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. Tree canopy will never reach woodland density due to the interaction of climate and edaphic factors, even in the absence of fire. This system burns occasionally, but the vegetation is sparse enough that fires are typically not carried through the stand. Fire frequency is estimated to be 30 to 50 years. However, the invasion of cheatgrass (Bromus tectorum) is changing fire frequency, spread, and severity. 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. Periodic burning is used to expose mineral soils, provide nutrient availability, reduce competition, and stimulate native grass and forb production.

Excessive grazing can result in the depletion of the most common perennial grasses and lead to an abundance of exotic grasses in this system.

Successful restoration of native grasses within this system may be limited to sites where pre-fire cheatgrass (Bromus tectorum) cover was low. Cheatgrass establishment in low-elevation ponderosa pine and Douglas-fir forests can be enhanced by disturbance that opens the understory, removes litter, or both. (Mack and Pyke1983). Fall germination and rapid elongation of roots provide cheatgrass with a competitive advantage over native perennial species (Harris, 1967). Prolific seed production also contributes to the competitive advantage of this grass over native grasses and associated perennial forbs.