Pinus edulis Engelm. Pinyon Pinaceae Pine family Frank P. Ronco, Jr. Pinyon (Pinus edulis) is a small, drought-hardy, in much demand, making them its most valuable long-lived tree widespread in the southwestern product. United States. Its common name is derived from the Spanish pifion which refers to the large seed of pino Habitat (pine). For this reason the tree is known in the Southwest and throughout its range by this Spanish Native Range equivalent (49). Other common names are Colorado pinyon, nut pine, two-needle pinyon, and two-leaf As a codominant with juniper species (Juniperus pinyon (50). Its heavy, yellow wood is used primarily spp.), pinyon trees (fig. 1) predominate in pinyon- for fuel. Because of their delicate flavor its seeds are 1 0 100 200 300 400 MILES I1 I1 I I I I I I I I I I I I I l”‘l”‘l”‘l”‘l”‘l”‘l 0 100 200 300 400 500 600 KILOMETERS Figure l-The native range ofpinyon. The author is Principal Silviculturist (retired), Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO. 327 Pinu8 edulis juniper woodlands of the semidesert zone, which The mean annual temperature in pinyon-juniper cover nearly 24.7 million ha (61 million acres), ex- woodlands varies from 4” to 16” C (40” to 61” F); tending from Texas to California (9). Woodlands in extremes may fall to -35” C (-31” F) and reach 44” which pinyon is the major pine species cover about C (112” F). January means may be as low as -10” C 14.9 million ha (36.9 million acres) in Arizona, (14” F) in the more northerly portion of the type, and Colorado, New Mexico, and Utah. Outliers in Califor- about 6” C (43” F) near the southern limits. Mean nia, Oklahoma, Texas, and Wyoming contribute a July temperatures are less variable, ranging from relatively insignificant acreage to the total (48). How- 20” to 27” C (68” to 81” F). The frost-free period ever, the outlier in California has been considered a ranges from about 90 to 205 days, the shorter period population of 2-needled individuals of single-leaf typifying more northerly latitudes and higher eleva- pinyon (Pinus monophylla), and more recently, a new tions (59,67,73). species, California single-needle pinyon (Pinus Because of wide variation in temperature and the californiarum) (5,44). amount and distribution of precipitation, the follow- ing classification has been proposed to better charac- terize the climate of pinyon-juniper woodlands in Climate Arizona and New Mexico (62): Precipitation The pinyon-juniper type occupies the lowest and warmest forested zone in the United States, with a Climate Winter Summer climate generally characterized as semiarid, and lo- mm cally as dry subhumid (65). Summers are hot and Cool, moist 230 to 280 180 to 230 winters relatively cold, especially in northern loca- Warm, moist 250 to 330 150 to 230 tions and at high elevations. A high percentage of Cool, winter dry 130 to 180 180 to 230 clear days, intense solar radiation, and windy condi- Warm, winter dry 100 to 180 200 to 280 tions favor high evapotranspiration rates (67). Cold, winter dry 100 to 150 200 to 250 Cold, summer dry 180 to 230 100 to 150 Annual precipitation, which varies widely Warm, summer dry 180 to 230 100 to 150 throughout the type because of differences in eleva- tion, topography, and geography, ranges from 250 in mm (10 in) at low elevations where the type adjoins Cool, moist 9to11 7 to 9 the desert or grassland vegetation to 560 mm (22 in) Warm, moist 10 to 13 6 to 9 Cool, winter dry 5 to 7 7 to 9 or higher at the upper reaches (62,67,73). Locally, Warm, winter dry 4 to 7 8to 11 amounts as high as 690 mm (27 in) have been Cold, winter dry 4 to 6 8tolO recorded, as along the Mogollon Rim in northern Cold, summer dry 7 to 9 4 to 6 Arizona (15). Warm, summer dry 7 to 9 4 to 6 Seasonal distribution, which also varies consider- ably, is related to prevailing storm patterns. In east- Soils and Topography ern New Mexico, for example, approximately 75 per- cent of the annual precipitation occurs during the Pinyon-juniper woodlands are located mainly on warm season (April through September) from storms the more rocky plateaus, mesas, foothill terraces, and originating in the Gulf of Mexico, whereas the per- lower mountain slopes; shrubs or grasses grow on centage decreases as these summer storms lose in- finer soils in intervening valleys, canyons, or shallow tensity during their northwesterly movement (62). washes. Such discontinuities have been attributed to Nevertheless, summer precipitation throughout fire history and soil-related differences. The oldest much of northern Arizona and the south-central and pinyons are frequently found on steep, rocky sites eastern portions of Utah is still about equal to, or where fire occurrence and severity are probably slightly greater than, winter moisture (14,42). Fur- lower than in intervening areas, and consequently thermore, as much as one-third of the rainfall may less damaging to trees (37,41,67,73). occur during July and August (67). In contrast, wood- Discontinuities ascribed to soil differences may in lands of Nevada and northern Utah receive more fact be related to the greater amounts of water that precipitation during the cool season (October through coarser soils make available to the tree, as the dif- March), primarily from Pacific winter and spring ferent soils are adjacent and there are no obvious storms (13,14). Snow depths are not great, except at differences in climatic factors. Furthermore, pinyon- higher elevations and more northerly latitudes, but juniper woodlands are found on a wide variety of soil even then, melt generally occurs within a few days, depths and textures that range from coarse, rocky especially on south-facing slopes (62). gravels to fine, compacted clays, indicating little if 328 ‘Pinus edulis any correlation between these conditions and the (Type 210), Rocky Mountain Juniper (Type 2201, In- presence of pinyon. Depth and texture, however, terior Ponderosa Pine (Type 2371, Arizona Cypress could affect productivity (37,45,62,67). (Type 240), and Western Live Oak (Type 241). It is Woodlands also are associated with a broad range an integral component in Pinyon-Juniper (Type 239) of soil Great Groups; of which Haplustalfs of the over a large area. However, as the type extends order Alfisols, Ustochrepts of the order Inceptisols, westward, pinyon is replaced by singleleaf pinyon and Ustorthents of the order Entisols are the most (Pinus monophylla) in Nevada and some localities in common (37,52,76). Parent materials are equally western Utah and northwestern Arizona (4,67). varied, Sedimentary sandstones, limestones, and Southward along the Mexican border, Mexican shales are most common, but materials of igneous pinyon (I? cembroides var. bicolor), recently given origin, such as cinders and basalt, and those from separate species status as border pinyon (I! discolor), metamorphic sources, also are found (40,62,67). In becomes the dominant tree in the woodlands some soils, carbonates may accumulate and form a (6,48,49). petrocalcic horizon (hardpan) that may extend as Common associates of pinyon over most of its deep as 1.5 m (5 ft), but is usually much shallower. range are oneseed juniper (Juniperus monosperma) Upper layers of woodland soils generally exhibit pH and Utah juniper (J. osteosperma); redberry juniper values ranging from about 7 to 8.4, but at higher and (J. erythrocarpa), also a one-seeded juniper, is con- wetter elevations, soils tend to be slightly acid in fined to the southern portion. Alligator juniper (J. reaction, approaching 6.5 (31,37,43,45,69). deppeana) and Rocky Mountain juniper (J. Pinyon-juniper woodlands are found between the scopulorum) are also found in some localities (1,4,67). low plains covered by grassland, desert shrub, or Oneseed juniper predominates in east-central chaparral vegetation and the high mountains just Arizona and most of New Mexico, and extends into below the zone dominated by either submontane western Texas and southcentral Colorado. Rocky (Pinus ponderosa). shrubs or ponderosa pine The Mountain juniper is also a common component in lower limit of growth is probably related more to the northern New Mexico and the western half of inability of pinyon trees-especially seedlings-to tolerate water stress arising from decreasing Colorado, but it is found over most of the woodlands precipitation and subsequent reduction of total mois- as well. It usually grows at higher elevations and is ture, rather than to soil or temperature factors. In seldom dominant in the stand. Utah juniper is the contrast, the upper limit appears to be a function of codominant associate in Utah, northern Arizona, greater biotic competition resulting from increased western Colorado, and northwestern New Mexico. At moisture (10,671. higher, more mesic elevations in southern and In elevation, the woodlands lie mostly between western New Mexico and westward into central 1370 and 2440 m (4,500 and 8,000 ft) (67). Individual Arizona, alligator juniper commonly forms a com- pinyons, however, may extend up to 3200 m (10,500 ponent of stands. ft) on south- and west-facing slopes in the mixed Although pinyon-juniper woodlands consist of conifer forests of Arizona (70), while scattered relatively few tree species, stands exhibit consider- juniper trees may descend to 910 m (3,000 ft) (41). able diversity in appearance and composition (4). Although the range in any given locality is consider- Some have nearly closed canopies of a single tree ably narrower, the elevational band occupied by species with little or no understory vegetation.