Vegetation Transition Model
Spruce-fir Forest
Description.--Spruce-fir forest consists of sparse to dense stands of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). 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), but aspen (Populus tremuloides) is largely absent. 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).
Distribution.–Spruce-fir forest is found on all aspects at the highest elevations (Figure-distribution map; Figure-elevation chart; Table-areal coverage), similar to its distribution throughout 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 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.
2. Stem exclusion (SE) - Sparse ground cover of grasses, forbs and shrubs, and a moderate to dense cover of pole-sized trees 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.
3. Understory re-initiation (UR) - Heterogeneous ground cover of grasses, forbs and shrubs, and varying density of trees 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 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 30, after which stands begin transitioning to the SE condition. Stands may persist in the SI condition for as long as 200 years on some sites, after which all stands will have transitioned to SE, and the likelihood of delayed transition increases with elevation.
2. SE - persists until age 150, after which stands begin transitioning to the UR condition. Stands may persist in the SE stage until age 300, after which all stands will have transitioned to UR, and the likelihood of delayed transition increases with elevation. Following low mortality wildfire or any non-stand-replacing insect outbreak stands will succeed to UR if they are at least 100 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, and the likelihood of delayed transition increases with elevation. 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. 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 >100 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 forests are subject to two different insect disturbance processes in RMLANDS: spruce beetle and western spruce budworm. Because spruce-fir 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), as follows (Figure-model):
1. SI - high mortality outbreak of both insects recycles the stand through the SI condition, while any other combination of insect outbreak maintains the stand in this condition.
2. SE - high mortality outbreak of both insects returns the stand to the SI condition, while any other combination of insect outbreak accelerates the succession transition to the UR condition if the stand age is >100 years.
3. UR - high mortality outbreak of both insects returns the stand to the SI condition, while any other combination of insect outbreak accelerates the succession transition to the SM condition if the stand age is >300 years.
4. SM - high mortality outbreak of both insects returns the stand to the SI condition, while any other combination of insect outbreak maintains the stand in the SM condition.
*Note, see Succession Rules and Disturbance Rules for the formal implementation of these rules in RMLANDS.
