Prehistoric Fires Not Caused by Understory Grasses

Determining prehistoric understory vegetation of giant sequoia (Sequoiadendron giganteum) stands by soil phytolith analysis

Photo by yourmap, Flickr Creative Commons
Photo by yourmap, Flickr Creative Commons

Grassy fuels on the forest floor were not the cause of frequent prehistoric fires in giant sequoia (Sequoiadendron giganteum) groves, according to UC Berkeley researchers and California State Park ecologists.

Fire scars in the rings of giant sequoias describe fire history dating back to the birth of the tree—sometimes as far back as 2,500 years ago. Analysis of tree-ring scars indicates that before settlers came to California low-intensity surface fires, ignited by native people or lightening, burned every five years on average. These small fires cleared only the understory brush and small trees, leaving the larger trees only partially burned and on the whole unharmed. They maintained gaps in the canopy, provided space for developing sequoia saplings, replenished soil, and most likely made hunting and moving around in the forest easier for native people.


Today, park managers are finding it difficult to mimic the frequency and intensity of prehistoric fires. They have developed the practice of introducing similar low-intensity fires into giant sequoia understories, usually in late summer or fall, to remove excess fuels as a guard against the more dangerous, high-intensity fires capable of eliminating entire stands of trees. At best, it takes the forest 15 years to regenerate enough fuel for managers to start another prescribed fire. What has happened to change this interval?

Some believe that gaps in giant sequoia stands allowed enough sunlight to reach the forest floor to support an understory of native grass species, which during the dry season fed frequent surface fires. UC Berkeley’s James Bartolome and his team have found very strong evidence to prove this untrue.

To test theory, they sampled soil in Calaveras Big Trees State Park in the Sierra Nevada Mountains for phytoliths—rigid microscopic silica bodies found in plants that provide structural stability to leaves and stalks. Thousands of years after a plant dies, its phytoliths remain preserved in the soil, fossilized marks of its existence. Grasses in particular have easily identifiable and diagnostic phytoliths. Soils that were long ago covered with grasses are expected to retain a high concentration of grass phytolyths regardless of historic vegetation changes. The number of grass phytoliths per unit of measurement in Dr. Bartolome’s samples were low enough for him to conclude that the giant sequoia understory was not covered by grasses.

Two possible explanations exist to account for park managers’ lack of success in mimicking prehistoric fires, both of which could be true. First, it is likely that trees in forest stands long ago were more widely spaced. This would cause litter to collect unevenly on the forest floor, leading to very patchy, low-intensity annual fires. Park managers today, however, attempt to burn throughout an entire stand. In addition, park managers do not burn under extreme summer conditions as a safety precaution, but it’s very likely that prehistoric fires ignited during extreme summer heat.

Past logging and fire suppression in Calaveras Big Trees State Park have left an environment in which giant sequoia groves do not regenerate naturally. Tightly crowded thickets of fir and pine trees that have developed in the absence of surface fires put large mature trees at risk of being wiped out by high-intensity crown fires. Park managers expect to use the results of this study to revise their use of prescribed fire as a restoration tool in the giant sequoia forest.

The researchers’ report has been accepted for publication in the journal Madrono.

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