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), and in western Montana low-severity fires occur at mean return intervals of 15 to 23 years (Smith and Fischer, 1997). Fires in this system generally occur in the early spring or late fall when conditions are wet or when trees are dormant, increasing their fire resistance (Habeck, 1992). In comparison to the Great Plains Ponderosa Pine Woodland and Savanna system in central and eastern Montana, this system experiences a larger percentage of mixed-severity fires (U.S. Department of Agriculture, 2012). 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. The thick bark and open crown structure of ponderosa pines allows them to withstand fire, a necessity in this system to control competition and allow for successful regeneration. In the absence of fire, forest density increases, and fire-intolerant species like Douglas-fir become more common (Gayton et al., 2006). Increased forest density also leads to increased incidence of high severity fires and greater susceptibility to insect attack (Jenkins et al., 2014; Habeck, 1992).

Many biotic disturbances affect this system. Seedlings and saplings can be deformed by shoot borers (Eucosma sonomana) and tip moths (Rhyacionia bushnelli), and defoliation can occur as a result of insects like the pine butterfly (Neophasia menapia). The insects that cause the most extensive mortality in this system are of the genus Dendroctonus, and include the western pine beetle (Dendroctonus brevicomis) and the mountain pine beetle (Dendroctonus ponderosae) (Graham and Jain, 2005; Habeck, 1992). Bark beetles generally occur at endemic levels in this system. However, in recent years, prolonged summer drought, milder winters, and exclusion of low severity fires resulting in increased stand density have contributed to greater susceptibility to and increasing severity of bark beetle outbreaks (Kolb et al., 2007). Fire may also decrease tree defenses to beetle attack immediately post-fire, resulting in increased vulnerability to insect attack (Davis et al., 2012). Additionally, pathogens like dwarf mistletoe (Arceuthobium campylopodum) influence ponderosa pine growth rates and interact with the effects of beetle attacks to increase downed woody surface fuels (Klutsch et al., 2014). Ponderosa pine mortality is high in many areas of Montana, especially in the west-central area of the state. Increased incidence of beetle outbreaks in ponderosa pine stands can have ecosystem-level effects including changes to carbon cycling (Kurz et al., 2008), hydrology (Bearup et al., 2014; Mikkelson et al., 2013), and fuel structure and flammability (Hicke et al., 2012; Jolly et al., 2012).

Grazing by domestic livestock may reduce bunchgrasses, and in cases of intensive overgrazing, cheatgrass (Bromus tectorum) may be dominant in the understory. Disturbances including wind, snow, and ice result in decreased stand density and provide canopy openings that allow for regeneration (Graham and Jain, 2005).

In the absence of natural fire, periodic prescribed burning in late fall, 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. Habeck (1992) suggests prescribed burning at intervals of 20 to 25 years to maintain ponderosa pine dominance and nutrient cycling within this system. Prescribed burning may increase long-term mortality of old growth ponderosa pine, particularly when combined with increased drought stress (Kolb et al., 2007). Thinning of understory trees and removal of ladder fuels and accumulated organic matter from the base of large trees may be necessary to protect old growth from mortality during prescribed burns (Kolb et al., 2007). Mechanical thinning has the added benefit of decreasing stand susceptibility to mountain pine beetle attack and dwarf mistletoe as outbreaks are generally associated with high stand densities (Jenkins et al., 2014; Klutsch et al., 2014).

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. Management actions that increase basal diameter growth may also favor resistance to bark beetle attacks (Kolb et al., 2007). In some cases, especially on sites heavily infested with cheatgrass, frequent prescribed burning may stimulate greater cheatgrass cover following fire, especially if the burn does not eliminate the seed bank. Increasing time between prescribed fires may inhibit cheatgrass by increasing surface fuels (both herbaceous and litter) which directly inhibit establishment. 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.

Post-fire restoration strategies will depend largely on fire severity. Because fire creates favorable seedbeds for seedling establishment, lightly burned areas recover quite quickly from fire and restoration practices are generally not necessary. In cases where severe, stand-replacing fires have occurred, reseeding or replanting efforts may be necessary. When supplemental seeding or planting is necessitated, germination and seedling survival are enhanced by a period of cold stratification and relatively moist conditions (Habeck, 1992). To ensure successful establishment, repeated browsing by deer or trampling by livestock should be minimized (Habeck, 1992).

Wildfire severity and frequency are expected to increase in this system threatening old growth ponderosa pine stands that historically experienced frequent low-severity fires. In the absence of natural fire, restoration treatments such as mechanical thinning or prescribed burning reduce fuels that accumulate in the understory leading to high intensity, stand replacing fires. Additional benefits of thinning include a reduction of competition stress on old trees, which may in turn reduce mortality associated with drought and mountain pine beetles (Jenkins et al., 2014; Kolb et al., 2007).