Common (Translated Scientific) Name: White Fir - Douglas-fir - Blue Spruce Forest Macrogroup
Colloquial Name: Southern Rocky Mountain Lower Montane Forest
Hierarchy Level: Macrogroup
Type Concept: These are forests and woodlands, or fire-maintained savannas of the lower montane to lower treeline ecotone of the Rocky Mountains from Wyoming south into southern New Mexico, and west into scattered locations of the Great Basin. The characteristic trees are predominantly conifers, including Abies concolor, Juniperus scopulorum, Pinus edulis, Pinus ponderosa (primarily var. scopulorum and var. brachyptera), Pseudotsuga menziesii, and the less extensive Picea pungens. Occasionally cold-deciduous trees (Populus tremuloides) mix in the canopy, or in some locations are dominant (Acer grandidentatum). Other conifers that may be present include Abies lasiocarpa var. arizonica, Abies lasiocarpa var. lasiocarpa, Picea engelmannii, Pinus contorta, and Pinus flexilis. In the mountains of southern Arizona and New Mexico, associated trees may include Pinus cembroides, Pinus discolor, Pinus edulis, Pinus strobiformis, and in far southern stands Juniperus deppeana may also be common. Cold-deciduous broad-leaved shrubs are common in the undergrowth in most occurrences, and grasses or forbs can be abundant to sparse. The composition and structure of the overstory are dependent upon the temperature and moisture relationships of the site and the successional status of the occurrence. This macrogroup is widespread throughout the southern Rocky Mountains, occurring on all aspects and across a range of landforms and substrates. Generally it is found below the subalpine zone down to the lower treeline. Elevation ranges from 1200-3300 m (3936-10,824 feet), but generally decreases with increasing latitude. 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. Communities in this macrogroup vary somewhat in fire regimes and adaptations. In general, fire suppression has led to the encroachment of more shade-tolerant, less fire-tolerant species (e.g., climax) into occurrences and an attendant increase in landscape homogeneity and connectivity (from a fuels perspective). This has increased the lethality and potential size of fires.
Diagnostic Characteristics: Forests and woodlands, or fire-maintained savannas of the lower montane to lower treeline ecotone of the Rocky Mountains from Wyoming south into southern New Mexico, and west into scattered locations of the Great Basin. The characteristic trees are predominantly conifers, including Abies concolor, Juniperus spp., Pinus ponderosa (primarily var. scopulorum and var. brachyptera), Pseudotsuga menziesii, and the less extensive Picea pungens. Occasionally cold-deciduous trees (Populus tremuloides) mix in the canopy, or in some locations are dominant (Acer grandidentatum). Cold-deciduous broad-leaved shrubs are common in the undergrowth in most occurrences, and grasses or forbs can be abundant to sparse.
Rationale for Nominal Species or Physiognomic Features: Pseudotsuga menziesii is a widespread dominant or codominant tree species. Abies concolor and the more restricted Picea pungens are more abundant in mesic stands. However, diverse and variable mesic understory species are diagnostic. Also Acer grandidentatum may dominate some stands, but its distribution is too limited to include as one to the three nominal species. Pinus ponderosa is another widespread tree that is common in this macrogroup and could be an alternate to Pseudotsuga menziesii.
Classification Comments: The transition between this macrogroup and Central Rocky Mountain Dry Lower Montane-Foothill Forest Macrogroup (M501) will certainly have some floristic and biophysical similarities, and may need further clarification to distinguish the two macrogroups. In general, this macrogroup (M022) will be drier and has a different precipitation pattern than M501 (which receives more winter and spring rains), which will result in a floristic shift from south to north.
Similar NVC Types:
M011 Madrean Montane Forest & Woodland, note:
M020 Rocky Mountain Subalpine-High Montane Forest, note: "is where aspen-dominated associations are placed, along with those mixed with the high montane/subalpine conifers, such as Abies lasiocarpa, Picea engelmannii, and Pinus contorta, although Populus tremuloides can occur in M022, mixed with the lower montane conifers."
