Common (Translated Scientific) Name: Ponderosa Pine - Douglas-fir - Limber Pine Central Rocky Mountain Dry Forest Macrogroup
Colloquial Name: Central Rocky Mountain Dry Lower Montane-Foothill Forest
Hierarchy Level: Macrogroup
Type Concept: This macrogroup comprises conifer forests, woodlands and savannas found on dry settings of the lower montane to foothill zones of the interior Pacific Northwest, and extending east into the northwestern Great Plains regions. It is generally dominated by Pinus ponderosa var. ponderosa or Pinus ponderosa var. scopulorum, Pseudotsuga menziesii, Pinus flexilis, or Juniperus osteosperma or Juniperus scopulorum. Other occasional trees may include Pinus contorta, Picea engelmannii, Picea glauca (or their hybrid), and in the Great Plains, deciduous trees such as Acer negundo, Betula papyrifera, Fraxinus pennsylvanica, Populus tremuloides, Quercus macrocarpa, and Ulmus americana. Shrub and herbaceous components are widely variable, ranging from taxa found in the Great Plains mixedgrass region (such as Andropogon gerardii, Juniperus horizontalis, Prunus spp., Schizachyrium scoparium, or Yucca glauca) to those found across the Northern Rockies region into the eastern Cascades (e.g., Amelanchier alnifolia, Arctostaphylos uva-ursi, Artemisia spp., Cercocarpus spp., Juniperus communis, Physocarpus malvaceus, Spiraea betulifolia, Symphoricarpos spp., and graminoids such as Achnatherum spp., Bouteloua gracilis, Calamagrostis rubescens, Carex geyeri, Carex inops, Festuca idahoensis, Hesperostipa comata, Koeleria macrantha, Piptatheropsis micrantha, Poa secunda, or Pseudoroegneria spicata). This is a very wide-ranging macrogroup; it has common substrate and soil characteristics across its range, but from west to east shifts from a climate regime moderately influenced by maritime air masses, to one that is entirely continental. Generally these communities occur in lower montane to foothill settings, or on rock outcrops in the mixedgrass region of the Great Plains. Occurrences are found on all slopes and aspects; however, moderately steep to very steep slopes or ridgetops and plateaus are most common. Substrates tend to be composed of limestone, sandstone, dolomite, granite, colluvium, unweathered lava flows, pumice, cinders, or eolian sands. In most cases soil features include good aeration and drainage, coarse textures, circumneutral to slightly acidic pH, an abundance of mineral material, and periods of drought during the growing season.
Most of the communities in this macrogroup are fire-dependent types. Presettlement fire regimes may have been characterized by frequent, low-intensity surface fires that maintained relatively open stands of a mix of fire-resistant species. Pseudotsuga menziesii forests were probably subject to a moderate-severity fire regime in presettlement times, with fire-return intervals of 30-100 years. Many of the important tree species in these forests and woodlands are fire-adapted (Populus tremuloides, Pinus ponderosa, Pinus contorta), and fire-induced reproduction of Pinus ponderosa can result in its continued codominance in Pseudotsuga menziesii forests. A subset of the macrogroup includes woodlands and scrub patches that are edaphically controlled rather than fire-dependent. In the limber pine - juniper woodlands, fire is infrequent and spotty because the rocky substrates prevent development of a continuous vegetation canopy needed to spread fire. In other locations, where ponderosa pine is the predominant tree, periodic drought or areas of sand dunes, scablands, and pumice limit tree establishment. This climate-edaphic interaction results in widely scattered trees over "shrub-steppe" of sagebrush, bitterbrush, or sparsely distributed grasses. Tree growth is likely episodic, with regeneration episodes in years with available moisture. The expansion of Pinus ponderosa woodlands from the Black Hills montane zone into the central Great Plains may be due to fire suppression.
