Abstract
Giant sequoias (Sequoiadendron giganteum [Lindl.] J. Buchholz) preserve a detailed history of fire within their annual rings. We developed a 3000 year chronology of fire events in one of the largest extant groves of ancient giant sequoias, the Giant Forest, by sampling and tree-ring dating fire scars and other fire-related indicators from 52 trees distributed over an area of about 350 ha. When all fire events were included in composite chronologies, the mean fire intervals (years between fires of any size) declined as a function of increasing spatial extent from tree, to group, to multiple groups, to grove scales: 15.5 yr (0.1 ha), 7.4 yr (1 ha.), 3.0 yr (70 ha), and 2.2 yr (350 ha), respectively. We interpreted widespread fires (i.e., fire events recorded on ≥2 trees, or ≥25% of all trees recording fires within composites) to have occurred in areas of 70 ha to 350 ha at mean intervals ranging from about 6 yr to 35 yr. We compared the annual, multi-decadal and centennial variations in Giant Forest fire frequency with those documented in tree-ring and charcoal-based fire chronologies from four other giant sequoia groves in the Sierra Nevada, and with independent tree-ring-based reconstructions of summer drought and temperatures. The other giant sequoia fire histories (tree rings and charcoal-based) were significantly (P < 0.001) correlated with the Giant Forest fire frequency record and independent climate reconstructions, and confirm a maximum fire frequency during the warm and drought-prone period from 800 C.E. to 1300 C.E. (Common Era). This was the driest period of the past two millennia, and it may serve as an analog for warming and drying effects of anthropogenic greenhouse gases in the next few decades. Sequoias can sustain very high fire frequencies, and historically they have done so during warm, dry times. We suggest that preparation of sequoia groves for anticipated warming may call for increasing the rate of prescribed burning in most parts of the Giant Forest.
Highlights
Giant sequoia (Sequoiadendron giganteum [Lindl.] Buchholz) trees in California, USA, are among the most magnificent and awe inspiring living things on Earth
Using our observations from the full cross section and the partial sections that we brought to our laboratory in Tucson, Arizona, we developed the most comprehensive, complete fire scar dating and analysis of a single giant sequoia tree
A total of 122 fire events were recorded on this tree, and 87 of these dates were recorded as fire scars
Summary
Giant sequoia (Sequoiadendron giganteum [Lindl.] Buchholz) trees in California, USA, are among the most magnificent and awe inspiring living things on Earth. Beyond the astounding scale and aesthetic beauty of giant sequoias, another characteristic inspires our wonder: some sequoias are very ancient, exceeding 3200 years in age. They contain a rich and detailed history of fire and climate within their annual tree rings. The giant sequoia chronology that he developed was long enough, with the oldest specimen dating back to 1306 B.C.E., but it was unhelpful in crossdating the archeological specimens because of climatic (and ring-width pattern) differences between the Sierra Nevada and southwest US. Douglass’ sequoia work was invaluable to our studies, providing information on where to find the oldest trees and a ring-width chronology for crossdating our numerous tree-ring and fire scar specimens
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