M501 Central Rocky Mountain Dry Lower Montane-Foothill Forest, note: "also has Pinus ponderosa and Pseudotsuga menziesii as important trees; however, the sub-taxa are different for each, and associated shrub and herb floristics will differ somewhat."
Physiognomy and Structure: Single species- or mixed conifer-dominated savannas, woodlands and forests with shrub, grass or sparse understories. Occasionally broad-leaved deciduous trees are intermixed with the conifers in mesic environments, or are dominant in some locations (cool ravines). In some communities, open savannas (<25% cover of trees) have a parklike understory strongly dominated by fire-resistant graminoids. Shrubs are few or absent from these communities, although there may be a mid-level canopy of shrubs, copses of deciduous shrubs or trees.
Floristics: These are forests and woodlands, and some fire-maintained savannas, of predominantly conifers, often mixed, but also of single species. The characteristic trees include Abies concolor (= var. concolor), Juniperus scopulorum, Pinus ponderosa (primarily var. scopulorum and var. brachyptera), Pseudotsuga menziesii, and the less extensive Picea pungens. The deciduous Populus tremuloides or Acer grandidentatum are early-seral species that may be codominant in some stands; Acer grandidentatum-dominated stands are included in this macrogroup as well. Other conifers that may be present include Abies lasiocarpa var. arizonica, Abies lasiocarpa var. lasiocarpa, Picea engelmannii, Pinus contorta, Pinus edulis, and Pinus flexilis. In the mountains of southern Arizona and New Mexico, associated trees may include Pinus strobiformis, and in far southern stands Juniperus deppeana may also be common. The composition and structure of the overstory are dependent upon the temperature and moisture relationships of the site and the successional status of the occurrence (DeVelice et al. 1986, Muldavin et al. 1996).
Shrub and herb taxa are variable; in the mesic occurrences occurring outside the riparian floodplains, hence not considered wetlands or true riparian areas, scattered riparian and facultative wetland species may be present. Some of the important shrub species in these more mesic stands include Acer glabrum, Alnus incana, Betula occidentalis, Cornus sericea, Holodiscus dumosus, Jamesia americana, Physocarpus malvaceus, Quercus gambelii, Robinia neomexicana, Vaccinium membranaceum, and Vaccinium myrtillus. Common herbaceous species include Bromus ciliatus, Carex geyeri, Carex rossii, Carex siccata, Erigeron eximius, Fragaria virginiana, Luzula parviflora, Muhlenbergia straminea (= Muhlenbergia virescens), Osmorhiza berteroi, Packera cardamine, Pseudoroegneria spicata, Thalictrum fendleri, and Thalictrum occidentale.
In drier occurrences, there are a number of common shrubs, including Amelanchier alnifolia, Arctostaphylos patula, Arctostaphylos uva-ursi, Holodiscus dumosus, Jamesia americana, Juniperus communis, Mahonia repens, Paxistima myrsinites, Physocarpus monogynus, Quercus gambelii, Quercus x pauciloba, Robinia neomexicana, Rubus parviflorus, Symphoricarpos oreophilus, and Vaccinium myrtillus. Where soil moisture is favorable, the herbaceous layer may be quite diverse, including graminoids Bromus ciliatus (= Bromus canadensis), Calamagrostis rubescens, Carex geyeri, Carex rossii, Carex siccata (= Carex foenea), Festuca occidentalis, Koeleria macrantha, Muhlenbergia montana, Muhlenbergia straminea, Poa fendleriana, Pseudoroegneria spicata, and forbs Achillea millefolium, Arnica cordifolia, Erigeron eximius, Fragaria virginiana, Linnaea borealis, Luzula parviflora, Osmorhiza berteroi, Packera cardamine (= Senecio cardamine), Thalictrum occidentale, Thalictrum fendleri, Thermopsis rhombifolia, Viola adunca, and species of many other genera, including Arenaria, Galium, Lathyrus, Penstemon, Lupinus, Vicia, and others.