Diagnostic Characteristics: Forests, woodlands and savannas found on dry settings of the lower montane to foothill zones of the interior Pacific Northwest, and extending east into the northwestern Great Plains regions. Generally dominated by Pinus ponderosa var. ponderosa or Pinus ponderosa var. scopulorum, Pseudotsuga menziesii var. glauca, Pinus flexilis, or Juniperus osteosperma or Juniperus scopulorum. Other occasional trees may include Pinus contorta, Picea engelmannii, Picea glauca (or their hybrid), and in the Great Plains, deciduous trees such as Acer negundo, Betula papyrifera, Fraxinus pennsylvanica, Populus tremuloides, Quercus macrocarpa, and Ulmus americana.
Rationale for Nominal Species or Physiognomic Features: Woodlands dominated by Pinus ponderosa are very widely distributed in the western U.S. and southwestern Canada. They have now been broken into macrogroups generally corresponding to the distribution of varieties, which do have distinct ecotype characteristics. This macrogroup contains associations found where Pinus ponderosa var. ponderosa (interior Pacific Northwest) occurs. Pinus flexilis has a broad distribution in the western U.S., occurring from rock outcrops in the western Great Plains all the way up to the transition to the alpine in the Rocky Mountains and California.
Classification Comments: There are a number of classification issues pertaining to groups within this macrogroup, which may eventually result in conceptual changes to either this macrogroup, or to other related macrogroups. Below are some of the relevant comments. In addition, this description certainly needs review and additions for interior British Columbia or southern Alberta characteristics.
How to treat Pinus flexilis in the Rocky Mountains is still somewhat uncertain. For now, there are three groups which have limber pine as a component. The limber pine group included in this macrogroup is composed predominantly of limber pine or juniper that is elevationally below the zone of continuous lower montane forests found in the main Rocky Mountain cordillera. The associations placed in this group are restricted to foothill settings on rock outcrops, or to escarpments in the Great Plains, and in Montana, these are limestone outcrops (L. Vance pers. comm. 2014). Associations extending from the foothill zone into the subalpine, such as Pinus flexilis / Arctostaphylos uva-ursi Woodland (CEGL000802), are included in Rocky Mountain Subalpine-Montane Limber Pine - Bristlecone Pine Woodland Group (G221). Additionally, there are Juniperus osteosperma-dominated stands included in this group from the Pryor, Big Horn, and Laramie mountain ranges because these stands are significantly disjunct from the main distribution of Juniperus osteosperma in the Colorado Plateau and Great Basin regions and have floristic similarities to the Great Plains.
Forests and woodlands of mixed Pinus ponderosa and Pseudotsuga menziesii in this macrogroup need some review of concepts in relation to other groups and macrogroups, including Central Rocky Mountain Mesic Grand Fir - Douglas-fir Forest Group (G211) (in Central Rocky Mountain Mesic Lower Montane Forest Macrogroup (M500)) and East Cascades Mesic Grand Fir - Douglas-fir Forest Group (G212) (placed in Vancouverian Coastal Rainforest Macrogroup (M024)). In PNV (PAGs) concept, this is mostly Pseudotsuga menziesii, moist Pinus ponderosa series, dry Abies grandis, or warm, dry Abies lasiocarpa series in the Canadian Rockies, northern Middle Rockies, eastern Cascades and Okanagan ecoregions. Everett et al. (2000) indicate that in the eastern Cascades of Washington, this group forms fire polygons due to abrupt north and south topography with presettlement fire-return intervals of 11-12 years typically covering less than 810 ha. Currently, fires have 40- to 45-year return intervals with thousands of hectares in size. East Cascades Mesic Grand Fir - Douglas-fir Forest Group (G212) has a North Pacific floristic composition and is mostly found in the East Cascades ecoregion, peripheral in Okanagan ecoregion, and West Cascades. For now, this macrogroup does not include any Abies grandis-named associations. It may be that some of the drier end of Abies grandis forests should be included in this macrogroup, as they may well be mixed with Pinus ponderosa or Pseudotsuga menziesii.
