Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 55rainfall was proportionally (and most likelyabsolutely) less than at present. Whilemore mesic than now, the San Juan Basinwas probably colder and more arid thanareas further south and east where evidenceof successful Paleoindian adaptations ISmore abundant. (Gillespie 1985:27)The Middle Holocene (8,000 to 4,00012,000B.P.) probably had temperatures that were generallyhigher than any time before or since. The debate overspecific characteristics-e.g., whether there was a hot,dry period as described by Antevs (1955) or arelatively mesic period with more summer rainfall asdescribed by Paul Schultz Martin (1963)-wasunresolved. Macrobotanical data indicated the developmentof pinon and juniper woodlands to replaceearlier, more mesic conifers (Betancourt and VanDevender 1980,1981; Betancourt 1984; Betancourtetal. 1983). Palynological evidence indicated reducedpine pollen between 5,500 and 2,999 B.P. andprobably less regional woodland coverage (Hall 1981and Fredlund 1984). Petersen (1981) and Pippin(1979) attributed lower pine pollen counts to moremesic conditions; Petersen saw increased pinon woodlandsaround 4,000 to 3,000 B.P. Faunal remainsfrom around 5,000 B.P. indicated a fauna similar thatfound today, but with several nonlocal smallvertebrates that are found in better developedgrasslands. Gillespie (1985:30) suggested that thegrasslands respond to increased summer rainfall.Geological data (e.g., Wells et al. 1983) suggestedthat the period was more arid than at present. Theclimatic reason for the shift from more montane mixedconifers to pinon and juniper on these soils and fromcold desert scrub to desert grassland in open habitatswas thought to be a m~or shift in the seasonality ofprecipitation, with increased summer precipitation dueto strong monsoonal circulation during the thermalmaximum. Trees such as pinon and ponderosa thatdepend on adequate summer moisture were replacedby species that were better adapted to winter-dominantprecipitation. By ca. 5,500 B.P., the change to morexeric conditions began and these lasted until about2,000 B. P. -a period that may have been the most aridtime in the entire post-glacial period.During the past 2,000 years, there has been anincrease in pine pollen, which Hall (1977) interpretedas representing more mesic conditions. Gillespie(1985) indicated that studies of pollen for theBasketmaker III through Pueblo III period (Euler et al.1979; Schoenwetter and Dittert 1968) suggest favorableclimatic conditions. Samuels and Betancourt(1982) suggested that the need for fuel andconstruction materials caused the removal of localpinon-juniper growth. Even though Judd (1954) andothers assumed that the few relict pines indicated thepresence of forests i..'"1 the area, ponderosa and othermontane conifers had always been rare in the canyon.Palynologists (Hall 1977; Euler et al. 1979; andPetersen 1981) differed in interpreting their data. Hallfavored a period of more aridity until A.D. 1100,when there was an increase in pine pollen; Euler et al.(1979), and Petersen (1981) thought the periodbetween A.D. 950 and 1150 was more mesic. Thetree-ring data (Rose et al. 1982) provided the bestinformation on small-scale climatic fluctuations; thesepointed out the drought between A.D. 1130 and 1180.An extended period of cool arid summers thenpersisted from ca. A.D. 1300 until the 1800s.Geological evidence was equivocal; Bryan (1954) andHall (1977) both thought the arroyo formed ca. A.D.1100. Bryan attributed it to drought, while Hallthought it was due to increased precipitation thatcaused arroyo-cutting. D. Love (1983b) indicatedthere were numerous buried channels during thisperiod.Regional pollen sequences (Euler et al.1979; Petersen 1981) suggest that fromabout A.D. 950 to mid-HOOs relativelywarm temperatures with generally highsummer precipitation characterized thesoutheastern Colorado Plateau. Anasaziculture development at Chaco Canyonflourished during this period, in partbecause of favorable conditions foragricultural production. The decline ofChaco Anasazi is well correlated with aperiod of low summer rainfall in themiddle of the twelfth century. This periodof drought may have been a majordestabilizing factor in the apparent declinein the Chaco-based regional economicsystem. (GiIIespie 1985:35)That data from these different scales are difficultto correlate, especially when used to interpret pastenvironments, is not surprising. Dean (1988) ex-