Pinus ponderosa woodlands at the lower treeline tend to be drier, and while some shrub or herb taxa are shared with the cooler, higher elevation mixed-conifer woodlands, others are more distinct to these woodlands. Common shrubs include Amelanchier alnifolia, Arctostaphylos patula, Arctostaphylos uva-ursi, Artemisia nova, Artemisia tridentata, Cercocarpus montanus, Fallugia paradoxa, Forestiera pubescens, Prunus virginiana, Purshia stansburiana, Purshia tridentata, Quercus gambelii, Ribes spp., Robinia neomexicana, Rosa spp., and Symphoricarpos spp. The herbaceous layer tends to vary inversely with shrub cover, but is composed primarily of graminoids. Important species include Achnatherum hymenoides (= Oryzopsis hymenoides), Achnatherum occidentale (= Stipa occidentalis), Bouteloua gracilis, Carex geyeri, Carex pensylvanica, Carex rossii, Elymus elymoides, Festuca arizonica, Festuca idahoensis, Hesperostipa comata (= Stipa comata), Koeleria macrantha, Leucopoa kingii (= Festuca kingii), Muhlenbergia montana, Muhlenbergia straminea, Poa secunda, and Pseudoroegneria spicata. Important or diagnostic forb species include Achillea millefolium, Aspidotis densa, Balsamorhiza sagittata, Maianthemum racemosum (= Smilacina racemosa), Sedum stenopetalum, Vicia americana, Wyethia mollis, and species of many other genera, such as Antennaria, Arenaria, Erigeron, Fragaria, Heterotheca, Lathyrus, and Lupinus.
In savannas of Pinus ponderosa, small trees and shrubs are poorly represented but can include scattered Artemisia tridentata, Chrysothamnus depressus, Juniperus spp., and Quercus gambelii. The understory is predominantly graminoid-dominated with species including Andropogon gerardii, Bouteloua gracilis, Carex rossii, Elymus elymoides, Festuca arizonica, Festuca idahoensis, Koeleria macrantha, Muhlenbergia straminea, Piptatheropsis micrantha (= Piptatherum micranthum), Poa fendleriana, Pseudoroegneria spicata, and Schizachyrium scoparium.
Dynamics: Forests, woodlands and savannas in this macrogroup vary somewhat in fire regimes and adaptations. In general, fire suppression has led to the encroachment of more shade-tolerant, less fire-tolerant species (e.g., climax) into occurrences and an attendant increase in landscape homogeneity and connectivity (from a fuels perspective). This has increased the lethality and potential size of fires.
Formerly, Abies concolor in the Utah High Plateaus and Colorado Plateau region was restricted to rather moist or less fire-prone areas by frequent surface fires. These areas experienced mixed fire severities, with patches of crowning in which all trees are killed, intermingled with patches of underburn in which larger Abies concolor survived (Mauk and Henderson 1984, Zouhar 2001a). With fire suppression, Abies concolor has vigorously colonized many sites formerly occupied by open Pinus ponderosa woodlands. These invasions have dramatically changed the fuel load and potential behavior of fire in these forests. In particular, the potential for high-intensity crown fires on drier sites now codominated by Pinus ponderosa and Abies concolor has increased. Increased landscape connectivity, in terms of fuel loadings and crown closure, has also increased the potential size of crown fires.
Pseudotsuga menziesii forests are generally "fire-tolerant." 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) (Burns and Honkala 1990a), and fire-induced reproduction of Pinus ponderosa can result in its continued codominance in Pseudotsuga menziesii forests (Moir et al. 1997). Seeds of the shrub Ceanothus velutinus can remain dormant in forest occurrences for 200 years (Steele et al. 1981) and germinate abundantly after fire, competitively suppressing conifer seedlings. Successional relationships in this group are complex. Pseudotsuga menziesii is less shade-tolerant than many northern or montane trees such as Tsuga heterophylla, Abies concolor, Picea engelmannii, and its 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 occurrences are second-growth forests dating from fire, logging, clearing for mineral exploration, or other occurrence-replacing disturbances (Mauk and Henderson 1984, Veblen and Lorenz 1986).
