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SPECIES: Pinus monophylla

INTRODUCTORY

AUTHORSHIP AND CITATION:
Zouhar, Kristin L. 2001. Pinus monophylla. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.usda.gov /database/feis/plants/tree/pinmon/all.html []

ABBREVIATION:
PINMON

SYNONYMS:
Pinus monophylla Torr. and Frem. var. fallax (Little) Silba
Pinus monophylla Torr. and Frem. var. monophylla [104]

NRCS PLANT CODE [217]:
PIMO

COMMON NAMES:
singleleaf pinyon
nut pine
piñon

TAXONOMY:
The currently accepted scientific name of singleleaf pinyon is Pinus monophylla Torr. & Frem. (Pinaceae) [92,103,142,232]. It is a member of the subgenus Strobus, section Parrya, subsection Cembroides of Pinus [120].

Singleleaf pinyon hybridizes with Colorado pinyon (P. edulis), Parry pinyon (P. quadrifolia), and Sierra Juárez piñon (P. juarezensis) [15,73,118,119,121,246,247].

LIFE FORM:
Tree

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
No entry

DISTRIBUTION AND OCCURRENCE

SPECIES: Pinus monophylla
GENERAL DISTRIBUTION:
Pinyon-juniper woodlands cover more than 55.6 million acres in the western U.S. [153]. Singleleaf pinyon has a large area of distribution and, therefore, probably a large degree of genetic variation [120]. It is the predominant tree species in the isolated mountain ranges of the Great Basin, ranging from southern Idaho, western Utah and northwestern Arizona, through most of Nevada (it's Nevada's state tree) and eastern and central California to northern Baja California [56,92,120,133,142]. There is about 2.5 billion ft3 of singleleaf pinyon in Nevada, and about 5 billion ft3 in Utah [162]. It is also found in the Mojave Desert borderlands of southern California and in small, fragmented populations in a belt across Arizona south of the Mogollon Rim into southwestern New Mexico [121,122]. The U.S. Geological Survey provides a distributional map of singleleaf pinyon.  

The distribution of singleleaf pinyon has undergone many changes in both prehistoric and historic times [35], and Everett [59] points out that any assessment of pinyon and juniper woodland distribution is only a snapshot of a woodland in motion. Historic changes in distribution are reported by several authors [79,172,199,244]. Much has been written on the evolutionary distribution of singleleaf pinyon [7,121,128,138,161,194,198,207,213,219,220,231] and pinyon in general [10,11,57], and may provide information pertinent to predicting potential changes in distribution resulting from climate change.

Hybrids of Colorado and singleleaf pinyon occur in 3 geographical zones along the margin where their distributions overlap. The most extensive zone is that along the eastern margin of the Great Basin in central Utah. Another zone is in the canyons of the Colorado River in southern Utah, and a 3rd zone is among the scattered pine groves south of the Mogollon Rim in central Arizona [75,119]. Hybrids of Parry pinyon and singleleaf pinyon are reported in southern and Baja California [246,247]. Hybrids of singleleaf pinyon and Sierra Juarez piñon are found in scattered places in northern Baja [118].

ECOSYSTEMS [76]:
FRES21 Ponderosa pine
FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES40 Desert grasslands

STATES:

AZ CA ID NV NM UT
MEXICO

BLM PHYSIOGRAPHIC REGIONS [9]:
3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range
9 Middle Rocky Mountains
12 Colorado Plateau

KUCHLER [116] PLANT ASSOCIATIONS:
K019 Arizona pine forest
K022 Great Basin pine forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K030 California oakwoods
K031 Oak-juniper woodland
K032 Transition between K031 and K037
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K039 Blackbrush
K040 Saltbush-greasewood
K041 Creosote bush
K042 Creosote bush-bursage
K058 Grama-tobosa shrubsteppe

SAF COVER TYPES [70]:
209 Bristlecone pine
211 White fir
217 Aspen
219 Limber pine
235 Cottonwood-willow
237 Interior ponderosa pine
238 Western juniper
239 Pinyon-juniper
240 Arizona cypress
241 Western live oak
247 Jeffrey pine
249 Canyon live oak

SRM (RANGELAND) COVER TYPES [181]:
101 Bluebunch wheatgrass
102 Idaho fescue
104 Antelope bitterbrush-bluebunch wheatgrass
105 Antelope bitterbrush-Idaho fescue
107 Western juniper/big sagebrush/bluebunch wheatgrass
209 Montane shrubland
210 Bitterbrush
211 Creosote bush scrub
212 Blackbush
235 Cottonwood-willow
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
405 Black sagebrush
406 Low sagebrush
408 Other sagebrush types
411 Aspen woodland
412 Juniper-pinyon woodland
413 Gambel oak
414 Salt desert shrub
415 Curlleaf mountain-mahogany
422 Riparian
501 Saltbush-greasewood
504 Juniper-pinyon pine woodland
506 Creosotebush-bursage
509 Transition between oak-juniper woodland and mahogany-oak association

HABITAT TYPES AND PLANT COMMUNITIES:
Classifications describing plant communities in which singleleaf pinyon is a dominant species are as follows:

Arizona [129,154,190]
California [87,93,94,150,166,224,238]
Idaho [175]
Nevada [14,16,17,18,19,132,164,238]
New Mexico [129,154,190]
Utah [238]

Pinyons (Cembroides) typically grow in association with juniper (Juniperus spp.), with juniper dominating the lower elevations of their range and pinyons the upper. In fully stocked pinyon-juniper stands, understory species comprise a very small portion of the total biomass, though they may be important forage species and the plants most likely to reclaim the site following disturbance [62]. In general, coverage of understory vegetation associated with singleleaf pinyon woodlands varies with geographic area, site quality, and successional stage. Singleleaf pinyon occurs in 3 general regions: the Great Basin, southern California and northern Baja, and parts of Arizona and New Mexico south of the Mogollon Rim.

In the Great Basin, singleleaf pinyon is found primarily with Utah juniper (J. osteosperma), sagebrush (Artemisia spp.), rabbitbrush (Chrysothamnus spp.), Arizona joint-fir (Ephedra fasciculata), antelope bitterbrush (Purshia tridentata), granite pricklygilia (Leptodactylon pungens), and a variety of other shrubs, herbs, and cryptobiotic or cryptogamic crusts [78,88,122,164,199,235]. The pinyon-juniper type lies between desert shrub or grassland below and ponderosa pine (Pinus ponderosa) associations above, with broad ecotones [57]. Utah juniper is absent from some singleleaf pinyon stands in extreme western and southern Nevada and adjacent California [57,235]. In southern Nevada, singleleaf pinyon shares a broad lower ecotone with blackbrush (Coleogyne ramosissima), where creosotebush (Larrea tridentata) and white bursage (Ambrosia dumosa) occupy the lower slopes and valley floors, and an upper ecotone with ponderosa pine communities with white fir (Abies concolor), limber pine (P. flexilis) and quaking aspen (Populus tremuloides) [130,131]. Curlleaf mountain-mahogany (Cercocarpus ledifolius) is associated with singleleaf pinyon in many areas, particularly near its upper elevational limit [57].

