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The Sedimentary Record of Fire in Sierran Montane Meadows
Introduction
The relationship between climate, vegetation type, and fire occurrence has attracted considerable interest from ecologists, land managers, and climate and vegetation modelers. Historical and tree-ring records indicate a strong linkage between climatic factors and forest-fire disturbance; less direct but equally compelling evidence is provided by pollen records. These records demonstrate the importance of regional and global climatic patterns on ecosystem processes.
Examination of charcoal and plant remains in stratigraphic sediments can extend fire disturbance records to more ancient times. Construction of these longer term records, and comparison with independent records of climate change, should provide additional confirmation of fire-vegetation relationships at the local level. Ultimately, our goal is to resolve climate-fire patterns and predictions at the landscape level.
The approach we used to examine long-term fire history consists of documenting charcoal deposition in montane meadows. Our comparisons of sedimentary charcoal, pollen, and plant macrofossil data from these meadows strongly suggests that climate is the driving force for change in Holocene fire regimes. We hypothesize that climate has influenced fire regimes in the Sierra Nevada through biomass accumulation (in the form of fine fuels) in the understory, with greater biomass accumulation during the late Holocene than that of earlier periods.
Study sites
The seven charcoal data study sites occur between 1786 and 2206 m elevation on the west flank of the Sierra Nevada (Figure 1 and Table 1). Six of the sites are elongated meadows, varying in length from 500 to 1500 m and completely surrounded on the sideslopes by closed mixed conifer forest. Three of these sites (Hightop, Circle, Dogwood) occur within giant sequoia groves, three sites (Huckleberry, Weston, Long) are outside the groves. JB Swale is a small, circular meadow also surrounded by mixed conifer forest.
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| Meadow | County | Elevation (m) | Latitude (ºN) | Longitude (ºW) |
| JB Swale | Mariposa | 1786 | 37º 30' | 119º 37' |
| Hightop | Fresno | 1908 | 36º 48' | 118º 57' |
| Huckelberry | Fresno | 1987 | 36º 46' | 118º 57' |
| Weston | Tulare | 2036 | 36º 43' | 118º 53' |
| Long | Tulare | 2206 | 36º 35' | 118º 44' |
| Circle | Tulare | 2085 | 36º 34' | 118º 45' |
| Dogwood | Tulare | 1987 | 36º 12' | 118º 40' |
All meadows in this study were cored for sediments between July 1987 and June 1991. Detailed core description and pollen, macrofossil, and charcoal-particle analyses were performed on each core. Charcoal particles for each 1 mm depth increment were identified and measured; cumulative particle area was determined for each transect. Core chronologies were estimated from conventional bulk sediment radiocarbon dates.
Results
Figure 3.
Charcoal records
Figure 3 details individual charcoal records
for the seven meadows; each charcoal record consists of a series of
"spikes". Individual fires or fire periods are recorded as major
increases in charcoal abundance (large "spikes"). Four of the five
records (Hightop, Huckleberry, Circle, Dogwood) have charcoal peaks between 8700
and 9200 yr BP. Four records (Hightop, Huckleberry, Weston, Circle) reveal
elevated charcoal amounts centered near 6000 yr BP. Major charcoal peaks occur
from 3800 to 4500 (Hightop, Weston, Circle, Dogwood), 2000 to 2300 (JB, Weston,
Long), 1400 to 1500 (Hightop, Huckleberry, Long, Dogwood), 1000 to 1100 (JB,
Hightop, Weston, Long, Circle), and 700 to 800 yr BP (JB, Huckleberry, Weston,
Circle, Dogwood).
Combining records allows us to examine the pattern of burning in Sierran meadows during the entire Holocene. Figure 4 provides a composite record of charcoal deposition. The composite record shows little charcoal in sediments deposited during the first millennium of the Holocene, although a fire event may have occurred at 10000 yr BP, and at least one is recorded in the interval from 8700 to 9100 yr BP. Low background charcoal with some small peaks occur in the interval from 6300 to 8700 yr BP. The period from 4500 to 6300 yr BP shows slightly increased charcoal. After 4500 yr BP, however, charcoal increases substantially and, with the exception of a short period centered around 3000 yr BP, remains high until the most recent century of fire suppression.
Discussion
We believe that climate has influenced the long-term natural fire regime in the Sierra Nevada by two or more factors acting individually or in combination. These two factors include (1) a dry early Holocene, which produced forests with too little biomass to carry significant fires, and (2) development of El Nino conditions in this part of North America during the middle Holocene.
Our data suggest that forests in the Sierra Nevada were more open prior to 5000 to 6000 yr BP than they were during the latter Holocene. This condition suggests less biomass, especially coniferous biomass that tends to carry fire. The change toward cooler or wetter conditions after 5000 yr BP allowed for increases in the coniferous species, which produce the abundant fine fuels that burn readily, especially during extended periods of summer drought.
At present California lies at the northernmost extension of ENSO conditions. Intensification of El Nino conditions (winter/spring precipitation) in western North America would increase soil moisture during the summer growth season and inhibit fire activity during the dry summer. It would also allow for greater biomass production so that during the alternate La Nina years fire would be promoted. Although direct evidence is lacking, the fire record from the Sierra Nevada may be a proxy for El Nino conditions in North America.
Conclusions
The composite record of meadow charcoal demonstrates a distinctive pattern of burning, which, when considering the wide geographic area of the study, is strongly suggestive of a climatic cause. We believe that the temporal pattern of fire can be explained most simply by the occurrence of an early to middle Holocene xerothermic period in which forests were too open to carry large fires. With the climatic changes that accompanied the late Holocene driven by increases in winter storms from the north Pacific or an intensification of El Nino-like conditions the modern mixed conifer forest developed allowing for greater biomass accumulations thus encouraging greater fire occurrence.
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R. Scott
Anderson
Center for Environmental Sciences & Education Telephone: (520) 523-5821 Fax: (520) 523-7423 Email: Scott.Anderson@nau.edu
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This page last updated: Thursday, March 22, 2007