Forests and woodlands of the middle Rocky Mountains dominated by Pseudotsuga menziesii, which are included in this macrogroup, need to be clarified as to placement; they are related to groups currently in other macrogroups; e.g. Central Rocky Mountain Mesic Grand Fir - Douglas-fir Forest Group (G211) in Central Rocky Mountain Mesic Lower Montane Forest Macrogroup (M500), and East Cascades Mesic Grand Fir - Douglas-fir Forest Group (G212) in Vancouverian Coastal Rainforest Macrogroup (M024). Also, its transition in the south to Southern Rocky Mountain White Fir - Douglas-fir Dry Forest Group (G226) in Southern Rocky Mountain Lower Montane Forest Macrogroup (M022) needs to be clarified. Certainly this macrogroup is outside the range of distribution of either Picea pungens or Abies concolor, which are major components of the southern Rocky Mountain forest macrogroup. It also does not overlap with major interior Pacific Northwest forest types which are affiliated with a more maritime climate regime, where trees such as Tsuga heterophylla, Thuja plicata, Larix occidentalis, Abies grandis, or Pinus monticola occur. The floristic "transition" from middle to southern Rocky Mountains is not yet clear. Woodlands dominated by Pseudotsuga menziesii found in breaks along rivers and on escarpments in central and eastern Montana and Wyoming are included in this macrogroup.
Regarding Pinus ponderosa, this macrogroup includes the northern race of Interior Ponderosa Pine old-growth (USFS Region 6, USFS Region 1). The FEIS site describes different varieties of Pinus ponderosa and associated species. This macrogroup of the Central Rockies is primarily Pinus ponderosa var. ponderosa (Habeck 1992). Johansen and Latta (2003) have mapped the distribution of two varieties (Pinus ponderosa var. scopulorum and Pinus ponderosa var. ponderosa) using mitochondrial DNA. They hybridize along the Continental Divide in Montana backing up the FEIS information. Southern Rocky Mountain Lower Montane Forest Macrogroup (M022) includes forests and woodlands where Pinus ponderosa var. scopulorum, Pinus ponderosa var. brachyptera, and Pinus arizonica var. arizonica are the predominant ponderosa varieties. The transition between these two macrogroups (M501 and M022) is now defined to occur in the montane zones of the Bighorns (USFS section M331B) and Laramie Range (USFS section M331I) and to the east and south of these mountains. The southern Rocky Mountain macrogroup (M022) will also occur in other isolated mountain ranges of central Wyoming, but not in eastern Wyoming. It does not occur farther north than Wyoming; all western Montana ponderosa pine woodlands are placed into this macrogroup (M501). Ponderosa pine woodlands and "steppes" in eastern Wyoming, eastern and central Montana, including the Missouri River Breaks, are also included in this macrogroup, and are predominantly Pinus ponderosa var. scopulorum. Ponderosa pine woodlands found in the Great Plains do show some floristic similarities to these found within the forested mountains of the Southern Rockies, but typically have herbaceous floristics related to the Great Plains "mixedgrass." The southern extent of the ponderosa pine woodlands included in this macrogroup is hard to determine, but farther south in Colorado, there is more Juniperus, Pinus edulis, and Quercus gambelii. Stands of ponderosa pine at Black Mesa in western Oklahoma and in southeastern Colorado are currently included with the southern Rocky Mountain ponderosa pine woodlands in M022. South of the Modoc Plateau in California, Pinus ponderosa forests and woodlands are included in Southern Vancouverian Montane-Foothill Forest Macrogroup (M023).
Similar NVC Types:
M020 Rocky Mountain Subalpine-High Montane Forest, note:
M022 Southern Rocky Mountain Lower Montane Forest, note: "includes forests and woodlands where Pinus ponderosa var. scopulorum, Pinus ponderosa var. brachyptera, and Pinus arizonica var. arizonica are the predominant ponderosa varieties."
M500 Central Rocky Mountain Mesic Lower Montane Forest, note:
Physiognomy and Structure: These are physiognomically variable conifer forests, woodlands or savannas, ranging from very sparse patches of trees on drier sites, to nearly closed-canopy forest stands on north slopes or in draws where available soil moisture is higher. They can have grassy or shrubby understories. In places these are patchy woodlands dominated by relatively short conifers (scrub woodlands). Occasionally broad-leaved deciduous trees are intermixed with the conifers in mesic settings, or as seral components. Shrubs can be broad-leaved deciduous, or needle-leaved or microphyllous evergreen (sagebrush), while the graminoids are primarily bunch grasses, along with rhizomatous grasses. Understories are generally low to moderate in cover, especially in the most droughty and rocky sites. In some cases due to a climate-edaphic interaction, the structure is that of widely scattered trees over "shrub-steppe" of Artemisia spp., Purshia tridentata, or sparsely distributed grasses.
