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.