Common (Translated Scientific) Name: Douglas-fir - Western Hemlock - Subalpine Fir Forest & Woodland Division
Colloquial Name: Rocky Mountain Forest & Woodland
Hierarchy Level: Division
Type Concept: This division is composed of conifer forests, woodlands and savannas of the lower montane to subalpine zones of the continental temperate climates of western North America. These communities occur in the interior Pacific Northwest, the southern Rocky Mountains, and extend east of the Continental Divide into the northwestern Great Plains region, and south at high elevations of the Sierra Madre Mountains. Strong diagnostic conifers (needle-leaved trees) are Abies concolor, Abies lasiocarpa, Abies religiosa, Juniperus spp. (Juniperus osteosperma, Juniperus scopulorum), Larix lyallii, Larix occidentalis, Picea engelmannii, Picea x albertiana, Picea pungens, Pinus albicaulis, Pinus aristata, Pinus contorta var. latifolia, Pinus flexilis, Pinus hartwegii, Pinus longaeva, Pinus ponderosa (var. brachyptera, var. ponderosa, var. scopulorum), and Pseudotsuga menziesii var. glauca. Other conifers that are common in these forests and woodlands are Abies grandis, Thuja plicata, Tsuga heterophylla, and Tsuga mertensiana. Deciduous hardwoods (broad-leaved deciduous trees) are infrequent and include Acer grandidentatum, Betula papyrifera, and Populus tremuloides. Savannas and woodlands occur commonly in dry climates and on dry sites, and in some cases may be dominated by short trees in a "scrub woodland" form. Woodlands also predominate at high elevations, and at the highest elevations stands are composed of tree clumps or ribbons, with intervening grasslands or shrublands. Evergreen conifers dominate stands overall. Deciduous hardwoods can occur, intermingling with conifers, and deciduous conifers can dominate some areas. Stands can be composed of just one tree species, but more often are of mixed composition, sometimes of a diverse mix. Shrub and herb layers vary widely. Sometimes the shrub layer is dominant, with tall or short broad-leaved deciduous or needle-leaved evergreen shrubs; in other cases, perennial forbs and graminoids (grasses or sedges) are the predominant growth forms. Nonvascular species (mosses, liverworts, lichens, fungi, or soil cryptogams) also vary considerably in abundance, but many forests have a high cover of mosses.
The climate is cool temperate and continental, although many areas are influenced somewhat by Pacific maritime air masses. Temperature regimes vary considerably across the range, and between seasons. Precipitation ranges from 25-240 cm annually. All areas receive winter snow, but winter rain is also possible in most areas. In many areas, a seasonal drought period occurs. In most of the range, more arid grassland climates occur at elevations below this type and alpine tundra occurs at elevations above. Elevations range considerably. Landforms are variable and can include canyons, plateaus, draws, benches, hills, mesas, rolling plains, cinder cones, ravines, ridgetops, shoulders, sideslopes and toeslopes. Slopes can be gentle to extremely steep. Bedrock geology includes volcanic, intrusive, metamorphic, sedimentary and ultramafic rocks. Fractured rock, colluvium, and eolian materials are common substrates. Glacially-derived substrates are typical in mountainous and northern areas, and volcanically-derived substrates are common in central and southern areas. Fire is the predominant natural disturbance factor of this type. There is a strong correlation between climate and the fire regime: drier climates are dominated by fire-dependent vegetation with stand-maintaining fires common and/or short interval stand-replacing fires (as short as 30-50 years), whereas wetter climates are characterized by long fire-return intervals for stand-replacing fire events (up to 500 years or more). In recent times, fire suppression has changed the fire dynamics of natural forests. Other natural disturbance agents are insect outbreaks, disease, occasional windthrow, and avalanches. Forest harvesting is also a major factor over most of the type.
Diagnostic Characteristics: Forests, woodlands and savannas found in the lower montane to subalpine zones of the interior Pacific Northwest, southern Rocky Mountains, and extending east into the northwestern Great Plains regions. Strong diagnostic conifers are Abies concolor, Abies lasiocarpa, Juniperus spp. (Juniperus osteosperma, Juniperus scopulorum), Larix lyallii, Larix occidentalis, Picea engelmannii, Picea x albertiana hybrids, Picea pungens, Pinus albicaulis, Pinus aristata, Pinus contorta var. latifolia, Pinus flexilis, Pinus longaeva, Pinus ponderosa (var. brachyptera, var. ponderosa, var. scopulorum), and Pseudotsuga menziesii var. glauca. Associated conifers common in this division are Abies grandis, Thuja plicata, Tsuga heterophylla, and Tsuga mertensiana. Deciduous hardwoods include Acer grandidentatum, Betula papyrifera, and Populus tremuloides. Some strong diagnostic understory species are Calamagrostis rubescens, Vaccinium myrtillus, Vaccinium membranaceum, Vaccinium scoparium, and Xerophyllum tenax.
