Central Rockies Douglas-fir Forest

This ecological system occurs 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. It extends north into Montana on the east side of the Continental Divide, north to about the McDonald Pass area, and into the Rocky Mountain Front region of Montana. This is a Pseudotsuga menziesii-dominated system without the maritime floristic composition; these are forests and woodlands occurring in the Central Rockies where the southern monsoon influence is lessened and maritime climate regime is not important. This system includes 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 absent in these occurrences, but Picea engelmannii can occur in some stands. Understory components include shrubs such as Physocarpus malvaceus, Juniperus communis, Symphoricarpos oreophilus, and Mahonia repens, and graminoids such as Calamagrostis rubescens, Carex rossii, and Leucopoa kingii. The fire regime is of mixed severity with moderate frequency. This system often occurs 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, this system occurs as isolated stands of Pseudotsuga menziesii, with Artemisia tridentata, Pseudoroegneria spicata, Leucopoa kingii, and Carex rossii.

This is a Pseudotsuga menziesii-dominated system without the maritime floristic composition; it includes extensive Pseudotsuga menziesii forests, occasionally with Pinus flexilis on calcareous substrates and Pinus contorta at higher elevations. Picea engelmannii can occur in some stands; however, true firs, such as Abies concolor, Abies grandis, and Abies lasiocarpa, are absent. Understory components include shrubs such as Artemisia tridentata, Physocarpus malvaceus, Juniperus communis, Symphoricarpos oreophilus, and Mahonia repens, and graminoids such as Calamagrostis rubescens, Carex rossii, Leucopoa kingii, and Pseudoroegneria spicata. 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.

These are forests and woodlands occurring in the Central Rockies where the southern monsoon influence is lessened and maritime climate regime is not important. These Pseudotsuga menziesii forests occur under a comparatively drier and more continental climate regime, and at higher elevations than in the Pacific Northwest. Elevations range from less than 1000 m in the central Rocky Mountains to over 2400 m in the Wyoming Rockies. Lower-elevation stands typically 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.

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.

Soils are highly variable and derived from diverse parent materials. Pseudotsuga menziesii forests are reported by most studies (Pfister et al. 1977, Steele et al. 1983, Mauk and Henderson 1984) to show no particular affinities to geologic substrates. Rock types can include extrusive volcanics in the Yellowstone region, and sedimentary rocks elsewhere in the Rockies. The soils are typically slightly acidic (pH 5.0-6.0), well-drained, and well-aerated. They can be derived from moderately deep colluvium or shallow-jointed bedrock and are usually gravelly or rocky.

Successional relationships in this group are complex. Pseudotsuga menziesii is less shade-tolerant than some montane trees such as Abies concolor or 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 flexilis, which helps to minimize drought stress. In some locations, much of these forests have been 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 contorta) (Pfister et al. 1977). Some stands may have higher tree-stem density than historically, due largely to fire suppression (Steele et al. 1983).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 2111660). These are summarized as:

A) (10% of type in this stage) Tree cover is 0-100%. Dominated by graminoids and seedling/sapling Douglas-fir and possibly lodgepole pine. Understory may be dominated by Calamagrostis rubescens and/or Carex spp. Shrub species such as Symphoricarpos spp. may be present. Succession occurs in approximately 40 years, and the class moves to a mid-open state. Replacement fire occurs every 500 years, and mixed fire occurs every 200 years. If this class experiences no fire in 20 years, it will move to class B, a mid-closed state. Wind/weather events occur infrequently (probability of 0.001), but the class is maintained in this state.

B) Mid Development 1 Closed (tree-dominated - 10% of type in this stage): Tree cover is 41-100%. Relatively dense pole and some medium Douglas-fir and possibly lodgepole pine. The understory is open and relatively depauperate. Understory may be dominated by Calamagrostis rubescens and/or Carex spp. This class persists for 80 years, then moves to a late-closed stage. Replacement fire occurs every 200 years, and mixed fire every 50 years, causing a transition to a mid-open stage. Insect/disease outbreaks occur with a probability of 0.005 and can move the class to a mid-open state. Also, wind/weather stress causes a change to a mid-open state with a probability of 0.001. Although reviewers recommended removing insects/disease from this class, it was decided by Region 1 insect experts that some insect damage is likely for the class B forest types. The insects to be concerned about at low levels are Douglas-fir pole beetle and western spruce budworm.

C) Mid Development 1 Open (tree-dominated - 10% of type in this stage): Tree cover is 21-40%. Open pole and medium Douglas-fir that may have lodgepole pine with patchy graminoid cover and dispersed shrubs such as Symphoricarpos spp. Understory may be dominated by Calamagrostis rubescens and/or Carex spp. Conifer heights range between 5-20 m but adjusted to eliminate class overlap. This class can persist for 60 years, then moves to a late-open stage. Replacement fire occurs every 200 years, and mixed fire every 40 years. Without fire for 58 years, this class can move to a mid-closed state. Insect/disease outbreaks and wind/weather events occur with a probability of .005, and maintain this class in a mid-open state.

D) Late Development 1 Open (conifer-dominated - 50% of type in this stage): Tree cover is 21-40%. Open canopy of medium to large Douglas-fir with a graminoid and shrub understory with highly variable understory cover. Lodgepole pine may be present. Understory may be dominated by Symphoricarpos spp., Calamagrostis rubescens, and/or Carex spp. Heights can exceed 25 m up to approximately 30 m. Replacement fire occurs every 500 years, and mixed fire every 50 years. Without fire for 45 years, this class can move to a late-closed state. Insect disturbance occurs every 10 years but does not move this class to another class. Wind/weather stress also occurs, with a probability of 0.008, but does not cause a transition to another class.

E) Late Development 1 Closed (conifer-dominated - 20% of type in this stage): Tree cover is 41-100%. Multi-storied Douglas-fir, sometimes with lodgepole pine present. Understory with variable cover often dominated by Calamagrostis rubescens, Carex spp., Symphoricarpos spp., and/or Physocarpus malvaceus. Heights can exceed 25 m up to approximately 30 m. Replacement fire occurs every 200 years, and mixed fire every 30 years, causing a transition back to a late-open state. Insect outbreaks occur frequently, probability of 0.01, and cause a transition to an open state. Wind/weather stress occurs with a probability of 0.005 and causes a transition to a late-open state.

Fire regime is predominantly mixed-severity (Fire Regime III) with a MFI of approximately 20-50 years (Houston 1973, Arno and Gruell 1983, Fischer and Clayton 1983, Littell 2002, Korb et al. in prep.). Mixed-severity fires are generally characterized as spatially heterogeneous (LANDFIRE 2007a, BpS 2111660). Fire regime in more northern stands is predominantly mixed with a MFI of approximately 35-50 years (Crane and Fischer 1986, Bradley et al. 1992) (LANDFIRE 2007a, BpS 1911660).

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. These disturbances can cause significant soil loss/erosion and negatively impact the water quality within the immediate watershed. 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) 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 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. It extends north into Montana on the east side of the Continental Divide to the Rocky Mountain Front and includes all of the Beaverhead Mountains Section (M332E) (Bailey et al. 1994). It may also occur in scattered patches in southeastern Oregon.