Montana Field Guide

This inland Pacific Northwest ecological system occurs in the foothills of the Montana Rocky Mountains, where it forms a broad ecotone between true forests ad true steppe, shrublands, or grasslands, typically on warm, dry, exposed sites too droughty to support a closed tree canopy. This is not a fire-maintained system. The "steppe" character results from a climate-edaphic interaction that results in a graminiod-dominated landscape with widely scattered trees; even in the absence of fire, a "woodland" or "forest" structure will not be obtained. Occurrences are found on all slopes and aspects; however, moderately steep to very steep slopes or ridgetops on southerly or western aspects are most common. They can be found on glacial till, glacio-fluvial sand and gravel, dune, basaltic rubble, colluvium, deep loess or volcanic ash-derived soils, with characteristic features of good aeration and drainage, coarse texture, and an abundance of mineral material. Ponderosa pine (Pinus ponderosa) or Douglas-fir (Pseudotsuga menziesii)are the predominant conifers. Limber pine (Pinus flexilis)may be present in some occurrences. In fire-protected transition areas with big sagebrush steppe systems, antelope bitterbrush (Purshia tridentata), Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), big sagebrush (Artemisia tridentata ssp. tridentata), and three-tip sagebrush (Artemisia tripartita) may be common. Deciduous shrubs such as common ninebark (Physocarpus malvaceus), commonsnowberry (Symphoricarpos albus), or birch leaf spiraea (Spiraea betulifolia)may be abundant in occurrences west of the Continental Divide. Important grass species include bluebunch wheatgrass (Pseudoroegneria spicata), Sandberg’s bluegrass (Poa secunda), needle and thread (Hesperostipa comata), needlegrass (Achnatherumspecies), and bottlebrush squirreltail (Elymus elymoides). This system is very similar to Northern Rocky Mountain Ponderosa Pine Woodland and Savanna, but with more widely scattered trees.

Woody and herbaceous, aridic and shallow soils, short disturbance interval, Pinus ponderosa, Pseudotsuga menziesii

Ponderosa pine (Pinus ponderosa) or Douglas-fir (Pseudotsuga menziesii) are the most common conifers. Limber pine (Pinus flexilis) may be present in some occurrences. In transition areas with sagebrush steppe systems, antelope bitterbrush (Purshia tridentata), Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), big sagebrush (Artemisia tridentata ssp. tridentata), and three-tip sagebrush (Artemisia tripartita) may be common in fire-protected sites such as rocky areas. Common ninebark (Physocarpus malvaceus), common snowberry (Symphoricarpos albus), or birch leaf spiraea (Spiraea betulifolia) can be abundant in occurrences on northern or eastern aspects, or sites west of the Continental Divide in northwestern Montana. Important grass species include bluebunch wheatgrass (Pseudoroegneria spicata), Sandberg’s bluegrass (Poa secunda), needle and thread (Hesperostipa comata), needlegrass (Achnatherum species), prairie junegrass (Koeleria macrantha) and bottlebrush squirreltail (Elymus elymoides).

Common forbs include yarrow (Achillea millefolium), nodding onion (Allium cernuum), rosy pussytoes (Antennaria rosea), hairy golden aster (Heterotheca villosa), elk thistle (Cirsium undulatum), fleabane (Erigeron species), buckwheat (Eriogonum species), Indian blanket flower (Gaillardia aristata), stoneseed (Lithospermum ruderale), silky lupine (Lupinus sericeus), silky crazyweed (Oxtropis sericea), and Hood’s phlox (Phlox hoodii). Arrowleaf balsamroot (Balsamorhiza sagittata) can be abundant in some occurrences.

Dry conditions and droughty soils that limit tree establishment are the driving factors in this system. Tree growth is likely episodic, with regeneration episodes in years with available moisture. Tree density is limited in some areas by available growing space due to the rocky conditions of the site. Disturbances in this system include fire, soil erosion, and biotic vectors. This system burns occasionally, although it does not require fire to prevent succession to a closed canopy forest structure due to the interaction of climate and edaphic factors. Fire frequency is estimated to be 30 to 50 years (Landfire, 2007). While tree seedlings may be killed by fire, tree species in this system exhibit moderate to high fire resistance (Steinberg, 2002; Howard, 2003), and fuel loads are generally light due to open rocky terrain. Fires are consequently typically of low severity and cause minimal damage. Frequent low-intensity surface fires promote grass production (Howard, 2003), but may also encourage invasion by non-native species (Symstad et al., 2014). Consequently, invasion of non-native cheatgrass (Bromus tectorum) is changing fire frequency, spread, and severity, and cheatgrass invasion risk in Montana is expected to increase under future climatic conditions (Bradley, 2009).

Increasing drought frequency and severity subjects conifers in this system to increased incidence of biotic disturbance. Drought affects Douglas-fir susceptibility to Douglas-fir tussock moth (Orgyia pseudotsugata), Douglas-fir bark beetle (Dendroctonus pseudotsugae), and western spruce budworm (Choristoneura occidentalis) outbreaks (Steinberg, 2002). Both drought-stressed ponderosa pine and limber pine have an increased likelihood of attack by the mountain pine beetle (Dendroctonus ponderosae) (Graham and Jain, 2005; Jackson et al., 2010). However, outbreaks may be less likely to reach epidemic levels as stands in this system exhibit extremely low stand densities (Howard, 2003). White pine blister rust (Cronartium ribicola) additionally infects limber pine in this system, causing reduced regeneration and mortality (Jackson et al., 2010). Trees that occupy this edge environment and exhibit resistance to abiotic and biotic stressors are important genetic resources to maximize adaptive potential to current and continued environmental change.

In the absence of natural fire, periodic prescribed burns can be used to maintain and restore this system to similar pre-settlement conditions, although results may be variable due to insufficient ground fuels and the rocky terrain typical of this system. Periodic burning is used to expose mineral soils, provide nutrient availability, reduce competition, and stimulate native grass and forb production. When insect outbreaks are active, a combination of control actions including thinning, removal of infested trees, and application of targeted insecticides can be used, although control efforts are best utilized at small scales and for high-value individuals (Howard, 2003). In light of blister rust prevalence, stands can be managed to maintain limber pine forest composition, and to diversify age structure to include regeneration, thereby encouraging natural selection of rust resistant individuals (Jackson et al., 2010).

Excessive grazing can result in the depletion of the most common perennial grasses and lead to an abundance of exotic grasses in this system. In some cases, especially on sites heavily infested with cheatgrass, frequent prescribed burning at low intensities may stimulate greater cheatgrass cover following fire, especially if the burn fails to eliminate the seed bank. Controlling invasive species like cheatgrass prior to prescribed burning may help to reduce post-burn outbreaks (Symstad et al., 2014), and increasing the time between prescribed fires may inhibit cheatgrass by increasing surface fuels (both herbaceous and litter) which directly inhibit cheatgrass establishment (Keeley and McGinnis, 2007). On sites with slopes and forage that support domestic livestock, use can be monitored to maintain soil integrity.