Codominants of singleleaf pinyon found throughout the Great Basin include mountain big sagebrush (Artemisia tridentata ssp. vaseyana), Wyoming big sagebrush (A. t. ssp. wyomingensis), low sagebrush (A. arbuscula), blackbrush, antelope bitterbrush, desert bitterbrush (P. glandulosa), curlleaf mountain-mahogany, desert snowberry (Symphoricarpos longiflorus), Utah snowberry (S. oreophilus var. utahensis), rubber rabbitbrush (Chrysothamnus nauseosus), low rabbitbrush (C. viscidiflorus), pale serviceberry (Amelanchier pallida), greenleaf manzanita (Arctostaphylos patula), Gambel oak (Quercus gambelii), shadscale (Atriplex confertifolia), green ephedra (E. viridus), bluebunch wheatgrass (Pseudoroegneria spicata), Idaho fescue (Festuca idahoensis), Thurber needlegrass (Achnatherum thurberianum), desert needlegrass (A. speciosum), Indian ricegrass (A. hymenoides), Sandberg bluegrass (Poa secunda), mutton grass (P. fendleriana), blue grama (Bouteloua gracilis), bottlebrush squirreltail (Elymus elymoides), bullgrass (Elymus simplex), arrowleaf balsamroot (Balsamorhiza sagittata), and cheatgrass (Bromus tectorum) [14,16,17,18,19,59,106,132]. Singleleaf pinyon reaches the northern extent of its range at the northern end of the Great Basin, in southern Idaho, where it occurs in rare communities with the following species: Utah juniper, curlleaf mountain mahogany, mountain big sagebrush, black sagebrush (Artemisia nova), Sandberg bluegrass, and bluebunch wheatgrass [175]. Singleleaf pinyon is codominant with narrowleaf cottonwood (Populus angustifolia), big sagebrush, red-osier dogwood (Cornus sericea), narrowleaf willow (Salix exigua), and rabbitbrush species in some of the riparian ecosystems in Great Basin National Park, Nevada [184].

Everett and Koniak [62] describe several other important understory species found with singleleaf pinyon in the Great Basin of Nevada, and note that annual forbs are best adapted to coexist with the singleleaf pinyon and associated juniper species in fully stocked stands. Because of the ephemeral nature of this group, understory cover tends to be variable and response to disturbance less predictable. Understory vegetation varies with microsites, being most abundant in the transition zone between the duff under the tree canopies and the space between trees. Goodrich and others [77] describe endemic and endangered plants of pinyon-juniper communities in Utah.

In the Inyo and White mountains of eastern California, Utah juniper occurs with singleleaf pinyon primarily at lower elevations, with Utah juniper density decreasing at upper elevations. The shrub layer is sparse and composed primarily of big sagebrush, antelope bitterbrush and desert bitterbrush, intermixed with Ephedra spp., rubber and low rabbitbrush, and cactus (Opuntia and Echinocereus spp.). Singleleaf pinyon is bordered by Great Basin bristlecone pine (P. longaeva) and limber pine at higher elevation [157]. Singleleaf pinyon is also found with Great Basin bristlecone pine on several mountain ranges in Nevada, and in the San Francisco Mountains of southwestern Utah [142]. On the eastern slopes of the Sierra Nevada singleleaf pinyon is found with western juniper (J. occidentalis), Utah juniper, Jeffrey pine (P. jeffreyi), ponderosa pine, big sagebrush, curlleaf mountain-mahogany, rubber rabbitbrush, antelope bitterbrush, desert bitterbrush, bluebunch wheatgrass, Idaho fescue, and bottlebrush squirreltail [43,93,142,224]. Singleleaf pinyon may also be found with bigcone Douglas-fir (Pseudotsuga macrocarpa) at upper elevations in southern California [140].

In southern California, singleleaf pinyon is a common component of the desert montane landscape on arid slopes [187], and is most commonly found with California juniper (J. californica) [124,142]. In the Mojave Desert mountain ranges, it intergrades at higher elevations with white fir or Great Basin bristlecone pine forest, and at lower elevations with California juniper and Joshua tree (Yucca brevifolia) woodland with creosotebush and white bursage. Here it occurs with big sagebrush, curlleaf mountain-mahogany, blackbrush, Stansbury cliffrose (Purshia mexicana var. stansburiana), Nevada ephedra (Ephedra nevadensis), shrub live oak (Quercus turbinella), rubber rabbitbrush, low rabbitbrush, blue sage (Salvia pachyphylla), Wright buckwheat (Eriogonum wrightii var. trachygonum), hollyleaf gilia (Gilia latiflora), and cheatgrass [87,93]. In the San Bernardino Mountains singleleaf pinyon occurs primarily with California and western juniper, curlleaf mountain-mahogany, and big sagebrush [94,150]. Singleleaf pinyon may also be found in redshanks (Adenostoma sparsifolium) chaparral communities with desert ceanothus (Ceanothus greggii) [135]. In the Sierra Juárez in northern Baja, singleleaf pinyon forms extensive forests in association with California juniper, sugar sumac (Rhus ovata), Palmer oak (Quercus dunnii), Muller oak (Q. cornelius-mulleri), redberry buckthorn (Rhamnus crocea), Mojave yucca (Yucca schidigera), and chaparral yucca (Y. whipplei), with Parry pinyon dominant on the wetter western margin and singleleaf pinyon dominant on the eastern rim [151]. Singleleaf pinyon may also be found with Jeffrey pine and peninsular oak (Quercus peninsurlaris) [120,156]. On the eastern flank of the Sierra San Pedro Mártir, singleleaf pinyon forms extensive forests with desert chaparral understory dominated by peninsular manzanita (Arctostaphylos peninsularis) and peninsular oak and bordered by Parry pinyon at upper elevations. Elsewhere in Northern Baja, near the southern end of its distribution, singleleaf pinyon occurs in scattered patches on high ridges and northern aspects [151].

In the southern Arizona and southwestern New Mexico, singleleaf pinyon occurs at mid-elevations with ponderosa pine above and oak woodlands below. In the Hualapai Mountains of western Arizona, singleleaf pinyon is dominant at upper elevations with occasional occurrence of Utah juniper and ponderosa pine, and is codominant with Utah juniper at lower elevations. Understory plants include green ephedra, red barberry (Mahonia haematocarpa), broom snakeweed (Gutierrezia sarothrae), turpentine bush (Ericameria laricifolia), kingcup cactus (Echinocereus triglochidiatus var. melanacanthus), greenflower nipple cactus (Mammillaria viridiflorus), prickly-pear (Opuntia spp.), shrub live oak, ceanothus (Ceanothus spp.), skunkbush sumac (Rhus aromatica var. trilobata), banana yucca (Y. baccata), yerba de pasmo (Baccharis pteronioides), and perennial grasses including grama (Bouteloua spp.), fringed brome (Bromus ciliatus), California brome (B. carinatus), New Mexico feathergrass (Hesperostipa neomexicana), and purple threeawn (Aristida purpurea) [30]. In Arizona and New Mexico singleleaf pinyon is codominant with shrub live oak, Emory oak (Q. emoryi), pointleaf manzanita (A. pungens), blue grama, crucifixion thorn (Canotia holacantha), and banana yucca, and also occurs with Utah juniper, alligator juniper (J. deppeana), mimosa (Mimosa aculeaticarpa var. biuncifera), sumac (Rhus spp.), silktassel (Garrya spp.), curlymesquite (Hilaria belangeri), and prairie junegrass (Koeleria macrantha) [154,190].

MANAGEMENT CONSIDERATIONS

SPECIES: Pinus monophylla
WOOD PRODUCTS VALUE:
Wood of singleleaf pinyon is used primarily for fuel wood and fence posts [95,115]. It is not suitable for lumber because of its small size, irregular shape, and lack of self-pruning [71,83,142]. It may be used for particle board and cement board [71,159]. The old wood burns hot and served as an important fuel source for railroads and a major charcoal source for silver smelters in the late 1800s [110,122].

IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Pinyon-juniper woodlands provide shelter and forage for numerous species of wildlife, some of which may be obligate to these woodlands such as pinyon mice and woodrats [142,143,157,182]. These forests have value as habitat for several large mammals such as mule deer, white-tailed deer, pronghorn, desert bighorn sheep, elk, wild horses, mountain lions, and bears [143,157,182,192,206]. Gray foxes, bobcats, coyotes, weasels, skunks, badgers, and ringtails search for prey here [182,248]. Many species of birds [4,20,54,143,171,229] and reptiles [157] find food and shelter here. Pinyon-juniper forests are important wintering areas for Clark's nutcrackers [223]. The quantity and variety of species using the pinyon-juniper woodlands changes with succession [4,182].

Pinyon mice, deer mice, woodrats, squirrels, chipmunks, deer, black bears, and desert bighorn sheep eat singleleaf pinyon seeds [23], as do scrub, Steller's, and pinyon jays and Clark's nutcrackers [64,122,123]. Many of these animals cache seeds for winter use and are critical for regeneration of singleleaf pinyon [35,123,180,211]. Although it is not favored, mule deer eat pinyon foliage [121,124], using the foliage moderately in winter, spring, and summer [117]. Singleleaf pinyon comprised 1 to 2% of the winter diet of pronghorn in Utah, while cattle and domestic sheep did not use it [183]. Cows may feed on pinyon in the winter, and it is thought that this can cause them to abort [121]. The inner bark is a major food of porcupines, and is also eaten by squirrels, raccoons, rabbits, ringtails, coyotes, and gray foxes [182], as well as the larvae of the mountain pine beetle and the fungus causing pinyon blister rust [121]. Limbs are attacked by pinyon dwarf-mistletoe (Arceuthobium divaricatum). The pitch is a staple food of pitch midges and is used by Dianthidium bees to build their nests. Sawflies feed on needles and pollen. Gall midges live in the needles by causing 2-needle fascicles to develop and living in the fascicle crotch [121,124].

PALATABILITY:
The seed or "nut" of singleleaf pinyon has a lower fat content than that of Colorado pinyon, giving it a more starchy or mealy taste by comparison [134]. The palatability of singleleaf pinyon browse to livestock and wildlife species in Nevada and Utah is rated as follows [47,121]:

Nevada Utah
Cattle Poor Poor
Domestic sheep Poor Poor
Horses Poor Poor
Pronghorn --- Fair
Elk --- Fair
Mule deer --- Fair
Small mammals Good Good
Small nongame birds Good Good
Upland game birds Good Good
Waterfowl --- Poor

NUTRITIONAL VALUE:
Pinyon nuts have outstanding nutritional value. Shelled nuts of singleleaf pinyon have approximately 9.5% protein, 23% fat, and 54% carbohydrate. They are comparable in nutritional value to pecans. They contain all 20 amino acids, with 8 of the 9 essential amino acids are more abundant in pinyon nuts than in amaranth (Amaranthus spp.), another important food plant of Great Basin Native Americans. Singleleaf pinyon seeds are especially rich in tryptophan and cystine [121]. They have a large amount of oleic and linoleic unsaturated fatty acids, comprising 42.5% and 42% of the fatty acid total, respectively. Eighty percent of the fatty acids are polyunsaturated [125]. The nuts also provide significant amounts of vitamin A, thiamine, riboflavin, and niacin [114].

COVER VALUE:
Singleleaf pinyon provides cover and shelter for numerous birds and animals. Game animals favor areas where pinyon-juniper woodlands form mosaics with browse shrubs [74].

VALUE FOR REHABILITATION OF DISTURBED SITES:
Singleleaf pinyon has not been regarded historically as a species to be used for rehabilitation of disturbed sites, but more often as a disturbance agent and an invasive species [34]. Some literature recommends rehabilitating pinyon-juniper sites by managing for old-growth structure with an emphasis on pine nut production [110].

OTHER USES AND VALUES:
Pinyon-juniper ecosystems have had subsistence, cultural, spiritual, economic, aesthetic and medicinal value to Native American peoples for centuries [6,148,149], and singleleaf pinyon has provided food, fuel, medicine and shelter to Native Americans for thousands of years [6,99,121,122,210,249]. The pitch of singleleaf pinyon was used as adhesive, caulking material, and a paint binder [6]. It may also be used medicinally and chewed like gum [210]. Pinyon seeds are a valuable food source for humans [52,95], and a valuable commercial crop [110,134,149,232].

OTHER MANAGEMENT CONSIDERATIONS:
Past management of singleleaf pinyon has ranged from heavy use to planned eradication. Continued removal of old-growth trees could potentially eliminate them and promote reverse genetic selection by diminishing the quality of the trees left [110]. Multiresource management of pinyon-juniper woodlands for sustained production of a variety of products including tree products, forage for livestock, habitat for wildlife species, watershed protection, recreation, and archeological values is gaining support [58,81,100].

Singleleaf pinyon can be managed for tree products such as fuel, fenceposts, resins, and pine nuts. Production of these resources requires healthy functioning of whole ecosystems, providing economic incentive for sustainable ecological practices. Management of woodlands for nut production will yield 100 times more income than will management for livestock forage, and the 2 can both be done on the same land and not interfere with one another [110]. Fisher and others [72] give management suggestions for improving nut crops in pinyon.

Some authors report no appreciable increase in spatial distribution of singleleaf pinyon in California [136] and the Great Basin. In fact, its range in the Great Basin has substantially decreased since 1987 due to a large die-off of singleleaf pinyon in Utah [21]. However, most authors have reported increases in spatial extent and tree dominance and reductions of understory cover and diversity in pinyon-juniper woodlands [15,39,42,60,62,65,67,81,96,167,180,199,204,218,235]. These changes may be attributed to the interactions of many variables such as seed distribution by birds, centuries of domestic livestock grazing, changes in fire frequency, and climate change [5,15,48,58,81,85,90,124,197,204]. Some managers have been concerned about the subsequent loss of forage for livestock and big game and have supported efforts to remove trees and seed non-native grasses to create artificial grasslands [49,57,102,122,134,149,232]. Removal of trees tends to result in increased growth of understory species (e.g., see [65,66,67,216]); however, many "control" operations fail to meet their objectives or fall short of projected increases in forage production [40,122,169,188,244]. Furthermore, tree removal can have serious impacts on hydrology, nutrient cycling, soil fertility [81,204], and wildlife and archaeological resources [57,81,122,124,165]. Often the resulting slash from these operations is piled and burned, resulting in high heat levels that leave some areas sterilized and free of vegetation for over 20 years [165].

While tree encroachment may adversely affect habitat by reducing forage for some wildlife species, the total removal of trees may not be an effective alternative. Periodic removal of some singleleaf pinyon trees using prescribed fire or silvicultural treatments, while leaving areas of trees close by, creates more effective habitat for wildlife [34,182,186,206]. It is important that a variety of age classes be represented on the landscape, since each successional stage supports different species in different amounts. Old growth pinyon-juniper woodlands, for example, are home to many species of reptiles, and any changes in woodland tree density may have an effect on lizard populations [157]. Also, while exotic grasses may be more successful than native species at regeneration after tree removal, seeding exotics may "break the chain" successionally by replacing early successional hosts of mycorrhizae shared by later successional species [206]. Singleleaf pinyon removal and woodland conversion projects have the potential to increase cover of undesirable exotic species such as cheatgrass at the expense of the more desirable native forage species [193].