Rationale for Nominal Species or Physiognomic Features:
Classification Comments: In the western range of this division, where it abuts 1.B.2.Nd Vancouverian Forest & Woodland Division (D192), forests and woodlands are divided between the two divisions based on the predominance of species characteristic of each division. As there is overlap between the two in some tree species such as Abies grandis, Pinus contorta, Pinus ponderosa, Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla, understory species are helpful in distinguishing them. Understory species such as Gaultheria shallon, Rubus spectabilis, Mahonia nervosa, Polystichum munitum, and Rhytidiadelphus loreus are found in D192, whereas Vaccinium membranaceum, Vaccinium scoparium, Calamagrostis rubescens, Xerophyllum tenax, and Pleurozium schreberi predominate in D194. In addition, there are variety differences in several of the tree species: for Pinus contorta, var. latifolia is this found in this division (D194), whereas var. contorta occurs in D192. For Pseudotsuga menziesii, var. glauca occurs in D194, and var. menziesii in D192, although some var. menziesii occurs in the western edge of this division. Pinus ponderosa has several varieties in this division, var. brachyptera, var. ponderosa, and var. scopulorum, whereas var. benthamiana occurs in D192. Note that a recent paper describes five subspecies for Pinus ponderosa; in that treatment (Callaham 2013), ssp. critchfieldiana is the Pacific subspecies.
In the north, this division abuts forests and woodlands of 1.B.4.Na North American Boreal Forest & Woodland Division (D014). Again, forests and woodlands are divided between the two divisions based on the predominance of species characteristic of each division. D014 is characterized by Picea glauca, instead of Picea glauca x engelmannii or Picea engelmannii of this division. Pseudotsuga menziesii extends into northern forests of this division, on warm-aspect sites but is absent from D014. Larix laricina is of increasing importance in D014. In addition, many of the understory species that are common in the northern forests of D194, e.g., Vaccinium membranaceum, Oplopanax horridus, and Gymnocarpium dryopteris, are mostly absent from the D014. Ledum groenlandicum, Vaccinium vitis-idaea, Arctostaphylos rubra, and Dasiphora fruticosa ssp. floribunda are more prevalent in upland forests in D014.
The southernmost forests of this division occur at upper elevations of the Sierra Madre Mountains. These conifer-dominated subalpine forests occur at elevations above 1.B.1.Nd Madrean-Balconian Forest & Woodland Division (D060), which includes Madrean species of pinyon, juniper and oak, or conifer-oak mixed forests. The characteristic conifer species of this division are Abies concolor, Abies guatemalensis, Abies religiosa, Pinus hartwegii, and/or Pinus montezumae.
Similar NVC Types:
D060 Madrean-Balconian Forest & Woodland, note: "includes warmer climate evergreen, conifer or broad-leaved forests, woodlands or savannas of Madrean species of pinyon, juniper and oak, or conifer-oak mixed forests that occur to the south of D194."
D010 Western North American Pinyon - Juniper Woodland & Scrub, note:
D007 Californian Forest & Woodland, note: "includes warmer climate forests, woodlands and savannas dominated by oak and conifer species that occur south and west of D194."
D014 North American Boreal Forest & Woodland, note:
D192 Vancouverian Forest & Woodland, note: includes predominately conifer forests and woodlands of temperate maritime (coastal) areas that occur to the west of D194.
D195 Rocky Mountain-Great Basin Montane Flooded & Swamp Forest, note: includes floodplain and swamp forests over the range of D194.
