Montana Field Guide

This group occurs on warm, dry, exposed sites in the foothills and lower montane zone of the Rocky Mountains in west-central and central Montana, at the ecotone between lower elevation grasslands or shrublands and more mesic coniferous forests. It is an extensive, lower elevation forest type with small to large patch sizes depending on topography. Elevations range from 3,500-5,500 feet, with the higher elevation examples occurring in central Montana. Occurrences are found on all slopes and aspects. Habitats are characteristically an open forest with a grassy understory. 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 particularly west of the Continental Divide. In central Montana, limber pine (Pinus flexilis) is occasionally present. Understory vegetation is typically grasses and forbs that resprout following low to moderate intensity surface fires. Prolonged drought, beetle kill and invasion of non-native species are rapidly changing the dynamics of these habitats.

This Group includes the Rocky Mountain Ponderosa Pine Woodland and Savanna Ecological System and a portion of the Rocky Mountain Foothill Woodland-Steppe Transition Ecological System.

Ponderosa Pine (Pinus ponderosa); Xeric Conifer Forest and Woodland; Tree Cover generally from 10-50%, Rocky Mtn Foothills and Montane Zone; Soils with an A Horizon <10 cm; Low Intensity, Frequent Fire Regime.

In Montana, this group occurs in the foothills and montane zone in mountainous areas on both sides of the Continental Divide, excluding most of southwest Montana and along the Rocky Mtn Front where Ponderosa Pine is largely absent. East of the Continental Divide in central Montana, it occurs along the lower slopes of several of the isolated mountain ranges, including the Bears Paw and Little Rockies.

In MT, G213 occurs within these Level III Ecoregions: 15 (Northern Rockies), 16 (Idaho Batholith), 17 (Middle Rockies), and 41 (Canadian Rockies).

In Montana, G213 occurs within these Major Land Resource Areas: 43A - Northern Rocky Mountains, 43B - Central Rocky Mountains, 44A - Northern Rocky Mountain Valleys, 44B - Central Rocky Mountain Valleys, 46 - Northern and Central Rocky Mountain Foothills.

Matrix

In western and central Montana, this group forms a belt on warm, dry, exposed sites between lower elevation grasslands and Douglas-fir (Pseudotsuga menziesii) forests. Elevations range from 3,500-5,500 feet (Pfister etal 1977). Stands 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 (Pinus ponderosa) is the dominant conifer. Douglas-fir (Pseudotsuga menziesii) and Western Larch (Larix occidentalis) may be present in the tree canopy west of the Continental Divide and the former is also present east of the Divide in areas such as the foothills around Helena. In central Montana, limber pine may be a component of some stands. The understory may have a shrub component though habitats are mostly dominated by grasses. Shrubs such as 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. Total shrub cover in these sites is relatively low. Understory vegetation is more typically perennial grasses and forbs. These more open stands support grasses such as bluebunch wheatgrass (Elymus spicatus), Idaho fescue (Festuca idahoensis), rough fescue (Festuca campestris), prairie junegrass (Koeleria macrantha) and needle and thread (Stipa comata), as well as dryland sedges like threadleaf sedge (Carex filifolia) and sun sedge (Carex inops ssp. heliophila). Common forbs include yarrow (Achillea millefolium), rosy pussytoes (Antennaria rosea), arrowleaf balsamroot (Balsamorhiza sagittata), Indian blanket flower (Gaillardia aristata), and silky lupine (Lupinus sericeus). Invasive species such as Cheatgrass (Bromus tectorum) and Spotted Knapweed (Centaurea stoebe) are prevalent at some sites.

In Montana, this Group is represented by 2 Alliances and 9 Associations. These are generally well-described and likely represent the diversity of types on the landscape.

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). In comparison to the Great Plains Ponderosa Pine Woodlands in central and eastern Montana, this group 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 fires have 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 less fire tolerant 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 etal. 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 etal. 2007). Fire may also decrease tree defenses to beetle attack immediately post-fire, resulting in increased vulnerability to insect attack (Davis etal. 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 etal. 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 etal. 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 etal. 2014; Klutsch etal. 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 etal. 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 etal. 2014; Kolb et al. 2007).

M.S. Reid, K.A. Schulz and M. Manning

S. Mincemoyer

12/4/2024