Ecological dynamics

The subalpine fir cool moist shrub (AF -CMS) ecological site (ES) is made up of the modal subalpine fir (Abies lasiocarpa)/big huckleberry (Vaccinium membranaceum) plant association (code named ABLA/VAME). The less extensive subalpine fir/Queen’scup beadlily (Clintonia uniflora) and subalpine fir/twinflower (Linnea borealis) are included in this ecological site group (respectively coded as ABLA/CLUN and ABLA/LIBO).
A very limited amount of Subalpine fir/common beargrass (Xerophyllum tenax), coded as ABLA/XETE, may be found in association with this ecological site. ABLA/XETE is not described in the Blue-Ochoco plant association reference, and no data is available for this plant association in the local references used for the other plant association(s). ABLA/XETE is most likely cooler than the other named associations.
These plant associations are identified in the plant association group as “cool moist”, and in the broader plant vegetative type group as “moist upland forest.” This ecological site is transitional to the grand fir cool moist forb ecological site (GF-CMF: BpS model 910470) at lower elevations, and to the subalpine fir cold dry shrub AF-CDS: BpS 910550) found at higher elevations on very cold, relatively dry sites. The ecological site on all aspects, over a wide variety of slope positions and gradients. Stand canopy coverage is broadly variable, even in the late development community plant phases.
The historic long term climatic expression of this ecologic site is characterized by cold winters and cool summers, where frost can occur in any month of the summer season. Snowfall amounts are high and the accumulated snowpack lingers long into the summer months.
As the global climate continues to change, a shift towards a warmer and drier environment will occur, changing the nature and expression of the forests included in the current ES.
Engelmann spruce (Picea engelmanii) often dominates post-disturbance stands in mid- to late community development stages, but subalpine fir easily establishes in the understory shade of the mature spruce forest. For these reasons, both subalpine fir and Engelmann spruce (which is a persistent, long lived conifer) are common overstory associates in mature, late seral forests. Although these two species are typically co-dominant within the stand, subalpine fir is considered the climax series. As a species, subalpine fir is well adapted to long fire free intervals, where fires that do occur are either stand replacement or mixed fire events. Conversely, individual subalpine fir trees rarely survive any type of fire because of the thin bark, shallow rooting structure, and dense branching characteristics which often continue down to the surface of the ground. Needle foliage and crowns are highly flammable and many older trees contain abundant lichen growth.
Although it is very shade tolerant, subalpine fir is often the pioneer conifer species on harsh sites, and it establishes well in the absence of plant competition under partial sunny conditions.
Engelmann spruce is well adapted to recover rapidly following mixed or stand replacement fire occurrences. It produces abundant seed crops which have good viability and are well dispersed by wind. Engelmann spruce requires mineral soils in order to achieve optimum levels of seedling germination and subsequent establishment, and it will germinate in all light conditions although it does best under shady conditions. Young spruce seedlings growing under full sun exposure will often succumb to the late summer impacts of solar radiation, which results in drought or heat girdling losses.
Engelmann spruce shares many of the same attributes as subalpine fir regarding individual tree susceptibility to wildfire.
Douglas-fir (Pseudotsuga menziesii) is a mid-seral component of this ecological site. The species is approaching the upper limits of its ecological tolerance (i.e. ecological “amplitude”) to colder temperature regimes, but Douglas-fir is often a viable component of mid and late seral stands on the more favorable sites.
Lodgepole pine (Pinus contorta), western larch (Larix occidentalis), and limited levels of quaking aspen (Populus trichocarpa) are all early seral tree species which can aggressively occupy severely disturbed sites when conditions are favorable. Disturbances (i.e. wildfire or endemic insect outbreaks) initiates the process of lodgepole pine regeneration. Lodgepole pine produces viable seed crops at a relatively early age, and both serotinous and non-serotinous cones are common, allowing regeneration to occur with and without fire. Pure stands of disturbance driven lodgepole (mainly following stand replacement fire) are generally restricted to slopes of 15% or less, where cold air pooling enhances lodgepole dominance.
