Dendrochronological Sourcing of Timbers from the Great Houses of Chaco Canyon, New Mexico: Building a millennium-length ponderosa pine tree-ring chronology from the greater Bandelier-Jemez Landscape

Guiterman, Christopher; Swetnam, Thomas; Dean, Jeffrey; Allen, Craig

The cultural history of Chaco Canyon is characterized by connections with distant landscapes. The “Chaco Phenomenon” during which the Chacoan Anasazi peoples inhabited and utilized the canyon lasted for roughly three centuries (A.D. 850-1130). During this period, Chaco Canyon was the center of a civilization that spread across the San Juan Basin of present day northern New Mexico (Figure 1). On the canyon floor was Chaco’s “downtown,” where great house structures hosted many festivals and activities. Constructing the great houses of Chaco Canyon required enormous investments in labor and materials, on a scale far greater than most other North American construction efforts at the time. Dean and Warren (1983) estimated that 240,000 trees were harvested to build the great houses. Early archaeologists in Chaco believed these timbers came from a prehistoric forest that blanketed the canyon and was entirely cut down for the civilization (Douglass, 1935; Judd, 1954, 1964). Evidence in support of this hypothesis came from remnant logs and live trees located in secluded areas of the canyon, and from a lone ponderosa pine (Pinus ponderosa) log discovered in the west courtyard of Pueblo Bonito, the main great house structure (Judd 1954). Recent research, however, has challenged the interpretation that the plaza tree grew in Pueblo Bonito (Guiterman, Swetnam, Baisan, & Dean, n.d.; Wills, 2012). Paleoenvironmental reconstructions in Chaco Canyon from plant macrofossils in packrat middens revealed that ponderosa pine, the most common architectural timber after ca. A.D. 920, was not abundant enough in the canyon to provide the necessary quantity of timber for great house construction (Betancourt & Van Devender, 1981). This discovery led to a new interpretation of timber procurement: the immediate environs of Chaco Canyon were not the primary source for timber, and almost all timber must have come from the forested uplands and mountain ranges surrounding the San Juan Basin. Supporting evidence for the long-distance transport of timbers comes from the identification of spruce (Picea) and fir (Abies) species within the collections of great house timbers (Betancourt et al. 1986). As was the case through the late Holocene, these species are constricted to high elevations where growing conditions are cooler and wetter than at lower sites. Thus only a few candidate mountains at some distance from Chaco are the likely harvest sites for timbers. These areas have been identified through geochemical provenance methods that identify the source of construction beams by comparing their geochemical signatures (e.g., the strontium isotope ratio [87Sr/86Sr]) to those of modern trees growing in potential harvesting areas. Strontium isotopes of spruce and fir beams strongly suggest as source areas the Chuska Mountains > 75 km to the west and the San Mateo Mountains (i.e., Mount Taylor) > 80 km to the south of Chaco Canyon (English, Betancourt, Dean, & Quade, 2001). The objective of this study is to investigate the potential for building a millennium-length ponderosa pine tree-ring chronology from the greater Bandelier-Jemez region. Such a chronology would not only benefit efforts to source Chacoan timbers, it would also provide an invaluable record of past environmental and climatic change in the region.