Intermountain Aspen and Conifer Forest

This ecological system occurs on montane slopes and plateaus in Utah, western Colorado, northern Arizona, eastern Nevada, southern Idaho, western Wyoming, and in north-central Montana in the Big Snowy Mountains. It also occurs in localized settings in the Klamath Mountains of California, as well as in the Sierra Nevada and adjacent Great Basin mountains (Inyo, White, Warner, and Modoc Plateau). Elevations range from 1700 to 2800 m. Occurrences are typically on gentle to steep slopes on any aspect but are often found on clay-rich soils in intermontane valleys. Soils are derived from alluvium, colluvium and residuum from a variety of parent materials but most typically occur on sedimentary rocks. The tree canopy is composed of a mix of deciduous and coniferous species, codominated by Populus tremuloides and conifers, including Pseudotsuga menziesii, Abies concolor, Abies lasiocarpa, Abies magnifica, Picea engelmannii, Picea x albertiana, Picea pungens, Pinus contorta, Pinus flexilis, Pinus jeffreyi, Pinus contorta var. murrayana, and Pinus ponderosa. As the stands age, cover of Populus tremuloides may be slowly reduced until the conifer species become dominant. Common shrubs include Amelanchier alnifolia, Prunus virginiana, Acer grandidentatum, Symphoricarpos oreophilus, Juniperus communis, Paxistima myrsinites, Rosa woodsii, Spiraea betulifolia, Symphoricarpos albus, or Mahonia repens. Herbaceous species include Bromus carinatus, Calamagrostis rubescens, Carex geyeri, Elymus glaucus, Poa spp., and Achnatherum, Hesperostipa, Nassella, and/or Piptochaetium spp. Achillea millefolium, Arnica cordifolia, Asteraceae spp., Erigeron spp., Galium boreale, Geranium viscosissimum, Lathyrus spp., Lupinus argenteus, Mertensia arizonica, Mertensia lanceolata, Maianthemum stellatum, Osmorhiza berteroi, and Thalictrum fendleri. Most occurrences at present represent a late-seral stage of aspen changing to a pure conifer occurrence. Nearly a hundred years of fire suppression and livestock grazing have converted much of the pure aspen occurrences to the present-day aspen-conifer forest and woodland ecological system. This is the typical meadow edge aspen-conifer setting in the Sierra Nevada where frequently, due to fire suppression, the conifers are replacing aspens.

The open to moderately closed canopy is composed of a mix of deciduous and coniferous species, codominated by Populus tremuloides and conifers, including Pseudotsuga menziesii, Abies concolor, Abies lasiocarpa, Picea engelmannii, Picea x albertiana (= Picea glauca x engelmannii), Picea pungens, Pinus contorta, Pinus flexilis, and Pinus ponderosa. The sparse to moderately dense understory may be structurally complex and includes tall-shrub, short-shrub and herbaceous layers, or it may be simple with just an herbaceous layer or sparse. Because of the open growth form of Populus tremuloides, more light can penetrate the canopy than in a pure conifer occurrence. If present, the tall-shrub layer may be dominated by Amelanchier alnifolia, Prunus virginiana, or Acer grandidentatum, and short-shrub layer by Symphoricarpos oreophilus, Juniperus communis, or Mahonia repens. Other common shrubs include Paxistima myrsinites, Rosa woodsii, Spiraea betulifolia, Symphoricarpos albus, and in wet areas Salix scouleriana. Where the herbaceous layer is dense, it is often dominated by graminoids such as Bromus carinatus, Calamagrostis rubescens, Carex geyeri, Elymus glaucus, Poa spp., and species of Achnatherum, Hesperostipa, Nassella, and/or Piptochaetium. Sparse herbaceous layers are generally a more even mixture of forbs such as Achillea millefolium, Arnica cordifolia, Eucephalus engelmannii (= Aster engelmannii), Erigeron speciosus, Fragaria vesca, Galium boreale, Geranium viscosissimum, Lathyrus spp., Lupinus argenteus, Mertensia arizonica, Mertensia lanceolata, Maianthemum stellatum, Osmorhiza berteroi (= Osmorhiza chilensis), and Thalictrum fendleri. Annuals are typically uncommon. The exotic species Poa pratensis and Taraxacum officinale are more common in livestock-impacted occurrences (Mueggler 1988). The vegetation description is based on several references, including DeByle and Winokur (1985), Mueggler (1988), Howard (1996), Reid et al. (1999), Bartos (2001), Comer et al. (2002), Tuhy et al. (2002), and Sawyer et al. (2009).

