Montana Field Guide

This ecological system occurs in foothill and lower montane zones in the northern Rocky Mountains and island mountain ranges of Montana and on escarpments extending out to the western Great Plains grasslands. Elevation ranges from 1,219 to 2,286 meters (4,000-7,500 feet), occasionally higher in southwestern Montana. At higher elevations, it is limited to sites with thin soils on rock outcrops. Some of the most ecologically interesting examples occur along and within the mountains of the Rocky Mountain Front where it occurs most commonly on west and north facing aspects. At lower elevations, it can occur on all aspects and on relatively level terrain. Fire is infrequent and spotty because rocky substrates inhibit growth of the continuous canopy that would be needed to spread. This system occurs on sites that are characterized by extreme winter weather and droughty summer conditions. It is typically dominated by limber pine (Pinus flexilis) or Rocky Mountain juniper (Juniperus scopulorum). This systemis usually found below continuous forests of Douglas-fir (Pseudotsuga menziesii), or rarely, ponderosa pine (Pinus ponderosa) or lodgepole pine (Pinus contorta) in the foothills. Rocky Mountain juniper stands often occur in complex transitional zones or grow on exposed or severe sites within other forest systems. These juniper stands can exhibit a savanna-like character in southwestern Montana. In the system as a whole, because sites are so marginal for tree growth, limber pine mortality from abiotic and biotic stresses may be high. East of the Continental Divide, limber pine can occur at the upper tree line, with whitebark pine (Pinus albicaulis) in Glacier National Park and the Sweetgrass Hills. The climtate characteristic of these systems is marked by a relatively small amount of precipitation, with the wettest months during the growing season, very low humidity, and wide annual and diurnal temperature ranges. Winter conditions may be very cold but relatively dry, and often include rapid fluctuations in temperature associated with chinook winds. In Montana, limber pine and Rocky Mountain juniper stands are found mainly on calcareous substrates. Soils have a high rock component (generally over 50% cover) and are coarse- to fine-textured, often gravelly. Slopes are moderately steep to steep.

Forest and woodland, aridic, sandy, shallow soils, organic A horizon less than 10 cm, Pinus flexilis, Juniperus scopulorum

The systems is usually found below continuous forests of Douglas-fir (Pseudotsuga menziesii) or lodgepole pine (Pinus contorta) in the foothills,and can occur in large stands well within the zone of continuous forests in the northern Rocky Mountains. Along the Northern Rocky Mountain Front, this system is found on west and north facing aspects in the foothills and within the mountains. At lower elevations, it can occur on all aspects and on relatively level terrain. Rocky Mountain juniper stands are often found in complex transitional zones or growing on exposed or severe sites within other forest systems. Climate is characterized by a relatively small amount of precipitation, with the wettest months during the growing season, very low humidity, and wide annual and diurnal temperature ranges. Winter conditions may be very cold but relatively dry, and often include rapid fluctuations in temperature associated with chinook winds.

In Montana, this system occurs grows mainly on limestone substrates, where roots follow the pattern of fractured and weathered rock (Burns and Honkala, 1990). Soils have a high rock component (typically over 50% cover) and are coarse- to fine-textured, often gravelly and calcareous. Soils are generally poorly developed, shallow, have low moisture holding capacity and are easily erodable, so in some occurrences, little topsoil is present. Although the system can be seen on gently rolling terrain, limestone cliffs, and exposed bluffs, it is most often found on rocky ridges and steep rocky slopes, and can survive in extremely windswept areas at both lower and upper tree line. Slopes are typically moderately steep to steep. Elevation ranges from 1,219-2,286 meters (4,000-7,500 feet) (Pfister et al, 1977). In the Beaverhead Mountain range, it occurs at higher elevations (Cooper et al, 1999).

Vegetation is characterized by an open-tree canopy or patchy woodland that is dominated by either limber pine or Rocky Mountain juniper. In the Pryor Mountains Utah juniper (Juniperus osteosperma) is sometimes seen at its northernmost extent within this system. Douglas-fir regularly occurs, but ponderosa pine and lodgepole pine are comparatively rare within these forests. Prolonged drought and white pine blister rust (Cronartium rubicola) have decimated limber pine along the Rocky Mountain Front (and elsewhere), resulting in a skeleton woodland with scattered Douglas-firs as the only living trees. At the northern end of its range in Montana, limber pine is mostly associated with common juniper (Juniperus communis) and creeping juniper (Juniperus horizontalis), whereas in the southern end of its range, it is associated with Rocky Mountain juniper. A sparse to moderately dense short-shrub layer is usually present. Within north-central and northwestern Montana, the most common shrubs include bearberry (Arctostaphylos uva-ursi), creeping juniper, shrubby cinquefoil (Dasiphora fruticosa ssp. floribunda),and Canadian buffaloberry (Shepherdia canadensis). Other shrubs that may be present in west-central and southern Montana includebig sagebrush (Artemisia tridentata), black sagebrush (Artemisia nova), curl-leaf mountain mahogany (Cercocarpus ledifolius), rubber rabbitbrush (Ericameria nauseosa), skunkbush sumac (Rhus trilobata), Woods’ rose (Rosa woodsii), common snowberry (Symphoricarpos albus), or western snowberry (Symphoricarpos occidentalis). Herbaceous layers are generally sparse, but range to moderately dense; they are typically dominated by perennial graminoids such as blue grama (Bouteloua gracilis), Idaho fescue (Festuca idahoensis), rough fescue (Festuca campestris), poverty oatgrass (Danthonia intermedia), spike fescue (Leucopoa kingii), needle and thread (Hesperostipa comata), prairie junegrass (Koeleria macrantha), Indian ricegrass (Oryzopsis hymenoides), Sandberg’s bluegrass (Poa secunda), or blue bunch wheatgrass (Pseudoroegneria spicata). Common forbs include yarrow (Achillea millefolium), fringed sage (Artemisia frigida), arrowleaf balsamroot (Balsamorhiza sagittata), prairiesmoke (Geum triflorum), hymenopappus (Hymenopappus species), four-nerve daisy (Hymenoxys species), dotted gayfeather (Liatris punctata), stone seed (Lithospermum ruderale), silver lupine (Lupinus argenteus), pricklypear (Opuntia species), crazyweed (Oxytropis species), and cushion plants such as draba (Draba species), phlox (Phlox species), Rocky Mountain douglasia (Douglasia montana) and Howard’s alpine forget-me-not (Eritrichium howardii).