Physiognomy and Structure: The communities of this type are mostly forests and woodlands but include savannas and tree islands. Savannas and woodlands occur commonly in dry climates and on dry sites, and in some cases may be dominated by short trees in a "scrub woodland" form. Woodlands also predominate at high elevations, and at the highest elevations stands are comprised of tree clumps or ribbons, with intervening grasslands or shrublands. Evergreen conifers dominate stands overall. Deciduous hardwoods can occur, intermingling with conifers stands, and deciduous conifers can dominate some areas. Stands can be composed of just one tree species, but more often are of mixed composition, sometimes of a diverse mix. Shrub and herb layers vary widely, with tall or short deciduous or evergreen shrubs dominating the undergrowth, or in some cases with few or no shrubs, and perennial forbs, grasses or sedges are the predominant growth forms. Nonvascular species (mosses, liverworts, lichens, fungi, or soil cryptogams) also vary considerably in abundance, but many forests have a high cover of mosses.
Floristics: The forests and woodlands of this type include characteristic western North American conifers (needle-leaved trees) such as Abies grandis, Abies lasiocarpa, Larix occidentalis, Picea engelmannii, Pinus contorta var. latifolia, Pinus flexilis, Pinus ponderosa, Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla. Many of these species are wide-ranging but occur in specific environments influenced by climate, site, and historical conditions.
Pinus ponderosa forests and woodlands occur in dry climatic areas and on dry rocky sites or warm aspects over much of the central and southern range. Pinus ponderosa often occurs in pure stands, but can also occur as mixed stands with other conifer or hardwood species, e.g., Pseudotsuga menziesii in slightly moister climates, or Quercus macrocarpa in the northwestern Great Plains. The understory of Pinus ponderosa woodlands is varied and can be dominated by broad-leaved shrubs, e.g., Amelanchier alnifolia, Physocarpus malvaceus, Purshia tridentata, Symphoricarpos albus, or grasses, e.g., Festuca idahoensis and Pseudoroegneria spicata. Pinus ponderosa can also occur in savannas mixed with grasslands or big sagebrush steppe.
The limber pine - juniper woodlands dominated by Pinus flexilis, Juniperus osteosperma, or Juniperus scopulorum are included in this type, as are woodlands or "savannas" of the deciduous conifer Larix occidentalis.
Pseudotsuga menziesii is common in forests of this division. It can dominate many dry climate areas and sites, either in pure stands or mixed with Pinus ponderosa, Pinus contorta, or Larix occidentalis. The understory can be shrubby or grassy, and sometimes dominated by mosses or lichens. A variety of shrubs occur in these stands, such as Acer glabrum, Juniperus communis, Physocarpus malvaceus, Symphoricarpos albus, and Spiraea betulifolia. Graminoids are common, e.g., Calamagrostis rubescens, Carex geyeri, and forbs are variable, but typical taxa include Arnica cordifolia, Osmorhiza berteroi, Thalictrum occidentale, and species of many other genera, including Erigeron, Fragaria, Lathyrus, Lupinus, Penstemon, and Vicia. Pseudotsuga menziesii can persist in more mesic stands as a long-lived seral species.
Mesic conifer forests of the lower montane regions are characteristically mixed stands dominated by two or more of Abies grandis, Pseudotsuga menziesii, Thuja plicata, Tsuga heterophylla, Larix occidentalis, and Pinus contorta. Other conifers that often comprise part of the stand are Abies lasiocarpa, Picea engelmannii, Picea glauca x engelmannii, and Pinus monticola. Deciduous hardwood species such as Populus tremuloides or Betula papyrifera also occur, but typically are not dominant. These stands typically have a well-developed shrub and/or forb understory, as a result of the more mesic conditions, but can be sparse due to a dense canopy. Common shrubs are Acer glabrum, Amelanchier alnifolia, Paxistima myrsinites, Rubus parviflorus, Spiraea betulifolia, Symphoricarpos albus, Taxus brevifolia, and Vaccinium membranaceum. Oplopanax horridus occurs in depressional areas with high water tables. Composition of the herbaceous layer reflects local climate, site, and degree of canopy closure and can include Adenocaulon bicolor, Aralia nudicaulis, Clintonia uniflora, Cornus canadensis, Goodyera oblongifolia, Linnaea borealis, Tiarella trifoliata, Viola orbiculata, and Xerophyllum tenax. Graminoids are generally only a very minor component. Ferns and fern allies form an important component of the understory on moist sites and commonly include Athyrium filix-femina, Dryopteris filix-mas, Equisetum spp., and Gymnocarpium dryopteris. A dense moss layer often forms on the forest floor, particularly in northern forests.