Floristics: In the northwestern Great Plains, the tree canopy is primarily dominated by Pinus ponderosa var. scopulorum but may include a sparse to relatively dense understory of Juniperus scopulorum or Cercocarpus with just a few scattered trees. Deciduous trees are an important component in some areas (western Dakotas, Black Hills) and are sometimes codominant with the pines, including Acer negundo, Betula papyrifera, Fraxinus pennsylvanica, Populus tremuloides, Quercus macrocarpa, and Ulmus americana. Important or common shrub species with ponderosa pine can include Amelanchier alnifolia, Arctostaphylos uva-ursi, Juniperus communis, Juniperus horizontalis, Mahonia repens, Physocarpus monogynus, Prunus virginiana, Rhus trilobata, Symphoricarpos spp., and Yucca glauca. The herbaceous understory is variable and can range from a sparse to dense layer with species typifying the surrounding prairie group, with mixedgrass species common, such as Andropogon gerardii, Bouteloua curtipendula, Carex inops ssp. heliophila, Carex filifolia, Koeleria macrantha, Nassella viridula, Oryzopsis asperifolia, Pascopyrum smithii, Piptatheropsis micrantha (= Piptatherum micranthum), and Schizachyrium scoparium. Higher-elevation stands often have herbaceous species more typical of the Rocky Mountains such as Achillea millefolium, Antennaria rosea, Balsamorhiza sagittata, Cerastium arvense, Danthonia intermedia, Fragaria spp., Galium boreale, Pulsatilla patens, and Lathyrus ochroleucus.
Further west, Pinus ponderosa-dominated woodlands include two physiognomic phases: true woodlands of Pinus ponderosa with shrubby or grassy understories, and "wooded steppes" with widely spaced, scattered Pinus ponderosa trees over generally shrubby but sparse understories. The former are generally fire-maintained, while the later are often too dry and with widely spaced vegetation to carry fire. Pinus ponderosa var. ponderosa is the predominant conifer (west of the Continental Divide); Pseudotsuga menziesii or Pinus flexilis may be present in the tree canopy but are usually absent. The understory can be shrubby, with Amelanchier alnifolia, Arctostaphylos patula, Arctostaphylos uva-ursi, Ceanothus velutinus, Cercocarpus ledifolius, Physocarpus malvaceus, Purshia tridentata, Rosa spp., Symphoricarpos oreophilus or Symphoricarpos albus, and Vaccinium cespitosum common species. In transition areas with big sagebrush steppe, Artemisia arbuscula, Artemisia tridentata ssp. tridentata, Artemisia tridentata ssp. wyomingensis, Artemisia tripartita, and Purshia tridentata may be common in fire-protected sites such as rocky areas. Deciduous shrubs, such as Physocarpus malvaceus, Symphoricarpos albus, or Spiraea betulifolia, can be abundant in more northerly sites or more moist climates. Understory vegetation in the true savanna occurrences is predominantly fire-resistant grasses and forbs that resprout following surface fires; shrubs, understory trees and downed logs are uncommon. These more open stands support grasses such as Achnatherum spp., dry Carex species (Carex inops), Festuca campestris, Festuca idahoensis, Hesperostipa spp., or Pseudoroegneria spicata. More mesic sites may include Calamagrostis rubescens or Carex geyeri, species more typical of the higher elevation Douglas-fir - pine forests.
