Annually resolved radiocarbon (¹⁴C) measurements on tree rings led to the discovery of abrupt variations in ¹⁴C production attributed to large solar flares. We present new results of annual and subannual ¹⁴C fluctuations in tree rings... more
Annually resolved radiocarbon (¹⁴C) measurements on tree rings led to the discovery of abrupt variations in ¹⁴C production attributed to large solar flares. We present new results of annual and subannual ¹⁴C fluctuations in tree rings from a middle-latitude sequoia (California) and a high-latitude pine (Finland), analyzed for the period 1030–1080 CE, to trace a possible impact of the Crab supernova explosion, occurring during the Oort minimum of solar activity. Our results indicate an increase of Δ¹⁴C around 1054/55 CE, which we estimate is higher in magnitude than the cyclic variability due to solar activity at a 2σ significance level. The net signal appears to be synchronized in the studied locations. Several sources of this event are possible including γ-rays from the Crab supernova, an unusually weak solar minimum or a solar energetic particle incident. More data are needed to provide more insight into the origin of this ¹⁴C event
Research Interests: Tree Rings and Supernova
Research Interests: Climate Change, Fire Ecology, Ecology, Fire History, Climate, and 15 moreFire regimes, Topography, Forests, Climatic Change, Vegetation, Growth rings, Spatial Variation, Tree Ring, Ecological Applications, Conservation Priorities, Age Structure, Temporal Variability, Fire regime, ENVIRONMENTAL SCIENCE AND MANAGEMENT, and ponderosa pine
We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km2 drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised... more
We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km2 drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised the valley-filling alluvial sediments in the lower basin, and created exposures of fire-related of deposits of late-Holocene age. Tree-ring and alluvial sediment fire history records were created separately, and then compared and integrated to create a ~ 3000 year record of past fire activity. The tree-ring record showed that from AD 1679 to 1879, there were frequent surface fires, while patches of high-severity fire occurred during widespread fire years. The alluvial record showed that a low- to moderate-and mixed-severity fire regime has likely been dominant over the past ~ 2600 calibrated calendar years before present, as shown by locally episodic deposition of charcoal-rich, fine-grained sediments. Radiocarbon dating suggested that in two strat...
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ABSTRACTAnnually resolved radiocarbon (14C) measurements on tree rings led to the discovery of abrupt variations in 14C production attributed to large solar flares. We present new results of annual and subannual 14C fluctuations in tree... more
ABSTRACTAnnually resolved radiocarbon (14C) measurements on tree rings led to the discovery of abrupt variations in 14C production attributed to large solar flares. We present new results of annual and subannual 14C fluctuations in tree rings from a middle-latitude sequoia (California) and a high-latitude pine (Finland), analyzed for the period 1030–1080 CE, to trace a possible impact of the Crab supernova explosion, occurring during the Oort minimum of solar activity. Our results indicate an increase of Δ14C around 1054/55 CE, which we estimate is higher in magnitude than the cyclic variability due to solar activity at a 2σ significance level. The net signal appears to be synchronized in the studied locations. Several sources of this event are possible including γ-rays from the Crab supernova, an unusually weak solar minimum or a solar energetic particle incident. More data are needed to provide more insight into the origin of this 14C event.
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We performed a new series of measurements on samples that were part of early measurements on radiocarbon (14C) dating made in 1948–1949. Our results show generally good agreement to the data published in 1949–1951, despite vast changes in... more
We performed a new series of measurements on samples that were part of early measurements on radiocarbon (14C) dating made in 1948–1949. Our results show generally good agreement to the data published in 1949–1951, despite vast changes in technology, with only two exceptions where there was a discrepancy in the original studies. Our new measurements give calibrated ages that overlap with the known ages. We dated several samples at four different laboratories, and so we were also able to make a small intercomparison at the same time. In addition, new measurements on samples from other Egyptian materials used by Libby and co-workers were made at UC Irvine. Samples of tree rings used in the original studies (from Broken Flute Cave and Centennial Stump) were obtained from the University of Arizona Laboratory of Tree-Ring Research archive and remeasured. New data were compared to the original studies and other records.
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Two radiocarbon excursions (AD 774–775 and AD 993–994) occurred due to an increase of incoming cosmic rays on a short timescale. The most plausible cause of these events is considered to be extreme solar proton events (SPE). It is... more
Two radiocarbon excursions (AD 774–775 and AD 993–994) occurred due to an increase of incoming cosmic rays on a short timescale. The most plausible cause of these events is considered to be extreme solar proton events (SPE). It is possible that there are other annual 14C excursions in the past that have yet to be confirmed. In order to detect more of these events, we measured the 14C contents in bristlecone pine tree-ring samples during the periods when the rate of 14C increase in the IntCal data is large. We analyzed four periods every other year (2479–2455 BC, 4055–4031 BC, 4465–4441 BC, and 4689–4681 BC), and found no anomalous 14C excursions during these periods. This study confirms that it is important to do continuous measurements to find annual cosmic-ray events at other locations in the tree-ring record.
