Montana Field Guide

This system occurs on warm, dry, exposed sites in the foothills of the Rocky Mountains in west-central and central Montana, at the ecotone between grasslands or shrublands and more mesic coniferous forests. Elevations range from 1,066 to 1,676 meters (3,500-5,500 feet), with higher elevation examples mostly confined to central Montana. Occurrences are found on all slopes and aspects; however, moderately steep to very steep slopes or ridgetops are most common. True savanna types are infrequent; the system is more characteristically an open forest with a grassy understory. In the western part of the state, this system is seen mostly on dry slopes in the rainshadow of the Bitterroot Mountains. East of the Continental Divide, it is most widespread around Helena and Lewistown, although it occurs throughout mountain ranges as far east as the Little Rocky and Bearpaw Mountains. Ponderosa pine (Pinus ponderosa) is the dominant conifer. Douglas-fir (Pseudotsuga menziesii) and western larch (Larix occidentalis) may be present in the tree canopy in the more western areas, but are usually absent. In central Montana, limber pine (Pinus flexilis) and horizontal juniper (Juniperus horizontalis) are frequently components. Although the understory of ponderosa pine forests is often shrubby in other states, in Montana, habitats are mostly dominated by graminoids, although bitterbrush (Purshia tridentata), white snowberry (Symphoricarpos albus), and skunkrush (Rhus trilobata) occur in forests on benchlands and rocky slopes in the central portion of the state. Understory vegetation is more typically grasses and forbs that resprout following low to moderate intensity surface fires. Prolonged drought, beetle kill and exotic invasion are rapidly changing the dynamics of this system.

In western and central Montana, this ecosystem forms a belt on warm, dry, exposed sites between grasslands and Douglas-fir (Pseudotsuga menziesii) forests. Elevations range from 1,066 to 1,676 meters (3,500-5,500 feet) (Pfister et al, 1977). This system can occur at higher elevations in central Montana. It is generally found on gravelly soils with good aeration and drainage and a neutral to slightly acidic pH.

Ponderosa pine is the dominant conifer. Douglas-fir and western larch may be present in the tree canopy in the more western areas, but are usually absent. In central Montana, limber pineand horizontal juniperare often components. Although the understory for ponderosa pine forests is often shrubby in other states, in Montana, habitats are mostly dominated by grasses, although antelope bitterbrush, snowberry, serviceberry (Amelanchier alnifolia), bearberry (Arctostaphylos uva-ursi), common juniper (Juniperus communis) and skunkbush occur in forests on benchlands and rocky slopes in the central portion of the state. Understory vegetation is more typically fire-resistant grasses and forbs that resprout following surface fires. High shrub cover, understory trees, and downed logs are uncommon. These more open stands support grasses such as bluebunch wheatgrass (Pseudoroegneria spicata), which is usually dominant, prairie junegrass (Koeleria macrantha) and needle and thread (Hesperostipa comata), as well as dryland sedges likethreadleaf sedge (Carex filifolia) and sun sedge (Carex inops ssp. heliophila). On more mesic sites,bluebunch wheatgrass occurs as the dominant graminoid species with Idaho fescue (Festuca idahoensis) and rough fescue (Festuca campestris). In central Montana, soapweed yucca (Yucca glauca), Pennsylvania sedge (Carex pennsylvanica), grama (Bouteloua spp.) and bluestem (Andropogon spp.) occur on especially dry sites. Common herbaceous forbs include yarrow (Achillea millefolium), pink pussytoes (Antennaria rosea), arrowleaf balsamroot (Balsamorhiza sagittata), Indian blanket flower (Gaillardia aristata), and silky lupine (Lupinus sericeus).

Under natural conditions, ponderosa pine woodlands and grasslands are maintained by frequent surface fires (Arno, 1980). The threat of stand-replacing fires is high in areas where periodic fire has been suppressed. A study at the Sawmill Resource Natural Area in the Bitterroot National Forest found an average fire return interval of 13 years prior to the initiation of systematic fire suppression policies. In the absence of fire, forest density increases, and fire-intolerant species like Douglas-fir become more common (Gayton et al., 2006). Grazing by domestic livestock may reduce bunchgrasses, and in cases of intensive overgrazing, cheatgrass (Bromus tectorum) may be dominant in the understory. Wind is not generally an issue, except when trees have been damaged by lightning strikes.

In the absence of natural fire, periodic prescribed burns, selective thinning, and reduction of ladder and basal fuels to prevent crown fires can be used to maintain and restore this system to similar pre-settlement conditions. Thinning understory trees and manually removing ladder fuels and heavy fuels from the base of large trees may be necessary to protect old growth from mortality during prescribed burns.

Periodic burning is used to expose mineral soils, provide nutrient availability, reduce competition, stimulate native grass and forb production, increase basal diameter growth of overstory ponderosa pine, and provide favorable seedbeds. 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 did not kill the seed bank. Increasing time between prescribed fires may inhibit cheatgrass by increasing surface fuels (both herbaceous and litter) which directly inhibit establishment, and by creating higher intensity fires capable of killing a much greater fraction of the seed bank. Postfire cheatgrass dominance has been shown to be most strongly controlled by the pre-fire seed bank, soil moisture, fire intensity, soil nitrogen, and duration of direct sunlight (Keeley and McGinnis 2007). Excessive grazing can lead to the loss of the most common perennial grasses and increased abundance of exotic grasses.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 Pyke 1983). Prolific seed production also contributes to the competitive advantage of this grass over native grasses and associated perennial forbs.

In recent years, prolonged summer drought conditions and milder winters have led to increasingly severe outbreaks of mountain pine beetle (Dendroctonus ponderosae). Ponderosa pine mortality is high in many areas of Montana, especially in the west-central area of the state. Beetle kill is creating conditions for severe, stand replacing fires and is rapidly changing the dynamics and potential recovery of this system.