The Western San Juan Mountains: Their Geology, Ecology, and Human History

11. Biotic Communities of the Semiarid Foothills and Valleys

CHAPTER II BIOTIC COMMUNITIES OF THE SEMIARID FOOTHILLS AND VALLEYS LISA FLOYD-HANNA, ALBERT W. SPENCER, AND WILLIAM H. ROMME The semiarid foothills and valleys lying between 4,500 and 9,000 feet (1,400-2,750 01) occupy the largest area of the San Juan region . We will discuss the low-lying and broadest valleys, the mesas and cuestas, and the slopes and narrow valleys of the true foothills in turn, as though there were a uniform progression from one to the other. (In actuality, travelers continually pass from one to another as they drive the Skyway.) Five types ofcommunities will be discussed: greasewood-shadscale shrub-steppe, Great Basin sagebrush shrub-steppe, pinon-juniper woodland, the mountain shrub community, and ponderosa pine-oak-Douglas fir forest. The communities, in the order listed, tend to occupy progressively higher elevations or better soil conditions. Pinon-juniper woodland, Great Basin sagebrush-steppe, and greasewood-shadscale communities occur along less than 10 percent of the San Juan Skyway loop, but they are characteristic ofthe major area ofthe Colorado Plateau and the canyon country lying south, west, and north of the San Juan Mountains. Within this broad geographic region, the total area covered by arid vegetation types dwarfs that encompassed by the montane and alpine zones that dominate the Skyway scenery. The shrub-steppe habitats are also the ones most heavily exploited and altered by humans. From 143 144 The Western San juan Mountains prehistoric times, people have lived, foraged, and farmed in the areas covered by these communities, which include the greatest expanses of arable land in the Four Corners region. The streams passing through them have been dammed and the waters spread out on irrigated fields so that cultivated grains and forage have displaced hundreds ofthousands ofacres ofbig sagebrush vegetation. Generations of cattle and sheep have stripped away many of the native grasses and forbs over much of this area. Indeed, many of the herbs found here are exotic weed species that have invaded the denuded ground. For instance, cheatgrass (Bromus tectorum), a Eurasian weed, is a dominant grass in many parts ofthe Four Corners region. SHRUB-STEPPE COMMUNITIES Two kinds of semiarid shrub-steppe communities cover most of the area at the lowest elevations. Steppe is a Russian word for semiarid grassland. Low places in the major valleys, especially those carved out ofmarine shales such as the Mancos, have heavy, poorly drained, saline soils. These soils support greasewood (Sarcobatus vermiculatus), shadscale (Atriplex confertifolia), and other halophytic (salt-tolerant) plants such as sea blight (Suaeda spp.) and saltgrass (Distichlis spicata). This greasewood-shadscale shrubsteppe community is widespread along the lower reaches of the San Juan Mountains but along the Skyway occurs only east of Cortez. To accommodate the additional water stress salts impose, halophytic plants secrete excess salt from their leaves and/or selectively exclude salt as their roots take up water. Both of these survival adaptations have a cost in metabolic energy, so the halophytes usually grow more slowly than plants that do not need to cope with excessive salt in the soil. Pluck and chew a leaf from the shadscale; you will taste the salt unless recent rains have washed away the deposits. Annual species also constitute an important portion of the biota of both the greasewood and the sagebrush communities. The seeds ofannuals very effectively meet the problem ofavoiding water stress; they can lie dormant in the soil for months or even years until adequate rainfall stimulates their germination. Biotic Communities ofthe Semiarid Foothills and Valleys 145 Great Basin sagebrush shrub-steppe communities develop in low-lying areas having deep, well-drained soils. The dominants are mainly grasses such as blue grama (Bouteloua gracilis) and galleta (Hilaria jamesii), a variety ofother herbs, and big sagebrush (Artemisia tridentata). The sagebrush is easily recognized by its silvery foliage and pungent odor, whereas greasewood has pale green leaves and long, straw-colored spines. Before the arrival of European agriculturalists, sagebrush shrub-steppe generally covered all of the low areas having deep, rich, well-drained soils in the San Juan region. Such soils often have been deposited over centuries by wind, most often on sandstone strata such as the Dakota Sandstone or on alluvial terraces along the major streams. The red soils exposed in the dryland wheat and bean fields north and west of Cortez and south ofDurango are representative; they were once covered mainly by sagebrush . Sagebrush still dominates much...