Rocky Mountain Foothill Limber Pine-Juniper Woodland

This ecological system occurs in foothill and lower montane zones in the Rocky Mountains from northern Montana south to central Colorado and on escarpments across Wyoming extending out into the western Great Plains. Elevation ranges from 1000-2440 m. It occurs generally below continuous forests of Pseudotsuga menziesii or Pinus ponderosa and can occur in large stands well within the zone of continuous forests in the northeastern Rocky Mountains. It is restricted to shallow soils and fractured bedrock derived from a variety of parent material, including limestone, sandstone, dolomite, granite and colluvium. 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. At lower montane elevations, it is limited to the most xeric aspects on rock outcrops, and at lower elevations to the relatively mesic north aspects. Fire is infrequent and spotty because rocky substrates prevent a continuous vegetation canopy needed to spread. Vegetation is characterized by an open-tree canopy or patchy woodland that is 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 Arctostaphylos uva-ursi, Artemisia nova, Artemisia tridentata, Cercocarpus ledifolius, Cercocarpus montanus, Dasiphora fruticosa ssp. floribunda, Ericameria nauseosa, Juniperus horizontalis, Purshia tridentata, Rhus trilobata, Rosa woodsii, Shepherdia canadensis (important in Montana stands), Symphoricarpos albus, or Symphoricarpos oreophilus. Herbaceous layers are generally sparse, but range to moderately dense, and are typically dominated by perennial graminoids such as Bouteloua gracilis, Festuca idahoensis, Festuca campestris, Danthonia intermedia, Leucopoa kingii, Hesperostipa comata, Koeleria macrantha, Piptatheropsis micrantha, Poa secunda, or Pseudoroegneria spicata. Within this ecological system, there may be small patches of grassland or shrubland composed of some of the above species.

Vegetation is characterized by an open-tree canopy or patchy woodland that is dominated by either 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 Arctostaphylos uva-ursi, Artemisia nova, Artemisia tridentata, Cercocarpus ledifolius, Cercocarpus montanus, Dasiphora fruticosa ssp. floribunda, Ericameria nauseosa, Juniperus horizontalis, Purshia tridentata, Rhus trilobata, Rosa woodsii, Shepherdia canadensis (important in Montana stands), Symphoricarpos albus, or Symphoricarpos oreophilus. Herbaceous layers are generally sparse, but range to moderately dense, and are typically dominated by perennial graminoids such as Bouteloua gracilis, Festuca idahoensis, Festuca campestris, Danthonia intermedia, Leucopoa kingii, Hesperostipa comata, Koeleria macrantha, Piptatheropsis micrantha (= Piptatherum micranthum), Poa secunda, or Pseudoroegneria spicata.

This ecological system occurs in foothill and lower montane zones in the Rocky Mountains from northern Montana south to central Colorado and on exposed, windswept escarpments and other geographic breaks across Wyoming extending out into the northwestern Great Plains. Elevation typically ranges from 1000-2400 m. It occurs generally below continuous forests of Pseudotsuga menziesii or Pinus ponderosa but can occur in large stands well within the zone of continuous forests in the northeastern Rocky Mountains. In Wyoming, some limber pine stands are found up to 2440 m (8000 feet) elevation and are still included in this system.

Climate: This woodland system occurs in a semi-arid, cool-temperate climate. Annual precipitation patterns and amounts are variable but are typically below 500 mm annual precipitation with much occurring in winter as snow or spring rain.

Physiography/landform: Stands occur on moderately steep to steep slopes on all aspects but are most common on dry south- and west-facing slopes. At higher elevations, it is limited to the most xeric aspects on rock outcrops, and at lower elevations to the relatively mesic north aspects.

Soil/substrate/hydrology: It is restricted to shallow soils and fractured bedrock derived from a variety of parent material, including limestone and calcareous sandstone, but also dolomite, granite, gneiss, quartzite, rhyolite, schist, shale and colluvium. Some stands are on eroded substrates and resemble "badlands" while others may occur on lava flows. Soils are typically shallow and have a high rock component (skeletal) with typically over 50% cover of surface rock. They are often coarse-textured, such as gravelly, sandy loams or loams, but may include alkaline clays. Exposed soil is common, and many stands have over 50% cover of bare soil. Soil pH is typically neutral or slightly alkaline, but ranges from acidic to alkaline.

The processes shaping the distribution and persistence of scarp woodlands is not well understood (CNHP 2010). The interaction of wind, fire, and topography is thought to have played a major role in the current pattern of occurrences. These woodlands are not physiologically limited to a particular substrate, but are generally found on larger, relatively high escarpments, and not on smaller or more gently sloping breaks. The abrupt topographic changes may act as natural firebreaks. In addition, the typically sparse vegetation of the breaks in comparison with the adjacent deeper soils does not allow grassland fires to carry into the woodland understory (CNHP 2010).

