Montana Field Guide

This forested system is widespread in upper montane to subalpine zones of the Montana Rocky Mountains, and east into island ranges of north-central Montana and the Bighorn and Beartooth ranges of south-central Montana. These are montane to subalpine forests where the dominance of lodgepole pine (Pinus contorta) is related to fire history and topoedaphic conditions. In Montana, elevation ranges from 975 to 2,743 meters (3,200-9000 feet). These forests occur on flats to slopes of all degrees and aspect, as well as valley bottoms. Fire is frequent, and stand-replacing fires are common. Following stand-replacing fires, lodgepole pine will rapidly colonize and develop into dense, even-aged stands. Most forests in this ecological system occur as early- to mid-successional forests persisting for 50-200 years on warmer, lower elevation forests, and 150-400 years in subalpine forests. They generally occur on dry to intermediate sites with a wide seasonal range of temperatures and long precipitation-free periods in summer. Snowfall is heavy and supplies the major source of soil water used for growth in early summer. Vigorous stands occur where the precipitation exceeds 533 millimeters (21 inches). These lodgepole forests are typically associated with rock types weathering to acidic substrates, such as granite and rhyolite. In west-central Montana ranges such the Big Belts and the Rocky Mountain Front, these forests are found on limestone substrates. These systems are especially well developed on the broad ridges and high valleys near and east of the Continental Divide. Succession proceeds at different rates, moving relatively quickly on low-elevation, mesic sites and particularly slowly in high-elevation forests such as those along the Continental Divide in Montana.

Forest and woodland, acidic, shallow ustic soils, organic A horizon greater than 10 cm, Pinus contorta

These forests are dominated by lodgepole pine with shrub, grass, or barren understories. At montane elevations east of the Continental Divide, lodgepole pine stands succeed to Douglas-fir (Pseudotsuga menziesii) forests. In western Montana, there are a number of commonly occurring tree species in later seral stages, including Douglas-fir, western larch (Larix occidentalis), western white pine (Pinus monticola), western red cedar (Thuja plicata), grand fir (Abies grandis) and western hemlock (Tsuga heterophylla). In the subalpine zone, Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa) and mountain hemlock (Tsuga mertensiana) commonly succeed lodgepole pine following stand mortality (Pfister et al.,1977). In the productive habitats of western Montana, lodgepole pine stands often decline in a wave of mortality, usually before they are 120 years old.

The shrub stratum may be conspicuous to absent. Common species include bearberry (Arctostaphylos uva-ursi), snowbrush ceanothus (Ceanothus velutinus), twinflower (Linnaea borealis), creeping Oregon grape (Mahonia repens), antelope bitterbrush (Purshia tridentata), birch leaf spiraea (Spiraea betulifolia),Canadian buffaloberry (Shepherdia canadensis), dwarf huckleberry (Vaccinium caespitosum), grouse whortleberry (Vaccinium scoparium), mountain huckleberry (Vaccinium membranaceum), snowberry (Symphoricarpos species) and currant (Ribes species).

Herbaceous layers are generally sparse, but can be moderately dense, and are typically dominated by perennial graminoids such as Columbia needlegrass (Achnatherum nelsonii), pinegrass (Calamagrostis rubescens), Geyer’s sedge (Carex geyeri), Ross’ sedge (Carex rossii), California oatgrass (Danthonia californica), blue wildrye (Elymus glaucus), and Idaho fescue (Festuca idahoensis). Common forbs include yarrow (Achillea millefolium), arnica (Arnica spp.), American pathfinder (Adenocaulon bicolor), queen’s cup beadlily (Clintonia uniflora), silky lupine (Lupinus sericeus) and beargrass (Xerophyllum tenax). Saprophytic species such as coralroot orchid (Corallorhiza spp.), Indian pipe (Moneses uniflora), pinesap (Monotropa hypopithys), and pinedrops (Pterospora andromedea) are often associated with lodgepole pine forests.

