Pinyon-juniper woodlands are located mainly on the more rocky plateaus,  mesas, foothill terraces, and lower mountain slopes; shrubs or grasses  grow on finer soils in intervening valleys, canyons, or shallow washes.  Such discontinuities have been attributed to fire history and soil-related  differences. The oldest pinyons are frequently found on steep, rocky sites  where fire occurrence and severity are probably lower than in intervening  areas, and consequently less damaging to trees (37,41,67,73).

    Discontinuities ascribed to soil differences may in fact be related to  the greater amounts of water that coarser soils make available to the  tree, as the different soils are adjacent and there are no obvious  differences in climatic factors. Furthermore, pinyon-juniper woodlands are  found on a wide variety of soil depths and textures that range from  coarse, rocky gravels to fine, compacted clays, indicating little if any  correlation between these conditions and the presence of pinyon. Depth and  texture, however, could affect productivity (37,45,62,67).

    Woodlands also are associated with a broad range of soil Great Groups,  of which Haplustalfs of the order Alfisols, Ustochrepts of the order  Inceptisols, and Ustorthents of the order Entisols are the most common  (37,52,76). Parent materials are equally varied. Sedimentary sandstones,  limestones, and shales are most common, but materials of igneous origin,  such as cinders and basalt, and those from metamorphic sources, also are  found (40,62,67). In some soils, carbonates may accumulate and form a  petrocalcic horizon (hardpan) that may extend as deep as 1.5 m (5 ft), but  is usually much shallower. Upper layers of woodland soils generally  exhibit pH values ranging from about 7 to 8.4, but at higher and wetter  elevations, soils tend to be slightly acid in reaction, approaching 6.5  (31,37,43,45,69).

    Pinyon-juniper woodlands are found between the low plains covered by  grassland, desert shrub, or chaparral vegetation and the high mountains  just below the zone dominated by either submontane shrubs or ponderosa  pine (Pinus ponderosa). The lower limit of growth is probably  related more to the inability of pinyon trees- especially seedlings- to  tolerate water stress arising from decreasing precipitation and subsequent  reduction of total moisture, rather than to soil or temperature factors.  In contrast, the upper limit appears to be a function of greater biotic  competition resulting from increased moisture (10,67).

    In elevation, the woodlands lie mostly between 1370 and 2440 m (4,500  and 8,000 ft) (67). Individual pinyons, however, may extend up to 3200 m  (10,500 ft) on south- and west-facing slopes in the mixed conifer forests  of Arizona (70), while scattered juniper trees may descend to 910 m (3,000  ft) (41). Although the range in any given locality is considerably  narrower, the elevational band occupied by woodlands is a rather uniform  span of about 610 m (2,000 ft). There is a tendency, however, for the  entire band to decrease in elevation in a southeasterly direction (72). In  Arizona, the majority of the type is found between 1370 and 1980 m (4,500  and 6,500 ft), whereas in Colorado, the band extends from 1830 to 2440 m  (6,000 to 8,000 ft). The bulk of the woodland in New Mexico and Utah  occupies a zone from 1520 to 2130 m (5,000 to 7,000 ft).