It remains unclear what type of stand structure provides the most desirable watershed conditions in pinyon-juniper woodlands. Some managers suggest that a high density of trees causes erosion, while others suggest that centuries of grazing and destruction of cryptobiotic crusts have caused erosion in these woodlands. No studies support the claim that deforestation reduces erosion and protects watersheds [48].

Insects and diseases affecting singleleaf pinyon include pinyon needle scale, which weakens trees and subjects them to killing attacks by bark beetles (pinyon ips) [215,240]. Pinyon pine sawfly can also be a serious defoliator [215]. Singleleaf pinyon is parasitized by the pinyon dwarf-mistletoe [91]. Black stain root disease causes extensive mortality in single leaf pinyon in portions of the San Bernardino Mountains in southern California [187]. Cone and seed insects associated with singleleaf pinyon, such as the pinyon cone beetle (Conophthorus monphyllae), coneworms (the larval stage of Dioryctria moths), and the pinyon cone borer may need to be considered if nut production is an objective [38,147,160]. Variability of stand conditions within a forest or region may reduce the risk of widespread mortality due to insect pests and diseases and may be achieved by reintroduction of fire to reduce biomass and fuel loads in some areas [51]. Removal of all pinyon slash larger than 3 inches (8 cm) will usually prevent pinyon ips populations from reaching epidemic proportions [22,143].

Nearly 70 taxa of narrow endemic plants, many of which are listed as threatened, endangered, or sensitive, or have been considered for listing, are found in pinyon-juniper communities in Utah, and may be somehow dependent on the pinyon-juniper association. An understanding of the habitat (geology, soils, hydrology) and biology of these protected species is also an important aspect of management decisions [77]. Cryptobiotic crusts are important features of pinyon-juniper ecosystems, as they are thought to increase infiltration, reduce erosion, contribute to soil organic matter, and fix nitrogen [8,233]. It is important to consider impacts of management activities on these crusts, as they are fragile and recover slowly [82]. Slash management is also an important consideration in the nutrient-poor environment typical of singleleaf pinyon communities [58], and the culling of dead and dying trees through fuel wood and salvage operations removes nutrients as well as habitat for insects and wildlife [42]. However, Thran and Everett [208] found little change in soil nutrients after complete tree removal in Nevada.

Management of singleleaf pinyon involves a complex ecosystem, sometimes requiring landscape-scale objectives [110]. Because stand characteristics of singleleaf pinyon woodland vary with geography, topography, local site conditions, understory community and general climate, effective management must work within the community ecology of the woodland sites of interest, including all the species present and the history of use and change on the site, as well as its current ecological status [53,81,204]. Gottfried and Severson [81] provide a detailed review of past management objectives and procedures and an evaluation of their relative successes. Ellenwood [53] suggests some generalized management objectives commonly applied to pinyon-juniper forests (sustaining grassland, sustaining woodland, and sustaining woodland savannah), and silvicultural systems that may be used to achieve these objectives. Other silvicultural recommendations for singleleaf pinyon woodlands are available [80,82,178]. Miller [148] presents examples of holistic woodland management which take into account the human element and cultural and spiritual requirements of the Native peoples who live in pinyon-juniper ecosystems. He also suggests "community forestry" practices as good templates for woodland planning [149].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Pinus monophylla
GENERAL BOTANICAL CHARACTERISTICS:
Mature singleleaf pinyon is typically a short tree (20-40 feet (6-12 m) tall), with a rounded to flat-topped crown and multiple, upswept branches due to lack of self-pruning. It may occasionally be multi-stemmed from simultaneous establishment from seed caches [213]. It generally forms open woodlands [124,142]. Bark is smooth and thin (0.4-0.8 inch (1-2 cm)) on young trees, forming deep, irregular fissures and ridges with thin scales, and is up to an inch thick with age [83]. The wood is light and soft, not strong or resinous [168]. Singleleaf pinyon has an extensive lateral root system, giving it the ability to penetrate into open areas between tree canopies and extract water and nutrients. Its ability to invade adjacent shrublands may also be related to the tree's ability to maintain a seasonally stable xylem water potential and thereby to endure drought better than the associated shrubs [58]. 

The needles of singleleaf pinyon are solitary, rigid, and 1 to 1.4 inches (2.5-3.5 cm) long. The sheaths deciduous [232]. The single needle (leaf) is unique among pines of the world [121]. Singleleaf pinyon needles are long-lived (5-12 years) [83,139]. Their longevity is considered an extreme expression of "evergreenness," giving the tree the ability to conserve nutrients to take advantage of short favorable conditions within a generally unfavorable period [139]. The allelopathic effects of singleleaf pinyon needle litter on germination and growth of herbaceous plants has been documented [60], and is attributed to terpene hydrocarbons [241]. These compounds also impede decomposition, are highly flammable, and are readily volatilized by fire [239].

Singleleaf pinyon bears ovulate cones, 1.4 to 2.2 inches (3.5-5.5 cm) long, with thick scales. Cones bear large (0.4-0.7 inch (10-17 mm) long), heavy (0.01 oz (400 mg)), moderately thin-shelled, edible, wingless seeds [137,211,232]. Cones that dry and open in the fall drop to the ground in winter or spring and may form a conspicuous litter layer [83,124,142]. 

Singleleaf pinyon is long- lived. On fire-safe sites, large trees can monopolize site resources over a life span of 350 years or more [63]. Dominant pinyons are often 400 years old and have been known to reach 800 to 1000 years [105,174]. Mature singleleaf pinyon trees (over 200 years) are relatively uncommon [21,98]. Of the 20 million acres of pinyon-juniper stands in Nevada and Utah, about half are estimated to be between 40 and 120 years old, with almost 20% in Utah and 9% in Nevada over 200 years of age [162,163]. Biomass estimates for singleleaf pinyon are available for Nevada [36,37,146].

Hybrids are diagnosed by frequency of monophylly and leaf resin canal number [75,119,127]. Hybrids of singleleaf and Colorado pinyon have a mixture of 1- and 2-needle fascicles on the same branchlets, fewer resin ducts than singleleaf pinyon but more than Colorado pinyon, and intermediate cone size [121].

RAUNKIAER [170] LIFE FORM:
Phanerophyte

REGENERATION PROCESSES:
Reproduction in singleleaf pinyon is by seed and does not occur naturally by vegetative means [142]. It is monoecious and wind pollinated [58]. Cone and seed development require 3 seasons and about 26 months [58,105,121]. Strobili or cone primordia are initiated the growing season prior to their appearance the following spring, undergoing a period of dormancy through the winter prior to opening. Pollen release is controlled by local conditions. Once pollinated, growth of the pollen tube undergoes another winter dormancy period prior to fertilization. After fertilization, seeds develop rapidly, mature, and disperse about 6 months later, the 2nd autumn after pollination. With so much time between so many stages, cone and seed crops are exposed to a number of  variables that affect the seed crop, such as weather, predation, and internal competition for resources between the previous and current year's cones; therefore, seed production is highly variable from tree to tree, year to year, and place to place [105].

Singleleaf pinyon generally begins bearing cones at about 35 years of age, begins producing good seed crops at about 75 to 100 years, and reaches maximum production at about 160 to 200 years [58,142]. Singleleaf pinyon exhibits region-wide synchrony in cone production, masting every 2 to 3 years [35,215]. Some seeds may be produced every year [99], and good seed crops occur somewhere over a geographical area (e.g. the Great Basin) nearly every year [142]. A 5-year study reported per acre cone production as follows: 1975, 765 cones; 1976, 0 cones; 1977, 2,560 cones; 1978, 2,325 cones; 1979, 585 cones [99]. Mast years in singleleaf pinyon may be related to the polar front jet stream [161]. Singleleaf pinyon is said to be more productive and predictable than Colorado pinyon, with seed production being predicted fairly accurately 2 years in advance and more accurately 1 year in advance [99]. Seed production and survival may be affected by several insect pests of cones and seeds. For example, the pinyon cone beetle can destroy more than 50% of the crop of its host pinyon, while coneworms tunnel in cones and shoots but are of minor importance [38]. For other pests, such as the pinyon cone borer, the magnitude of the effects is unknown [147].

