Montana Field Guide

This group is composed of montane coniferous forests dominated by Douglas-fir (Pseudotsuga menziesii) often with a Ponderosa Pine (Pinus ponderosa) component; these forests have an affinity to the Interior Pacific Northwest in terms of distribution and floristic affinities. As such, it occurs predominantly west of the Continental Divide in Montana, though some of the Associations contained within it also occur east of the Divide. This is a common and widespread group in western Montana. However, the distinction between this group and G215 Rocky Mtn Douglas-fir Forest, which occurs more on the eastside of the Continental Divide in Montana is not clearcut and additional review and refinement of the two groups is still needed.

Other seral conifer species typical of these stands include, Lodgepole Pine (Pinus contorta), Western White Pine (Pinus monticola), and Western Larch (Larix occidentalis). Grand Fir (Abies grandis), a fire-sensitive, shade-tolerant species has increased on many sites once dominated by Douglas-fir. These were historically maintained by low-severity wildfire. Understories may be dominated by shrubs or graminoids. A potential realignment of Associations with G215 is needed before a comprehensive summary of understory vegetation can be provided that better distinguishes between the two groups.

This Group incorporates portions of the Rocky Mountain Dry-Mesic Montane Mixed Conifer Forest Ecological System and the Rocky Mountain Montane Douglas-fir Forest and Woodland.

Conifer Forest and Woodland; Montane Zone; Inland Pacific Northwest and Rocky Mtn Region

Typical Dominants: Douglas-fir (Pseudotsuga menziesii) with Ponderosa Pine (Pinus ponderosa) as a codominant.

In Montana, this Group occurs primarily west of the Continental Divide where it may be found in all the major mountain ranges and major river drainages within this area. East of the Divide, it is found in more localized patches where it overlaps with G15.

In MT, G210 occurs within these Level III Ecoregions: 15 (Northern Rockies), 16 (Idaho Batholith), 17 (Middle Rockies), and 41 (Canadian Rockies).

In Montana, G210 occurs within these Major Land Resource Areas: 43A-Northern Rocky Mountains; 43B-Central Rocky Mountains, and 44A - Northern Rocky Mountain Valleys.

Matrix

This group is associated with a submesic climate regime typical of the Inland Pacific Northwest. It is typified by warm, dry summers and cold winters. Annual precipitation ranges from 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 2,000 to 6,700ft.

This montane coniferous forest group is dominated by Douglas-fir (Pseudotsuga menziesii), often with Ponderosa Pine (Pinus ponderosa), though pine is sometime absent from these sites. Other conifer species may occur in the tree canopy, including Western Larch (Larix occidentalis), Lodgepole Pine (Pinus contorta), and Western White Pine (Pinus monticola). Stands of this group often occur as large patches. Grand Fir (Abies grandis), a fire-sensitive, shade-tolerant species has increased on many sites once dominated by Douglas-fir (Pseudotsuga menziesii) and Ponderosa Pine (Pinus ponderosa), which were formerly maintained by low-severity wildfire. The floristic affinities of this group are allied with the maritime-influenced climate of the Interior Pacific Northwest.

Understories may be dominated by shrubs or graminoids. The Douglas-fir/pinegrass (Calamagrostis rubescens) type is the most ubiquitous association found within this group in Montana. A potential realignment of Associations with G215 is needed before a comprehensive summary of understory vegetation can be provided that better distinguishes between the two groups.

In Montana, this group is currently composed of 11 Associations in 2 Alliances. However, the relationship of this group to G215 requires further evaluation. At a minimum, several Associations within these groups need to be realigned. It may be better to combine these two groups into one group due to the large amount of geographic and floristic overlap.

Disturbances in these forests 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 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 of 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 ladder fuels, 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).

In the absence of natural fire, periodic prescribed burns can be used to maintain this system. Low-severity burning decreases fuel loading, probability of stand-replacing fires, and increases available nutrients in the soil (Arno et al. 1995). Prescribed fire may additionally be useful for limiting invasion of Douglas-fir into sagebrush steppe and grassland ecosystems in southwestern Montana (Steinberg 2002; Heyerdahl et al. 2006). It may be important to consider the consequences of prescribed burning on stand susceptibility to Douglas-fir beetles, as beetles respond quickly to available resource pulses (i.e. fire damaged trees) following fire (Negron et al. 1999; Six and Skov 2009).

Post-fire Douglas-fir forest restoration strategies will depend largely on the severity of the fire. Because these forests recover well following light to moderate intensity burns, restoration practices are generally not necessary. Recovery may be slow after large-scale stand-replacing fires, as seeds are wind-dispersed and rely on nearby stands to provide a seed source. Severely burned sites on steep slopes will require sediment retention and erosion control actions before and during restoration. When supplemental seeding or planting is necessitated, Douglas-fir establishment is best on mineral soil or thin organic seedbeds, and survival is greatest at relatively dry sites with partial shading (Steinberg 2002). Prescribed fire may be an additional valuable restoration strategy in stands with high levels of dwarf mistletoe. High severity burning controls Douglas-fir dwarf mistletoe by eliminating infected trees and promoting regeneration of uninfected individuals (Alexander and Hawksworth 1976). However, when utilizing prescribed burning as a restoration strategy, the effects of fire on local Douglas-fir beetle populations should be considered.

M.S. Reid and K.A. Schulz

S. Mincemoyer

12/4/2024