This ecological system is found on montane slopes and high plateaus in Utah, western Colorado, northern Arizona, eastern Nevada, southern Idaho, and western Wyoming from 1700 to 2800 m elevation. Climate is temperate with cold winters. Mean annual precipitation is greater than 38 cm and typically greater than 50 cm. Although often drier, sites are similar to Rocky Mountain Aspen Forest and Woodland (CES306.813) with regards to environmental characteristics. Topography is variable, with sites ranging from level to steep slopes. Aspect varies according to the limiting factors. Occurrences at high elevations are restricted by cold temperatures and are found on warmer southern aspects. At lower elevations aspen is restricted by lack of moisture and is found on cooler north aspects and mesic microsites such as seeps and drainages. Soils are derived from alluvium, colluvium and residuum from a variety of parent materials and may include sedimentary, metamorphic or igneous rocks, but it appears to grow best on sedimentary rocks such as limestone and calcareous or neutral shales, or basalt (Mueggler 1988). Soil texture ranges from sandy loam to clay loam. This system represents a stable mixed aspen - conifer woodlands typically found on broad plateaus where periodic disturbance such as die-back from drought is thought to maintain the mixed deciduous-conifer composition. It is sometimes confused with the relatively short-lived, mid-seral stages of conifer-dominated forest and woodland systems such as Rocky Mountain Subalpine Dry-Mesic Spruce-Fir Forest and Woodland (CES306.828), Rocky Mountain Subalpine Mesic-Wet Spruce-Fir Forest and Woodland (CES306.830), or Southern Rocky Mountain Mesic Montane Mixed Conifer Forest and Woodland (CES306.825). Distribution of this ecological system is primarily limited by adequate soil moisture required to meet its high evapotranspiration demand (Mueggler 1988). Secondarily, its range is limited by the length of the growing season or low temperatures (Mueggler 1988). The environmental description is based on several other references, including DeByle and Winokur (1985), Mueggler (1988), Howard (1996), Reid et al. (1999), Bartos (2001), Comer et al. (2002), Tuhy et al. (2002), and Sawyer et al. (2009).

Populus tremuloides is a fast-growing deciduous tree that reaches 20 m in height and forms clones that can be ancient, although the stems are relatively short-lived (up to 150 years in the western U.S.) (Howard 1996, Sawyer et al. 2009). It is thin-barked and stems are readily killed by fire, although the clone will usually resprout after burning or other disturbance (Howard 1996). It is a fire-adapted species that generally needs a large disturbance to establish and maintain dominance in a forest stand. Mixed aspen - conifer forests are generally seral and, in the absence of stand-replacing disturbance such as fire, will slowly convert to a conifer-dominated forest (Mueggler 1988). Although the young conifer trees in these occurrences are susceptible to fire, older individuals develop self-pruned lower branches and develop a thick corky bark that makes them resistant to surface fires. The natural fire-return interval is approximately 20 to 50 years for seral occurrences (Hardy and Arno 1996). Intervals that approach 100 years are typical of late-seral occurrences (Hardy and Arno 1996).

However, this system represents stable mixed aspen - conifer woodlands typically found on broad plateaus in the interior western U.S. where periodic disturbance such as die-back from drought or other disturbance is thought to maintain the mixed deciduous-conifer composition and not allow conifers to dominate and shade out the aspen (Tuhy et al. 2002). Sudden aspen decline (SAD) results in root mortality with subsequent effects on tree canopy and clone persistence. It appears to be triggered by severe drought (Worrall et al. 2010). This may have increasing impact on these forests. More research is needed to clarify the dynamics of this system as it is sometimes confused with the relatively short-lived, mid-seral stages of conifer-dominated forest and woodland systems such as Rocky Mountain Subalpine Dry-Mesic Spruce-Fir Forest and Woodland (CES306.828), Rocky Mountain Subalpine Mesic-Wet Spruce-Fir Forest and Woodland (CES306.830), or Southern Rocky Mountain Mesic Montane Mixed Conifer Forest and Woodland (CES306.825).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has five classes in total (LANDFIRE 2007a, BpS 1810610). The model represents a fire maintained, seral mixed aspen - conifer types that succeeds to a conifer dominated types without mixed-severity fire (mean FRI of 20 years). The classes are summarized as:

A) Early Development 1 All Structures (14% of type in this stage): Grass/forb and aspen suckers <12 feet tall. Generally, this is expected to occur 1-3 years post-disturbance. Fire is absent. Succession to class B after 10 years.