In the Central Rockies forests are typically dominated by a mix of Pseudotsuga menziesii and Pinus ponderosa in the tree canopy, although either can be absent. Other seral trees may occur, including Pinus contorta, Pinus monticola, and Larix occidentalis (neither in central Montana). Picea engelmannii (or Picea glauca or their hybrid) becomes increasingly common to the east. In the eastern Cascades, Pinus contorta may be the codominant pine, rather than Pinus ponderosa. Abies grandis (a fire-sensitive, shade-tolerant species not occurring in central Montana) has increased on many sites once dominated by Pseudotsuga menziesii and Pinus ponderosa, which were formerly maintained by low-severity wildfire. Abies concolor and Abies grandis x concolor hybrids in central Idaho (the Salmon Mountains) may occur in some stands, but have very restricted ranges in this area. Abies concolor and Abies grandis in the Blue Mountains of Oregon are probably hybrids of the two and mostly Abies grandis. Understories are typically dominated by graminoids, such as Calamagrostis rubescens, Carex geyeri, Carex rossii, and Pseudoroegneria spicata, and a variety of shrubs, such as Acer glabrum, Juniperus communis, Physocarpus malvaceus, Symphoricarpos albus, Spiraea betulifolia, or Vaccinium membranaceum on mesic sites.
The foothill-rock outcrop limber pine - juniper woodlands are dominated by Pinus flexilis, Juniperus osteosperma, or Juniperus scopulorum. Pinus edulis is not present. A sparse to moderately dense short-shrub layer, if present, may include a variety of shrubs, such as Artemisia nova, Artemisia tridentata, Cercocarpus ledifolius, Cercocarpus montanus, Ericameria nauseosa, Juniperus horizontalis, Purshia tridentata, Rhus trilobata, or Rosa woodsii. Herbaceous layers are generally sparse, but range to moderately dense, and are typically dominated by perennial graminoids such as Bouteloua gracilis, Hesperostipa comata, Koeleria macrantha, Leymus innovatus (in Alberta), Piptatheropsis micrantha, Poa secunda, Pseudoroegneria spicata, or Schizachyrium scoparium.
In the middle Rocky Mountains are found extensive Pseudotsuga menziesii forests, occasionally with Pinus flexilis on calcareous substrates, and Pinus contorta at higher elevations. True firs, such as Abies concolor, Abies grandis, and Abies lasiocarpa, are generally absent in these occurrences, but Picea engelmannii can occur in some stands. Pinus ponderosa is also not common in this region. Understory components include shrubs such as Amelanchier alnifolia, Juniperus communis, Linnaea borealis, Mahonia repens, Physocarpus malvaceus, Purshia tridentata, Spiraea betulifolia, Symphoricarpos oreophilus, and Symphoricarpos albus. Common graminoids include Calamagrostis rubescens, Carex rossii, and Leucopoa kingii. Forbs are variable, but typical taxa include Arnica cordifolia, Osmorhiza berteroi, Thalictrum occidentale, Viola adunca, and species of many other genera, including Arenaria, Erigeron, Fragaria, Galium, Lathyrus, Lupinus, Penstemon, Vicia, and others. These Douglas-fir forests often occur at the lower treeline immediately above valley grasslands, or sagebrush steppe and shrublands. Sometimes there may be a "bath-tub ring" of Pinus ponderosa at lower elevations or Pinus flexilis between the valley non-forested and the solid Pseudotsuga menziesii forest. In the Wyoming Basins, there are isolated stands of Pseudotsuga menziesii, with Artemisia tridentata, Pseudoroegneria spicata, Leucopoa kingii, and Carex rossii.
Dynamics: Most of the communities in this macrogroup are fire-dependent types. Presettlement fire regimes may have been characterized by frequent, low-intensity surface fires that maintained relatively open stands of a mix of fire-resistant species. Pseudotsuga menziesii forests were probably subject to a moderate-severity fire regime in presettlement times, with fire-return intervals of 30-100 years. Many of the important tree species in these forests and woodlands are fire-adapted (Populus tremuloides, Pinus ponderosa, Pinus contorta) (Pfister et al. 1977), and fire-induced reproduction of Pinus ponderosa can result in its continued codominance in Pseudotsuga menziesii forests (Steele et al. 1981).
