Montana Field Guide

These ponderosa pine (Pinus ponderosa) occurrences differ from the Rocky Mountain Ponderosa Pine Woodland and Savanna systems in that they are typically found within the matrix of the Great Plains grassland systems east of the Continental Divide on sites that are relatively dry and nutrient-poor (Howard, 2003). They are often surrounded by mixed-grass prairie, in places where available soil moisture is comparatively higher or soils are more coarse and rocky. Soils in this system are typically derived from both igneous and sedimentary parent materials including volcanic cinder, basalt, sandstone, and limestone (Howard, 2003). This system occurs at elevations that range from 1,189 meters (3,900 feet) in southeastern Montana to 1,646 m (5,400 feet) in north-central Montana. At the upper elevational portion of its range in central Montana, this system merges with Rocky Mountain Foothill Limber Pine-Juniper Woodlands, while in southeastern Montana, ponderosa pine is the only forest tree species (Howard, 2003; Arno, 1979). Occurrences are usually on east- and north-facing aspects.

Common overstory associates of this system include Douglas-fir (Pseudotsuga menziesii), limber pine (Pinus flexilis), and Rocky Mountain juniper (Juniperus scopulorum). Understories are commonly comprised of shrubs such as western snowberry (Symphoricarpos occidentalis) and skunkbush sumac (Rhus trilobata), and grassland species such as blue grama (Bouteloua gracilis), side-oats grama (B. curtipendula), and prairie Junegrass (Koeleria macrantha) (Howard, 2003; Arno, 1979). 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. 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 this system.

Ridge-summit-upper slope, aridic-sand soil texture, organic A horizon less than 10 cm, Pinus ponderosa with graminoid understory, Pinus ponderosa with shrub understory

Along the Missouri Breaks in north-central Montana, woodlands dominated by Douglas-fir (Pseudotsuga menziesii) are found in the same ecological settings as ponderosa pine, and so are included in this system. In the breaks where it occurs, Douglas-firhas a very open canopy over graminoid undergrowth, predominantly composed of bluebunch wheatgrass (Pseudoroegneria spicata), with few to no shrubs present. In most of the Missouri Breaks, however, ponderosa pine dominates and Rocky Mountain juniper (Juniperus scopulorum) is a common associate. Shrubs associated with ponderosa pine dominated forests include bearberry (Arctostaphylos uva-ursi), creeping Oregon grape (Mahonia repens), soapweed yucca (Yucca glauca), snowberry (Symphoricarpos species), chokecherry (Prunus virginiana), common juniper (Juniperus communis), horizontal juniper (Juniperus horizontalis), serviceberry (Amelanchier alnifolia), skunkbush sumac (Rhus trilobata) and ninebark (Physocarpus species). The herbaceous understory can range from a sparse to a dense layer of species typical of the surrounding prairie system. Mixed-grass species are usually common, such as big bluestem (Andropogon gerardii), sideoats grama (Bouteloua curtipendula), sun sedge (Carex inops ssp.heliophila), threadleaf sedge (Carex filifolia), poverty oatgrass (Danthonia intermedia), prairie junegrass (Koeleria macrantha), green needlegrass (Nassella viridula), roughleaf ricegrass (Oryzopsis asperifolia), and western wheatgrass (Pascopyrum smithii). Common herbaceous forbs include yarrow (Achillea millefolium), pussytoes (Antennaria species), boreal sagewort (Artemisia frigida), arrowleaf balsamroot (Balsamorhiza sagittata), Indian blanket flower (Gaillarida aristata), silky lupine (Lupinus argenteus), crazyweed (Oxytropis species), alpine sweetvetch (Hedysarum alpinum), penstemon (Penstemon species), prairie cinquefoil (Potentilla gracilis), goldenrod (Solidago species) and smooth aster (Symphyotrichum laeve).

Frequent low-intensity surface fires are common within this system due to generally sparse understories and the accumulation of litter at the base of mature trees (Graham and Jain, 2005; Pfister et al., 1977). Fire return intervals average approximately 15 years, although infrequent mixed- to high-severity fires may also occur at longer time scales (U.S. Department of Agriculture, 2012). More mesic sites, as well as higher elevation stands are typically more dense and experience infrequent higher severity fires (every 100-200 years). 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 may be 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 this system than those occurring west of the Continental Divide (Howard, 2003), although the 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 discontinued 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 the 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 this system will depend on fire severity or grazing 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 quite 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 to restore nutrient cycling, and reduce the probability of beetle attack and stand replacing fires is necessary.