Vegetation Transition Model

Spruce-fir-Aspen Forest

      Description.--Spruce-fir-aspen forest consists of sparse to dense stands of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and aspen (Populus tremuloides). Stands also may contain Douglas fir (Pseudotsuga menziesii), white fir (Abies concolor), blue spruce (Picea pungens), limber pine (Pinus flexilis), or southwestern white pine (Pinus strobiformis). A well-developed layer of shrubs, including blueberry (Vaccinium myrtillus and scoparium), thimbleberry (Rubacer parviflorum), currant (Ribes montigenum), elderberry (Sambucus microbotrys), and Lonicera involucrate is often present. A rich mixture of mesophytic herbs also is often present, including fleabane (Erigeron eximius), Richardson geranium (Geranium richardsonii), one-sided wintergreen (Orthilia secunda or Ligusticum porteri), bluebells (Mertensia ciliata), arnica (Arnica cordifolia), columbine (Aquilegia elegantula), lousewort (Pedicularis racemosa), sagewort (Artemisia franseriodes), violet (Viola canadensis), rattlesnake plantain (Goodyera oblongifolia), strawberry (Fragaria vesca), goldenweed (Oreochrysum parryi), peavine (Lathyrus leucanthus), wintergreen (Pyrola minor), false Solomon's seal (Maianthemum stellatum), Luzula parviflora, death camas (Anticlea elegans), bromegrass (Bromopsis canadensis), elk sedge (Carex geyeri), and sweet cicely (Osmorhiza depauperata). Species composition in this vegetation type is very similar to that of the spruce-fir forest cover type, described elsewhere, with the exception that aspen is also an important component of this type. Aspen is usually the dominant canopy species for several decades after a major disturbance, but is gradually replaced by conifers over time. In late-successional stands, aspen may be represented only by scattered large canopy stems, with little or no understory regeneration.

      Distribution.–Spruce-fir-aspen forest is found on all aspects at the highest elevations (Figure-distribution map; Figure-elevation chart; Table-areal coverage). Spruce-fir-aspen is found at slightly lower elevations than spruce-fir forest, although the distributions of these two cover types overlap greatly; the reason why aspen is an important component of some stands but not others is unknown. Similar communities are elsewhere in the southern Rocky Mountain region (Peet 1988, Jamieson et al. 1996, Johnston and Huckaby 2001, Romme et al. 2003).

      Stand conditions.–We recognized four separate stand conditions following the model of Oliver (1981) and Oliver and Larson (1990). Although this particular model has been criticized for its failure to incorporate the full range of spatial and temporal variability in forest stand development (Franklin et al. 2002), it is widely recognized and understood among ecologists and silviculturalists.

      1.   Stand initiation (SI) - Dense ground cover consisting of grasses, forbs and low shrubs, and a moderate to dense cover of tree seedlings and saplings (aspen much greater abundance than conifers) with an open canopy. This condition is characterized by the recruitment of a new cohort of early successional, shade-intolerant tree species into an open area created by a stand-replacing disturbance. This condition was classified as aspen for purposes of FRAGSTATS analysis and wildlife habitat models.

      2.   Stem exclusion (SE) - Sparse ground cover of grasses, forbs and shrubs, and a moderate to dense cover of pole-sized trees (aspen greater abundance than conifers) with a closed canopy. This condition occurs once the pioneer cohort has occupied all of the open area and formed a closed canopy, thereby excluding further tree recruitment because of intense competition for light and other resources. This condition was classified as aspen for purposes of FRAGSTATS analysis and wildlife habitat models.

      3.   Understory re-initiation (UR) - Heterogeneous ground cover of grasses, forbs and shrubs, and varying density of trees (conifers greater abundance than aspen) of variable size classes. The older (larger) trees from the original cohort form a patchy closed canopy, but there is an understory of uneven aged trees which begins when the pioneer cohort reaches maturity and individual stems begin to die, creating gaps in the canopy into which new stems can be recruited.

      4.   Shifting mosaic (SM) - Heterogeneous ground cover of grasses, forbs and shrubs, and variable density of trees (conifers much greater abundance than aspen) of all size classes that maintain a patchy closed canopy. This condition begins when all or nearly all of the pioneer cohort has died, and the stand becomes dominated by fine-scale gap dynamics, which leads to great structural complexity. The stand persists in this condition until a stand-replacing disturbance. Note, stand age in this stage is not particularly meaningful, as there typically exist trees in all age classes. In addition, the oldest trees in this stage do not necessarily indicate the time since stand origin, as most or all trees will have been established after the initial stand-replacing disturbance event. In general, the oldest trees will reflect the longevity of the species (in this case, 300-450 years), not necessarily the time since stand origin.

