Montana Field Guide

These ponderosa pine (Pinus ponderosa) stands differ from the Rocky Mountain Ponderosa Pine Forest and Woodlands in that they are typically found within the matrix of the Great Plains grasslands east of the Continental Divide on sites that are relatively dry and nutrient-poor (Howard, 2003). They are often surrounded by mixed-grass prairie. Ponderosa Pine dominates these open woodlands with Rocky Mountain Juniper (Juniperus scopulorum) occurring on or co-dominating some sites, rarely Juniper may be the dominant species. These woodlands can be physiognomically variable, ranging from very sparse patches of trees on drier sites, to nearly closed-canopy forest stands on north slopes or in draws where available soil moisture is higher. The understory may be shrub or graminoid dominated, depending on the site. On sites with greater available soil moisture understories may be dense and support species commonly associated with Ponderosa Pine woodlands west of the Continental Divide. Fire suppression of the past century, increasingly severe drought and insect outbreaks have altered the natural processes of these habitats. Ponderosa Pine has expanded and/or increased in abundance in some areas due to fire suppression and has been reduced in others from high-intensity wildfires over the last several decades.

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

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

This Group occurs east of the Continental Divide in the Great Plains Region and extending to the foothills of several of the island ranges where it grades into G213. It occurs extensively along the Missouri River breaks, near the Little Belt Mtns and the Snowy Mtns, and in south-central Montana between the Bighorns and the Black Hills (along the Tongue and Powder Rivers), and in other areas of eastern Montana.

In Montana, G216 occurs in Level III Ecoregions 42 (Northwestern Glaciated Plains) and 43 (Northwestern Great Plains).

In Montana, G216 occurs or potentially occurs within these Major Land Resource Areas: 46 - Northern and Central Rocky Mountain Foothills; 52 - Brown Glaciated Plains; 58A,B,C,D - Northern Rolling High Plains; 60A,B – Pierre Shale Plains

Large Patch-Matrix

This woodland group occurs primarily on gentle to steep slopes along escarpments, buttes, hills, canyons, rock outcrops, or ravines. Soils typically range from well-drained loamy sands to sandy loams formed in colluvium, weathered sandstone, limestone, scoria, or eolian sand. Elevations of stands in this group range primarily from 2,500 to 4,500ft with some outliers beyond this range.

Ponderosa Pine dominates these open woodlands of the Great Plains with Rocky Mountain Juniper (Juniperus scopulorum) occurring on or co-dominating some sites, rarely Juniper may be the dominant species. The understory may be shrub or graminoid dominated, depending on the site. Shrubs associated with ponderosa pine dominated forests include bearberry (Arctostaphylos uva-ursi), creeping Oregon grape (Mahonia repens), soapweed yucca (Yucca glauca), Western Snowberry (Symphoricarpos occidentalis), chokecherry (Prunus virginiana), common juniper (Juniperus communis), horizontal juniper (Juniperus horizontalis), serviceberry (Amelanchier alnifolia), and skunkbush sumac (Rhus trilobata). The herbaceous understory can range from a sparse to a dense layer of species typical of the surrounding prairie system. Mixedgrass Prairie species are usually common, such as big bluestem (Andropogon gerardii), Little Bluestem (Schizachyrium scoparium) Blue Grama (Bouteloua gracilis), sideoats grama (Bouteloua curtipendula), sun sedge (Carex inops ssp.heliophila), threadleaf sedge (Carex filifolia), prairie junegrass (Koeleria macrantha), green needlegrass (Stipa viridula), Littleseed Ricegrass (Oryzopsis micrantha), and western wheatgrass (Elymus smithii). Common herbaceous forbs include yarrow (Achillea millefolium), pussytoes (Antennaria species), Fringed Sage (Artemisia frigida), Blanketflower (Gaillardia aristata), silky lupine (Lupinus argenteus), crazyweed (Oxytropis species), alpine sweetvetch (Hedysarum alpinum), penstemon (Penstemon species), prairie cinquefoil (Potentilla gracilis), and goldenrod (Solidago species).

