DOCSLIB.ORG

Dendrochronology 459

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Dendrochronology 459 DENDROCHRONOLOGY 459 Taylor, R. E. (1987). Radiocarbon Dating: An Archaeological Wintle, A. G. (1973). Anomalous fading of thermoluminescence in Perspective. Academic Press, New York. mineral samples. Nature 245, 143–144. Tuniz, C., Bird, J. R., Fink, D., and Herzog, G. F. (1998). Wohlfarth, B., Bjo¨ rck, S., Possnert, G., et al. (1993). AMS dating Accelerator Mass Spectrometry: Ultasensitive Analysis for of Swedish varved clays of the last glacial/interglacial transition Global Science. CRC Press, Boca Raton, FL. and potential difficulties of calibrating Late Weichselian ‘‘abso- Wagner, G. A. (1995). Age Determination of Young Rocks and lute’’ chronologies. Boreas 22, 113–128. Artifacts. Springer, Berlin. Zreda, M., and Phillips, F. (2000). Cosmogenic nuclide buildup in Wehmiller, J. F., and Miller, G. H. (2000). Aminostratigraphic dating surficial materials. In Quaternary Geochronology, Methods methods in Quaternary geology. In Quaternary Geochronology, and Applications (J. S. Noller et al., Eds.), pp. 61–76. Methods and Applications (J.S.Nolleret al., Eds.), pp. 187–222. American Geophysical Union, Washington, DC. American Geophysical Union, Washington, DC. Westgate, J. A., and Naeser, N. D. (1995). Tephrochronology and fission-track dating. In Dating Methods for Quaternary Relevant Website Deposits (N. W. Rutter and N. R. Catto, Eds.), pp. 15–28. Geological Survey of Canada, Ottawa. http://www.gg.rhul.ac.uk – Royal Holloway: Department of Geography. Dendroarchaeology see Plant Macrofossil Methods and Studies: Dendroarchaeology DENDROCHRONOLOGY D Smith and D Lewis, University of Victoria, Victoria, precipitation (Stallings, 1937). The first recorded Canada application of dendrochronology occurred in 1737 ª 2007 Elsevier B.V. All rights reserved. when Du Hamel and De Buffon noted a prominent frost damaged tree ring in recently felled trees that was later recorded in both Sweden and Germany Dendrochronology (Studhalter, 1955, 1956). By the mid-1800s, the eco- logical foundations of dendrochronology were estab- Dendrochronology is the science that deals with the lished following Theodore Hartig’s studies of wood dating and study of the annual growth layers, or tree structures and annual tree-ring development rings, in woody trees and shrubs. In temperate cli- (Schweingruber, 1988). By the end of the nineteenth mates, these layers of wood (tree rings) contain sea- century, Hartig’s son, Robert, had published numer- sonal cell structures (earlywood and latewood) that ous papers on the anatomy and ecology of tree-rings, signify one annual growth ring. When all the trees at and had used tree-rings to date hail, frost, and insect a site are affected by a common environmental fac- damage in trees (Schweingruber, 1988). tor, such as climate, crossdating provides an accurate Astronomer Andrew Ellicott Douglass pioneered chronological record that can be used to date events the science of dendrochronology in North America. or describe variations in environmental conditions. In 1904, he began examining the annual rings of Due to the annual resolution possible throughout an Ponderosa pine trees in the southwestern United entire tree-ring record, dendrochronological analyses States as a potential source for preinstrumental cli- provide both reliable and ubiquitous archives for mate records. He soon established that narrow tree paleoenvironmental reconstruction. rings were formed in very dry years over large geo- graphical areas, thereby confirming that the cross- dating of tree-ring patterns between sites could be Origins and History used as a chronological tool to identify the exact Although observations of tree-ring structures and calendar year a ring was produced (Douglas, 1909). related phenomena have a lengthy history, dendro- By 1914, Douglass had developed a 500-year cross- chronology as a scientific discipline emerged relatively dated pine chronology and established a positive cor- recently. As early as the fifteenth century, Leonardo da relation between ring width and precipitation of the Vinci noted the annual nature of tree-rings, recogniz- preceding winter (Douglass, 1914). Douglass subse- ing a relationship between tree ring widths and quently devoted the next 15 years to developing long 460 DENDROCHRONOLOGY tree-ring