A symposium at the American Association for the Advancement of Science Southwestern and Rocky Mountain Division 80th annual meeting (Progress and Sustainability in the 21st Century): April 13-16, 2005, University of Arizona, Tucson, AZ.
April 14, 8:00AM to Noon
Andrew Ellicott Douglass (1867-1962) was one of America's great men of science. Primarily an astronomer, Douglass's interests were wide ranging and voracious. In addition to helping establish four American observatories, Douglass made substantial contributions to the fields of climatology and archaeology and established the field of dendrochronology. In 1901, he became intrigued with the growth rings visible in the trunks of trees. Speculating that they might indicate a relation between solar variation and terrestrial climate, Douglass designed and created a number of increasingly complex instruments to help him study cycles. His research helped him establish a chronology that enabled the prehistoric Pueblo ruins of the American Southwest to be accurately dated. The University of Arizona, where Douglass worked for more than fifty years, has preserved a number of these instruments, and my paper will describe several of them and discuss their significance.
The American West is uniquely rich both in ancient trees and in wood preserved for millennia. Since the inception of dendrochronology by Andrew Ellicott Douglass in Arizona during the first years of the 20th century, scientists have sought to decode the various records of past environment stored in these natural archives, particularly as they relate to climate. Some, such as the bristlecone pines, cover up to 9000 years, most of the time since the end of the last Ice Age. Some dozens of records between 1000 and 3000 years long have been developed, and many hundreds that cover the last 400 to 500 years. These records have played a central role in helping uncover the nature of climate variability on several timescales. The scientific basis for this will be discussed, as will the implications of the results. Particular attention will be given to the history of winter half-year precipitation since AD 1000 in Arizona and New Mexico.
A long and productive history of dendrohydrology at the Laboratory of Tree-Ring Research began with the pioneering work of Edmund Schulman in the 1940s and 50s. Schulman demonstrated that runoff variability for basins in the Southwest could be effectively studied with networks of properly developed tree-ring chronologies. In the 1970s, Charles Stockton fused this idea with modern statistical methods to generate a reconstruction of the Colorado River at Lees Ferry, and concluded that the waters of the Colorado had been allocated based on statistics for the wettest period in the last 500 years. Tree rings have since been successfully applied to estimate the natural variability of runoff in several other basins in the Southwest. Emphasis in reconstructions has shifted noticeably in recent years toward stakeholder-driven questions. Much emphasis is now directed to finding ways that water managers can incorporate tree-ring information in planning and operation of river systems. The recent severe drought that lowered water levels drastically in reservoirs throughout the Southwest has in particular spurred research on long-term reliability of water supply and on multi-basin aspects of supply. Current studies in the Upper Colorado River Basin and Salt and Verde River Basins are aimed at providing information for basin management on various time scales. In this presentation we review the scientific basis for dendrohydrology, summarize some major historical landmarks of studies conducted at the Laboratory of Tree-Ring Research, and describe ongoing studies.
To improve understanding of the full range of interactions between human groups and volcanic eruptions, we have begun a re-analysis of the dating of the last eruption of Sunset Crater, northern Arizona, long thought to have erupted in AD 1064. While the original dating was based on dendrochronological analysis of ring widths, our re-analysis uses dendrochemical techniques, namely the measurement and analysis of various chemical elements in dated tree rings. To discover and calibrate potential dendrochemical signals from cinder cone eruptions generally, we started our re-analysis by investigating rings of trees growing around Volcano Parícutin, Michoacán, México, which last erupted from 1943 to 1952 in an event that is considered to have been geologically similar to the Sunset Crater eruption. Of particular note in Parícutin tree-ring samples that extend back in time to prior to the eruption, ring levels of phosphorus increased abruptly beginning in 1943 and dropped back again after 1952, although not to pre-eruption levels. In archeological tree-ring samples from nearby Sunset Crater, ring levels of phosphorus also changed during the 11th century, though not exactly at AD 1064. If changes in ring phosphorus can truly be interpreted as evidence of cinder cone activity, then perhaps Sunset Crater had a more complex eruption sequence than is indicated by just the single-year date of 1064. Much work remains to verify these preliminary findings, but dendrochemistry might be a powerful tool to help refine dating certain past volcanic eruptions, thereby improving the archeological interpretations of human responses to them.
Dendroarchaeology began with Andrew Ellicott Douglass' demonstration in 1917 that parts of the Anasazi site of Pueblo Bonito in Chaco Canyon, New Mexico, were built 50 years before parts of the Aztec Ruin fifty miles to the north. The discipline came into its own in 1929 when Douglass joined his archaeological and living-tree ring chronologies and assigned absolute calendar dates to the prehistoric sites that contributed wood samples to the archaeological segment of the sequence. Since then, the Laboratory of Tree-Ring Research at The University of Arizona has produced more than 60,000 dates from more than 5,000 sites. Given their high accuracy, precision, and resolution, these dates provide the Southwest with the finest prehistoric temporal controls in the world. In addition, archaeological tree-ring sample collections illuminate past human behavior, specify past environmental conditions and variations, and provide data for dendroclimatic reconstructions of precipitation, temperature, drought indices, streamflow, and other climate variables. Analyses of individual site and locality chronologies, the implications of human wood use behavior, and the impacts of past environmental variability on human societies exemplify dendroarchaeology's chronometric, human behavioral, and cultural ecological contributions to understanding sociocultural stability, variation, and change in the Southwest over the past 2,000 years. The recent expansion of archaeological tree-ring dating into the Great Basin, Sonoran Desert, and northern Mexico adumbrate future dendroarchaeological improvements in areas outside the method's traditional range.
Archaeological tree-ring dating has played an important role in delineating aspects of the Navajo occupation of the Southwest. Major changes in our interpretations of Navajo history and prehistory have occurred in the past decade due to intensive dendroarchaeological research in the traditional Navajo homeland of Dinétah in northwestern New Mexico. Recently, we have been able to expand out of the traditional Dinétah and explore early Navajo land-use practices and community organization in the Rio Puerco of the East river valley. Although slightly later in time, and much closer to the Spanish Colony, the 18th century Navajo archaeological sites on La Ventana Mesa exhibit strong similarities to sites farther north. This new research suggests that early Navajo adaptations to increasing conflict did not change the fundamental nature of Navajo subsistence or social organization.
Dendrochronology has played a central role in providing better understanding of past forest history in the Southwest, leading to several changes in land management goals and practices. One the most influential topics of research has been on forest fire history. Very detailed records of wildfires can be reconstructed from tree-ring samples of fire-scarred trees. By combining and comparing fire-scar records from many forest stands, watersheds, and mountain ranges throughout the Southwest, we have learned about the causes and consequences of past land management practices (e.g., livestock grazing and fire suppression), as well as the increasingly important role of climatic variation and change. In addition to fire history, tree-ring studies have also provided insights on the role of past and current insect outbreaks in forests and woodlands. The recent Southwestern drought, enormous wildfires, and massive bark beetle infestation and tree die-off have greatly increased the interest in historical insights provided by dendrochronology. The beauty of tree rings is that they enable us to approach environmental science questions from multiple angles, with multiple lines of evidence: ecological, climatic, and cultural.