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. They are often surrounded by mixed-grass prairie, in places where available soil moisture is higher or soils are more coarse and rocky. Elevation ranges from 1,189 meters (3,900 feet) in southeastern Montana to 1,646 m (5,400 feet) in north-central Montana. Occurrences are usually on east- and north-facing aspects. 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.

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).

Surface fires can be frequent on drier sites and aspects. More mesic sites likely have infrequent stand-replacing fires (every 100-200 years). Grazing by domestic livestock may reduce associated grasses; in cases of extreme overgrazing, cheatgrass (Bromus tectorum) may become established. 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 in order to protect old growth from death 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 the fire, especially if the burn did not kill the seed bank. Increasing the time between prescribed fires may inhibit cheatgrass by increasing surface fuels (both herbaceous and litter), which directly inhibits cheatgrass establishment, and by creating higher intensity fires capable of killing a much greater fraction of the seed bank (Keeley and McGinnis, 2007).

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 and Douglas-fir forests will be 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.