This system often occurs in complex ecotones on severe sites within other forest systems. It often intergrades with Rocky Mountain Dry-Mesic Montane Mixed Conifer, Rocky Mountain Lower Montane, Foothill and Valley Grassland, Mountain Mahogany Woodland and Shrubland, and Montane Sagebrush Steppe.

Major disturbances in this system include fire, soil erosion from over-used range, and biotic vectors. Clark's nutcrackers have co-adapted an important mutualism with limber pine, and are the primary harvester and disperser of its seeds. These woodlands often originate with and are likely maintained by fire. Regeneration on burns is largely from germination of seedlings from Clark's nutcracker seed caches. Fire can easily kill young limber pine and Rocky Mountain juniper because of their thin bark (Fischer and Clayton, 1983), however, fuel loads in this system are generally light due to open rocky terrain, and fires do not generate severe damage or considerably alter vegetation composition. In comparison with upper treeline limber pine communities, foothill populations are thought to experience greater disturbance frequency (Schuster et al., 1995), and fire return intervals vary between 50 and 400 years (U.S. Department of Agriculture, 2012). Over time limber pine woodlands have shifted both up- and down-slope in response to changing climate and drought, and are considered to be particularly sensitive to future change (Means 2010).

This system occurs on dry, rocky sites that are typified by extreme winter weather and droughty summer conditions that offer marginal conditions for tree growth. Consequently, mortality from abiotic and biotic stressors is high in some areas. Limber pine is highly susceptible to white pine blister rust (Cronartium ribicola), the pine needle pathogen (Dothistroma septospora), and mountain pine beetles (Dendroctonus ponderosae). It can also be heavily infected or killed by limber pine dwarf-mistletoe (Arceuthobium cyanocarpum), particularly in south-central and southwestern Montana (Jackson et al., 2010), and is susceptible to infestation by cone beetles (Conophthorus contortae), the ponderosa pine cone worm (Dioryctria auranticella), and the western conifer seed bug (Leptoglossus occidentalis). Resistance to blister rust and mountain pine beetle is lower for limber pine than for other North American pines affected by these biotic agents (Hoff and McDonald 1993; Means 2010), and drought can exacerbate vulnerability to pine beetle attack (Jackson et al., 2010). The most significant damage due to biotic factors appears to occur at locations on the Lewis and Clark National Forest along the northern Rocky Mountain Front, the Gravelly range and sections of the Yellowstone ecosystem in southwestern Montana. Large numbers of trees have very thin crowns and poor terminal growth, and severe mortality is occurring in some areas.

Under natural conditions, Rocky Mountain juniper seedlings become established on moist sites in partial shade (Burns and Honkala 1990). Stands found in southwestern Montana may exhibit a savanna-like character due to the inability of Rocky Mountain juniper to establish on drier micro-sites. Rocky Mountain juniper is generally shallow rooted, and forest health can be negatively affected by heavy grazing, especially on exposed sites with erodible soils. Although Rocky Mountain juniper is relatively resistant to disease and tolerant of insects, several insect pests do attack this species including cedar flathead borers (Chrysobothris spp.) and bark beetles (Phloeosinus spp.) (Scher, 2002). Additionally, mistletoes (Phoradendron spp.), a blight caused by Cercospora sequoia, and cedar apple rust (Gymnosporangium juniper virgiananae) can be especially problematic (Burns and Honkala 1990).

Reintroduction of prescribed fire fosters limber pine restoration because it provides open sites and exposed mineral soils that are suitable for Clark's nutcracker to cache seeds, and for seedlings to establish. Augmenting natural limber pine regeneration with seed sources that exhibit some resistance to blister rust or, in some cases, with nursery stock, will be necessary in areas where seed sources are absent or greatly reduced. Blister rust-resistant or tolerant trees can be identified in previously infected stands, or by screening for an identified resistance gene (Schoettle and Sniezko 2007; Schoettle et al. 2014). Direct seeding may be the most practical restoration method on most sites, however seed-transfer guidelines should be considered when the seed source is a considerable distance from the seeding site (Schoettle and Sniezko 2007). Germination will take place during the first growing season after fall caching or direct seeding.

Because this system is characterized by shallow soils, outplanting of nursery stock may be limited to microsites with deeper soil pockets. Out-planted seedling survival has been shown to be higher when seedlings are planted on microsites with some cover, when seedlings are planted in clumps rather than singly, and when competition from surrounding vegetation is minimized (Asebrook et al. 2011). Seedling health was marginally improved in burned areas compared to unburned areas within an experimental area in Waterton Lakes National Park, Alberta (Asebrook et al., 2006).