Montana Field Guide

In Montana, this ecological system occurs on the east side of the Continental Divide, north to about the McDonald Pass area, and along the Rocky Mountain Front. This system is associated with a dry to submesic continental climate regime with annual precipitation ranging from 51 to 102 centimeters (20-40 inches), with a maximum in winter or late spring. Winter snowpacks typically melt off in early spring at lower elevations. Elevations range from valley bottoms to 1,980 meters (6500 feet) in northern Montana and up to 2,286 meters (7500 feet) on warm aspects in southern Montana. It occurs on north-facing aspects in most areas, and south-facing aspects at higher elevations. This is a Douglas-fir (Pseudotsuga menziesii) dominated system without any maritime floristic composition. Fire disturbance intervals are as infrequent as 500 years, and as a result, individual trees and forests can attain great age on some sites (500 to 1,500 years). In Montana, this system occurs from lower montane to lower subalpine environments and is prevalent on calcareous substrates. Common understory shrubs include common ninebark (Physocarpus malvaceus), common juniper (Juniperus communis), Rocky Mountain juniper (Juniperus scopulorum), birch-leaf spiraea (Spiraea betulifolia), snowberry (Symphoricarpos species), creeping Oregon grape (Mahonia repens) and Canadian buffaloberry (Shepherdia canadensis). The Douglas-fir/pinegrass (Calamogrostis rubescens) type is the most ubiquitous association found within this system in Montana.

This system is dominated by Douglas-fir (Pseudotsuga menziesii) forests. Limber pine (Pinus flexilis) can co-occuron calcareous substrates, and lodepole pine (Pinus contorta) is often frequent in these stands at higher elevations. Engelmann spruce (Picea engelmannii) is found in some stands within the upper montane zone, occurring just below or intergrading with whitebark pine (Pinus albicaulis) forests. Common understory shrubs include common ninebark (Physocarpus malvaceus), common juniper (Juniperus communis), Rocky Mountain juniper (Juniperus scopulorum) birch-leaf spiraea (Spiraea betulifolia), snowberry (Symphoricarpos species), creeping Oregon grape (Mahonia repens) and Canadian buffaloberry (Shepherdia canadensis). Dwarf huckleberry (Vaccinium caespitosum) or mountain huckleberry (Vaccinium membranaceum) are foundon colder, mesic sites. Common graminoids include pinegrass (Calamagrostis rubescens), Ross’ sedge (Carex rossii), and Geyer’s sedge (Carex geyeri). Bluebunch wheatgrass (Pseudoroegneria spicata) and Idaho fescue (Festuca idahoensis) are often common on sites adjacent to upper elevation montane grasslands. The Douglas-fir/pinegrass (Camogrostis rubescens) type is the most ubiquitous association found within this system in Montana. Common forbs within these forests include yarrow (Achillea millefolium), lanceleaf arnica (Arnica latifolia), pussytoes (Antennaria racemosa), wild strawberry (Fragaria virginiana), twinflower (Linnaea borealis), and beargrass (Xerophyllum tenax).

Disturbances in this system are primarily caused by fire and insects. Fire return intervals generally range from 20-45 years, although fire-free periods may be longer at more mesic or higher elevation sites (Steinberg, 2002; U.S. Department of Agriculture, 2012). The majority of fires in this system are mixed-severity, although stand replacement and low-severity fires may also occur (U.S. Department of Agriculture, 2012). Douglas-fir regenerates well following fire, with higher regeneration following low-severity burns (Harvey et al., 2013). After about 40 years, trees develop fire-resistant bark that promotes tolerance low to moderate intensity surface fires (Steinberg, 2002). In the absence of disturbance, Douglas-fir will continue to regenerate under shaded conditions, and becomes dominant in undisturbed stands.

Pre-settlement fire regimes may have been characterized by frequent, low intensity ground fires that maintained relatively open stands. Under present conditions, the fire regime is mixed severity and more variable, with stand-replacing fires more common, encouraging forest homogeneity. With vigorous fire suppression, longer fire-return intervals are now common, and multi-layered stands provide fuel "ladders," making these forests more susceptible to high-intensity, stand-replacing fires. In some areas, these forests have been priorities for timber harvesting and grazing. An additional consequence of fire suppression has been the establishment of Douglas-fir in sagebrush steppe and grassland ecosystems in southwestern Montana. Historically, frequent fires confined Douglas-fir growth to particularly rocky or moist microsites. (Arno and Gruell, 1986; Heyerdahl et al., 2006).

Fire additionally interacts with biotic disturbance dynamics in this system. The Douglas-fir bark beetle (Dendroctonus pseudotsugae) causes abundant damage, and fire-affected stands tend to be more vulnerable to attack (Negron et al., 1999; Hood and Bentz, 2007; Six and Skov, 2009). A study from southwestern Montana found that large trees with high levels of crown scorch in dense stands were most vulnerable to beetle attack post-fire (Hood and Bentz, 2007). In some cases, post-fire beetle infestations may spread to nearby trees not injured by fire (DeNitto et al., 2000). Post-fire mortality may also occur as the result of Douglas-fir tussock moth (Orgyia pseudotsugata), western spruce budworm (Choristoneura occidentalis), and wood borers. Trees infected with Douglas-fir dwarf mistletoe (Arceuthobium douglasii) may be more susceptible to fire due to the accumulation of dense brooms that serve as ladder fuels (Steinberg, 2002).

In recent years, these forests have been subjected to prolonged periods of drought, creating conditions where stands are susceptible to outbreaks of Douglas-fir tussock moth and Douglas-fir bark beetle. The combination of prolonged drought and fire suppression have also contributed to an increase in the intensity and duration of western spruce budworm epidemics, increased Douglas-fir bark beetle populations, and increased dwarf mistletoe infestation (Steinberg, 2002). In addition to the effects of drought, disturbances such as snow breakage and windthrow may increase stand susceptibility to insect attack (Negron et al., 1999).