chronologies to date wood samples from quantitative approach to dendrochronology. Huber prehistoric sites of the American southwest. His suc- developed the statistical test of similarity, the cessful crossdating of a 585-year floating (undated) Gleichlaeufigkeit (Huber, 1943; Eckstein and archeological tree-ring sequence provided precise Pilcher, 1990) that remains an integral component dates of construction for numerous previously of statistical crossdating in European laboratories. undated pueblo sites, bringing the science of dendro- In the early 1960s, Hal Fritts established the basic chronology to age as a dating tool (Dean, 1997). physiology of radial tree growth by employing a Prior to pioneering the subfields of dendroclima- dendrograph (Haasis, 1933) to measure the daily tology and dendrohydrology, Edmund Schulman growth of tree-rings. As detailed in his seminal pub- began working with Douglass to develop better lication Tree Rings and Climate (1976), Fritts was methods of tree-ring analysis (Schulman, 1945). In able to describe many of the complex connections his quest for improving the length and sensitivity of between tree-rings and climate, and used these tree-ring records, Schulman also discovered the old- insights to provide annually-resolved climate recon- est living trees in the world, the over 4,000-year-old structions for eastern and western North America bristlecone pines of the California White Mountains (Fritts, 1991). During the same time period, Polge (Schulman, 1954). This discovery was key to cali- (1966) applied X-ray densitometry to the study of brating atmospheric 14C records and set the founda- tree rings and illustrated how latewood density pro- tion for precision radiocarbon dating methods vided higher resolution dendroclimatic insights than (LaMarche and Harlan, 1973). was possible from an analysis of the total annual The basic premise of radiocarbon dating relies on ring-width increment (Lenz et al., 1976). the fixation of an unstable atmospheric carbon iso- In the last 50 years, dendrochronology has evolved tope (carbon-14) by organisms (e.g., trees) in the from a discipline primarily applied to the dating of biosphere. As long as plants and animals are living monuments and archaeological pueblos, to one with and continue to take up air in their biophysical pro- a variety of applied applications including studies of: cesses, their tissues absorb carbon-14. Once the the history and effects of pollution (Schweingruber, organism dies, its intake of carbon atoms ceases and 1988); geobotanical dating (Winchester and the amount of carbon-14 within their tissue begins to Harrison, 2000); earthquakes (Jacoby et al., 1997); decrease logarithmically at a fixed rate, which can snow avalanches (Smith et al., 1994); glacier move- then be used to determine the time since death (Libby ment (Luckman and Villalba, 2001); volcanoes et al., 1949). It was soon realized, however, that raw ( Jones et al. , 1995 ); rive r flooding (St George an d radiocarbon measurements were not equivalent to Nielso n, 2003); and insect life cycles ( Swe tnam and calendar dates, as the level of atmospheric carbon- Lynch, 1993). In addition to the continued applica- 14 has not been constant with time. Fortunately, tion of the techniques first pioneered by Douglass, long-lived trees, like the bristlecone pines studied by the introduction of new analytical techniques (i.e., Sherman (1954), retain an annual measure of the X-radiography, X-ray densitometry, neutron activa- amount of carbon-14 present in the atmosphere. By tion analysis) are providing insights into climate- crossdating the tree-ring records from both living and induced morphological and chemical changes within dead trees, and measuring the amount of carbon-14 tree-rings and wood cells, and are being used to present in each tree ring, researchers have developed described trace-element and isotopes signatures annually resolved time-series of radiocarbon concen- (Kuniholm, 2001). trations, or radiocarbon calibration curves. The raw radiocarbon date of any sample within this time- frame can then be converted to a calendar date Biological Foundation of using the calibration curves (Damon et al., 1974). Dendrochronology These tree-ring based calibrations have successfully Tree-ring formation increased the accuracy of radiocarbon dating and presently provide annually-resolved carbon-14 