Long-distance wood procurement and the Chaco florescence.

First documented during a military reconnaissance in 1849, Chaco Canyon and the Ancestral Pueblo society that flourished therein [850–1140 Common Era (CE)] have been the focus of archaeological investigation for more than a century. Beginning in the early ninth century CE, the inhabitants of farming communities lining a 14-km stretch of the lower canyon in present-day northwest New Mexico erected massive free-standing masonry pueblos, or “great houses.” Great houses are monumental in scale and are among the most iconic and remarkable feats associated with Chacoan societal development. The construction of one multistory great house, Pueblo Bonito, with at least 650 masonry rooms, required the Chacoan people to quarry 50,000 tons of sandstone and harvest and transport more than 50,000 pine (Pinus sp.), fir (Abies sp.), and spruce (Picea sp.) trees from distant (60−80 km) montane forests (Fig. 1). Guiterman et al.’s (1) analysis of a spatially broad sample of construction timbers from 7 of 11 great houses in Chaco Canyon documents the chronology and spatial extent of wood harvesting strategies and reveals a significant and previously unknown wood source. Chacoan Wood Sources Bordered by forested monoclines and uplifts, the central San Juan Basin of northwest New Mexico is arid, grass-covered, predominantly treeless, and punctuated by buttes, mesas, and canyons. Extending from east to west across the central basin, the entire 32-km length of Chaco Canyon (1,850–1,950 m elevation) is bounded on the north and south by high mesas that are sparsely populated with stands of pinyon (Pinus sp.) and juniper (Juniperus sp.). Larger tree species such as ponderosa pine (Pinus ponderosa) and Douglas fir (Pseudotsuga menziesii) are rare in Chaco Canyon and limited to higher-elevation portions of the San Juan Basin distant from the canyon. Although early great house construction relied on locally available wood species, it is estimated that by the mid-12th century CE, the Chacoan people had harvested more than 240,000 ponderosa pine, Douglas fir, spruce, and possibly aspen (Populus tremuloides) trees for the construction of their great houses. In addition to the staggering demand for construction timbers well in excess of postulated local timber resources, tree-ring data indicate selective harvesting of specific size classes of trees, a behavior thatwould have been possible only in heavily forested areas. Taken together, this evidence has prompted many scholars to theorize that Chacoan builders obtained wood from the uplands of the San JuanBasin, most likely the SanMateoandChuskaMountain regions to the south and west, respectively (2–4). Subsequent stable isotope analysis initially supported this interpretation (5). In particular, strontium isotope (Sr/Sr) ratios measured for Chacoan great house architectural timbers were compared with strontium assays of modern tree samples from the Chuska, San Mateo, and San Pedro Mountains. Analysis of the strontium data suggested that the Chuska and San Mateo ranges were primary foci of tree harvesting, whereas the San Pedro/Nacimiento region east of Chaco was not a significant source. Moreover, the results of this previous research revealed that, at times, wood was simultaneously procured from disparate regions. Fig. 1. North wall masonry and construction timbers of Pueblo Bonito great house.