Common (Translated Scientific) Name: Singleleaf Pinyon - Two-needle Pinyon - Juniper species Woodland & Scrub Division
Colloquial Name: Western North American Pinyon - Juniper Woodland & Scrub
Hierarchy Level: Division
Type Concept: This division is composed of woodland, savanna, and scrub characterized by an open to closed tree and shrub canopy of Pinus monophylla, Pinus edulis, Juniperus occidentalis, Juniperus osteosperma, Juniperus monosperma, or Juniperus scopulorum. Typically one, or combinations of two, of these species dominate the canopy. Stands are typically open-canopied (10-30% cover), but closed-canopy conditions with a sparse understory are not uncommon. Pinyon and juniper stands may occur as persistent woodlands, open savannas, and wooded shrublands. Persistent woodlands occur where climate substrates support pinyon pines and junipers; typically with infrequent wildfire. Open savannas occur where landform and substrates support both trees and grasses, and where more frequent surface fires were likely the historical norm. Wooded shrublands occur where climate and soils support shrublands, but where trees increase in abundance under favorable climate and limited disturbance, and decrease during droughts and following disturbance. Understory layers are variable and may be dominated by shrubs, graminoids, or be absent, especially on rocky substrates. Except for in the extreme eastern and southern portion of its range, shrub layers are frequently dominated by Artemisia tridentata, which in places can form a moderately dense shrub canopy. Other common associated shrub species include Arctostaphylos patula, Artemisia arbuscula, Artemisia bigelovii, Artemisia nova, Atriplex canescens, Cercocarpus intricatus, Cercocarpus ledifolius (in tree or shrub form), Cercocarpus montanus, Chrysothamnus viscidiflorus, Coleogyne ramosissima, Ericameria nauseosa, Fraxinus anomala, Fraxinus cuspidata, Glossopetalon spinescens, Krascheninnikovia lanata, Purshia stansburiana, Purshia tridentata, Quercus gambelii, Quercus turbinella, Quercus x pauciloba, Ribes cereum, Rhus trilobata, Tetradymia spp., Yucca baccata, and Yucca glauca. The most frequent succulents are Cylindropuntia imbricata, Opuntia phaeacantha, and Opuntia polyacantha. The herbaceous layer may be sparse to dense depending on overstory density, substrate, landscape position, and disturbance history, with the densest graminoid layer in open-tree or shrub savanna. Common graminoid associates include Bouteloua curtipendula, Bouteloua gracilis, Carex filifolia, Festuca idahoensis, Hesperostipa comata, Leymus cinereus, Leymus salinus, Pleuraphis jamesii, Pseudoroegneria spicata, Poa fendleriana, and Poa secunda. Forb species may be diverse but typically have low canopy cover values, and can include Astragalus spp., Castilleja integra, Cryptantha cinerea var. jamesii, Erigeron divergens, Eriogonum jamesii, Hymenopappus filifolius, Ipomopsis multiflora, Mentzelia spp., Penstemon spp., and Petradoria pumila.
Diagnostic Characteristics: Structurally complex, low woodland, scrub, and savanna with pinyon pine and or juniper dominance in uppermost canopy, especially Pinus monophylla, Pinus edulis, Juniperus occidentalis, Juniperus osteosperma, Juniperus monosperma, or Juniperus scopulorum. The understory is dominated by either shrub or grass species of cool-temperate affinity.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: This division occurs over a very broad biogeographic range with many diagnostic species. The southern end of the range of this division occurs over a very broad transition zone with 1.B.1.Nd Madrean-Balconian Forest & Woodland Division (D060) which is distinguished by the presence of other Madrean tree species, such as Juniperus coahuilensis, Juniperus deppeana, Juniperus pinchotii, Pinus cembroides, Pinus discolor, or evergreen oaks such as Quercus emoryi, Quercus grisea, or Quercus mohriana along with Madrean grasses, forbs, and characteristically warm-temperate subshrubs such as Nolina spp. and Dasylirion spp. Classifying open juniper savanna stands from grasslands with scattered trees is challenging, especially where juniper trees are invading grasslands. The invasive juniper savanna stands are typically characterized by younger, shorter (<3 m tall), pointed-crown trees of lower density.
Similar NVC Types:
D039 North American Warm Desert Scrub & Grassland, note:
D060 Madrean-Balconian Forest & Woodland, note:
D194 Rocky Mountain Forest & Woodland, note:
Physiognomy and Structure: This broadly defined evergreen division is composed of a woodland, savanna (or open woodland), and scrub structure. Stands are typically short (2-10 m tall), with an open to closed (10-60% cover), evergreen needle-leaved or scale-leaved or broad-leaved, sclerophyllous tree and/or shrub canopy. The understory is variable with lush grass cover and occasionally scattered shrubs in the savanna stands to a sparse to dense short-shrub layer and/or herbaceous layer in woodland stands. This division encompasses savanna that has widely spaced, short (2-10 m tall), mature (>150-year-old) trees with a moderately dense to dense herbaceous layer dominated by perennial graminoids. On extremely xeric sites, diagnostic juniper and pinyon trees species may only attain 2 m in height and have a more shrub form. However, Juniperus occidentalis-dominated stands have two different tree canopy structures: (1) an old-growth woodland with large, fairly well-spaced trees with rounded crowns, and (2) relatively young, often dense junipers trees with pointed crowns. Cover of understory species sharply declines when tree canopy cover exceeds 40% (Young et al. 1982). Many of the tree savannas have a sparse shrub layer present. Herbaceous layers are variable depending on the density of woody canopy, substrate, landscape position, and disturbance history. Perennial graminoids typically dominate most herbaceous layers with most species individually contributing low cover.
