Culture and Ecology of Chaco Canyon and the San Juan Basin

52 Chaco Project Synthesis1981 [sic]). Change in local woodlandswould be better seen in pollen recordsfrom the escarpments, which would havesupported these woodlands. Such locallysensitive pollen records could be obtainedfrom caves or rockshelters where aeoliansediments and scree have accumulated.(Fredlund 1984:191)After reviewing the variation among thevariables that could affect the palynological data andinterpretations of them, plus his own analysis ofmaterial from Sheep Camp and Ashislepah shelters,Fredland (1984:205) concluded that palynological dataprovide good evidence for regional environmentalreconstructions. Unfortunately, the records from theLate Holocene (after 1,200 B.P.) were obscure, andpreclude useful additions to the interpretationsavailable. For Late Pleistocene and Early Holocenevegetation, interpretations were constrained by twomajor problems: the identification of Pinus species,and the ability to distinguish local pollen sources fromregional pollen rain, especially for samples taken fromshelters along escarpments. As a result, reconstructionof Late Pleistocene vegetation from pollendata in Chaco Canyon is difficult. Fredland indicatedthat the Mid-Holocene (5,800 to 2,200 B.P.) was aperiod of sustained aridity, an explanation that isinternally consistent with all data and conformsexternally with published results by Hall (1977). Hesuggested that the ponderosa and pinon woodlandswere significantly larger at the beginning of thisperiod than they are today, but decreased thereafter.Relict stands of pinon and juniper remained until2,200 B.P., at which time they began to recover.Fredlund (1984:209) acknowledged that the period ofaridity proposed is too broad in scope, but that thelack of understanding of climatic records makes itdifficult to appreciate pollen data with regard tospecific environments, such as that in Chaco Canyon.There seem to be differences in events by area.Dendroclimatological ReconstructionThe NPS was a major sponsor of the SouthwestPaleoclimate Project, which was funded by contractsfrom 1967 to 1971 and again from 1974 to 1976, toconstruct a network of climate-sensitive tree-ringchronologies based on archaeological materials andsamples from living trees. One result of this studywas a series of maps that indicated the relativevariation in tree-ring growth in space and time, as wellas fluctuations in rainfall and temperatures (Dean andRobinson 1977). A second project, jointly sponsoredby the Chaco Center (NPS), the Dolores Project(BLM), and the Puerco Project (ENMU), studied thedendrochronology of the Southwest Plateau in andaround Chaco Canyon (Rose 1979; Rose et al. 1982).Building on earlier work, Rose et a1. (1982; and Rose1979) used tree-ring data to reconstruct theenvironment from A.D. 900 through 1970. See Roseet al. (1982:Table 1, Figure 1.2) for locations used toconstruct expanded tree-ring chronologies.Rose et al. (1982) acknowledged that there werea number of methodological problems that could beaddressed by using statistical tests to determine therepresentativeness of the tree-ring chronologies fromseven different prehistoric and historic locations.They incorporated the assumptions that characterize alldendroclimatic work (e.g., no long-distance transportof wood used in archaeological sites, uniformitarianismin climatic factors, chronological accuracy,ability to incorporate different species of trees, andhistoric and prehistoric sample populations possesssimilar time and frequency domain characteristics).Using monthly temperature and precipitation data,they reconstructed regional precipitation and temperaturevalues, as well as calculated the Palmer DroughtSeverity Index (PDSI). This is an index of meteorologicaldrought-"an anomaly characterized by a prolongedabnormal moisture deficiency, with its severitydepending on the duration and magnitude of theabnormality" (Rose et al. 1982: 109). This meteorologicalmeasure was used rather than agriculturaldrought or hydrological drought to avoid the problemsthat must be considered when other variables comeinto play; e.g., the economic factors in the localcommunity and responses available to agriculturaliststo overcome them, or the engineering problems thatthe disciplines of hydrology, geology, and geophysicsaddress. Rose et al. (1982) selected the July PDSIbecause it was the period when most demands onwater supply are high and when droughts tend to peak,thus having the greatest stress affects on tree growth.Table 2.3 indicates the values used to determinedrought severity.