Fire Regimes in Sierrian Mixed-Conifer Forests

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Why is fire important in forest ecology?

Forest ecosystems throughout the world have evolved over millions of years with fire as a common disturbance process. Fires caused by lightning have been an evolutionary factor since plants first arrived on land. Lightning is still the most common source of fire in many terrestrial ecosystems. Since the recent evolution of humans and their acquisition of fire as a tool, they too have become a common source of fire. Past human uses of fire for manipulating vegetation, however, was highly variable from one place and time to another. In many remote mountain areas, and before the 20^th century, other biotic and physical features of the landscape were probably more important than people in controlling fire patterns. These factors include vegetation, topography, and climate. Many species of plants in fire-prone habitats have evolved special adaptations and survival strategies that help them persist and thrive with fire. In fact, without fire, many of these species are unable to regenerate and compete effectively within their community. Semi-arid forests, woodlands, grasslands and chaparral are particularly well adapted to fire with species that are either resistant to fire damage or are able to re-sprout from undamaged root systems or buried seeds.

What can tree-ring studies tell us about the role of fire in forests?

Tree-rings and fire scars
Many trees in temperate regions (those with a strong seasonal climate) produce annual growth layers that appear as rings in a cross sectional view of a tree stem. Variations in growing conditions from year-to-year produce a sequence of wide, narrow, and average ring widths. Over time the sequence forms a unique pattern that can be used like a fingerprint to determine the calendar year in which each ring was produced. This procedure is called crossdating. Events in a trees life that have a recognizable impact on its growth may also be dated once the dates of the annual rings are known. Low to moderate intensity fires that burned through a forest may injure or scar surviving trees, leaving a clear record of their passage. Records of fires from many fire-scarred trees can be compared to provide a history of the frequency, extent, and character of this process through time. Since the events can be dated to the exact calendar year, and in some cases to the season, records from one area can be compared to records from any other, as well as with independent historical records and reconstructions of past climate.

Fire in the sequoia groves
Sierra sequoia mixed-conifer forests are a typical example of a fire-adapted forest complex. The dominant tree species have elevated canopies and thick fire-resistant bark. They can easily survive low-intensity surface fires that were common in past centuries. Giant sequoias, for example, have bark up to half a meter thick, great height (up to 90 meters), cones that open to drop their seeds following heating by fires, and seedlings that require mineral soil and abundant light to survive. These characteristics allow adults to survive all but the most intense fires, and their seedlings to colonize areas cleared of forest litter and potential competitors.

Sequoias are also excellent sources of information on the history of fires in their vicinity. Surface fires often scar trees around the base, wounding small areas of tissue that are subsequently grown over, enclosing the wound within the new wood. With individuals able to live more than 3,000 years the trees can contain many centuries of environmental history within their rot-resistant wood.

Wood sections cut from down sequoia logs and standing snags were used to reconstruct the fire history in five sequoia groves for the past 1,500 years. In two groves (the Giant Forest in Sequoia National Park and in Mountain Home State Forest) the data extends back more than 3,000 years. Due to the remarkable preservation of the wood a very detailed history was compiled with seasonal resolution and information on fire extent and severity for the groves.

Findings
The detailed fire histories reconstructed for the groves show that fire was a frequent and pervasive influence in these forests for the entire length of the record. Fires of some size occurred every few years with larger fires occurring once or twice per decade. Regional fire years were also apparent in the record, with four or five groves recording the same fire dates at a greater frequency than would be expected by chance. Frequencies of these regional events varied from 2 to 5 times per century. This variability in fire synchrony and frequency through time appears to typify the record and suggests that factors controlling fire regimes varied through time. Regional fire synchrony indicates the importance of climate in landscape pre-conditioning and ignition rates. Wet conditions probably inhibited fire ignition and spread while accelerating fuel production and accumulation. Dry conditions favored effective ignition and spread. Persistence of cool/moist conditions for extended periods seemed to result in decreased fire frequency while increasing typical fire size and intensity. This probably occurred because longer intervals between fires resulted in more abundant and continuous fuels and, therefore, larger areas were burned during dry/warm conditions that eventually arrived. Long-term warm/dry conditions may have had the opposite effect, with more frequent fires, but less spatial continuity of fuels and, consequently smaller fires.
Observed temporal fluctuations in fire frequency and synchrony correspond to independent estimates of past climate. A maximum in fire frequency occurred from about AD 1000 to about AD 1300 during the Medieval Warm Period, bracketed by minima in fire frequency and temperature. The latter relatively cool period is known as the Little Ice Age. In addition to the century-scale patterns, decadal fluctuations in fire frequency also match similar changes in summer temperature reconstructions for the Sierra Nevada. At the year-to year or seasonal time scale, winter-spring drought was the most important factor influencing fire occurrence. 