Tree-ring dating is possible due to the physiological records for the last 12,000 years (Friedrich et al., behavior of trees growing in locations with seasonal 2005). climates. In temperate climates, this annual growth After the Second World War, German botanist appears as ‘rings’ that vary in width in response to Bruno Huber established an ecological approach to fluctuating environmental variables (Fritts, 1976). dendrochronology in Europe. His discovery that the Although there are differences in the anatomical year-to-year variation in tree-rings was not as pro- structure of tree rings between angiosperms (decid- nounced in temperate tree species, as it was in those uous) and gymnosperms (coniferous), both tree types of semiarid zones, led to the development of a more are commonly used in dendrochronology. DENDROCHRONOLOGY 461 Radial growth in trees is achieved by the division
Load more

Recommended publications
  • Dendrochronology, Fire Regimes
    Encyclopedia of Scientific Dating Methods DOI 10.1007/978-94-007-6326-5_76-4 # Springer Science+Business Media Dordrecht 2013 Dendrochronology, Fire Regimes Peter M. Brown* Rocky Mountain Tree-Ring Research, Inc., Fort Collins, CO, USA Definition Use of tree-ring data and methods to reconstruct past fire timing, fire regimes, and fire effects on individuals, communities, and ecosystems. Introduction Fire directly or indirectly affects woody plants in many ways, some of which will leave evidence in age or growth patterns in individual trees or community structure that can be cross-dated using dendrochronological methods. This evidence can be used to reconstruct past fire dates, fire regimes, and fire effects on individuals, communities, and ecosystems (“pyrodendroecology”). Fire regimes are defined as the combination of fire frequency, severity, size, seasonality, and relationships with forcing factors such as climate or changes in human land use. Fire Evidence in Trees and Community Structure A common type of fire evidence is from severe fire that kills all or most trees in an area which opens up space for new trees to establish. Even-aged forest structure is often used as evidence of past lethal fire, also referred to as “stand-replacing” fire. Such evidence provides a minimum date for the fire (i.e., the fire occurred sometime before the innermost dates of the oldest trees), and, by sampling multiple stands in an area, estimates of the fire size can be made based on broader- scale patterns of tree ages. Conversely, multiaged structure is considered to be evidence of less severe fires or other patchy mortality and recruitment events.
    [Show full text]
  • General Comments This Paper Brings Together Three Independent Dating
    General comments This paper brings together three independent dating techniques to improve age model reliability for a lake record in Alaska. Davies et al. help to advance two areas; firstly, to expand the tephrostratigraphic record for North America and, secondly, provide a working example of how useful tephrochronology is in validating other independent dating techniques with the use Bayesian statistics. The paper itself is clearly written and provides an example for using multiple chronometers in a lake record setting. The introduction sets out the problems of obtaining accurate and precise ages using radiocarbon dating, particularly in high latitude lakes, and how these could be resolved with the use of combining other dating techniques such as palaeomagnetism and tephrochronology with the appropriate methods of Bayesian statistics. The methods conducted by the authors is definitive though additional detail could be added to some sections to allow clear reproduction of certain processes and provide additional information as to why a certain process were chosen. The step-by-step assessment of which radiocarbon dates were appropriate for the overall age model was useful to see. The tephra correlations made by the authors look robust and clear. I agree with the comments made by RC1 for this part of the paper. There are a few additional comments made on the attached document around the presentation of data to justify primary deposition. The justification for including/excluding certain ages in the Bayesian age model were made clear and the age model results were repeatable. The discussion section brings together all the important points in a concise format.