Because singleleaf pinyon seeds are totally wingless, seed dispersal is dependent on vertebrate dispersers that store seeds in food caches, where unconsumed seeds germinate. This dispersal mechanism is a good example of a co-evolved, mutualistic, plant-vertebrate relationship [58,123,213]. The cone scales of singleleaf pinyon have a membranous tissue that holds the seed in place after the cones open, protecting them from ground-foragers and keeping them available to avian dispersers. Several corvids are responsible for dispersal of singleleaf pinyon seeds over distance. Scrub and Steller's jays forage alone or in pairs, carrying 1 to a few seeds less than a mile before burying them. Pinyon jays forage in flocks of hundreds, carrying about 40 nuts each up to 5 miles (8.3 km) before caching them in the soil. Clark's nutcrackers forage in somewhat smaller flocks, each carrying several dozen seeds a distance of up to 13 miles (22 km) and burying 1 to 15 seeds per cache, 1 to 3 cm deep in gravelly soil, mineral soil, or duff [122,123,211]. Seed caching by Clark's nutcrackers begins late August to early September [211]. In a good seed crop year, an individual Clark's nutcracker may scatter-hoard 17,900 singleleaf pinyon seeds [35,211]. The large range of singleleaf pinyon may be attributable, in part, to seed dispersal by these birds [137]. For further information, see the FEIS reviews of pinyon jay and Clark's nutcracker.

Chipmunks, squirrels, deer mice, pinyon mice, Great Basin pocket mice, and Panamint kangaroo rats all scatter-hoard singleleaf pinyon seeds locally [35,121]. These animals consume most of the seed, but some is left to germinate. Quantitatively, these rodents are less effective than avian dispersers, since it is only in mast years that large numbers of seeds fall to the ground and become available to rodents. The seed characteristics and the microhabitats in which seeds are placed are important in determining their fate after dispersal. Rodents are qualitatively effective dispersers of singleleaf pinyon seed since they tend to bury seeds under and adjacent to shrubs, whereas avian dispersers tend to cache seeds in interspace environments, a less suitable environment for singleleaf pinyon seedlings [35,180].

Seed dispersal by humans in the past has also been suggested [126].

In general, pinyon seeds are short-lived with little innate dormancy and thus form only a temporary seed bank [35,58,143]. Fresh seeds have 85 to 95% viability, but this decreases in a year or less [58]. Most seeds germinate the spring following dispersal [35], requiring 28 to 90 days of cold stratification for germination [125]. Germination and establishment are most likely when favorable moisture conditions follow a mast year. Seeds may germinate in the open, but seedling establishment in the open is rare [63,143]. 

Singleleaf pinyon seedling establishment is episodic. Population age structure is affected by drought, which differentially reduces seedling and sapling recruitment more than other age classes [185]. Top-growth of seedlings is slow (1.0 inch (2.5 cm) per year in height, and 0.012 inch (0.3 mm) per year in diameter). Root growth is more rapid, with the taproot reaching 6 inches (15 cm) 10 days after germination [142]. Seedlings can thereby withstand soil water below the wilting point for about 2 weeks. However, field drought conditions are often more severe than this, and so seedlings only survive in the most favorable microenvironments [58]. Singleleaf pinyon seedlings survive best in the microhabitat provided by nurse plants, where organic matter, nutrient concentrations, relative humidity, water infiltration, and water holding capacity tend to be higher, and irradiance and soil temperatures tend to be lower [31,32,35,63,105,143]. However, nurse plants also compete for water and nutrients, so the trade-off is slower seedling growth rate [32,35]. Seedlings maintain a more favorable water status and have greater drought avoidance than shrub nurse plants [44,50]. The complex interaction between seedlings and nurse plants is a balance between facilitation and competition on moisture and light gradients [33].

The ecotones between singleleaf pinyon woodlands and adjacent shrublands and grasslands provide favorable microhabitats for singleleaf pinyon seedling establishment since they are active zones for seed dispersal, nurse plants are available, and singleleaf pinyon seedlings are only affected by competition from grass and other herbaceous vegetation for a couple of years. This facilitates expansion of woodlands along these ecotones, with singleleaf pinyon seedlings eventually overtopping and shading out the shrubs. Conversely, singleleaf pinyon seedlings establishing under adult trees have little chance of maturing unless the adult tree is removed or dies [35,44,50].

Singleleaf pinyon is slow growing. A dominant tree requires about 60 years to reach 6.6 feet (2 m) in height, and about 150 years to attain 28 feet (8.5 m) in height and a stump height diameter of about 12 inches (30 cm) [142,143]. Average annual height and diameter growth of immature dominants is about 2 inches (5 cm), and 0.04 to 0.20 inch (1 to 5 mm), respectively. Growth rates vary considerably even among trees on identical sites, and are greatly influenced by competition for severely limited water supplies. Dominant trees may maintain constant diameter growth rates for more than 200 years. Observed reductions in growth rates with age are likely caused by increasing competition as stands develop, and no definite age of culmination of growth has been determined [141,142].

SITE CHARACTERISTICS:
Singleleaf pinyon is adapted to a wide variety of sites. It usually grows on pediments, dry, rocky slopes, ridges, and alluvial fans and is rarely found on valley floors [124,187]. It is frost resistant, tolerant of drought, and requires full sunlight for maximum growth [122]. On favorable sites where past mismanagement has not been severe, the woodland may form a dense cover with trees 30 to 40 feet (9-12 m) tall. On drier sites spacing widens and tree size diminishes [120]. Old-growth or climax stands of singleleaf pinyon often occupy rocky hillslopes where the sparse understory will not carry fire [110]. At the northern end of its range, singleleaf pinyon is found primarily on south-facing slopes and outcrops of decomposed granite. At the southern end of its range it occurs only on north-facing slopes [59,121,151]. 

Elevational range of singleleaf pinyon is generally 3,280 to 9,186 feet (1000-2800 m). In Baja and parts of southern California it may be found below 3,280 feet (1000 m). The lower elevational limit in the high desert of the Great Basin is just above the elevation of the adjacent valleys, varying from 4,987 to 6,988 feet (1520-2130 m). The upper limit of singleleaf pinyon varies with climate and competing tree species, but it has been found as high as 10,000 feet (3050 m) in the White Mountains of California [124]. The frequency of monophylly was found to be inversely related to elevation in Zion National Park, with a narrow elevational range up to about 5,600 feet (1860 m); Colorado pinyon occurred at higher elevations and hybrids of the 2 species were distributed throughout the elevational ranges of both [75,127]. The relationship of monophylly to elevation was also observed in the New York Mountains in California and was interpreted as a variation of character along a moisture-temperature gradient [243], with Colorado pinyon occupying higher elevation sites and singleleaf pinyon lower sites [214].