Northern forests of this macrogroup are dominated by Picea x albertiana (= Picea engelmannii x glauca), Abies lasiocarpa, and/or Pinus contorta, with Pseudotsuga menziesii occurring in warmer areas and on warm sites. The deciduous hardwood species Populus tremuloides and Betula papyrifera commonly occur, dominating forests near settlements and around agriculture areas. Picea mariana sometimes occurs in these forests. These forests are transitional between temperate and boreal forests. The understory is similar to that of other mesic conifer forests of this division but includes some northern species such as Rosa acicularis, Lonicera involucrata, Viburnum edule, Rubus pubescens, and Galium boreale.
Abies lasiocarpa - Picea engelmannii forests and woodlands characterize upper montane to subalpine zones over much of the range of this division. Pinus contorta is often present and can dominate dry climate areas; other associated tree species are Larix lyallii, Pinus albicaulis, Pinus aristata, Pinus flexilis, Pinus longaeva, Populus tremuloides, and Tsuga mertensiana. Canopies can be mixed or dominated by a single species. Shrub species are highly variable, and typically are cold-deciduous (sometimes evergreen), including Lonicera utahensis, Ribes inerme, several Vaccinium spp. (Vaccinium membranaceum (= Vaccinium globulare), Vaccinium myrtillus, Vaccinium scoparium), Ledum glandulosum, Menziesia ferruginea, Rhododendron albiflorum, and Phyllodoce empetriformis. Associated herbaceous species are especially diverse given the wide elevational and latitudinal range of these forests, with alpine species occurring near the upper treeline and montane and subalpine species below. Mesic stands include herbaceous species such as Clintonia uniflora, Eucephalus engelmannii (= Aster engelmannii), Gymnocarpium dryopteris, Heracleum maximum, Luzula glabrata var. hitchcockii, Pedicularis racemosa, Rubus pedatus, Senecio triangularis, Tiarella spp., Valeriana occidentalis, Valeriana sitchensis, and Xerophyllum tenax. Drier sites close to the alpine might include xeric graminoids, such as Calamagrostis purpurascens, Festuca arizonica, Festuca idahoensis, and Trisetum spicatum.
Mid-elevation forests and woodlands of the southern Rocky Mountains are characterized by Abies concolor, Juniperus scopulorum, Pinus ponderosa, Pseudotsuga menziesii, and Picea pungens. The deciduous Populus tremuloides or Acer grandidentatum are early-seral species that may be codominant in some stands. Other conifers that may be present include Abies lasiocarpa, Picea engelmannii, Pinus contorta, Pinus edulis, and Pinus flexilis. In the southernmost range, associated trees may include Pinus strobiformis and Juniperus deppeana.
High montane (subalpine) forests of the Sierra Madre Mountains, characterized by the conifers Pinus hartwegii or Abies religiosa, are included in this division. Associated trees include Abies concolor, Abies guatemalensis, Alnus firmifolia, Cupressus spp., Pinus montezumae, Pseudotsuga menziesii, and Quercus laurina.
Dynamics: Fire is the predominant natural disturbance factor of forests and woodlands of this division. There is a strong correlation between climate and the fire regime: drier climates are dominated by fire-dependent vegetation with stand-maintaining fires common and/or short interval stand-replacing fires (as short as 30-50 years), whereas wetter climates are characterized by long fire-return intervals for stand-replacing fire events (up to 500 years or more). In recent times, fire suppression has changed the fire dynamics of natural forests. Other natural disturbance agents are insect outbreaks, disease, occasional windthrow, and avalanches. Forest harvesting is also a major factor over most of the type.
Forests and woodlands of very dry climates and sites had historic fire regimes characterized by frequent, low-intensity surface fires that maintained relatively open stands of a mix of fire-resistant species, mostly Pinus ponderosa but including Pseudotsuga menziesii in some circumstances. Fire maintained the open canopies characteristic of savannas and open woodlands of these species. Mature trees can survive low-intensity surface fires. With human settlement and subsequent fire suppression, stands have become denser. Presently, many occurrences contain understories of more shade-tolerant species, as well as a greater density of younger cohorts. These altered structures have affected fuel loads and altered fire regimes. Presettlement fire regimes were primarily frequent (5- to 15-year return intervals), low-intensity surface fires triggered by lightning strikes or deliberately set by Native Americans. With fire suppression and increased fuel loads, fires are now less frequent and often become intense crown fires, which can kill mature trees. The result is a mixed-severity fire regime for many Pinus ponderosa and Pseudotsuga menziesii stands as stand-replacing fires are becoming more common.