Although some of the conifers that are typically codominant in Pinus flexilis stands are late-successional species, they are not likely to displace Pinus flexilis. This is because most of these stands occur on harsh sites where Pinus flexilis is more competitive than most other conifer species. These stands are generally considered to be topographic or edaphic "climax" stands (Cooper 1975, Eyre 1980). Even in stands at lower elevations, such as prairie breaks, it is unlikely that other coniferous species will become dominant (Eyre 1980). Because Pinus flexilis occurs over a broad range of elevations, it can also be important as a post-fire seral species on drier sites in the Rocky Mountains (Cooper 1975, Peet 1988). Peet (1978a) reported apparent competitive displacement with Pinus flexilis in Colorado. He noted that Pinus flexilis may dominate xeric sites from low to high elevations, except where Pinus aristata or Pinus albicaulis occur. There, Pinus flexilis is largely restricted to lower elevation, rocky sites. Peet (1978a) also reported that Pinus flexilis occurs in the less xeric Pinus contorta and Pinus ponderosa habitats. However, the higher elevation Pinus flexilis stands would be included in Rocky Mountain Subalpine-Montane Limber-Bristlecone Pine Woodland (CES306.819).

Birds and small mammals often eat and cache the large, wingless pine seeds. Most important is the Clark's nutcracker, which can transport the seeds long distances and cache them on exposed windswept sites (Lanner and Vander Wall 1980, Lanner 1985, 1996). This results in the regeneration of pines in clumps from forgotten caches (Woodmansee 1977, Eyre 1980, Steele et al. 1983).

Fire history information is lacking and has a wide range, making modeling difficult. As a whole, fire has occurred in this community in relation to fuel types adjacent to and within the woodland site. On shallow, rocky sites fire may have occurred less frequently. On deeper-soiled sites, the associated vegetation is more robust and would support a more frequent fire-return interval.

Given the uncertainty about the fire frequencies of this ecological system, it is predicted to vary from 30 to 80 years for mixed-severity fire and over 200 years for replacement fires (LANDFIRE 2007a). Fire is likely infrequent and spotty because rocky substrates prevent a continuous vegetation canopy that is needed for fire to spread.

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has three classes in total (LANDFIRE 2007a, BpS 2010490). These classes are summarized as:

A) Early Development 1 All Structures (30% of type in this stage): Grass/forb/shrub/seedling - usually post-fire. Cover is 0-30%. Shrub height 0-1.0 m. The first 25 years dominated by shrub/herbaceous. Toward end of class increasing pine/juniper. When pine/juniper becomes dominant it has 10-20% cover. Height of pine/juniper reaching 15 m (48 feet). On shallow, rocky sites, seedlings tend to establish in protected areas, such as sheltered spaces in rocky outcrops. On these sites there is little grass or herb competition. On deeper-soiled sites, there is a significant herbaceous component and seedlings are established from bird seed caches and seed from limber pine and juniper that were not killed. This class lasts for 50 years or less. Replacement fire occurs every 250 years.

B) Mid Development 1 Open (30% of type in this stage): Tree cover is 21-40%. Tree height <10 m. Trees are established, but typically short and widely spaced. Grasses and herbs are sparse in shallow, rocky soils. On deeper-soil sites grasses and shrubs are prevalent. This class lasts until trees are approximately 100 years old, and then succeeds to Class C. Other indicator species might be Cercocarpus montanus. Replacement fire occurs every 200 years.

C) Late Development 1 Closed (40% of type in this stage). Tree cover is 41-60%. Tree height <10 m. Mature trees greater than 100 years old. On shallow, rocky sites trees dominate the site with sparse shrub-grass understory. On deeper-soil sites mature trees are codominant with shrub-grass understory with an increasing component of younger age class limber pine and juniper that will shade out shrubs and eventually leave a woodland site dominated by pine or pine-juniper overstory and grass understory. It is possible that limber pine might not occur in this stage in some areas. Replacement fire occurs every 200 years. Insect/disease occur with a probability of 0.0016 (every 625 years, or 0.16% of this class each year), returning the class to class A.

Disturbance from firewood cutting, drought, and agricultural use may also influence the distribution and persistence of these woodlands (CNHP 2010).

Pinus flexilis is very susceptible to the non-native white pine blister rust (Cronartium ribicola) that infects and kills this tree (Hoff et al. 1980). There is long-term concern with the persistence of this species/system. Although the isolation of many stands on rocky outcrops and ranges has reduced that rate of spread, the only long-term solutions is propagating individuals that have high genetic resistance to blister rust (Steele et al. 1983, Burns and Honkala 1990a, Schmidt and McDonald 1990).

Other insect threats include epidemics of native mountain pine beetle (Dendroctonus ponderosae), which can attack and kill limber pine trees. The limber pine dwarf mistletoe (Arceuthobium cyanocarpum) is a common parasite of this tree, which can weaken but rarely kills it (Burns and Honkala 1990a).

This system occurs in foothill and lower montane zones in the Rocky Mountains from northern Montana south to central Colorado and on escarpments across Wyoming, extending out into the western Great Plains. Elevation ranges from 1000-2400 m. This system may also occur in southeastern Idaho, though it would not be common there. It is also very likely to occur north into Canada along the Front Range of Alberta, in similar ecological settings.