Culture and Ecology of Chaco Canyon and the San Juan Basin

38 Chaco Project SynthesisLybrook, Cuba, and Pueblo Pintado. Healthy Pinuspinon are found on Chacra Mesa and Southwest Mesa.Understanding the vegetative cover and diversitywas one goal of the Chaco Project. One of the firststudies to sample by transects was that of Jones(1970), whose lines crossed six physiographic areas(plains, rock outcrop, cliff, alluvium, canyon floor,and arroyo); he also classified vegetation into fourgroups (forbs and grasses, half-shrubs, shrubs, andtrees). Scientific and common names were providedfor various species (seven grasses, 11 forbs, one tree,and 11 shrubs); three unidentified plants and manyothers were noted to be present in the area. Jonescommented that for all plant associations, the plantdiversity was lower than expected. The four-wingsaltbush/greasewood association had the least speciesdiversity and was more sensitive to manmade disturbances."Based upon casual observations made duringthis project, it appears that all of the associations areecologically frail. The minor amount of visitationfrom our investigations is quite evident at eachsampling unit, and in a few cases minor erosion can beseen due to our activities" (Jones 1972:71). Thisobservation was reiterated one year later, when Jonescontinued research on small rodents (Potter 1974:115). The census of small animals in each unit isdiscussed below.On a broader scale, aerial photography andground-based studies were employed to prepare avegetative map and description of vegetative types forthe area encompassed within the former Chaco CanyonNational Monument. Using color transparencies takenat 1:6000 in 1973 and black-and-white photographstaken at 1:3000 in 1974, plus information fromJones's (1970) vegetational survey, a preliminaryvegetative map was prepared by N. Edmund Kelley.Vegetative data were correlated with relief and otherfeatures within 1 m units. Due to the scale of thephotography, it was not possible to recognize thespecies of grasses, but two types of grasslands weredesignated. It was also possible to detect areas wherethere had been watershed treatments or previouslycultivated fields. The resulting map (Figure 2.5) anddescription of the vegetative types (E. Kelley andPotter 1974; Potter and Kelley 1980) illustrated thezones, coded for either the dominant plant species orcombination of species, or physiographic area, asfollows:HB: Hilaria-Bouteloua grassland. This grasslandtype is found in the upland plateau where soils are thinand composed of silt and sand. Depending on thetexture and depth of soils, the associations of plantsvary. Galleta grass (Hilaria jamesii) is found inheavier soils. Blue grama grass (Bouteloua gracilis)is greater where soils are intermediate in texture;where soils tend to be sandier, sand drop seed(Sporobolus cryptadrus) and Indian ricegrass(Orozopsis hymenoides) increase. Other commonspecies include winterfat (Eurotia lanata), the exoticRussian thistle (Salsola kali), and a desert hiddenflower(Cryptantha angustifolia). In areas where therehad been sand dunes, there is an increase in Indianricegrass and four-wing saltbush; this was subdividedas HB(A). HB(J) denotes scattered junipers located inthis soil with fractured sandstone.Sp-Si: Special grassland dominated by alkalinesacaton (Sporobolus airoides) and bottlebrush squirreltailgrass (Sitanion hystrix). Only two small areaswhere there are low, level, clay soils with highalkalinity were placed in this category. During moistsummers, tansy mustard (Descurainia) is expected;other species include grasses and weedy speciesindicative of disturbed soils.AOS: Atriplex-Oryzopsis-Sporobolus. Whenfour-wing saltbush (Atriplex canescens) is co-dominantwith Indian ricegrass (Oryzopsis hymenoides) and sanddropseed (Sporobolus cryptandrus), the designationAOS was given. This zone is commonly associatedwith deep sandy soils that were once dunes; it is alsofound at the base of cliffs where outwash is relativelycoarse. On the lower slopes, where brush increases,it blends with the upper limits of black greasewood(Sarcobatus vermiculatis). Also appearing are avariety of grasses, forbs, and shrubs.AS: Atriplex-Sarcobatus. This code marks azone of transition from upper to lower slopes, sandyto clayey soil, high to low alkalinity, and greater tolesser depth of the water table. Four-wing saltbush(A trip lex canescens) and greasewood (Sarcobatusvermiculatus) extend from the banks of the ChacoWash to the intermediate and lower slopes of thecentral canyon, where they dominate the vegetativecover. Other species present include several grasses,shrubs, and forbs.