Fire in other Sierrian forest types

Fires were, and continue to be important within the entire range of forest types in the Sierra. Before the late 1800s, lower elevation pine-oak forests and woodlands were subject to relatively frequent (2-6 years), but low-intensity fires. As elevation increases and forest conditions become cooler and wetter, fire frequency decreased. For some areas along the west face of the Sierra, fire-frequency varied systematically with elevation. However, fire in the Yosemite area had a weak relationship with elevation; generally high fire frequencies occurred throughout the studied area. This may be due to the complexity of the terrain and its highly dissected nature. Another factor may have been increased burning of this landscape by Native American populations.

How have fire and climate interacted over the past several millennia?

The long sequoia record shows clearly that fire regimes were not constant through time but fluctuated as climate and forest conditions changed. Warm climatic episodes may result in higher fire frequencies, but fire-fuel feedbacks tend to result in smaller, patchier fires over time. Conversely, cooler wetter periods may decrease fire frequency, but may result in more extensive and severe fires as fuel accumulates between events. Furthermore, extreme events may be most likely during shifts of climatic state, such as in the warm-dry to cool-wet transition around the end of the thirteenth century. We have found evidence, for example, of widespread and unusually severe fires in some of the sequoia groves at this time.

What can we expect in the future?

We should expect fire regimes to continue evolving in tune with changes in forest conditions and fluctuations in climate. Altered fire regimes in a warmer climate (more frequent burning) could help push forest-type boundaries upslope and change the reproductive success of forest tree species leading to changes in species composition within types. Higher precipitation coupled with a warmer climate could lead to increases in fuel production, with corresponding increases in fire intensity and size. Warmer-drier conditions might initially lead to intense fires followed by a decrease in fire severity as fuel production declined. The paleofire reconstructions demonstrate that the forest-climate system is highly dynamic. Although relatively stable states persisted for centuries, large changes also occurred in the record. The past does not provide a perfect analog for expected future changes, but it does tell us how these dynamic systems responded to past climate changes. The past also helps us understand how we arrived at the current state of conditions.

Selected Publications:

Caprio, A.C. and T.W. Swetnam 1993. Historic fire regimes along an elevational gradient on the West slope of the Sierra Nevada, California A. C. poster paper presented at the Symposium on Fire in Wilderness and Park Management, March 30-April 1, 1993, University of Montana, Missoula, Montana.

Dieterich, J. and T.W. Swetnam. 1984. Dendrochronology of a fire-scarred ponderosa pine. For. Sci. 30:238-247.

Douglass, A.E. 1941. Crossdating in Dendrochronology. J. For. 39:825-831.

Fritts, H.C. 1976. Tree Rings and Climate. Academic Press, London. 567pp.

Fritts, H.C. 1991. Reconstructing Large-Scale Climatic Patterns from Tree-Ring Data. The University of Arizona Press, Tucson. 286 pp.

Fritts, H.C. and T.W. Swetnam. 1989. Dendrochronology: A tool for evaluating variations in past and present forest environments. Advances in Ecological Research. 19:111-188.

Stephenson, N.L., D.J. Parsons, and T.W. Swetnam 1989. Restoring natural fire to the sequoia-mixed conifer forest: should intense fire play a role? N. L. Proceedings 17th Tall Timbers Fire Ecology Conference. High Intensity Fire in Wildlands: Management Challenges and Options. May 18-21. Tallahassee, Florida. pp.321-337.

Stokes, M.A. and T.L. Smiley. 1968. An Introduction to Tree-Ring Dating. University of Chicago Press.

Swetnam, T.W. and J.L. Betancourt 1990. Fire-southern oscillation relations in the southwestern United States. Science 249:1017-1020.

Swetnam, T.W. 1993. Fire history and climate change in giant sequoia groves. Science. 262:885-889.

Van Pelt, N.S. and T.W. Swetnam. 1990. Conservation and stewardship of tree-ring resources: Living trees and sub-fossil wood. Natural Areas J. 10(1): 19-27

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This page last updated: Thursday, March 22, 2007