Singleleaf pinyon is the most xeric pine in the United States. Its mean annual precipitation range is 8 to 18 inches (200-460 mm), with most precipitation falling December through April. Its mean annual air temperature is 50 degrees Fahrenheit (10 °C), ranging from 21 degrees Fahrenheit (-6 °C) in January to 86 degrees Fahrenheit (30 °C) in July [142]. Annual stem growth, needle length and percentage of double-needled fascicles all had significant positive correlations (p<0.05) with annual precipitation received prior to the completion of each year's growth [202]. Mature singleleaf pinyon is not shade tolerant; however, water rather than light is the limiting factor in survival and growth of this species [143]. Results of a Nevada study suggest that the ability of singleleaf pinyon to exist on a wide range of environmental conditions is not a function of variable ecophysiological responses, but an opportunistic response to the availability of resources and conditions suitable for growth [97].

Elevation and precipitation ranges are as follows:

State Elevation Precipitation References
Utah 2,690-9,700 feet (820-2960 m)  >12 inches (300 mm) [41,232]
Nevada 4,900-9,500 feet (1500-2900 m)  >12inches (300 mm) [41]
California 3,000-10,000 feet (900-3050 m)   [124,185,187]
Arizona 4,500-7,500 feet (1370-2280 m) 12-22 inches (300-560 mm)  
New Mexico 4,500-7,500 feet (1370-2280 m) 12-22 inches (300-560 mm) [190]
Idaho 5,500-7,400 feet (1675-2255 m)   [175]
Mexico 3,500- 6,600 feet (1070-2000 m) 12-24 inches (300-600mm) [120,168]

Pinyon-juniper woodlands occur on many types of soils and parent materials. Singleleaf pinyon typically grows on shallow, well-drained, low fertility soils, although it has been found on more productive soils as well [58,80,81]. Singleleaf pinyon in shallow soils tend to grow more slowly than those in deeper soils [215]. Surface soil pH usually is between 6.0 and 8.0 [80]. A study in the Great Basin found singleleaf pinyon was most common on granitic parent material (33% cover), followed by alluvial parent material (9.5% cover), and finally limestone parent material (0.3% cover). It was not found on quartzite or sandstone. Singleleaf pinyon was most commonly found on soils that contained 15-35% skeletal material by volume [89].

While singleleaf pinyon can grow on a wide variety of soil types and environmental conditions, composition and distribution of associated understory species may be driven by the plants' position relative to the tree crown [69], soil type [59,237], the seasonality and effectiveness of precipitation [234], and/or the distribution of nutrients [209]. Summerfield and others [191] found that soils supporting singleleaf pinyon stands in western Nevada commonly had mollic epipedons, argillic horizons, shallow depth to bedrock, mesic temperature regimes, and low available water capacities. These soils are well suited for producing woodlands, but have low potential for forage production. A study in the Great Basin in Nevada found that singleleaf pinyon was absent from sites with hydrothermally altered andesite parent material. Researchers concluded that the absence of singleleaf pinyon was more likely due to the absence of big sagebrush nurse plants than to substrate-induced nutrient limitations, since it was able to grow on this soil in the greenhouse [32,45,177]. 

SUCCESSIONAL STATUS:
The pinyon-juniper woodland is generally a climax vegetation type throughout its range, reaching climax about 300 years after disturbance [58], with an ongoing trend toward increased tree density and canopy cover and a decline in understory species over time [59,142,182,201,236]. Woodlands may also expand into adjacent grass and shrublands over time [29,201,236]. The woodland type often occurs in a mosaic, with trees occupying the stonier soils where fires spread poorly and competition from shrubs and grass is minimal [1]. Fire may have kept trees out of grasslands in the past [200], and it is debated whether the lack of fire is now allowing woodlands to invade true grasslands [81,82,165]. A variety of natural and anthropogenic processes can lead to changes in the spatial distribution of pinyon-juniper woodlands over time. Among these are 1) tree seedling establishment during favorable climatic periods, 2) tree mortality (especially seedlings and saplings) during periods of drought, 3) expansion of trees into adjacent grassland in response to overgrazing and/or fire suppression, and 4) removal of trees by humans [101], fire, or other disturbance episodes. Specific successional pathways after disturbance in singleleaf pinyon stands are dependent on a number of variables such as plant species present at the time of disturbance and their individual responses to disturbance, past management, type and size of disturbance, available seed sources in the soil or adjacent areas, and site and climatic conditions throughout the successional process.

A general successional pattern in pinyon-juniper after overstory removal may be as follows: grasses and forbs dominate for the first 10 years; shrubs are well established within 20 years and dominate at 30 years; between 10 and 20 years, tree seedlings appear, their presence becoming important after about 50 years; site is again woodland with low understory cover after 70 to 80 years [200,201]. A fire chronosequence study in the Great Basin followed this general pattern for grasses, forbs, and shrubs over time, although grass and shrub cover was maintained as late as 115 years following fire, and singleleaf pinyon had less than 10% cover 115 years after fire [186]. After fire disturbance, several successional pathways from annual to shrub dominance are possible, including initial postfire dominance by shrubs [61,69].

Successional stages in pinyon-juniper woodlands often follow the model of initial floristics, having the same species present in different amounts and dominance on the landscape over time [58,68]. Singleleaf pinyon may be present in early to mid-succession, but slow growth and establishment preclude early dominance [69,112]. Postfire succession may be to native species present in the preburn community or seed bank, or may be to invasive exotics such as cheatgrass that subsequently inhibit shrub and tree establishment [60,142]. Early successional stages following fire or tree harvesting are often dominated by several weedy annuals and sprouting shrubs. Disturbed sites may also see the establishment of non-sprouting shrubs [142]. Later successional stages from shrub to tree dominance have been studied in a few specific sites [5,60,69,203,205]. Variability in tree- and shrub-dominated communities complicates extrapolation of these results to sites of different growing conditions [196].

Once trees establish, they continually increase their dominance of the site. Trees start competing with the understory when they are approximately double the size of the shrub nurse plant [60], and can exclude the understory within a 100-year period [60,113]. The litter of singleleaf pinyon inhibits establishment of understory species through allelopathy, especially affecting Idaho fescue and Sandberg bluegrass, with cheatgrass and bottlebrush squirreltail being the least affected. An understory may be maintained by periodic fire, reducing tree density [60].

Because a number of variables may affect succession, following a standard successional model to predict stand development may be unwise [60]. Most successional projections come from stands that have been grazed [5,60,69], and response on grazed and ungrazed sites will be floristically different [61]. Seed reserves in soil under singleleaf pinyon may also decrease in number and species diversity from early to late succession [113]. Koniak [112] found that aspect and elevation influenced postfire community after fire in a Great Basin singleleaf pinyon stand, with north and east slopes supporting a higher cover and occurrence of shrubs, perennial grasses and perennial forbs, while south and west slopes generally had high cover and occurrence of annual forbs and annual grasses. Goodrich and others [77] provide a general overview of succession in pinyon-juniper woodlands, and its application to management for different uses based on successional stage.

SEASONAL DEVELOPMENT:
The growing season of singleleaf pinyon is affected by winter cold, summer heat, and severe water deficits [105]. Tree growth usually starts in April and ceases in September or October. During most of this time, trees depend on moisture stored by winter precipitation [142]. Male and female strobili emerge from buds formed the previous summer in late spring or early summer. Conelets emerge during this period of elongation and spread their scales. Staminate cones usually begin to shed pollen by mid-June, when the ovulate cones are receptive to pollination. Wind-borne pollen sifts into the ovules, and the pollinated conelet then closes its scales, sealing the pollen grains within. Cones develop slowly the 1st growing season, being 1/4th their mature size by September, and pass the winter this way. The following spring, the yearling cone starts to grow again, the pollen grains inside fertilize the egg cells, and the process of seed formation begins, about 1 year after pollination. Cones grow rapidly throughout the summer of the 2nd growing season, the seeds maturing in late summer. Cones mature and dry in early September and turn from green to brown. The scales part and the seeds are displayed in September to November, depending on environmental conditions and elevation [121,142].

