One of the most striking things about the environment of Chaco Canyon today is its vegetation, which is dominated by greasewood and saltbush and features an almost total lack of larger trees on the canyon floor. There are some cottonwoods and tamarisks in the arroyo of the wash, which were mostly planted in the 1930s by the Civilian Conservation Corps, and on top of the mesas there are quite a few junipers and some piñon pines, but for the most part it’s a shrubby, treeless landscape.
The Chacoan style of architecture, however, features an enormous amount of wood, mostly in the ceilings of rooms but also in the lintels of doorways and vents and in some cases as intramural beams inside the walls. It has been estimated that about 200,000 beams were used in construction in the canyon. Because of the arid climate and the strength of the construction, a considerable amount of this wood has survived quite well and can be seen by visitors today. One of the questions those visitors ask most frequently is where all this wood came from. It’s a very reasonable question, and one that has resulted in a considerable amount of research that, unusually for Chaco research, has actually come up with some answers.
Early excavators, like many modern visitors, generally assumed that the wood must have been local, and that the climate therefore must have been significantly different in Chacoan times from how it is now. This view persisted as late as the 1960s, when most experts agreed that there had been a substantial forest in the canyon that had since retreated as a result of both human-induced deforestation and natural climate change. Considerable research since then, however, has demonstrated quite clearly that this is not the case and that the climate in the canyon has been more or less the same for at least the past 5000 years. Although there have been fluctuations from year to year and decade to decade in rainfall amounts and other climatic variables, the range of variation seen over the past 1000 years is basically the same as that seen in historical records over the past 100 years. That is, rainfall can vary by a couple of inches or so pretty regularly, but there hasn’t been the sort of rainfall that would support a large forest during any time that would be relevant to the study of the great houses.
It’s likely that there were a few isolated stands of ponderosa pine in the canyon before construction really got going in the 800s, and the famous tree found in the excavations of the west plaza of Pueblo Bonito was presumably a remnant of one of these, but once construction began in earnest these would have been depleted very quickly. After they were gone, it would have just been the juniper and cottonwood seen today. These are found frequently as charcoal in hearths, indicating that they were likely widely used as firewood, but structural wood is mostly ponderosa, douglas fir, spruce, fir, aspen, and other large, straight, high-elevation species.
Over the past twenty years the Park Service has been conducting a series of studies, collectively known as the Chaco Wood Project, to sample every piece of exposed wood in the canyon and try to use the samples to come up with answers to a variety of questions. The main purpose has been to create a very accurate and precise chronology using tree-ring dating, which has resulted so far in a substantial reinterpretation of the early history of Pueblo Bonito, but the samples have been used for other sorts of research as well. Among these is the use of isotope ratios to determine the sources of structural wood.
The main isotope ratio used in this sort of study is that of Strontium-86 to Strontium-87. Strontium is a trace element found in all sorts of living things, and it has the useful property of being absorbed while the living thing in question is living but being unaffected by any biological processes after it is absorbed. As a result, the amount of the various isotopes of strontium in a given organism is determined by the environment in which it grows and then remains constant throughout its lifetime and beyond.
Since the amounts of both Strontium-86 and Strontium-87 in an organism result from the environment in which it originated but don’t change after that, even if it is moved elsewhere after its death, measuring the ratio of the two in surviving samples of tissue and comparing it to the ratios found in various places where the organism might have originated can potentially reveal with considerable precision the area of origin. This technique has been applied to samples of structural wood from Chaco with considerable success.
The most successful study of this type looked at spruce and fir (which are generally indistinguishable in this context) from sites in the canyon. Although nowhere near as common as ponderosa in Chacoan construction, these species are from higher elevations and thus occur in fewer places in the region, which greatly simplifies this type of study since it reduces the number of possible areas of origin that need to be tested. The study measured the ratios in the samples and compared them to those found at the three nearest areas where these species grow. These are the Jemez Mountains to the east, the Mt. Taylor area to the southeast, and the Chuska Mountains to the west. Each of these areas is about 50 miles from Chaco Canyon.
The results of the study showed that both the Chuskas and Mt. Taylor were major sources of timber found in Chaco, but that the Jemez was not, despite being just as close. There is no obvious ecological or economic reason for this, but it is noteworthy that outlying communities showing extensive Chacoan influence are very numerous in the foothills of the Chuskas and Mt. Taylor but totally absent in the Jemez. Indeed, the east is the direction in which there is the least evidence for any Chacoan influence, and the evidence for any contact at all between Chaco and the areas east of the Rio Puerco is very scanty. This suggests that the sourcing of timber destined for Chaco was affected more by social factors than by geographic or ecological ones.
A subsequent study using the same methodology looked at possible sources for ponderosa, which is much more abundant in Chacoan construction than spruce and fir but much harder to investigate in this way because of its much more widespread distribution in the region, which is probably the reason for its greater abundance in construction. Since it grows at lower elevations than spruce or fir, ponderosa is all over the place, and it is very difficult to test a sufficiently wide range of possible sources to come up with definitive results. The results of this second study, therefore, while consistent with those of the first in indicating the Chuskas and Mt. Taylor, were more tentative than those of the first, and it’s quite possible that there were sources of ponderosa that were not tested in the study. Still, the basic congruence of the results in the two studies indicates pretty clearly that, whatever additional sources there may have been, the primary sources for structural wood at Chaco were the Chuskas and Mt. Taylor.
These two areas, but especially the Chuskas, in addition to having numerous outliers and considerable evidence of Chacoan influence, appear to have provided other goods to Chaco, such as pottery and chipping stone, in significant quantities as well. All of these lines of evidence, therefore, point to a very close connection between Chaco and the Chuskas, and probably to a similarly close connection to the Mt. Taylor area as well.
There are few questions about Chaco for which there are clear and relatively straightforward answers, but thanks to the application of modern geochemical techniques to archaeological questions this is one for which there are such answers. In the future, this type of research is likely to become more and more widespread, and so other questions may soon be added to the “answerable” list. In the meantime, however, the wood question stands as a beacon of hope in a sea of mystery.