    [Show full text]
  • An Experimental Evaluation of Fire History Reconstruction Using Dendrochronology in White Oak (Quercus Alba)
    1 An experimental evaluation of fire history reconstruction using dendrochronology in white oak (Quercus alba) Ryan W. McEwan, Todd F. Hutchinson, Robert D. Ford, and Brian C. McCarthy Abstract: Dendrochronological analysis of fire scars on tree cross sections has been critically important for understanding historical fire regimes and has influenced forest management practices. Despite its value as a tool for understanding histor- ical ecosystems, tree-ring-based fire history reconstruction has rarely been experimentally evaluated. To examine the effi- cacy of dendrochronological analysis for detecting fire occurrence in oak forests, we analyzed tree cross sections from sites in which prescribed fires had been recently conducted. The first fire in each treatment unit created a scar in at least one sample, but the overall percentage of samples containing scars in fire years was low (12%). We found that scars were cre- ated by 10 of the 15 prescribed fires, and the five undetected fires all occurred in sites where fire had occurred the previous year. Notably, several samples contained scars from known fire-free periods. In summary, our data suggest that tree-ring analysis is a generally effective tool for reconstructing historical fire regimes, although the following points of uncertainty were highlighted: (i) consecutive annual burns may not create fire scars and (ii) wounds that are morphologically indistin- guishable from fire scars may originate from nonfire sources. R6um6 : L'analyse dendrochronologique des cicatrices de feu sur des sections radiales de tronc d'arbre a it6 d'une im- portance cruciale pour comprendre les regimes des feux pass& et a influenck les pratiques d'amtnagement forestier.
    [Show full text]
  • Basic Tree-Ring Sample Preparation Techniques for Aging Aspen
    Utah State University DigitalCommons@USU Aspen Bibliography Aspen Research 2001 Basic tree-ring sample preparation techniques for aging aspen L.A. Asherin S.A. Mata Follow this and additional works at: https://digitalcommons.usu.edu/aspen_bib Part of the Forest Sciences Commons Recommended Citation Asherin, LA and Mata, SA. 2001. Basic tree-ring sample preparation techniques for aging aspen. Shepperd, WD; Binkley, D; Bartos, DL; Stohlgren, TJ; and Eskew, LG, compilers. Sustaining Aspen in Western Landscapes: Symposium Proceedings. Proceedings RMRS-P-18. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. Fort Collins, CO. This Contribution to Book is brought to you for free and open access by the Aspen Research at DigitalCommons@USU. It has been accepted for inclusion in Aspen Bibliography by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Basic Tree-Ring Sample Preparation Techniques for Aging Aspen Lance A. Asherin1 and Stephen A. Mata1 Abstract—Aspen is notoriously difficult to age because of its light-colored wood and faint annual growth rings. Careful preparation and processing of aspen ring samples can overcome these problems, yield accurate age and growth estimates, and concisely date disturbance events present in the tree-ring record. Proper collection of aspen wood is essential in obtaining usable ring data. Mounting of increment cores and sawn disk samples to wood backings holds samples rigid for easy surfacing. Sequential use of planers, belt sanders, and an assortment of sanding material on the surface of aspen core and disk samples can enhance visibility of tree rings. Application of stain on samples will color the late wood a dark brown and enhance the rings’ visibility.