      Succession Transitions.–In the absence of another disturbance, succession transitions occur as follows (Figure-model):

      1.   SI - persists from age 0 until age 20, after which stands begin transitioning to the SE condition. Stands may persist in the SI condition until age 40, after which all stands will have transitioned to the SE.

      2.   SE - persists until age 80, after which stands begin transitioning to the UR condition. Stands may persist in the SE stage until age 120, after which all stands will have transitioned to UR. Following low mortality wildfire or any non-stand-replacing insect outbreak stands will succeed to UR if they are at least 70 years old.

      3.   UR - persists until age 300, after which stands begin transitioning to the SM condition. Stands may persist in the UR condition until age 450, after which all stands will have transitioned to SM. Following low mortality wildfire or any non-stand-replacing insect outbreak stands will succeed to SM if they are at least 300 years old.

      4.   SM - persists until the next stand-replacing disturbance.

*Note, see Succession Rules for the formal implementation of these rules in RMLANDS.

      Wildfire Disturbance Transitions.-Wildfires tend to be high-mortality, stand-replacing fires that initiate a process of post-fire forest succession. High-mortality fires kill large as well as small trees, and may kill many of the shrubs and herbs as well, although below-ground organs of at least some individual shrubs and herbs survive and re-sprout. Aspen stems are very vulnerable to fire damage, but the root system usually survives even severe fires and promptly re-sprouts. Wildfires invoke the following potential transitions (Figure-model):

      1.   SI - high mortality wildfire recycles the stand through the SI condition, while a low mortality wildfire maintains the stand in this condition.

      2.   SE - high mortality wildfire returns the stand to the SI condition, while a low mortality wildfire accelerates the succession transition to the UR condition if the stand age is >70 years.

      3.   UR - high mortality wildfire returns the stand to the SI condition, while a low mortality wildfire accelerates the succession transition to the SM condition if the stand age is >300 years.

      4.   SM - high mortality wildfire returns the stand to the SI condition, while a low mortality wildfire maintains the stand in the SM condition.

*Note, see Succession Rules and Disturbance Rules for the formal implementation of these rules in RMLANDS.

      Insect/Pathogen Disturbance Transitions.–Spruce-fir-aspen forests are subject to two different insect disturbance processes in RMLANDS: spruce beetle and western spruce budworm. Because spruce-fir-aspen stands are assumed to contain a mixture of both spruce and fir, neither insect alone is sufficient to cause a stand-replacing event. Rather, an epidemic of one insect will simply shift the tree species composition of the forest in favor of the non-host species. Both insects working in concert, however, can result in near or complete overstory mortality and therefore invoke stand-replacement (although this is very uncommon). In addition, because early successional stages are dominated by aspen, insect disturbances can not cause stand replacement until the later conifer-dominated stages of development. Insect disturbances invoke the following potential transitions (Figure-model):

      1.   SI - any combination of insect outbreak (i.e., high or low mortality of either or both insects) maintains the stand in this condition. Note, high mortality outbreak of both insects does not recycle the stand through this condition (as with wildfire) because the aspen sprouts, which dominate during this stage, are not affected by these insect disturbances.

      2.   SE - any combination of insect outbreak (i.e., high or low mortality of either or both insects) accelerates the succession transition to the UR condition if the stand age is >70 years. Note, high mortality outbreak of both insects does not return the stand to the SI condition (as with wildfire) because the aspen trees, which dominate during this stage, are not affected by these insect disturbances.

      3.   UR - any combination of insect outbreak (i.e., high or low mortality of either or both insects) accelerates the succession transition to the SM condition if the stand age is >300 years. Note, any outbreak will likely cause the loss of many or most of the dominant conifer trees and accelerate the transition to the SM condition. Even high mortality outbreak of both insects does not return the stand to the SI condition (as with wildfire) because the aspen trees, although declining during this stage, are still present in sufficient numbers to maintain a cover of large trees if the conifers are lost.

      4.   SM - high mortality outbreak of both insects returns the stand to the SI condition, while any other combination of insect outbreak (i.e., high or low mortality of either or both insects) maintains the stand in the SM condition. Note, the remnant aspen stems in this stage are no longer sufficient to maintain a tree canopy if the conifers are lost.

*Note, see Succession Rules and Disturbance Rules for the formal implementation of these rules in RMLANDS.