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Introduction Fire is a primary disturbance process in sagebrush steppe communities, influencing plant dynamics, composition, and structure. The balance between woody and herbaceous vegetation is highly influenced by the length of fire... more
Introduction Fire is a primary disturbance process in sagebrush steppe communities, influencing plant dynamics, composition, and structure. The balance between woody and herbaceous vegetation is highly influenced by the length of fire return intervals. Fire regimes have changed since ...
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Fire is a dominant ecological process in almost all landscapes of La Frontera. Fire histories from throughout the region on the US side of the border show that, before ca. 1900, extensive surface fires occurred within pine-dominant... more
Fire is a dominant ecological process in almost all landscapes of La Frontera. Fire histories from throughout the region on the US side of the border show that, before ca. 1900, extensive surface fires occurred within pine-dominant forests at about the same frequency ...
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EXECUTIVE SUMMARY The purpose of this project was to develop a detailed, high-resolution, millennial-length regional reconstruction of precipitation for the southern New Mexico/Rio Grande Basin area, with particular reference to Fort... more
EXECUTIVE SUMMARY The purpose of this project was to develop a detailed, high-resolution, millennial-length regional reconstruction of precipitation for the southern New Mexico/Rio Grande Basin area, with particular reference to Fort Bliss, White Sands Missile Range, and Holloman Air Force Base. Based on a network of sites containing trees where growth is largely determined by precipitation, this reconstruction is to be used by natural and cultural resources managers of these and other agencies to:(1) document changes in ...
Fire history researchers employ various forms of search-based sampling to target specimens that contain visible evidence of well preserved fire scars. Targeted sampling is considered to be the most efficient way to increase the... more
Fire history researchers employ various forms of search-based sampling to target specimens that contain visible evidence of well preserved fire scars. Targeted sampling is considered to be the most efficient way to increase the completeness and length of the fire-scar record, but the accuracy of this method for estimating landscape-scale fire frequency parameters compared with probabilistic (i.e. systematic and random) sampling is poorly understood. In this study we compared metrics of temporal and spatial fire occurrence reconstructed independently from targeted and probabilistic fire-scar sampling to identify potential differences in parameter estimation in south-western ponderosa pine forests. Data were analysed for three case studies spanning a broad geographic range of ponderosa pine ecosystems across the US Southwest at multiple spatial scales: Centennial Forest in northern Arizona (100ha); Monument Canyon Research Natural Area (RNA) in central New Mexico (256ha); and Mica Mou...
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ABSTRACT The main objective of this project was to reconstruct past fire history in different giant sequoia groves distributed along a north·south transect from Yosemite National Park southward to Mountain Home State Forest. We planned to... more
ABSTRACT The main objective of this project was to reconstruct past fire history in different giant sequoia groves distributed along a north·south transect from Yosemite National Park southward to Mountain Home State Forest. We planned to develop millennia-length fire-scar chronologies for different sequoia mixed-conifer groves and to compare the fire histories (fire frequency, size, intensity, and season) within and among these groves. Earlier fire-scar studies in the Redwood Mountain Grove by Kilgore and Taylor (1979) demonstrated that pre-settlement (pre·1875) surface fires were relatively frequent with two to three year mean fire intervals within watersheds of about 800 to 1000 ha., and five to nine year mean fire intervals in smaller sites of 3 to 16 ha. These fire-scar chronologies, based upon specimens from pines, fir, and incense cedar, were used to estimate fire frequency variations within two watersheds back to about A.D. 1700, although a few samples had scars extending back into the late 1400s. Our goal has been to spatially expand and temporally extend the knowledge base of fire history to other sequoia mixed-conifer groves in the Sierra Nevada by sampling ancient fire-scarred sequoias.
Research Interests: Tree Ring and Fire regime
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We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km2 drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised... more
We used tree-ring and alluvial sediment methods to reconstruct past fire regimes for a mixed conifer forest within a 1 km2 drainage basin which was severely burned by a wildfire near Durango, Colorado. Post-fire debris flow events incised the valley-filling alluvial sediments in the lower basin, and created exposures of fire-related of deposits of late-Holocene age. Tree-ring and alluvial sediment