Floristics: This division includes woodland, savanna (or open woodland), and scrub characterized by an open to closed tree and shrub canopy of Pinus monophylla and/or Pinus edulis, Juniperus occidentalis, Juniperus osteosperma, Juniperus monosperma, or Juniperus scopulorum. Typically one, or combinations of two, of these species dominate the canopy. In the Mohave Desert mountains, Yucca brevifolia may be an associate of Pinus monophylla. Shrub species include Arctostaphylos patula, Artemisia arbuscula, Artemisia bigelovii, Artemisia nova, Artemisia tridentata, Atriplex canescens, Cercocarpus intricatus, Cercocarpus ledifolius (in tree or shrub form), Cercocarpus montanus, Chrysothamnus viscidiflorus, Coleogyne ramosissima, Ericameria nauseosa, Fraxinus anomala, Fraxinus cuspidata, Glossopetalon spinescens, Krascheninnikovia lanata, Purshia stansburiana, Purshia tridentata, Quercus gambelii, Quercus turbinella, Quercus x pauciloba, Ribes cereum, Rhus trilobata, Tetradymia spp., Yucca baccata, Yucca brevifolia, and Yucca glauca. Shrub species more characteristic of 1.B.1.Nc ~Californian Forest & Woodland Division (D007)$$, such as Juniperus californica, Quercus chrysolepis, and/or Quercus john-tuckeri, may occur near the southwestern limits of the range. The most frequent succulents are Cylindropuntia imbricata, Opuntia phaeacantha, and Opuntia polyacantha. The herbaceous layer may be sparse to dense depending on overstory density, substrate, landscape position, and disturbance history, with the densest graminoid layer in open-tree or shrub savanna. Common graminoid associates include Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Carex filifolia, Hesperostipa comata, Hesperostipa neomexicana, Festuca idahoensis, Leymus cinereus (= Elymus cinereus), Leymus salinus, Muhlenbergia pauciflora, Pleuraphis jamesii, Poa fendleriana, Poa secunda, Pseudoroegneria spicata, and the non-native invasive annual Bromus tectorum. Forb species may be diverse but typically have low canopy cover values, and can include Astragalus spp., Castilleja integra, Cryptantha cinerea var. jamesii (= Cryptantha jamesii), Eriogonum jamesii, Erigeron divergens, Hymenopappus filifolius, Ipomopsis multiflora, Mentzelia spp., Penstemon spp., and Petradoria pumila.
Dynamics: Key dynamic processes are drought, fire, herbivory, and insect/disease outbreaks. Characteristic Pinus spp. and Juniperus spp. are relatively short (generally <15 m tall), shade-intolerant, drought-tolerant, slow-growing, long-lived trees (especially Juniperus osteosperma can reach 650 years old). Pinus spp. are non-sprouting and may be killed by fire. The effect of a fire on these stands is largely dependent on the tree height and density, fine fuel load on the ground, weather conditions and season. Large trees generally survive unless the fire gets into the crown due to heavy fuel loads in the understory. Fire acts to open stands, increase diversity and productivity in understory species, and create a mosaic of stands of different sizes and ages across the landscape while maintaining the boundary between woodlands and adjacent shrublands or grasslands.
As modeled by Landfire, this division is generally characterized by a spectrum of fire regimes, including frequent non-lethal fires, mixed-severity mosaic fires (mean FRI of 50-200 years), and very infrequent replacement fires (mean FRI of 200-500 years). Surface fire was likely the most frequent in savannas. Frequently, fire spreads from adjacent vegetation. Severe climatic events occurring during the growing season, such as frosts and drought, are thought to limit the distribution and density of pinyon and/or juniper stands to relatively narrow altitudinal belts on mountainsides and foothills. Weather-related stress thins trees in more closed stands. Insects/disease has a similar effect, but with a greater frequency in closed stands (mean return interval of 100 years) than open ones (mean return interval of 1000 years). Competition from grasses and older trees in late open stands is also included as a disturbance that maintains open woody canopies.
Environmental Description: Stands of this division in the Colorado Plateau, Great Basin, and Mohave Desert mountains occur between 1500 and 2600 m elevation on warm, dry sites of lower mountain slopes, hills, mesas, plateaus, ridges, and more recently on basins and flats where trees are expanding into semi-desert grasslands and steppe. Substrates are variable, but are generally shallow, cobbly, gravelly, or sandy loams to clay loam or clay. Parent materials are variable. Juniper stands in the Columbia Plateau range from under 200 m elevation along the Columbia River in central Washington to over 1500 m. In central Oregon, they occur on all aspects and slope positions. Stands of this division also occur in dry mountains and foothills in southern Colorado south into central New Mexico, extending east into the plains on breaks in the southwestern Great Plains. They are found in dry sites in lower slopes of mountains, plateaus and foothills and on limestone and shale breaks in the plains. In this portion of the range, stands are found at elevations from 1370 to 2900 m. Climate is cool-temperate. Severe weather events occurring during the growing season, such as frosts and drought, are thought to limit the distribution of pinyon-juniper woodlands to relatively narrow altitudinal belts on a given mountainside, and particularly influence the proportion of pinyon trees relative to juniper.
Geographic Range: This division generally occurs just above semi-desert shrublands and grasslands or shortgrass prairies and below montane forest vegetation throughout the semi-arid Intermountain West and western Great Plains of North America.
States/Provinces: AZ, CA, CO, ID, MT, NM, NV, OK, OR, TX, UT, WA, WY
|US Forest Service Ecoregions (2007)|
Confidence Level: High
Confidence Level Comments:
Concept Author(s): Faber-Langendoen et al. (2015)
Author of Description: P.J. Comer and C. Lea
Acknowledgements: J. Triepke
Version Date: 26Oct2015
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