The University of Arizona

Collections Background

Formally established in 1937, the Laboratory of Tree-Ring Research is an independent department at the University of Arizona. LTRR’s longstanding international renown derives from extensive contributions to research, teaching, and service in dendrochronology, a field founded in the early 20th century by LTRR's first director, Andrew Ellicott Douglass. The original contribution of dendrochronology was discovery of the exact calendrical dates of the construction and abandonment of the ancient cliff dwellings and other ruins of the American Southwest (Douglass 1929). Tree-ring based archaeological, biological, climatological, ecological, and geological applications have now expanded worldwide. Achievements of dendrochronologists at LTRR and other tree-ring laboratories around the world have been particularly notable in recent studies of climate change, historical assessments of river flows and water supplies, fire history and ecology, and historical human/environment interactions. LTRR's personnel study tree rings as natural chronometers and as recorders of change in the environment with which other multi-disciplinary research is inescapably linked.

        The LTRR currently houses over 2,000,000 specimens from all over the world. Most of these specimens are irreplaceable, and the collection functions essentially as a "wood herbarium" (Arnott 2008). Specimens include cross sections and cores from the world's largest and oldest trees, and timbers from ancient dwellings of the Southwest and elsewhere. Most, but not all of these specimens have been analyzed by various means. Yet, the specimens must be continuously and readily available for new observations and different types of measurements, including with new technologies. Over the past half century dendrochronology has embraced the use of various new tools and techniques, including isotopic and chemical analyses, and various methods of measuring wood density and micro-anatomy.

        Re-analyses of existing tree-ring collections with new tools has proven exceptionally productive. Perhaps the most spectacular example is the use of LTRR's exactly dated bristlecone pine tree-ring collection--spanning the past 9,000 years--as the standard for calibration of the radiocarbon time scale, with profound impacts on archaeology and earth sciences (Leavitt and Bannister 2009). Utilizing these "natural chronometers", what other analyses and discoveries will become possible in the future?

        Given the pace of technological innovation, there is good reason to believe that new observational and measurement tools will be developed, making tree-ring collections increasingly valuable for study by biologists, earth scientists, archaeologists, and a host of other scientists. The potential exists for much greater use of the collections. The metaphor of a "library" of biological, earth system and cultural history is apt, and most of the books have yet to be read.

        Dendrochronology is often compared to the assembly of a jigsaw puzzle. Time series or "chronologies" of tree growth responsive to climate, or fire scars representing forest fire history, or cutting dates of building timbers reflecting the rise and fall of societies are assembled piece by piece over a period of years and collected from a wide variety of independent sources. Long-term preservation and continued access to specimens is crucial to this field, as an individual sample may prove to be significant only years after its collection, when sufficient other samples have been obtained for crossdating, allowing completion of the puzzle. Moreover, it is extremely valuable in the context of regional to global-scale studies to unify and combine data from many collections distributed across broad spatial and long temporal scales (e.g., Cook et al. 2004).

        Collecting and preparing a usable tree-ring specimen requires a considerable initial investment of time and funds. The procedures outlined in this Manual are designed to ensure the long-term preservation of the specimens and their associated materials and provide greater access to and encourage the use of the Collection for legitimate research and educational purposes.