A subset of the macrogroup includes woodlands and scrub patches that are edaphically controlled rather than fire-dependent. In the limber pine - juniper woodlands, fire is infrequent and spotty because the rocky substrates prevent development of a continuous vegetation canopy needed to spread fire. In other locations, where ponderosa is the predominant tree, periodic drought limits tree establishment. This climate-edaphic interaction results in widely scattered trees over "shrub-steppe" of sagebrush, bitterbrush, or sparsely distributed grasses. Tree growth is likely episodic, with regeneration episodes in years with available moisture. Tree density is limited in some areas by available growing space due to rocky conditions of the site. Hence the tree canopy in these locations will never reach woodland density or close due to the interaction of climate and edaphic factors, even in the absence of fire. They burn occasionally, but the vegetation is sparse enough that fires are typically not carried through the stand. Fire frequency is speculated to be 30-50 years. Some stands also occur on areas of sand dunes, scablands, and pumice where the edaphic conditions limit tree abundance.
Pinus ponderosa is a drought-resistant, shade-intolerant conifer which usually occurs at lower treeline in the major ranges of the western United States. Historically, surface fires and drought were influential in maintaining open-canopy conditions in these woodlands. With settlement and subsequent fire suppression, occurrences have become denser. Presently, some occurrences contain understories of more shade-tolerant species, such as Pseudotsuga menziesii and/or Abies spp., as well as younger cohorts of Pinus ponderosa. These altered structures have affected fuel loads and 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, fire regimes are now less frequent and often become intense crown fires, which can kill mature Pinus ponderosa (Reid et al. 1999). Establishment is erratic and believed to be linked to periods of adequate soil moisture and good seed crops as well as fire frequencies, which allow seedlings to reach sapling size. Longer fire-return intervals have resulted in many occurrences having dense subcanopies of overstocked and unhealthy young Pinus ponderosa, along with Pseudotsuga menziesii on moist sites (Reid et al. 1999).
A meeting of Pacific Northwest ecologists for Landfire concluded that the "true savannas" of high-frequency / low-intensity fires and grassy understories are now rare in the central and northern Rocky Mountains. Most areas that may have been savanna in the past are now more nearly closed-canopy woodlands/forests. Louisa Evers (pers. comm. 2006) notes that she has not found any evidence that ponderosa pine savanna existed historically in north-central and central Oregon. In north-central Oregon, the savanna would have been oak or pine-oak. In central Oregon, it may well have been western juniper. Condition surveys of the Cascades Forest Reserve and General Land Office survey notes suggest that ponderosa pine formed a woodland with grassy understories, but still was often referred to as open-parklike.
In mixed stands of Pinus ponderosa and Pseudotsuga menziesii, presettlement fire regimes may have been characterized by frequent, low-intensity surface fires that maintained relatively open stands of a mix of fire-resistant species. Under present conditions, the fire regime is mixed-severity and more variable, with stand-replacing fires more common, and the forests are more homogeneous. With vigorous fire suppression, longer fire-return intervals are now the rule, and multi-layered stands of Pseudotsuga menziesii, Pinus ponderosa, and/or Abies grandis provide fuel "ladders," making these forests more susceptible to high-intensity, stand-replacing fires. They are productive forests which have been priorities for timber production.
Successional relationships in the middle Rocky Mountain Pseudotsuga menziesii forests are complex. Pseudotsuga menziesii is less shade-tolerant than many northern or montane trees such as Tsuga heterophylla, Abies concolor, Picea engelmannii, or Thuja plicata, and seedlings compete poorly in deep shade. At drier locales, seedlings may be favored by moderate shading, such as by a canopy of Pinus ponderosa, which helps to minimize drought stress. In some locations, much of these forests were logged or burned during European settlement, and present-day stands are second-growth forests dating from fire, logging, or other stand-replacing disturbances (Mauk and Henderson 1984). Pseudotsuga menziesii forests were probably subject to a moderate-severity fire regime in presettlement times, with fire-return intervals of 30-100 years. Many of the important tree species in these forests are fire-adapted (Populus tremuloides, Pinus ponderosa, Pinus contorta) (Pfister et al. 1977), and fire-induced reproduction of Pinus ponderosa can result in its continued codominance in Pseudotsuga menziesii forests (Steele et al. 1981). Seeds of the shrub Ceanothus velutinus can remain dormant in forest stands for 200 years (Steele et al. 1981) and germinate abundantly after fire, competitively suppressing conifer seedlings. Some stands may have higher tree-stem density than historically, due largely to fire suppression.
