Abstract

TEMPORAL AND SPATIAL PATTERNS OF GIANT SEQUOIA RADIAL GROWTH RESPONSE TO A HIGH SEVERITY FIRE IN A.D. 1297

Fire was a dominant disturbance process in the development and maintenance of most Sierra Nevadan forests, including the many unique giant sequoias groves. Individual severe fire events had important impacts on both the short-term dynamics and the long-term history of these forest stands. In the Mountain Home Grove of giant sequoias we observed that a fire in A.D. 1297, recorded as scars on many trees, was followed by a growth release of unusually large magnitude and duration. The number of scarred trees and a growth release of this size suggested this fire event was of unusually high severity, not equaled over the last 2,000 years. We hypothesize that this event was associated with mortality of most non-sequoia tree species and a considerable number of giant sequoias. In this study we examined the spatial extent of this event throughout the grove and the temporal response of the growth release by comparing ring-width series from samples from within the grove to a regional "control" tree-ring chronology. These findings were also compared to results from another recent study relating growth releases following contemporary prescribed burns and logging to impact severity (Mutch 1994).

Using the magnitude of the growth release as a proxy of fire severity, we found that the spatial distribution of the 1297 fire impacts indicated the fire was most severe in the northern and central portions of the grove. Sequoia trees in peripheral areas had a smaller post-fire growth release. The post-fire growth release at Mountain Home was apparent for about 100 years (to 1400) when pre-burn average growth was again equalled. The release was often proceeded by a short period of suppressed growth in many trees, that we attributed to the severity of the burn. Our comparison of the growth release to the regional "control" chronology showed significantly greater growth at Mountain Home during much of the release period. The comparison was somewhat confounded by the fact that there was an apparent growth increase at several of the individual "control" sites that may also have been associated with fire. Time series of precipitation, reconstructed from tree-rings, showed that a long drought occurred between 1292 and 1296. Hence, regional fire events may have occurred during this period and could have affected the "control" sites. The comparison of the 1297 event to modern disturbances showed that most recent prescribed burns were of much smaller impact, although a few may have responses that exceeded the effects of the 1297 event. Comparison to a logged area suggested that although there are some similar post-impact growth responses there may also be some differences.