FIRE ECOLOGY

SPECIES: Pinus monophylla
FIRE ECOLOGY OR ADAPTATIONS:
Because singleleaf pinyon occurs in a variety of habitat types with varying productivity, historic fire frequency is also varied. In the Great Basin, there is evidence of both frequent, low-severity fires carried by once-abundant perennial grasses, and less frequent, localized stand-replacement fires during extreme conditions. Fires burned in irregular patterns, producing a mosaic of burned and unburned landscape. On high-productivity sites where sufficient fine fuels existed, fires burned every 15 to 20 years, and on less productive sites with patchy fuels, fire intervals may have been in the range of 50 to 100 years or longer. Fire frequency in singleleaf pinyon communities varies with fuel loads and ignition source that, in turn, vary with habitat type, aspect, topography, stand history, and climatic conditions [85,165].

Singleleaf pinyon occurs in many xeric sites with infertile, shallow, rocky soils, where fires are infrequent and unpredictable and depend on exceptional rainfall years that lead to herbaceous growth sufficient to carry fire. In southern California the vegetation structure in singleleaf pinyon communities does not carry fire well, and fire return intervals of several hundred years are considered typical [187,227]. For example, singleleaf pinyon communities in the San Bernardino Mountains have experienced long-interval stand-replacement fires both before and during suppression with an estimated fire interval of 450 years, resulting in a mosaic of small scattered patches within uniform old-growth stands across the landscape [152,227]. Thin bark and lack of self pruning makes singleleaf pinyon very susceptible to intense fire [105], and the sharp ecotone between pinyon-juniper and mixed conifer communities may be due to the inability of singleleaf pinyon to withstand the frequency and intensity of surface fires in mixed-conifer communities [152].

FIRE REGIMES:
In mesic areas, north slopes, and canyon bottoms fires may have occurred at 15- to 90-year intervals, some through ignition by Native Americans, maintaining open or patchy stands [1]. This fire frequency favored older stands, since there is little recruitment and seedlings and saplings are more susceptible to fire. Older trees in open canopy structures are able to withstand surface fires that would have occurred with the fine fuel loads presumed to have been present before the huge influx of domestic livestock [85,197]. Higher densities of young trees have been observed at upper elevations, suggesting that singleleaf pinyon may not reach maturity on these sites because of susceptibility to periodic stand-replacement fires [115].

Burning in pinyon-juniper woodlands requires at least 600 to 700 lb/acre of fine fuel [58]. In the absence of fire and the presence of grazing, tree densities have increased and undergrowth is so sparse in many areas that surface fuels do not support fire [29,57,85,139,226]. Increased tree density and subsequent buildup of hot burning fuels causes a shift from low-severity to severe fires burning under extreme conditions of hot, dry weather and strong winds [1,85]. When stands are burned under these conditions, singleleaf pinyon tends to be eliminated from the site and recolonizes very slowly. In some areas, expansion of aggressive annual exotic grasses, such as cheatgrass, serve as fine fuels in a differently structured ecosystem, further altering the fire regime [111].

Fire return intervals for plant communities in which singleleaf pinyon occurs are summarized below. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [24]
basin big sagebrush Artemisia tridentata var. tridentata 12-43 [176]
mountain big sagebrush Artemisia tridentata var. vaseyana 20-60 [2,28]
Wyoming big sagebrush Artemisia tridentata var. wyomingensis 10-70 (40**) [225,245]
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus < 35 to < 100
desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100
cheatgrass Bromus tectorum < 10 [24]
curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [3,179]
mountain-mahogany-Gambel oak scrub Cercocarpus ledifolius-Quercus gambelii < 35 to < 100 
blackbrush Coleogyne ramosissima < 35 to < 100 
Arizona cypress Cupressus arizonica < 35 to 200 
western juniper Juniperus occidentalis 20-70 
creosotebush Larrea tridentata < 35 to < 100 
pine-cypress forest Pinus-Cupressus spp. < 35 to 200 
pinyon-juniper Pinus-Juniperus spp. < 35 [24]
Mexican pinyon Pinus cembroides 20-70 [155,195]
Colorado pinyon Pinus edulis 10-49 
Jeffrey pine Pinus jeffreyi 5-30
Rocky Mountain ponderosa pine* Pinus ponderosa var. scopulorum 2-10 
Arizona pine Pinus ponderosa var. arizonica 2-10 [24]
quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [24,84,145]
California oakwoods Quercus spp. < 35 
oak-juniper woodland (Southwest) Quercus-Juniperus spp. < 35 to < 200
canyon live oak Quercus chrysolepis <35 to 200 [24]
*fire return interval varies widely; trends in variation are noted in the species review
**mean

POSTFIRE REGENERATION STRATEGY [189]:
Tree without adventitious bud/root crown
Secondary colonizer (on-site or off-site seed sources)

FIRE EFFECTS

SPECIES: Pinus monophylla
IMMEDIATE FIRE EFFECT ON PLANT:
Thin bark and lack of self pruning make singleleaf pinyon very susceptible to intense fire [105]. Mature singleleaf pinyon can survive low-severity surface fires but is killed by more severe fires [1]. Most tree seedlings are killed by fire, but cached seeds may survive [35,63].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
The ability of singleleaf pinyon to survive fire is dependent on the size of the tree, intensity and severity of fire, and resulting damage to individual trees. Generally, as tree size increases, mortality decreases [115], and as fire severity increases, mortality increases.

Because mature singleleaf pinyon trees are short with open crowns, full of long-lived, highly flammable foliage, and do not self-prune their dead branches, individual trees are susceptible to fire [22]. On Nevada sites with prefire cover of 6 to 29% trees, 23 to 56% shrubs, and trace grasses and forbs, a spring prescribed fire killed all trees in all age classes  [228]. The effectiveness of tree kill depends on fire severity, which is largely determined by the amount of grass that carries the fire [158]. Low-severity fires in a mature pinyon-juniper stand will usually remove trees in the understory and a few overstory trees, while moderate-severity fires remove more of the overstory. In open stands with grass understories, trees less than 4 feet (1.2 m) tall (about 50 years old) are very susceptible to being killed by fire [22,27,58,85,245]. However, low-severity fire in open, pole-size stands may leave scattered individuals [22]. Trees taller than 4 feet (1.2 m) are difficult to kill unless there are heavy accumulations of fine fuel beneath the trees [22,85]. Fire scars have been observed on mature singleleaf pinyons, indicating survival [203], but are uncommon. This indicates that in some fire regimes, singleleaf pinyon is either killed outright or does not live long after being scarred because heart-rotting fungi enters the scar and hastens mortality [82].

Weise [230] found mortality of singleleaf pinyon varied with damage class and site after 2 wildfires in California in 1987. One year after wildfire, mortality of undamaged and slightly damaged trees differed between the 2 sites. It appears that singleleaf pinyon tolerates a small portion of crown scorch (<33%), while all trees with over 66% crown scorch and some crown consumption died within the 1st postfire year. Tree size did not appear to be a factor in initial survival, but may have been critical to long-term survival.