Marriot and Faber-Langendoen (2000) report different fire regimes for ponderosa pine communities in the Black Hills, with their "Dry Group" more typically having frequent surface fires and the "Mesic Group" having infrequent catastrophic fires (every 100-200 years). The Dry Group of associations includes lower elevation foothill savanna associations, and the mesic group somewhat higher elevation, north-slope, swale associations. Kelly Kindscher (pers. comm. 2007) believes that almost all of the stands in Nebraska were there at the time of settlement and are not a result of pine expansion due to fire suppression; in addition, at least some have disappeared, such as the one in southern Nebraska (Franklin County). It is possible, however, that some areas of Great Plains ponderosa have expanded in size due to fire suppression, but this needs substantiation.
Environmental Description: This is a very wide-ranging macrogroup; it has common substrate and soil characteristics across its range, but from west to east shifts from a climate regime moderately influenced by maritime air masses, to one that is entirely continental. The below summary of environmental characteristics has been split into 2 components, one for the "interior Pacific Northwest," the second for the component generally found east of the Continental Divide.
This macrogroup within the interior Pacific Northwest and Central Rockies regions often occurs at the lower treeline/ecotone between grasslands or shrublands and more mesic coniferous forests. Typically these plant communities are found in warm, dry, exposed sites at elevations ranging from 350 m in British Columbia to over 2400 m (1500-7875 feet) in the Wyoming Rockies. These interior Pacific Northwest woodlands receive winter and spring rains, and thus have a greater spring "green-up" than the drier woodlands in the Southern Rockies. In the middle Rocky Mountains, Pseudotsuga menziesii forests occur under a comparatively drier and more continental climate regime, and at higher elevations than in the coastal Pacific Northwest. However, these sites are often too droughty to support a closed tree canopy. They can occur on all slopes and aspects; often they occur on moderately steep to very steep slopes or ridgetops and plateaus. Lower elevation stands often occupy protected northern exposures or mesic ravines and canyons, often on steep slopes. At higher elevations, these forests occur primarily on southerly aspects or ridgetops and plateaus. Soils are highly variable and derived from diverse parent materials.
Climate: These interior Pacific Northwest woodlands receive winter and spring rains, and thus have a greater spring "green-up" than the drier woodlands in the Southern Rockies. The southern / southwestern monsoon influence is less and maritime climate regime is not important. Annual precipitation ranges from 50-100 cm with moderate snowfall and a greater proportion falling during the growing season. Monsoonal summer rains can contribute a significant proportion of the annual precipitation in the southern portion of the range. Winter snowpacks typically melt off in early spring at lower elevations.
Soil/substrate/hydrology: These sites are often too droughty to support a closed tree canopy. They can occur on all slopes and aspects; however, they commonly occur on moderately steep to very steep slopes or ridgetops and plateaus. Substrates include glacial till, glacio-fluvial sand and gravel, dunes, basaltic rubble and scablands, colluvium, or deep loess or volcanic ash-derived soils, all with characteristic features of good aeration and drainage, coarse textures, circumneutral to slightly acidic pH, an abundance of mineral material, rockiness, and periods of drought during the growing season. In the Oregon "pumice zone" stands occur as matrix-forming, extensive woodlands on rolling pumice plateaus and other volcanic deposits. Pseudotsuga menziesii forests are reported by most studies (Pfister et al. 1977, Steele et al. 1981, Mauk and Henderson 1984, Lillybridge et al. 1995) to show no particular affinities to geologic substrates.