Forests in dry to moist climates are characterized by stand-replacing fires with a variable return interval, ranging from about 50-150 years. Many of the important tree species in these forests are fire-adapted (e.g., Populus tremuloides, Pinus ponderosa, Pinus contorta) or fire-tolerant (e.g., Pseudotsuga menziesii) or given the right conditions, regenerate well after fire (e.g., Picea glauca x engelmannii). Other species, e.g., Abies spp. (Abies concolor, Abies grandis, Abies lasiocarpa), Thuja plicata, or Tsuga heterophylla, are not fire-adapted, but are shade-tolerant and become more prevalent in stands over time, if undisturbed, as the early-seral species die off. Establishment after fire is influenced by availability of seed source or other propagules, e.g., live aspen roots, of the various species in the area. Landscape and site position influence fire behavior as well as regeneration. The pattern of forest types and stand ages on a landscape is the result of the combined influence of seed source, fire behavior, and site conditions.
Forests in wetter climates tend to have long fire-return intervals, ranging from 150 to over 500 years for stand-replacing fires. Gap dynamics are important in older stands and pests and pathogens play a greater role in stand mortality. Thuja plicata - Tsuga heterophylla forests are an example of these forests; they can develop into very old forests with large, tall trees. Picea engelmannii - Abies lasiocarpa forests can also be very old. Although Abies lasiocarpa is not long-lived, it is very shade-tolerant and regenerates well in these upper elevation stands.
Insect pests, such as mountain pine beetle (Dendroctonus ponderosae), can cause significant stand and tree mortality and also influence stand development. Expansive stands of Pinus contorta that occur in many regions are particularly susceptible. Pinus albicaulis is a slow-growing, long-lived conifer that is common at higher elevations in the upper subalpine zone over much of the central and northern range of this division. The exotic pathogen white pine blister rust (Cronartium ribicola) is attacking and killing Pinus albicaulis trees. It is especially destructive in more mesic habitats that favor infection of its alternate host Ribes spp.
Two very slow-growing, long-lived pines in this division are Pinus longaeva and Pinus flexilis. Pinus longaeva may attain nearly 4900 years in age and 12 m in height, whereas Pinus flexilis may live 1000 years and attain 18 m in height.
Environmental Description: These forests and woodlands occur on upland sites of temperate continental regions of western North America. In most of the range, more arid grassland climates occur at elevations below this type and alpine tundra occurs at elevations above. In moist climate areas and in the far northern range, this type occupies all elevations below the alpine. As such, elevations range considerably. Valley forests in northwestern British Columbia as low as 100 m (330 feet) in elevation are included in this type; upper elevation transitions to alpine tundra or dwarf-shrublands occur at 1675 m (5500 feet) in the northern range, and up to 3670 m (12,000 feet) in the south.
Climate: The climate of this type is cool temperate and continental, although many areas are influenced somewhat by Pacific maritime air masses. Temperature regimes vary considerably across the range and between seasons. Precipitation ranges from 25-240 cm annually. All areas receive winter snow (50 - 900 cm), but winter rain is also possible in most areas. In many areas, a seasonal drought period occurs. In areas east of the Continental Divide and in the Southwest, summer precipitation predominates, whereas further west and north, winter storms from the west are important sources of precipitation. High snowpack can contribute significantly to early growing season soil moisture in the moister mountains. High winds are a common feature found to the east of the Continental Divide and out in the Great Plains.
Soils/substrate: Landforms are variable and can include canyons, plateaus, draws, benches, hills, mesas, rolling plains, cinder cones, ravines, ridgetops, shoulders, sideslopes and toeslopes. Slopes can be gentle to extremely steep. In much of the range of this division, closed to open forests occupy most of the landscape. Some areas and sites are too droughty to support a closed tree canopy, so open woodlands and savannas occur. At the highest elevations, the interaction between snow deposition, desiccating winds, soil and substrate characteristics, and the interacting effects of precipitation, temperature and both latitude and elevation/aspect influence the type of forest, creating krummholz or tree patches in the alpine transition. Occurrences at high elevations are restricted by cold temperatures and are found on warmer aspects, whereas, at lower elevations, occurrences are restricted by lack of moisture and are found on cooler north aspects and mesic microsites.