PLANT RESPONSE TO FIRE:
Reestablishment of singleleaf pinyon after fire requires seed survival or seed dispersal by birds or rodents into the burn area. Seedlings then require nurse plants (primarily sagebrush) for establishment, so initial postfire establishment may be delayed for 20 to 30 years until the establishment of the shrub layer [152]. Seedlings have been observed under the canopies of mature trees but they eventually die out unless the overstory is removed. Ward [228] found that singleleaf pinyon seedlings emerging under burned tree canopies died by the 2nd year. No singleleaf pinyon seedlings were recorded on prescribed burns for 5 years following fire in Nevada woodlands [63,69]. Moderate establishment was observed on 24-year-old burns, and singleleaf pinyon density was still low (5-10%) 115 years after fire in the Great Basin [186].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
The response of singleleaf pinyon to fire is a function of initial postfire damage and survival, postfire seed dispersal, and postfire succession of associated plant species. Response also varies by size, season, and type of burn [27,69]. Singleleaf pinyon grows very slowly, so the return to mature woodlands requires more than a century [186,227]. Where stands are closed or dense, the undergrowth declines significantly; fires become less and less common because fuels are scarcer [22] and require extreme conditions of hot, dry weather and strong winds to carry fire [1,85]. When stands are burned under these conditions, singleleaf pinyon tends to be eliminated from the site and recolonizes very slowly.

Perennial grass surface fires in mature pinyon-juniper woodlands result in light to moderate tree mortality and recovery to prefire conditions in less than 5 years, while woodlands with shrub understories commonly experience canopy fires that result in heavy tree mortality and require more than 100 years for recovery to prefire conditions [69,112,227].

Singleleaf pinyon seeds that are cached on the prefire site may survive [35]. Clark's nutcrackers have been observed caching seeds of whitebark pine in recently burned areas [212]; it is possible that pinyon seeds may also be cached in burn areas by these and other birds and rodents. On large burns, return of singleleaf pinyon may be slow due to greater distance from seed sources [12].

Wildfire and prescribed burning in singleleaf pinyon stands result in an initial increase in understory vegetation [5,13,69,200]. The species represented, their relative coverage, and patterns of succession are variable. Early (1-year) postfire succession in the Great Basin is generally dominated by annual and perennial forbs, giving way to shrubs and annual grass dominance in mid-succession (15-17 years), and to tree, shrub, and perennial grass dominance in late succession (22-60 years) [5,112]. In the San Bernardino Mountains in California, postfire succession to mature woodlands proceeds over a span of a century, beginning with the colonization of shrubs that increase in cover and density for 30 to 50 years and are joined by singleleaf pinyon recruits at 25 to 40 years. After 50 years, increases in singleleaf pinyon densities accompany a decline in the shrub layer, and mature woodlands with sparse understories return at 100 to 150 years [227]. Pinyon generally has a lower survival rate and slower establishment than juniper after a fire, but given time and more mesic site conditions, pinyon may dominate after about 60 years [25,27,203].

FIRE MANAGEMENT CONSIDERATIONS:
Prescribed fire may be used in singleleaf pinyon stands to reduce overstory trees and reestablish understory species in the successional cycle [25,60,107,109], to prevent invasion of trees into adjacent grass and shrublands [22,108,186], to reduce the effects of allelopathic chemicals in the litter [60,239,241], to improve big game winter range (mule deer, bighorn sheep, pronghorn, and elk) [108], to create mosaics of woodland and openings [165], to remove slash [58], to reduce fuel loads [85], and to control new tree regeneration [165]. Some primary considerations for prescribed burning in singleleaf pinyon stands are the number and species of trees present, nature of the understory vegetation, and weather conditions [58]. These elements determine if and how a stand will burn, and may help predict the ecological effects of burning.

Burning in singleleaf pinyon woodlands requires an understory adequate to carry a ground fire or extreme conditions to carry a crown fire. Pinyon-juniper stands most likely to burn have small scattered trees with abundant herbaceous fuel between them, or have a high density of mature trees capable of carrying crown fire during dry, windy conditions. More widely spaced trees with little understory are unlikely to burn [22]. It usually requires 600 to 700 pounds/acre (672-784 kg/ha) of fine fuels, or 20% sagebrush with 300 pounds/acre (336 kg/ha) of fine fuels, to carry a fire in the Great Basin [242]. Many singleleaf pinyon stands in the Great Basin are not productive enough to yield this amount of fine fuel, particularly if they have an overstory of trees [22,26]. Periodic growth of annual grasses in response to favorable weather conditions has become the primary fuel source in some rangelands that previously experienced low fire frequencies [111].

Research in the Great Basin suggests prescribed burning is likely to succeed on sites with scattered trees (9-23% cover) and in dense (not closed) stands (24-35% cover) [25]. Blackburn and Tueller [15] found that scattered and dense stands with 9 to 35% canopy cover burned well, whereas closed stands (>35% cover) were difficult to burn out of fire season. Many pinyon-juniper woodlands have advanced to a stage of succession where prescribed fire is no longer a viable option due to low fuel loads and large trees in open sites that are susceptible to weed invasion and erosion. Since pinyon burns more readily than juniper, stands burn better as the proportion of pinyon to juniper increases [22].

Crown fuels are typically around 3.6 tons/acre (8.1 t/ha) for foliage and 1.8 tons/acre (4.0 t/ha) for 0- to 0.25-inch branchwood in Colorado pinyon in Arizona [165]. Fuel biomass estimates in singleleaf pinyon using aerial photography are presented by Meeuwig and others [144]. Heat and ash content values, physical properties of woody fuel particles, and fuel bed characteristics for singleleaf pinyon in the Sierra Nevada are available [221,222].

A detailed site evaluation is critical in predicting the successional effects of fire in singleleaf pinyon stands. Qualitative prediction of postfire response is possible when we consider aspect, elevation, and the known response of individual species in the prefire community, but accurate predictions are difficult because of unknown soil seed reserves, species immigration potential, and postfire weather [60,65,69]. A low increase in forage production may be expected in sites that have greater than 20% canopy coverage of trees, while stands with fewer trees and high amounts of perennial grass cover respond quickly. Stands with high levels of tree coverage are often invaded by annuals following fire if perennials do not quickly occupy the site [26]. Response on grazed and ungrazed sites will be floristically different [61]. Current conditions in many pinyon-juniper stands may have little understory ground cover, seed production and soil seed reserves [60]. Burning a site with few desirable understory species may worsen the ground cover situation and only serve to destroy valuable wood [165]. Trees may be eliminated for 50 years [5], and it may take over 300 years for a climax stand to establish [55]. Prescribed burning to remove slash may damage residual trees and kill seedlings, sterilize soils where slash is piled, and remove valuable nutrients form the site [58]. Postfire seeding appears to be detrimental to many naturally occurring species [112].

Lack of native understory vegetation and presence of cheatgrass or other invasive plants within or adjacent to stands creates new postfire community types following fire in singleleaf pinyon communities [26,173]. Seeding burns with native species may be the only means of restoring the grass successional stage to many overgrazed woodlands, and has had variable success in the past [26,61,173]. Where understory is already composed of annual species, fire may only increase the cover of annuals and thus increase the fire hazard, resulting in more frequent fires that will prevent reestablishment of shrubs and trees [26]. For example, stands with good growth of cheatgrass are at higher risk for large fires that will likely result in increased yields of cheatgrass [12,22]. Several large fires in singleleaf pinyon stands in southern California and a reburn several years after a crown fire suggest that the spread of cheatgrass may be responsible for carrying these fires. This could have serious negative effects by shortening the fire return interval and thereby preventing tree establishment [187]. Severe burning in singleleaf pinyon communities in the San Gabriel and San Bernardino mountains of southern California can convert these woodlands to chaparral [86]. Burning areas invaded by cheatgrass with fires hot enough to remove litter and any stored seed may give managers a window of opportunity the 1st year after burning to plant more desirable perennial species [58].

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