The ponderosa pine, limber pine and juniper communities found in the northwestern Great Plains and along the foothills of the Rocky Mountains to the east of the Continental Divide occur in foothill and lower montane zones. Generally they occur on gentle to steep slopes along escarpments, buttes, canyons, rock outcrops or ravines and can grade into one of the Great Plains canyon groups or the surrounding mixedgrass prairie. Slopes are typically moderately steep to steep. In Montana, limber pine is restricted at low elevations to limestone outcrops. The Pinus ponderosa woodlands of the western Great Plains are found typically in the matrix of Great Plains grassland systems. They are often surrounded by mixedgrass or tallgrass prairie, in places where available soil moisture is higher or soils are more coarse and rocky. In some cases, these woodlands or savannas may occur where fire suppression has allowed trees to become established (in areas where deciduous trees are more abundant (Girard et al. 1987). These are typically not in the same setting as Rocky Mountain ponderosa pine, where ponderosa pine forms woodlands at lower treeline and grades into mixed montane conifer systems at higher elevations.
Climate: Climatologically, the region east of the Continental Divide is semi-arid and has a continental regime of hot summers and cold winters. High winds are a common feature found to the east of the Continental Divide and out in the Great Plains; limber pine is adapted to these winds with highly flexible branches which prevent breakage. Precipitation patterns are variable, but snow is common in winter, and spring rains are an important contributor.
Soil/substrate/hydrology: The Pinus flexilis and Juniperus spp. communities are restricted to shallow soils and fractured bedrock derived from a variety of parent material, including limestone, sandstone, dolomite, granite, and colluvium. An unusual community in Idaho occurs on relatively unweathered mafic lava flows, where it occurs in mesic pockets within the fractured lava. In all cases, soils have a high rock component (typically over 50% cover) and are coarse- to fine-textured, often gravelly and calcareous. Slopes are typically moderately steep to steep. Soils of the Pinus ponderosa woodlands range from well-drained loamy sands to sandy loams formed in colluvium, weathered sandstone, limestone, scoria or eolian sand.
Geographic Range: This widespread macrogroup occurs in Canada in southern British Columbia in the Fraser River drainage and eastward in valleys that drain into the Columbia and Kootenay rivers, and in southwestern Alberta east of the Continental Divide. Southward into the U.S. it occurs along the Cascades and central Rocky Mountains of Washington, Oregon and the Modoc Plateau of northeastern California. It also is found throughout the middle Rocky Mountains of central and southern Idaho (Lemhi, Beaverhead and Lost River ranges), south and east into the Greater Yellowstone region, and south and east into the Wind River, Gros Ventre and Bighorn ranges of Wyoming. In the northeastern part of its range, it extends across the central Rocky Mountains west of the Continental Divide into western Montana, south to the Snake River Plain in Idaho, and east of the Continental Divide into the foothills of west-central Montana. It extends east into the "sky island" ranges of central Montana, and from there east into the northwestern Great Plains along areas that border the Rocky Mountains and into the central Great Plains in a few scattered localities. Some associations placed in this macrogroup also occur in Colorado and northeastern Utah in the Uinta Mountains.
Nations: CA, US
States/Provinces: AB, BC, CA, CO, ID, KS?, MT, ND, NE, NV?, OR, SD, UT, WA, WY
|US Forest Service Ecoregions (2007)|
Southern Rocky Mountain Steppe - Open Woodland - Coniferous Forest - Alpine Meadow Province
Confident or certain
Yellowstone Highlands Section
Confident or certain
Confidence Level: Moderate
Confidence Level Comments:
Synonomy: > Black Hills pine forest (Pinus) (Küchler 1964)
> Douglas-fir forest (Pseudotsuga) (Küchler 1964)
> Eastern ponderosa forest (Pinus) (Küchler 1964)
>< Interior Douglas-fir: 210 (Eyre 1980)
>< Interior Ponderosa Pine: 237 (Eyre 1980)
>< Limber Pine: 219 (Eyre 1980)
>< Ponderosa Pine - Grassland (110) (Shiflet 1994)
>< Ponderosa Pine - Shrubland (109) (Shiflet 1994)
>< Rocky Mountain Juniper: 220 (Eyre 1980)
> Western ponderosa forest (Pinus) (Küchler 1964)
Concept Author(s): Faber-Langendoen et al. (2014)
Author of Description: M.S. Reid
Version Date: 15Oct2014
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