Culture and Ecology of Chaco Canyon and the San Juan Basin
Culture and Ecology of Chaco Canyon and the San Juan Basin
Culture and Ecology of Chaco Canyon and the San Juan Basin
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52 <strong>Chaco</strong> Project Syn<strong>the</strong>sis1981 [sic]). Change in local woodl<strong>and</strong>swould be better seen in pollen recordsfrom <strong>the</strong> escarpments, which would havesupported <strong>the</strong>se woodl<strong>and</strong>s. Such locallysensitive pollen records could be obtainedfrom caves or rockshelters where aeoliansediments <strong>and</strong> scree have accumulated.(Fredlund 1984:191)After reviewing <strong>the</strong> variation among <strong>the</strong>variables that could affect <strong>the</strong> palynological data <strong>and</strong>interpretations <strong>of</strong> <strong>the</strong>m, plus his own analysis <strong>of</strong>material from Sheep Camp <strong>and</strong> Ashislepah shelters,Fredl<strong>and</strong> (1984:205) concluded that palynological dataprovide good evidence for regional environmentalreconstructions. Unfortunately, <strong>the</strong> records from <strong>the</strong>Late Holocene (after 1,200 B.P.) were obscure, <strong>and</strong>preclude useful additions to <strong>the</strong> interpretationsavailable. For Late Pleistocene <strong>and</strong> Early Holocenevegetation, interpretations were constrained by twomajor problems: <strong>the</strong> identification <strong>of</strong> Pinus species,<strong>and</strong> <strong>the</strong> ability to distinguish local pollen sources fromregional pollen rain, especially for samples taken fromshelters along escarpments. As a result, reconstruction<strong>of</strong> Late Pleistocene vegetation from pollendata in <strong>Chaco</strong> <strong>Canyon</strong> is difficult. Fredl<strong>and</strong> indicatedthat <strong>the</strong> Mid-Holocene (5,800 to 2,200 B.P.) was aperiod <strong>of</strong> sustained aridity, an explanation that isinternally consistent with all data <strong>and</strong> conformsexternally with published results by Hall (1977). Hesuggested that <strong>the</strong> ponderosa <strong>and</strong> pinon woodl<strong>and</strong>swere significantly larger at <strong>the</strong> beginning <strong>of</strong> thisperiod than <strong>the</strong>y are today, but decreased <strong>the</strong>reafter.Relict st<strong>and</strong>s <strong>of</strong> pinon <strong>and</strong> juniper remained until2,200 B.P., at which time <strong>the</strong>y began to recover.Fredlund (1984:209) acknowledged that <strong>the</strong> period <strong>of</strong>aridity proposed is too broad in scope, but that <strong>the</strong>lack <strong>of</strong> underst<strong>and</strong>ing <strong>of</strong> climatic records makes itdifficult to appreciate pollen data with regard tospecific environments, such as that in <strong>Chaco</strong> <strong>Canyon</strong>.There seem to be differences in events by area.Dendroclimatological ReconstructionThe NPS was a major sponsor <strong>of</strong> <strong>the</strong> SouthwestPaleoclimate Project, which was funded by contractsfrom 1967 to 1971 <strong>and</strong> again from 1974 to 1976, toconstruct a network <strong>of</strong> climate-sensitive tree-ringchronologies based on archaeological materials <strong>and</strong>samples from living trees. One result <strong>of</strong> this studywas a series <strong>of</strong> maps that indicated <strong>the</strong> relativevariation in tree-ring growth in space <strong>and</strong> time, as wellas fluctuations in rainfall <strong>and</strong> temperatures (Dean <strong>and</strong>Robinson 1977). A second project, jointly sponsoredby <strong>the</strong> <strong>Chaco</strong> Center (NPS), <strong>the</strong> Dolores Project(BLM), <strong>and</strong> <strong>the</strong> Puerco Project (ENMU), studied <strong>the</strong>dendrochronology <strong>of</strong> <strong>the</strong> Southwest Plateau in <strong>and</strong>around <strong>Chaco</strong> <strong>Canyon</strong> (Rose 1979; Rose et al. 1982).Building on earlier work, Rose et a1. (1982; <strong>and</strong> Rose1979) used tree-ring data to reconstruct <strong>the</strong>environment from A.D. 900 through 1970. See Roseet al. (1982:Table 1, Figure 1.2) for locations used toconstruct exp<strong>and</strong>ed tree-ring chronologies.Rose et al. (1982) acknowledged that <strong>the</strong>re werea number <strong>of</strong> methodological problems that could beaddressed by using statistical tests to determine <strong>the</strong>representativeness <strong>of</strong> <strong>the</strong> tree-ring chronologies fromseven different prehistoric <strong>and</strong> historic locations.They incorporated <strong>the</strong> assumptions that characterize alldendroclimatic work (e.g., no long-distance transport<strong>of</strong> wood used in archaeological sites, uniformitarianismin climatic factors, chronological accuracy,ability to incorporate different species <strong>of</strong> trees, <strong>and</strong>historic <strong>and</strong> prehistoric sample populations possesssimilar time <strong>and</strong> frequency domain characteristics).Using monthly temperature <strong>and</strong> precipitation data,<strong>the</strong>y reconstructed regional precipitation <strong>and</strong> temperaturevalues, as well as calculated <strong>the</strong> Palmer DroughtSeverity Index (PDSI). This is an index <strong>of</strong> meteorologicaldrought-"an anomaly characterized by a prolongedabnormal moisture deficiency, with its severitydepending on <strong>the</strong> duration <strong>and</strong> magnitude <strong>of</strong> <strong>the</strong>abnormality" (Rose et al. 1982: 109). This meteorologicalmeasure was used ra<strong>the</strong>r than agriculturaldrought or hydrological drought to avoid <strong>the</strong> problemsthat must be considered when o<strong>the</strong>r variables comeinto play; e.g., <strong>the</strong> economic factors in <strong>the</strong> localcommunity <strong>and</strong> responses available to agriculturaliststo overcome <strong>the</strong>m, or <strong>the</strong> engineering problems that<strong>the</strong> disciplines <strong>of</strong> hydrology, geology, <strong>and</strong> geophysicsaddress. Rose et al. (1982) selected <strong>the</strong> July PDSIbecause it was <strong>the</strong> period when most dem<strong>and</strong>s onwater supply are high <strong>and</strong> when droughts tend to peak,thus having <strong>the</strong> greatest stress affects on tree growth.Table 2.3 indicates <strong>the</strong> values used to determinedrought severity.