Lodgepole pine is an aggressive colonizer and shade-intolerant conifer that occurs in the upper montane to lower subalpine forests throughout the major mountain ranges of Montana. Establishment is episodic and linked to stand-replacing disturbances, primarily fire. Historically, fire frequency varied between 50 and 300 years, depending on local climate and elevation, with fire frequency declining with increasing elevation (Schoenagel et al., 2003). In the Northern Rockies, severe fires have created large expanses of even-aged stands of lodgepole pine, although more frequent low- to mixed-severity burns may also occur in the intervals between stand-replacing fires, generating a matrix of mixed-age stands (Hardy et al., 2000; Arno et al., 1993). Occasionally, fire severity may be such that cones are destroyed and regeneration will rely on wind-dispersed seeds from nearby stands, resulting in slower regrowth (Anderson, 2003). Repeated fires allow lodgepole pine to persist as the climax species in this system by eliminating the potential for succession by more shade-tolerant species (Pfister et al., 1977).

Trees with closed, serotinous cones where seed release is a response to an environmental trigger, require high temperatures to release seeds and appear to be strongly favored by fire, allowing rapid colonization of fire-cleared substrates (Burns and Honkala, 1990). The incidence of serotinous cones varies within and between varieties of lodgepole pine, but within Rocky Mountain populations, serotiny varies both across regions and with stand age (Schoenagel et al., 2003). Stands that occur at lower elevations where fire return intervals are shorter exhibit greater serotiny, whereas higher elevation stands with greater fire return intervals favor non-serotinous cones which are more advantageous for successful regeneration (Schoenagel et al., 2003). Lodgepole pine stands exhibiting a multi-aged population structure also exhibit a higher proportion of trees bearing non-serotinous cones. Even-aged stands that establish after stand-replacing fires exhibit greater serotiny than those that establish after wind or insect disturbances (Anderson, 2003).

In fire-generated stands of similar age, trees become increasingly susceptible to both mountain pine beetle (Dendroctonus ponderosae) and lodgepole pine dwarf mistletoe (Arceuthobium americanum) infestations as they mature, often resulting in large-scale mortality. In general, pine beetles preferentially attack large individuals with greater nutritional resources (Cole and Amman, 1969). In this system, large scale, stand-replacing fires have occurred frequently throughout Montana during the past 20 years, increasing stand homogeneity favorable to beetle attack. Elevated temperatures and increasing drought severity additionally combine to favor beetle population growth and weaken lodgepole pine defense mechanisms, thereby increasing susceptibility to mountain pine beetle attack (Raffa et al., 2008). Interactions between biotic and abiotic disturbance agents in lodgepole pine systems are complex, and the widespread mortality associated with these disturbances alters ecosystem processes. Ecosystem-level effects of mountain pine beetle outbreaks include 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). However, at broad spatial scales it does not appear that pine beetle caused mortality increases stand susceptibility to fire (Hart et al., 2015; Simard et al., 2011). Alternatively, dwarf mistletoe increases host susceptibility to fire by altering fuel dynamics, and intensifies host vulnerability to insect attack (Hawksworth et al., 2002).

Low-frequency stand-replacing fires are characteristic of this system (Stephens et al., 2013), with higher frequency low- to mixed-severity burns occurring in the intervals between high-severity fires (Hardy et al., 2000). Restoration strategies will depend largely on management goals. Low intensity prescribed burning and selective thinning may be utilized as restoration strategies to restore historic fire regimes and increase long term resistance to mountain pine beetle attack (Hood and Sala, 2013; Fettig et al., 2006). Under favorable moisture conditions, seeds released from serotinous cones during fire germinate on exposed mineral soil and disturbed duff the following spring. Fire creates a favorable seedbed by removing loose organic matter and exposing mineral soil or decomposed organic matter, which encourages germination. Therefore, in most scenarios, additional post-fire restoration practices are not required. However, regeneration success may be marginal when stand-replacing fires are followed by years of severe drought (Stephens et al., 2013) and may require supplemental restoration efforts. When supplemental planting is necessary, germination rates for seeds from serotinous cones may be enhanced by short exposure to flame (Anderson, 2003). Early successional stages following fire in lodgepole pine forests are dominated by an understory of forbs and to a lesser extent, graminoids such as fireweed (Chamerion angustifolium), aster (Aster species), nettleleaf giant hyssop (Agastache urticifolia), and pinegrass (Calamagrostis rubescens).