been preserved at one location in the larger fan complex (Figure 4). Alluvial fan deposition is autogenic and controlled by fan topography, type of depositional process, and event magnitude. Therefore, multiple profiles within each fan complex are necessary to capture a complete fan deposition history.

Within each profile, individual deposits are described according to color, texture, depth, continuity, thickness, degree of sorting, sediment size, percent gravel (% > 2mm), and charcoal abundance. Detailed descriptions of deposit characteristics were then used to identify depositional process. Fire-related deposits were determined using charcoal abundance within the deposits and/or the presence of a burn surface directly below the deposit (Figure 4). Charcoal fragments to radiocarbon date were carefully chosen within each deposit based on their in-situ presence within the deposit, lack of bioturbation (visual appearance of mixing by animal disturbance), and the angularity of the charcoal fragment indicating if the charcoal had been reworked by multiple events (rounding the fragments). Modern erosional processes within the sub-basins are useful to compare with ancient deposits within the stratigraphic profiles (Figure 4). For example, recent large fire-related debris flows occurred following moderate to severe fires. Fire history, fire frequency, and fire severity are inferred from the timing of fire, the relative abundance of charcoal, number of events per timescale, and deposit characteristics (e.g. Meyer et al., 1995; Pierce et al., 2004).

The reconstructed fire history of the MFSR spans the last 14,000 cal yr BP (Figure 5). The number of dated deposits decreases with time. This is likely because there is a decreased likelihood of older deposits being preserved and exposed (Meyer et al., 1995). Records indicate that fires are grouped both spatially (multi-basin fires) and temporally (fire frequency) during the Holocene suggesting that the timing of fire is not random and fire synchronicity and periodicity are driven by changes in climate and vegetation distributions. Temporal spacing refers to fire frequency and was determined by the number of fires burning during a given time period. Spatially grouped fires were determined by comparing the timing of fire with the spatial extent of fire throughout the larger watershed. Times when fires were burning in multiple small basins are inferred as periods of widespread fires in the MFSR.

The late Pleistocene and early Holocene (14 - 8 ka) was a period of infrequent severe fires that produced large debris flows (Figure 6). This period was followed by an increase in fire frequency between 7–5.5 ka and an absence of large debris flows in the stratigraphic record. During this time, fires were region-wide, upper and lower basin ecosystems were burning, and sheetflooding was the dominant depositonal process identified (Figure 6). From 4–2.5 ka fires and fire-related debris flows were becoming relatively more frequent. At 3 ka, fires were synchronous, occurring in multiple (upper and lower) basins. Fires were burning in multiple sub-basins, determined by individual fires with the statistically same age distributions, at 110, 900, 1500, 2070, 3780, and 6460 cal yr BP. The frequency of synchronous fires across different ecosystems increased following 3 ka.

This increase in fire frequency, severity, and synchronicity around 3 ka roughly corresponds to the arrival of lodgepole pine to high elevation forests of the Middle Fork Salmon River (Whitlock et al., 2010) (Figure 6). Lodgepole pine forests have generally dense vegetation with abundant fuel. They are adapted to survive fire by having abundant seed supplies and being able to regenerate quickly following high severity stand-replacing crown fires. A shift in fire regimes appears at approximately 3 ka inferred from an increase in the number of fire-related debris flows in all study site sub-basins throughout the MFSR (Figure 6). An increase in fire activity across a range of ecosystems during the late Holocene suggests overall higher fuel loads, combined with severe summer drought, which likely triggered severe region-wide fires. In contrast, millennial-scale dry conditions of the mid-Holocene are associated with relatively high fire frequency and low fire severity (inferred from the absence of large fire-related debris flows).

Associated References

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