Bedrock geology includes volcanic, intrusive, metamorphic, sedimentary and ultramafic rocks. Fractured rock, colluvium, and eolian materials are common substrates. Glacial till is typical in mountainous and northern areas, which can also have other glacial parent materials, e.g., glaciolacustrine, glaciofluvial. Volcanic activity is common in central and southern areas with pumice or ash deposits occurring. Many soils have good aeration and drainage, with an abundance of mineral material of medium to coarse textures, and variable rockiness. Soils range from deep and well-developed to shallow and rocky.
Biogeography: The expression of the types of forests and woodlands of this division are in response to climatic gradients of temperature and moisture. As most of the mountain ranges are perpendicular to the prevailing winds, moisture gradients are strongly influenced by windward or leeward (rainshadow) positions on the mountain ranges, in conjunction with elevation in the mountains. As such, the forest types occur in elevational bands, e.g., in dry climatic areas of eastern Washington and southern British Columbia, Pinus ponderosa forests and woodlands occur at the lowest elevations, with the sequence with increasing elevation (cooler temperatures and more precipitation) from Pseudotsuga menziesii, to Pinus contorta, to Abies lasiocarpa - Picea engelmannii forests. In moister climate regions, e.g., Idaho, the elevation sequence is Pseudotsuga menziesii forests, to Tsuga heterophylla - Thuja plicata forests, to Abies lasiocarpa - Picea engelmannii forests.
The division occurs over a wide latitudinal range so that a particular forest type can occur at very different elevations throughout its range, e.g., Pinus ponderosa woodlands occur at 400-800 m in southern British Columbia but at 1800-2700 m in southern Utah. The division is only found at the highest forested elevations in its southern range in Mexico and Guatemala, whereas it occurs from valley bottom to mountaintop (excluding alpine tundra) over most of the southern and central interior of British Columbia.
Geographic Range: This division occurs throughout the southern and central Rocky Mountains, from western Texas and southern New Mexico north into southern Alberta and central British Columbia, west into mountain ranges of central British Columbia, Idaho, and Washington, through the Colorado Plateau, Great Basin and Mojave Desert to the eastern slopes of the Sierra Nevada, Cascades, and Coast Mountains, and then east of the Rocky Mountains to the mountains and highlands of South Dakota, the Greater Yellowstone region, and the Wind River, Gros Ventre and Bighorn ranges of Wyoming. This division also occurs at the high elevations of the Sierra Madre Mountains of Mexico and Guatemala.
Nations: CA, GT, MX, US
States/Provinces: AB, AZ, BC, CA, CO, ID, KS?, MT, ND, NE, NM, NV, OR, SD, TX, UT, WA, WY
|US Forest Service Ecoregions (2007)|
Confidence Level: Moderate
Confidence Level Comments:
Concept Lineage: D009 (former 1.C.2.Nb) split into 2 new divisions (D192 & D194).
Synonomy: < Forests and Meadows of the Rocky Mountains (Peet 2000) [Peet primarily discusses Rocky Mountain forests and woodlands, but does include one section on "Meadows and Parks." We exclude boreal Rocky Mountain forests and low-elevation warm-temperate pine-juniper-evergreen oak woodlands in Arizona, New Mexico and Mexico.]
Concept Author(s): R.K. Peet (2000)
Author of Description: D. Meidinger
Acknowledgements: M.S. Reid
Version Date: 02Nov2015
- Callaham, R. Z. 2013. Pinus ponderosa: A taxonomic review with five subspecies in the United States. Research Paper PSW-RP-264. USDA Forest Service, Pacific Southwest Research Station, Albany, CA. 52 pp.
- Faber-Langendoen, D., J. Drake, S. Gawler, M. Hall, C. Josse, G. Kittel, S. Menard, C. Nordman, M. Pyne, M. Reid, L. Sneddon, K. Schulz, J. Teague, M. Russo, K. Snow, and P. Comer, editors. 2010-2019a. Divisions, Macrogroups and Groups for the Revised U.S. National Vegetation Classification. NatureServe, Arlington, VA. plus appendices. [in preparation]
- Peet, R. K. 2000. Forests and meadows of the Rocky Mountains. Chapter 3 in: M. G. Barbour and W. D. Billings, editors. North American terrestrial vegetation. Second edition. Cambridge University Press, New York. 434 pp.