    [Show full text]
  • Scientific Dating of Pleistocene Sites: Guidelines for Best Practice Contents
    Consultation Draft Scientific Dating of Pleistocene Sites: Guidelines for Best Practice Contents Foreword............................................................................................................................. 3 PART 1 - OVERVIEW .............................................................................................................. 3 1. Introduction .............................................................................................................. 3 The Quaternary stratigraphical framework ........................................................................ 4 Palaeogeography ........................................................................................................... 6 Fitting the archaeological record into this dynamic landscape .............................................. 6 Shorter-timescale division of the Late Pleistocene .............................................................. 7 2. Scientific Dating methods for the Pleistocene ................................................................. 8 Radiometric methods ..................................................................................................... 8 Trapped Charge Methods................................................................................................ 9 Other scientific dating methods ......................................................................................10 Relative dating methods ................................................................................................10
    [Show full text]
  • Msc Or Phd Project in Urban Dendrochronology Resilient Urban Forests for Canadians Adapting to Climate Change for Enhanced Tree-Related Benefits
    MSc or PhD project in urban dendrochronology Resilient Urban Forests for Canadians Adapting to Climate Change for Enhanced Tree-Related Benefits Urban forests (UF) are associated with important ecosystem services (ES), such as heat reduction, improved air quality, and carbon storage, which benefit both environmental and human health. Yet it remains unclear how CC will impact the ability of our UF to provide these ES, how our UF will adapt, and how it could be better managed to maximize the generated benefits. We have secured significant funding from NSERC to better characterize UF with richer data, and to identify best practices for developing resilient UF that will support ES now and in the future. To do so, we are seeking excellent candidates for an MSc on the following research question. Extraordinary PhD candidates will also be considered (and project augmented accordingly). Measure the effects of climate change on urban forests using coring and dendrochronology to investigate the effects of past climatic events, such as drought, on growth. The selected candidate will integrate and benefit from a larger, interdisciplinary project framework which involves the participation of remote sensing, tree physiology, and human health specialists from both academic and governmental backgrounds, throughout Canada. The project involves travelling across Canada. Prospective students should contact us with the following information: letter of interest (1 page), CV, unofficial transcripts, and the contact information for three references. Informal inquiries are welcome. Position is based at UQAM in Montreal, Québec, Canada, a busy and lively multicultural city. UQAM is a French speaking university, but knowledge of the French language is not mandatory and Montreal is a friendly place for those hoping to learn a new language.
    [Show full text]
  • Summary of Sustainable Forestry and Other Research Fiscal Year 2015
    Michigan Department of Natural Resources www.michigan.gov/dnr SUMMARY OF SUSTAINABLE FORESTRY AND OTHER RESEARCH FISCAL YEAR 2015 Forest Resources Division IC4117 (Rev. 06/03/2016) MICHIGAN DEPARTMENT OF NATURAL RESOURCES MISSION STATEMENT "The Michigan Department of Natural Resources is committed to the conservation, protection, management, use and enjoyment of the State’s natural and cultural resources for current and future generations." NATURAL RESOURCES COMMISSION STATEMENT The Natural Resources Commission, as the governing body for the Michigan Department of Natural Resources, provides a strategic framework for the DNR to effectively manage your resources. The NRC holds monthly, public meetings throughout Michigan, working closely with its constituencies in establishing and improving natural resources management policy. The Michigan Department of Natural Resources provides equal opportunities for employment and access to Michigan's natural resources. Both State and Federal laws prohibit discrimination on the basis of race, color, national origin, religion, disability, age, sex, height, weight or marital status under the U.S. Civil Rights Acts of 1964 as amended, 1976 MI PA 453, 1976 MI PA 220, Title V of the Rehabilitation Act of 1973 as amended, and the 1990 Americans with Disabilities Act, as amended. If you believe that you have been discriminated against in any program, activity, or facility, or if you desire additional information, please write: Human Resources, Michigan Department of Natural Resources, PO Box 30028, Lansing MI 48909-7528, or Michigan Department of Civil Rights, Cadillac Place, 3054 West Grand Blvd, Suite 3-600, Detroit, MI 48202, or Division of Federal Assistance, U.S. Fish and Wildlife Service, 4401 North Fairfax Drive, Mail Stop MBSP-4020, Arlington, VA 22203 For information or assistance on this publication, contact the Forest Resources Division, Michigan Department of Natural Resources, P.O.
