Culture and Ecology of Chaco Canyon and the San Juan Basin

50 Chaco Project Synthesiswas probably cooler than it is today; precipitation fellmostly in winter and was probably twice as heavy(440 mm).Between 9,460 and 5,550 B.P., there was a gapin the data due to inadequate sampling of the pack ratmiddens in the canyon.Middle Holocene (or Early Archaic, 6,000 to4,000 B.P.): This period was thought to represent ashift from a mixed conifer forest to a well-developedpifion-juniperwoodland around 8,000 B.P. Summerswould have been slightly wetter than they are today.Although plant communities were dominated by pinonand juniper, small amounts of Douglas-fir and ponderosapine in one sample suggest that there was asubstantial decrease in effective moisture, probablystarting at the beginning of the Middle Holocene.Also appearing for the first time were several herbsthat respond to summer precipitation.Betancourt and Van Devender (1980:52) notedthat Hall (1977) suggested greater aridity during theperiod from 5,600 to 2,400 B.P., which resulted inmaximum reduction of the pinon-juniper woodlandsand ponderosa pine forests in northwestern NewMexico. However, they indicated that the piiionjuniperwoodland persisted well into the LateHolocene on the xeric exposures where pinon andjuniper are most vulnerable to increased aridity.Late Holocene (or Late Archaic, 3,940 to 1,230B.P.): A persistent pinon-juniper woodland wasinferred. In contrast with 87 percent found incontemporary middens that were examined, only 40percent of the perennial species recovered frommidden remains are found in the area today.Betancourt and Van Devender suggested that theunderstory was more like the modem desert scrubcommunities than the understory of the Chacra Mesawoodlands. There probably were limited stands ofponderosa pine in favorable sites on the south side ofthe canyon and on Chacra Mesa, but this possibilitycould not be inferred from the data. On the northside, a patchwork of desert scrub and grasslands withscattered junipers on exposed hillsides and piiionjuniperstands on cliff sides and sandy outcrops weredescribed.Anasazi (1,230 to 460 B.P.): There was a localreduction of piiion-juniper woodlands; Betancourt andVan Devender suggested that stands were slow toreproduce and that woodcutting by the prehistoricinhabitants eventually caused the disappearance ofthese species in the canyon.Recent (after 460 B.P.): No piiion or junipermacrofossils were recovered. The material that wasrecovered closely resembles the modem flora.Modern: Modem pack rat dens, located in closeproximity to the middens, revealed small traces ofjuniper in middens only in areas where isolated treesare present. There was no piiion in any of the modemdeposits. For the perennial species, 87 percent foundin the midden occurred within 30 m of the site. Todaythere are still isolated examples of Douglas-fir foundon Chacra Mesa, 20 to 25 km east of the main area ofthe canyon. Rocky Mountainjuniper are not found inthe lowlands of the San Juan Basin; they only occur inareas where annual precipitation is from 381 to 631mm annually. That range is above the 220 mmaverage that occurs in Chaco Canyon. Limber pine isfound only in the San Juan Mountains, approximately180 km to the northeast.The reduction in pinon-juniper after 1,230 B.P.was most difficult to understand. Because the regionalclimatic variability after 1,230 B.P. was probablywithin the range that occurred between 5,550 and1,230 B.P., Betancourt and Van Devender could notattribute it to climatic change alone. Instead, theysuggested that it resulted from an interaction betweenhumans and their resources, particularly the use offuel for fires, and the slow reproductive rate of thestands. To examine this proposal, Samuels andBetancourt (1982) used computer software designed todetermine fuel-harvesting impacts on a pinon-juniperwoodland; they compared the model and data fromChaco Canyon. To obtain the most generous estimatespossible, their figures overestimated the woodlandparameters (13,000 ha) and were conservativeregarding the amount of fuelwood/firewood procuredby the early Pueblo people (14.8 cords/ha/yr). Theyinitialized their model in A.D. 250 with a populationof 48 individuals reproducing at a 0.6 percent growthrate. Within the-200-year period between A.D. 950