Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 57occasional crop failure from unfavorablemeteorological conditions-in particular,frost-free seasons and inadequate precipitation.At the same time, it would be easyto overemphasize the marginality of thesituation. For example, by some standards(e.g., [E.] Adams 1979), the short, frostfreeseasons at Chaco Canyon wouldindicate "prohibitive" risks. Yet, the richarcheological record from there suggeststhat prehistoric farming was successfulover an extended period of time. On thisbasis, it would be difficult not to considerChaco to be closer to the optimal end of anoptimal-margin continuum or risks of yearto-yearsubsistence stress. Consideringother environmental variables in additionto meteorological characteristics, it seemslikely that Chaco was something of an"oasis" in the San Juan Basin, as suggestedby [Gwinn] Vivian (1970a). This in tumbrings into question the reliability ofassessing marginality or suitability forfarming on the basis of modem climaticdata. In particular, the accuracy of suchwidely cited criteria as minimum growingseason requirements needs to be betterestablished before their implications ofprehistoric adaptations can be wellunderstood. (Gillespie and Powers 1983:8)These investigators used data from the San JuanBasin Regional Uranium Study (SJBRUS) database toevaluate a model that predicts use of changing relativefrequencies of sites through time, depending on thegeology, elevation, and retrodicted temperature andprecipitation levels. With increased summer precipitation,expansion of settlements was expected.Between A.D. 900 and 1100, for example, there wasincreased site expansion at both high and lowelevations in the San Juan Basin, which occurs withincreased summer precipitation; during the mid A.D.IlOOs, growth halted when a 50-year period ofdecreased moisture impacted the San Juan Basin.Although settlements continued in limited areasaround the margins of the basin, the central areacollapsed. In the northern river basins, floodwaterfarming would have been an option; in the southeasthighlands, dry farming would have been possible.SummaryThere is little support for major climatic changeafter the transition from the Late Pleistocene to EarlyHolocene. There were no major changes in theenvironment during the Holocene. However, therehad been minor shifts in temperatures andprecipitation. The change to a more arid environmentduring the Middle Holocene brought with it a changein the species of trees available; except for isolatedstands, the ponderosa pine forest had disappeared fromChacra Mesa by 7,000 B.P. (Hall 1977). Dense pineforests were not present in the canyon, but someremnant forests did outcrop on the higher mesas, witha few trees appearing at higher elevations along theupper walls of the canyons.Pine, fir. and spruce were not abundant in thecanyon for construction purposes. During the Pueblooccupation, it was unlikely that the ponderosa pinelogs used in the construction of the great house sitescould have been obtained locally; rather, they wereprobably imported from some distance, where speciesgrew at high elevations; e.g., Mount Taylor, MountPowell, the Chuska Mountains, or the Cuba area (Hall1975:57, 1977). The studies of Betancourt and VanDevender (1980) and Samuels and Betancourt (1982)indicate that fir and spruce would have been importedfrom long distances, but that ponderosa pine wouldhave been available in scattered stands in favorablehabitats. Dense ponderosa stands were more distant,probably 40 km from Chaco Canyon. Because mostconstruction beams attributable to this species are 35em-plUS in diameter, it is unlikely that tree harvestingwould have destroyed these stands. The removal ofthe larger-sized, slow-growing specimens, however,would have forced the Pueblo people to travel fartherto obtain logs of appropriate size for construction(Betancourt et al. 1986:373). Samuels and Betancourt(1982) modeled the effects of timber-cutting, whichwould have deforested the area by A.D. 900.Harvesting of pinon and juniper in the canyon,however, probably caused their disappearance by thatdate.Rather than simple lists of plant and animalspecies, we now have a record of which ones occur inspecific zones. We know something about thedensities of some species and how they respond toeven small fluctuations in rainfall by season or