Soils and Topography
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).
- Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965). U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp. http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm
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