    [Show full text]
  • The Distal and Local Volcanic Ash in the Late Pleistocene Sediments Of
    geosciences Article The Distal and Local Volcanic Ash in the Late Pleistocene Sediments of the Termination I Interval at the Reykjanes Ridge, North Atlantic, Based on the Study of the Core AMK-340 Alexander Matul 1,* , Irina F. Gablina 2, Tatyana A. Khusid 1, Natalya V. Libina 1 and Antonina I. Mikhailova 2 1 Shirshov Institute of Oceanology, Russian Academy of Sciences, 117997 Moscow, Russia 2 Geological Institute, Russian Academy of Sciences, 119107 Moscow, Russia * Correspondence: [email protected] Received: 18 June 2019; Accepted: 29 August 2019; Published: 30 August 2019 Abstract: We made the geochemical analysis of the volcanic material from the sediment core AMK-340 (the Russian research vessel “Akademik Mstislav Keldysh” station 340), the central zone of the Reykjanes Ridge. Two ash-bearing sediment units within the interval of the Termination I can be detected. They correlate with the Ash Zone I in the North Atlantic Late Quaternary sediments having an age of 12,170–12,840 years within the Younger Dryas cold chronozone and 13,600–14,540 years within the Bølling–Allerød warm chronozone. The ash of the Younger Dryas unit is presented mostly by the mafic and persilicic material originated from the Icelandic volcanoes. One sediment sample from this unit contained Vedde Ash material. The ash of the Bølling–Allerød unit is presented mostly by the mafic shards which are related to the basalts of the rift zone on the Reykjanes Ridge, having presumably local origin. Possible detection of Vedde Ash could help to specify the timing of the previously reconstructed paleoceanographic changes for the Termination I in the point of the study: significant warming in the area might have occurred as early as 300 years before the end of the conventional Younger Dryas cold chronozone.
    [Show full text]
  • Determining Tree Growth in the Urban Forest
    Determining Tree Growth in the Urban Forest Abstract The benefits of the urban forest are likely to become increasingly important due to rising urban populations coupled with increased and intensified climatic events. Predicting the growth of tree species would enable researchers and urban foresters to predict ecosystem services over the lifetime of the urban forest whilst providing cost-benefit analysis, urban forest resilience and succession planning i.e. an improved understanding of urban tree growth would facilitate creation of valuable and sustainable urban forests. An approach to analysing tree growth in the urban forest is explored using dendrochronology. This approach allows relationships between tree growth and previous meteorological events to be investigated. Our study found that of four tree species investigated; sycamore (Acer pseudoplatanus), ash (Fraxinus excelsior), beech (Fagus sylvatica) and oak (Quercus robur), growth was greatest in ash followed by sycamore, oak and then beech. All species followed similar radial growth trends, however, sycamore cores were the most difficult to analyse due to indistinct growth rings. Sycamore cores were also the least sensitive to meteorological conditions, which indicated that this species is least affected by climate. It may also explain why sycamore is well adapted to UK urban areas. Beech showed a significant correlation between growth, annual average temperature, and sunlight hours. No significant relationship was found for growth in any of the four species when correlated with annual precipitation. Introduction Keywords: Background dendrochronology, dendroclimatology, In order to develop viable strategies for conserving eco-system services, it growth rates, urban is important to estimate the functional value of trees and woodlands so their forest importance can be demonstrated to stakeholders and beneficiaries (TEEB, 2009).
    [Show full text]
  • Advancing Dendrochronological Studies of Fire in the United States
    fire Perspective Advancing Dendrochronological Studies of Fire in the United States Grant L. Harley 1,* ID , Christopher H. Baisan 2, Peter M. Brown 3, Donald A. Falk 2,4, William T. Flatley 5, Henri D. Grissino-Mayer 6 ID , Amy Hessl 7, Emily K. Heyerdahl 8, Margot W. Kaye 9, Charles W. Lafon 10 ID , Ellis Q. Margolis 11 ID , R. Stockton Maxwell 12 ID , Adam T. Naito 4, William J. Platt 13 ID , Monica T. Rother 14, Thomas Saladyga 15, Rosemary L. Sherriff 16, Lauren A. Stachowiak 17, Michael C. Stambaugh 18, Elaine Kennedy Sutherland 19 and Alan H. Taylor 20 1 Department of Geography, University of Idaho, Moscow, ID 83844, USA 2 Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA; [email protected] (C.H.B.); [email protected] (D.A.F.) 3 Rocky Mountain Tree-Ring Research, Fort Collins, CO 80526, USA; [email protected] 4 School of Natural Resources and the Environment, University of Arizona, Tucson, AZ 85721, USA; [email protected] 5 Department of Geography, University of Central Arkansas, Conway, AR 72035, USA; wfl[email protected] 6 Department of Geography, University of Tennessee, Knoxville, TN 37916, USA; [email protected] 7 Department of Geology and Geography, West Virginia University, Morgantown, WV 26505, USA; [email protected] 8 Rocky Mountain Research Station, Fire Sciences Laboratory, USDA Forest Service, Missoula, MT 59808, USA; [email protected] 9 Department of Ecosystem Science and Management, The Pennsylvania State University, University Park, PA 16803, USA; [email protected] 10 Department of Geography, Texas A&M University, College Station, TX 77843, USA; [email protected] 11 U.S.