In Montana, this group is represented by 3 Alliances and 14 Associations which likely covers the diversity of vegetation types on the landscape.

Frequent, low-intensity surface fires were common within these habitats prior to European settlement (Graham and Jain 2005; Pfister etal. 1977). Fire return intervals averaged approximately 15 years, although infrequent mixed- to high-severity fires may also occur at longer time scales (U.S. Department of Agriculture, 2012). The thick, insulating bark and deep rooting habit characteristic of ponderosa pines allows individuals to withstand low-severity fires, although younger trees with thinner bark are more vulnerable (Graham and Jain 2005). Individuals in low density stands have a better likelihood of surviving surface fires than trees in higher density stands (Howard 2003).

Comparatively fewer insects attack ponderosa pines in the Great Plains Region than those occurring west of the Continental Divide (Howard, 2003). Mountain pine beetle (Dendroctonus ponderosae) is the most damaging insect to ponderosa pine throughout Montana. Mountain pine beetles generally occur at endemic levels and target trees with weakened defenses due to drought, lightning strikes, pathogen infections, and old age (Graham and Jain 2005). Outbreaks may reach epidemic levels when stands are characterized by high densities of large, stressed trees, a common attribute of stands in which low-intensity fires have been suppressed (Negron et al. 2008; Howard 2003).

Grazing by domestic livestock may reduce associated grasses, and in cases of extreme overgrazing, cheatgrass (Bromus tectorum) may become established. Frequent low-intensity surface fires promote grass production (Howard, 2003), but may also encourage invasion by non-native species (Symstad et al., 2014). Wind, ice, and snow also cause infrequent disturbance resulting in the formation of canopy openings where regeneration can occur, creating multi-aged stands (Graham and Jain 2005). In the absence of canopy-opening disturbance events, seedling establishment is significantly reduced approximately 70 years after a stand-replacing event (Lundquist and Negron, 2000).

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 in order to prevent old growth mortality during prescribed burning (Kolb et al., 2007). An additional benefit of thinning is to reduce the probability of insect and disease outbreaks, as these are more common in high density stands (Graham and Jain, 2005). 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 smaller scales and for high-value stands (Howard, 2003).

Periodic burning is used to expose mineral soils, increase 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 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 inhibits cheatgrass establishment (Keeley and McGinnis, 2007). Minimizing the extent of high severity burning by pre-fire thinning, or by applying prescribed burns in cooler, wetter conditions may also reduce the probability of post-burn outbreaks as higher burn intensity may correspond to greater probability of invasion by non-native species (Symstad et al., 2014).

Excessive grazing can result in the loss of the most common perennial grasses and lead to an abundance of exotic grasses in this system. Cheatgrass establishment in low-elevation ponderosa pine forests is enhanced by disturbance that opens the understory, removes litter, or both. (Mack and Pyke 1983). Fall germination and rapid elongation of roots provides 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.

Restoration strategies for these habitats will depend largely on the particular impacts. Fire generally creates favorable conditions for ponderosa pine regeneration as surface fires remove litter and duff that inhibit seedling establishment and reduce competition with understory species (Howard 2003). Because lightly burned areas recover quickly from fire, reseeding is usually not necessary, especially if an intact native herbaceous understory was present before the fire. However, to offset invasion of exotics such as cheatgrass, re-seeding with competitive native grasses may be desirable after low-intensity or prescribed surface fires. Intense fires that occur during summer months cause considerable damage to native, perennial grasses. Ponderosa pine seeds are typically only viable in the seed bank for the year after dispersal (Howard 2003). Therefore, in cases where severe, stand replacing fires occur, reseeding or replanting efforts may be necessary. Severe mountain pine beetle outbreaks may also necessitate restoration efforts as ponderosa pine does not establish well on unburned soils (Graham and Jain 2005). In order to slow the rapid loss of the oldest age classes of ponderosa pine in this system, mechanical thinning and the restoration of historic fire return intervals is necessary to remove competition and potential overstocking on a site.

M.S. Reid, K.A. Schulz and H. Marriott

S. Mincemoyer

12/4/2024