B) Mid Development 1 Closed (tree-dominated - 40% of type in this stage): Tree cover is 41-100%. Aspen saplings over 12 feet tall dominate. Canopy cover is highly variable. Replacement fire occurs every 60 years on average. Mixed-severity fire (average FRI of 40 years) does not change the successional age of these stands, although this fire consumes litter and woody debris and may stimulate suckering. Succession to class C after 30 years.

C) Mid Development 1 Closed (tree-dominated - 35% of type in this stage): Tree cover is 41-100%. Aspen trees 5-16 inches dbh. Canopy cover is highly variable. Conifer seedlings and saplings may be present. Replacement fire occurs every 60 years on average. Mixed-severity fire (mean FRI of 40 years), while thinning some trees, promotes suckering and maintains vegetation in this class. Insect/diseases outbreaks occur every 200 years on average with 80% of times causing stand thinning (transition to class B) and 20% of times causing stand replacement (transition to class A). Conifer encroachment causes a succession to class D after 40 years.

D) Late Development 1 Open (tree-dominated - 10% of type in this stage): Tree cover is 0-40%. Aspen dominate, making up ~80% of the overstory. Conifers which escape fire, or are the more fire-resistant species, are present in the understory and will likely cause the progressive suppression of aspen. Mixed-severity fire (20-year MFI) keeps this stand open, kills young conifers and maintains aspen (max FRI from Baker 1925). Replacement fire occurs every 60 years on average. In the absence of any fire for at least 100 years, the stand will become closed and dominated by conifers (transition to class E).

E) Late Development 1 Closed (conifer-dominated - 1% of type in this stage): Tree cover is 41-80%. Conifers dominate at 100+ years. Aspen over 16 inches dbh, uneven sizes of mixed conifer and main overstory is conifers. Greater than 50% conifer in the overstory. FRI for replacement fire is every 60 years. Mixed-severity fire (mean FRI of 20 years) causes a transition to class D. Insect/disease outbreaks will thin older conifers (transition to class D) every 300 years on average.

From (LANDFIRE 2007a, BpS 1810610): "This is a strongly fire-adapted community, more so than BpS 1011 (Rocky Mountains Aspen Woodland and Forest), with FRIs varying for mixed-severity fire with the encroachment of conifers. It is important to understand that aspen is considered a fire-proof vegetation type that does not burn during the normal lightning season, yet evidence of fire scars and historical studies show that native burning was the only source of fire that occurred mostly during the spring and fall. BpS 1061 has elements of Fire Regime Groups II, III and IV. Mean FRI for replacement fire is every 60 years on average in most development classes. Replacement fire is absent during early development (as for stable aspen, BpS 1011) and has a mean FRI of 100 years between 80 and 100 years in the open condition. The FRI of mixed-severity fire increases from 40 years in stands <100 years to 60 years in stands >100 years with conifer encroachment."

Under presettlement conditions, disease and insect mortality did not appear to have major effects; however, older aspen stands would be susceptible to outbreaks every 200 years on average. We assumed that 20% of outbreaks resulted in heavy insect/disease stand-replacing events (average return interval 1000 years), whereas 80% of outbreaks would thin older trees >40 years (average return interval 250 years). Older conifers (>100 years) would experience insect/disease outbreaks every 300 years on average (LANDFIRE 2007a, BpS 1810610).

In the western U.S., Populus tremuloides-dominated and -codominated forests have been utilized primarily for livestock grazing. Stands typically have lush understories because the Populus tremuloides tree canopy allows significant light to pass through and sites tend to be relatively mesic (DeByle and Winokur 1985, Howard 1996). Heavy grazing by livestock can deplete or convert an understory dominated by shrubs and forbs to an understory dominated by grazing-tolerant grasses. Degraded stands were often seeded to grazing-tolerant introduced forage species such as Bromus inermis, Dactylis glomerata, Phleum pratense, and Poa pratensis (DeByle and Winokur 1985). Excessive browsing by livestock or wildlife can also significantly impact regeneration by suckers (DeByle and Winokur 1985, Howard 1996).

Logging, prescribed fire or some other stand-replacing disturbance will convert these conifer - Populus tremuloides mixed canopy stands to Populus tremuloides-dominated stands because disturbance will generally favor Populus tremuloides regeneration (DeByle and Winokur 1985, Howard 1996).

Human development has impacted many locations throughout its range. High- and low-density urban and industrial developments also have large impacts. For example, residential development has significantly impacted locations within commuting distance to urban areas. Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

This system occurs on montane slopes and plateaus in Utah, eastern Nevada, southern Idaho, western and central Wyoming (in the Bighorn Mountains), and in north-central Montana in the Big Snowy Mountains. Elevations range from 1700 to 2800 m.