    [Show full text]
  • Dendrochronology-Based Fire History of Jeffrey Pine – Mixed Conifer Forests in the Sierra San Pedro Martir, Mexico
    Color profile: Generic CMYK printer profile Composite Default screen 1090 Dendrochronology-based fire history of Jeffrey pine – mixed conifer forests in the Sierra San Pedro Martir, Mexico Scott L. Stephens, Carl N. Skinner, and Samantha J. Gill Abstract: Conifer forests in northwestern Mexico have not experienced systematic fire suppression or logging, making them unique in western North America. Fire regimes of Pinus jeffreyi Grev. & Balf. mixed conifer forests in the Sierra San Pedro Martir, Baja California, Mexico, were determined by identifying 105 fire dates from 1034 fire scars in 105 specimens. Fires were recorded between 1521 and 1980 and median fire return intervals were less than 15 years at all compositing scales. Significant differences in mean fire return intervals were detected from 1700 to 1800, 1800 to 1900, and 1900 to 1997, most often at intermediate spatial compositing scales, and the proportion of trees scarred in the fires of the 1700s was significantly different from the fires of either the 1800s, the 1900s, or the combined post- 1800 period. Superposed epoch analysis determined that moderate and large spatial scale fires occurred on significantly dry years during the length of the record, but before 1800, these fires were preceded by significantly higher precipita- tion 1 year before the fire. The dominance of earlywood fires in the Sierra San Pedro Martir is similar to the seasonal- ity found in the southwest United States and is different from the western slope of the Sierra Nevada and Klamath Mountains of California. Résumé : Les forêts de conifères du Nord-Ouest du Mexique sont uniques dans l’Ouest de l’Amérique du Nord n’ayant pas été exploitées ni protégées contre les feux.
    [Show full text]
  • Radiocarbon Dating Tephra Layers in Britain and Iceland
    RADIOCARBON DATING TEPHRA LAYERS IN BRITAIN AND ICELAND A. J. DUGMORE,1 G. T. COOK,2 J. S. SHORE,2 A. J. NEWTON,1 K J. EDWARDS3 and GUDRUNLARSEN4 ABSTRACT. Layers of volcanic ash, or tephra form widespread chronostratigraphic marker horizons which are important because of their distinctive characteristics and rapid deposition over large areas. Absolute dating of prehistoric layers effec- tively depends upon 14C analysis. We focus here on Icelandic tephra layers at both proximal and distal sites and consider three strategies to obtain age estimates: 1) the conventional dating of individual profiles; 2) high-precision multisample techniques or "wiggle-matching" using stratigraphic sequences of peat; and 3) a combination of routine analyses from multiple sites. The first approach is illustrated by the dating of a peat profile in Scotland containing tephra from the AD 1510 eruption of Hekla. This produced a 14C age compatible with AD 1510, independently derived by geochemical correlation with historically dated Icelandic deposits. In addition, the ca. 2100 BP date for the Glen Garry tephra in Scotland, determined by a series of dates on a peat profile in Caithness, is supported 14C by its stratigraphic position within dated profiles in Sutherland, and may be applied over a very large area of Scotland. More precise dates for individual tephras may be produced by "wiggle-matching", although this approach could be biased by changes in peat-bog stratigraphy close to the position of the tephra fall. As appro- priate sites for "wiggle-match" exercises may be found only for a few Icelandic tephras, we also consider the results of a spa- tial approach to 14C dating tephra layers.
    [Show full text]