Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 51and 1150, the woodlands would have been completelydepleted.In 1982 Betancourt had the opportunity toparticipate in the study of Sheep Camp Shelter just tothe east of the park (from which four pack rat middenswere sampled) and Ashislepah Shelter located to thenorthwest of the monument. To fill in gaps in theinitial sequence (9,460 to 5,550 B.P.) obtained fromChaco Canyon, Betancourt also collected materialfrom 50 additional middens in the canyon, 35 ofwhich were dated. His interpretations and conclusions(Betancourt 1984) did not change, but he was able torefine his paleoecological reconstruction. An uncalibratedsample from a midden in Atlatl Cave dated8290 ± 150 B.P. contained abundant ponderosa,Douglas-fir, and piiion, with only a small amount ofRocky Mountain juniper lLnd one-se.ed juniper. Onerose microfossil was present. None of the last threeoccurred in later specimens (post-5,550 B.P.) and onlya few of the junipers occur today on Chacra Mesa.This supported his interpretation of the transitionbetween the Late Pleistocene and Early Holoceneenvironments. With regard to the existence of a hot,dry Altithermal period, the data were less clear. Noneof the assemblages dating prior to 5,000 B.P. weremore xeric than those dating later. The presence ofponderosa pine andlor Douglas-fir with pinon-juniperwould support a wetter period from 8,300 to 2,300B.P. than post-2,300 B.P., but there were no data tocontrol for site differences. Betancourt had notcollected samples younger than 2,830 B.P., either.The only middens in which ponderosa were found arenear Pueblo Pintado, where it grows today. Thus,there was no evidence to support a local origin formost ponderosa used to build the Chaco towns.All middens that had no evidence for piiion wereyounger than 500 B.P.; the youngest was the one eastofthe Gallo Wash that dated to 1,230 B.P. These datasupported the interpretation that the reduction in pinonwoodlands took place during the Anasazi occupation.Betancourt (1984: 185) suggested "The late Holocenewoodland in the Monument was likely restricted tocliffsides, further reducing the total cordage availableas fuelwood." This was not the case for Chacra Mesa,and although the Anasazi probably harvested ChacraMesa later in time and the impact was devastating, thevegetation was able to recover. It would have beenused later by the Navajo.Comparison of Pollen and Pack Rat MiddenResultsThe paleoenvironmental reconstructions thatresulted from analysis of pollen and pack rat middenmacrobotanical remains differ. The packat middenmaterial seems to indicate that both ponderosa andpiiion were expanding their ranges during the EarlyHolocene (10,000 to 7,000 B.P.), while spruce andlimber pine remained only as relict species from thePleistocene (Betancourt 1984; Betancourt and VanDevender 1981). Hall (1977), on the other hand,indicated that pollen analysis from the same middensshows that presence of ponderosa and pinon pine wassimilar to that seen today. Fredlund (1984: 187) evaluatedthe assumption that comparisons can be madebetween the alluvial and pack rat midden pollenassemblages and recognized that there are strengthsand weaknesses associated with these two complementarytechniques. Differences in pollen frequenciescan be due to to pollen production, transportation,preservation, recovery, and identification.The occurrence of high pine pollen percentagesin sediment samples taken from avery localized zone of woodland hasimportant implications. Hall (1977, 1981,1982) interprets variations in pine pollen inhis alluvial record as indicating regional,rather than local, vegetation change.Evidence from packrat middens, on theother hand, has documented the decimationof local woodlands at Chaco Canyonduring the height of the Anasazioccupation (Pueblo II and III) (Betancourtand Van Devender 1981: Samuels andBetancourt 1982; Betancourt, this volume).This apparent discrepancy has been blamedon the lack of sensititity of alluvial pollenrecords to local vegetation change(Betancourt and Van Devender 1981;Betancourt, this volume). This may becorrect, but for the wrong reasons.Alluvial pollen samples are overlysensitive to the local vegetation on thevalley floor. Evidence for changes inscarp woodlands of the area could beobscured by the over-representation offloodplain pollen taxa (Solomon et al.