Southern Rockies Mixed Conifer Forest

This is a highly variable ecological system of the montane zone of the Rocky Mountains. It occurs throughout the southern Rockies, north and west into Utah, Nevada, Wyoming and Idaho. These are mixed-conifer forests occurring on all aspects at elevations ranging from 1200 to 3300 m. Rainfall averages less than 75 cm per year (40-60 cm), with summer "monsoons" during the growing season contributing substantial moisture. 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. Pseudotsuga menziesii and Abies concolor are most frequent, but Pinus ponderosa may be present to codominant. Pinus flexilis is common in Nevada. Pseudotsuga menziesii forests occupy drier sites, and Pinus ponderosa is a common codominant. Abies concolor-dominated forests occupy cooler sites, such as upper slopes at higher elevations, canyon sideslopes, ridgetops, and north- and east-facing slopes which burn somewhat infrequently. Picea pungens is most often found in cool, moist locations, often occurring as smaller patches within a matrix of other associations. As many as seven conifers can be found growing in the same occurrence, and there are a number of cold-deciduous shrub and graminoid species common, including Arctostaphylos uva-ursi, Mahonia repens, Paxistima myrsinites, Symphoricarpos oreophilus, Jamesia americana, Quercus gambelii, and Festuca arizonica. This system was undoubtedly characterized by a mixed-severity fire regime in its "natural condition," characterized by a high degree of variability in lethality and return interval.

This highly variable ecological system comprises mixed-conifer forests at montane elevations throughout the Intermountain West region. The four main alliances in this system are found on slightly different, but intermingled, biophysical environments: Abies concolor dominates at higher, colder locations; Picea pungens represents mesic conditions; and Pseudotsuga menziesii dominates intermediate zones. As many as seven conifers can be found growing in the same occurrence, with the successful reproduction of the diagnostic species determining the association type. Common conifers include Pinus ponderosa, Pinus flexilis, Abies lasiocarpa var. lasiocarpa, Abies lasiocarpa var. arizonica, Juniperus scopulorum, and Picea engelmannii. Populus tremuloides is often present as intermingled individuals in remnant aspen clones or in adjacent patches. 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).

Several cold-deciduous shrub and graminoid species are found in many occurrences (e.g., Arctostaphylos uva-ursi, Mahonia repens, Paxistima myrsinites, Symphoricarpos oreophilus, Jamesia americana, Quercus gambelii, and Festuca arizonica). Other important species include Acer glabrum, Acer grandidentatum, Amelanchier alnifolia, Arctostaphylos patula, Holodiscus dumosus, Jamesia americana, Juniperus communis, Physocarpus monogynus, Quercus arizonica, Quercus rugosa, Quercus x pauciloba, Quercus hypoleucoides, Robinia neomexicana, Rubus parviflorus, 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 (= Muhlenbergia virescens), 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 Lathyrus, Penstemon, Lupinus, Vicia, Arenaria, Galium, and others.

These are mixed-conifer forests occurring on all aspects at elevations ranging from 1200 to 3300 m. Landforms are variable and can include canyons, plateaus, draws, benches, hills, mesas, ravines, shoulders, sideslopes and toeslopes. Slopes can be gentle to extremely steep. Rainfall averages less than 75 cm per year (40-60 cm), with summer "monsoons" during the growing season contributing substantial moisture. Geologic substrates include volcanic andesite, rhyolite, rhyolitic tuffs, colluvium, shale gneiss, granite, sandstone and limestone. Soils are variable from cobbles, clay loam, silt loam, sandy loam, sand, and gravel.

Forests in this ecological system represent the gamut of fire tolerance. Formerly, Abies concolor in the Utah High Plateaus were 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 (www.fs.fed.us/database/feis/). 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 the only true "fire-tolerant" occurrences in this ecological system. 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 occurrences for 200 years (Steele et al. 1981) and germinate abundantly after fire, competitively suppressing conifer seedlings. Successional relationships in this system are complex. Pseudotsuga menziesii is less shade-tolerant than many northern or montane trees such as Tsuga heterophylla, Abies concolor, Picea engelmannii, 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 have been logged or burned during European settlement, and present-day occurrences are second-growth forests dating from fire, logging, or other occurrence-replacing disturbances (Mauk and Henderson 1984, Chappell et al. 1997).

