Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 41I'SS: Sarcobatus vermiculatus-Sporobolus airoides.At the eastern end of the park, from where theChaco and Gallo washes cross the boundary to theroad that crosses the Chaco Wash from the south exitto the visitor center, are stands of black greasewood(Sarcobatus vermiculatus). Here the soils are characteristicallyalkaline silt and clay. Also in this areaare rubber rabbitbrush (Chrysothamnus nauseosus),several species of wheatgrasses (Agropyron), alkalisacaton (Sporobolus airoides), and spiny or shadscalesaltbush (Atriplex confertifolia).PJ: Pinon-juniper. This woodland zone, foundonly on Chacra Mesa at the southeastern edge of thepark, is dominated by one-seed juniper (Juniperusmonosperma), with pinon (Pinus edulis) below 2,133m (7,000 ft). Shrubs include true mountain mahogany(Cerocarpus montanus) and black sagebrush(Artemesia nova), plus a large variety of grasses andforbs that have no indicator value.RW: Riparian woody vegetation. The entirestream of the Chaco Wash was characterized as riparian,and woody, to correspond with soil piping anderosional studies being conducted by Malde. Severalspecies of rabbitbrush (Chrysothamnus spp.), greasewood(Sarcobatus sp.), coyote willow (Salix exigua),four-wing saltbush (Atriplex canescens), Fremont'scottonwood (Populus fremontii), and the exotictamarisk (Tamarix pentandra) are present.D: Dune area. Da: dune area-active. Dv: dunearea-vegetated. Rubber rabbitbrush (Chrysothamnusnauseosus) and sand sagebrush (Artemisiafilifolia) arethe dominant species present. Also present are severalherbs and grasses.BR: Bare rock.Wt: Watershed treatment. Manmade zone.Of: Old field succession. Manmade zone.Potter and Kelley (1980: 103) indicated thatchanges in vegetation could be linked to relativelyminor changes in environmental conditions. Manyspecies in the area, especially the annuals, fluctuategreatly in density and coverage due to changes inannual precipitation. There is also a close link tofeatures of geology, physiography, and soils. Insummary, a coarse vegetative classification map, witha detailed description of the different vegetative zones,was completed, and reasons for some changes invegetation were noted. A. Cully (1985a:Appendix A)published a comprehensive plant list based on existingcollections in the park and supplemented by thosefrom Ron Bronitsky in 1972 and those that she andMollie Struever Toll collected in association withpollen and flotation studies during the Chaco Project.After the monument became a park in 1980, theadditional lands were surveyed by Floyd-Hanna andHanna (1995) and Floyd-Hanna et al. (1993). Thevegetative map by Kelley and Potter was digitized,and inventory transects provided data, indicating that13 distinct vegetation types can be defined andaggregated into eight visually distinct vegetationcommunities. These include the riparian wash,Artemesia tridetatalChrysothamnus nauseosus, Sarcobatusassociations, pinon/juniper associations, Artemesianova-J.monosperma-C. visiiflorus, A. nova­Gutierrezia sarothrae-C. greenei, Sporobolus airoidesassociations, and Hilaria jamesii-Sarcobatus vermiculatus-Boutelouagracilis-Orozopsis hymenoides, aswell as the distinct formations of sandstone cliffs andactive dunes.Scott (1980) confirmed the variability in plantresponse to available moisture and supported observationsand explanations proposed by Potter (1974)regarding the annual variability in plant species due todifferences in rainfall. From 1975 through 1977,rainfall in Chaco was below normal (215 mm). In1975, it was 68 percent of average; in 1976, 62percent; and in 1977, 87 percent. When monthlyobservations on the phenology of plants in samplingareas were compared, seven of 11 observed species ofperennial plants reproduced during 1976, while 10 of11 reproduced in 1977. The flowering period (Aprilto October) contained two peaks: May-June, andAugust-October. Norman and Duke (section 2 of"Phenology of the Perennial Plants") recorded differingflowering dates for various species, some of whichflowered twice; e.g., gall eta grass (Hilaria james;;)and muhly (Muhlenbergia sp.). Scott (section 7 on"Perennial Plant Coverage") noted that some perennialspartially or completely died (e.g., snakeweed[Gutierrezia sp.]), depending on available moisture,while others (e. g. , Indian ricegrass [Oryzopsishymenoides], ephedra [Ephedra torreyi]) , blue gramagrass [Boutelous gracilis J, cactus [Opuntia polya-