Persistent, self-replacing lodgepole pine stands may develop in this ecologic site, often following replacement fire episodes coupled with the absence of subalpine fir and Engelmann spruce seed source(s). These stand(s) could persist for 100’s of years if they are not replaced by mid to late seral conifers, and if the cycle of stand replacement disturbance is relatively short. In this case lodgepole acts as a dominant seral species. Mixed fire maintains the stand in a dense population of pole sized lodgepole pine. However another very common route of succession introduces shade tolerant spruce and subalpine fir into the understory of mature lodgepole pine stands, eventually overtopping and dominating the older lodgepole stand in a longer term fire free environment.
Lodgepole pine and aspen can quickly establish following catastrophic wildfire events, and at times can be found together in mixed stands where lodgepole is the more dominant of the two species. In the very early post-disturbance phase, lodgepole pine regeneration will overtop and outcompete newly emerging Engelmann spruce or subalpine fir seedlings where seedlings of all three conifers have regenerated. Mountain pine beetle is the major biotic disturbance agent in lodgepole pine, especially when it reaches epidemic population levels.
Western larch is an extremely fast growing early seral conifer. It is the most fire adapted conifer in this ecologic site. Mature trees have thick bark, along with high open branches that are very resistant to torching. Western larch will persist into late seral community phases, and will typically survive even the most extreme level of fire, enabling it to seed the emerging post-fire forest communities. Larch establishes well under full sunlight.
In this ecologic site, small patches of aspen are scattered among the more extensive spruce-fir forests and within seral lodgepole pine stands. Quaking aspen stands are very resistant to fire because of the moist environment in which they occur. However individual aspen trees, smaller sized aspen patches, or stands in close proximity to flammable conifers are killed when wildfires sweep across the larger landscape. Aspen typically responds following fire to produce above ground stems that have sprouted (i.e. “sucker”) from underground roots.
Surface fires (or “underburns”) are essentially absent from this ecologic site. Mixed severity wildfire events are very rare, and replacement fire episodes will occur at three times the frequency of mixed severity fires. The average “mean fire return interval” (MFRI) of fires of all types is just under 100 years.
In addition to fire disturbance, the SF-CMS ecologic site is also impacted and changed by periodic windthrow, and by significant insect and disease disturbances (typically when these occur at an epidemic rather than an endemic level).
Other biotic and abiotic disturbance factors for the common conifers of the ecologic site:
• Engelmann Spruce is impacted by spruce beetle and by the western spruce budworm, wood rotting fungi, broom rusts and windthrow. Armillaria and laminated root disease susceptibility is moderate.
• Subalpine fir is impacted by western spruce budworm, as well as by bark beetles (such as the western balsam bark beetle and fir engraver), the non-native balsam aphid, and tussock moth. Anosus root disease, laminated root rot, Indian paintbrush and other fungi weaken subalpine fir. Subalpine fir is also prone to windthrow, especially at an advanced age or when weakened by root disease.
• Western larch is attacked by the dwarf mistletoe parasite. It is also impacted by needlecast and red ring rot, as well as by western spruce budworm and larch casebearer. Larch is not prone to windthrow except in rare instances, and is relatively free of most root disease.
• Douglas-fir suffers from tussock moth and other insect defoliators and dwarf mistletoe, and is susceptible to armillaria and laminated root disease.
• Lodgepole pine is attacked by mountain pine beetle, pine engraver, weevils, mistletoe and various fungi causing stem canker and gall rust. It is moderately susceptible to
armillaria root disease.
Understory species found in this ecologic include Grouse whortleberry, Big huckleberry (Vaccinium membranaceum), prince’s pine (Chimaphila umbellata), and heartleaf arnica (Arnica cardifolia). Twinflower occurs primarily in the ABLA/LIBO plant association.