Picea pungens is a slow-growing, long-lived tree which regenerates from seed (Burns and Honkala 1990a). It occurs more in microclimates in canyon bottoms and the edges of montane grassland valleys, cold-air drainages, and where there is more moisture. Seedlings are shallow-rooted and require perennially moist soils for establishment and optimal growth. Picea pungens is intermediate in shade tolerance, being somewhat more tolerant than Pinus ponderosa or Pseudotsuga menziesii, and less tolerant than Abies lasiocarpa or Picea engelmannii. It forms late-seral occurrences in the subhumid regions of the Utah High Plateaus. It is common for these forests to be heavily disturbed by grazing or fire.
Pinus ponderosa is a drought-resistant, shade-intolerant conifer which usually occurs at lower treeline in the major ranges of the western United States. Fire is a key factor in maintaining the open canopies characteristic of the Pinus ponderosa savannas. 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, many 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 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, fire are now less frequent and often become intense crown fires, which can kill mature Pinus ponderosa (Savage and Swetnam 1990).
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. Savage and Swetnam (1990) suggest that continuity of understory fuels, especially the grass layer, maintained high frequencies of low-intensity, surface fires along the entire gradient from ponderosa pine woodlands to spruce-fir forests. This hypothesis is supported by evidence that forests with grassy understories were once extensive and continuous over a large elevational range (Savage and Swetnam 1990, Moir et al. 1997). Descriptions of forests around the turn of the century noted open, large areas not confined to ponderosa pine forests. Most ecologists agree that hot crown fires were not extensive in these open ponderosa pine savannas, although small thickets would have been destroyed by spot crown fires. Mehl (1992) states the following: "Where fire has been present, occurrences will be climax and contain groups of large, old trees with little understory vegetation or down woody material and few occurring dead trees. The age difference of the groups of trees would be large. Where fire is less frequent, there will also be smaller size trees in the understory giving the occurrence some structure with various canopy layers. Dead, down material will be present in varying amounts along with some occurring dead trees. In both cases the large old trees will have irregular open, large branched crowns. The bark will be lighter in color, almost yellow, thick and some will like have basal fire scars."
Grace's warbler, pygmy nuthatch, and flammulated owl are indicators of healthy ponderosa pine woodlands. All of these birds prefer mature trees in an open woodland setting (Winn 1998, Jones 1998d, Levad 1998 as cited in Rondeau 2001).
Environmental Description: This macrogroup is widespread throughout the southern Rocky Mountains, occurring on all aspects and across a range of landforms and substrates. Generally it is found below the subalpine zone down to the lower treeline. Elevation ranges from 1200-3300 m (3936-10,824 feet), but generally decreases with increasing latitude. 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.
Climate: The quantity and timing of precipitation varies across the range of the macrogroup. Precipitation generally averages 25-60 cm annually, with some locations receiving up to 75 cm; most precipitation is through winter storms (with both snowfall and rains) and some monsoonal summer rains. Typically a seasonal drought period occurs. East of the Continental Divide and in the Southwest, summer precipitation predominates, whereas further west winter storms from the west are important. Monsoonal summer rains can contribute a substantial proportion to the annual precipitation totals in the Southwest. Temperatures range widely across the year; lower elevations can be hot in the summer, while in some areas east of the Continental Divide, winters can be very cold.
Soil/substrate/hydrology: Geologic substrates include volcanic andesite, basalt, rhyolite, rhyolitic tuffs, colluvium, shale gneiss, granite, sandstone and limestone (Youngblood and Mauk 1985). Soils are variable from cobbles, clay loam, silt loam, sandy loam, sand, and gravel. Characteristic soil features include good aeration and drainage, coarse textures, circumneutral to slightly acidic pH, and an abundance of mineral material. Some occurrences may occur as edaphic climax communities on very skeletal, infertile, and/or excessively drained soils, such as pumice, cinder or lava fields, and scree slopes. Surface textures are highly variable in this macrogroup, ranging from sand to loam and silt loam. Exposed rock and bare soil consistently occur to some degree in many sites.