Picea pungens is a slow-growing, long-lived tree which regenerates from seed (Burns and Honkala 1990a). 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.

In general, fire suppression has lead 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.

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2810510). These are summarized as:

A) Early Development 1 All Structures (15% of type in this stage): Shrub cover is 0-80%. Succession after a lethal fire will depend on what vegetation was on site before. In a general conifer-dominated scenario, some ponderosa pines are likely to survive. Fire will be an opportunity for new ponderosa pine establishment. On site Gambel oak will resprout. White fir will also be regenerating. If aspen cover is 50-100% prior to disturbance, the stand would regenerate back to aspen.

B) Mid Development 1 Closed (tree-dominated - 15% of type in this stage): Tree cover is 51-80%. If aspen is dominant the stand will achieve a mid-closed stage. Conifers such as white fir and Douglas-fir could be regenerating with it. Any surviving conifers such as ponderosa pine would be canopy dominants. If aspen canopy cover is 50-100%.

C) Mid Development 1 Open (tree-dominated - 10% of type in this stage): Tree cover is 21-50%. Ponderosa pine is the canopy dominant with an understory dominated by white fir. Douglas-fir present and some of its regeneration is entering the canopy. If aspen were present, the stand would have undergone some self-thinning that would have opened up the canopy. The conifers in the stand create a more flammable litter bed with their needles so that patchy surface fire could carry. Any fire would further open the stand by thinning aspen and fir. Eventually the aspen stand would become very open sharing the canopy with ponderosa pine and Douglas-fir.

D) Late Development 1 Open (conifer-dominated - 50% of type in this stage): Tree cover is 21-50%. Ponderosa pine is the canopy dominant. Douglas-fir can also be a canopy dominant. Recurrent fire maintains white fir as an understory tree, but a rare white fir will join the other two species in the canopy. If aspen is present, its numbers are few. Low levels of suckering may keep it in the stand. Open aspen stands are not common in the warm/dry mixed conifer.

E) Late Development 1 Closed (tree-dominated - 10% of type in this stage): Tree cover is 51-80%. Aspen stand is mature to over-mature with a heavy understory of conifers, mainly white fir and some Douglas-fir.

This BpS has a fire regime very similar to ponderosa pine. Frequent low-intensity surface fire is the dominant mode of disturbance. Fire intervals range from 2-71 years with a mean of 15 years. Lethal fires can occur on a limited scale, but this is not the norm unless aspen is involved. These will be characterized as mixed fires because they most likely occur as a part of a more widespread surface fire. Bark beetles may impact this BpS in isolated areas at small scales (LANDFIRE 2007a, BpS 2810510).

Nutrient cycling, specifically carbon cycling, is an important ecological process within many ecological systems. Biological decomposition in ponderosa pine forests is more limited than biological production, resulting in accumulation of organic materials, especially in the absence of fire (Harvey 1994, Graham and Jain 2005).

Threats and stressors to this forest and woodland system include altered fire regime, altered stand structure from fragmentation due to roads, logging, mining, or other human disturbances (CNHP 2010). These disturbances can cause significant soil loss/erosion and negatively impact the water quality within the immediate watershed (CNHP 2010). Invasive exotic species can become abundant in disturbed areas and alter floristic composition. Direct and indirect effects of climate change may alter dynamics of indigenous insects such as Douglas-fir beetle (Dendroctonus pseudotsugae) or mountain pine beetle (Dendroctonus ponderosae) causing a buildup in population size (with less extreme winters) leading to large outbreaks that can cause high mortality in mature trees.

This system occurs throughout the southern Rockies, north and west into Utah, Nevada, eastern Wyoming (very southern in the Laramie Range and possibly on Sheep Mountain) and Idaho. Although not common, it does occur in southeastern Oregon but does not extend farther west into the Cascades.