Vegetation Transition Model

Cool Moist Mixed-Conifer Forest

      Description.--Cool moist mixed-conifer forest consists of moderate to dense stands of a mixture of coniferous species, including Douglas fir (Pseudotsuga menziesii), white fir (Abies concolor), and/or blue spruce (Picea pungens). Stands also may contain Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa or bifolia), limber pine (Pinus flexilis), and southwestern white pine (Pinus strobiformis), but aspen (Populus tremuloides) is largely absent. A rich shrub understory is often present, including western serviceberry (Amelanchier alnifolia), Sambucus racemosa, Lonicera involucrata, and snowberry (Symphoricarpos rotundifolius). Common graminoids and forbs include elk sedge (Carex geyeri), fleabane (Erigeron eximius), one-sided wintergreen (Orthilia secunda or Ligusticum porteri), bluebells (Mertensia ciliata), columbine (Aquilegia elegantula), sagewort (Artemisia franseriodes), violet (Viola canadensis), rattlesnake plantain (Goodyera oblongifolia), strawberry (Fragaria vesca), goldenweed (Oreochrysum parryi), peavine (Lathyrus leucanthus), wintergreen (Pyrola minor), baneberry (Actaea rubra), false Solomon's seal (Maianthemum stellatum), Luzula parviflora, fringed bromegrass (Bromopsis canadensis), elk sedge (Carex geyeri), Rubus parviflorus, and sweet cicely (Osmorhiza depauperata).

      Distribution.–Cool moist mixed-conifer forest is found on north-facing slopes at middle elevations and on all aspects at high elevations (Figure-distribution map; Figure-elevation chart; Table-areal coverage). 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) – Grasses, forbs, low shrubs, and sparse to moderate cover of trees–seedlings/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; moderate to dense cover of trees–pole size 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 Reinitiation (UR) – Heterogeneous ground cover of grasses, forbs, and shrubs; varying density of trees–variable size classes with a patchy closed canopy. 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; variable density of trees (Pinus ponderosa declining or absent; no reproduction; but Pseudotsuga menziesii increasing in density in all size classes )–variable size classes, but mostly with a closed canopy (due to the prolonged absence of wildfire). 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 or low mortality fire transforms it into a fire-maintained open canopy condition. 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, 300-400 years), not necessarily the time since stand origin.

      Succession Transitions.–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 100 years on some sites, after which all stands will have transitioned to SE.

      2.   SE - persists until age 150, after which stands begin transitioning to the UR condition. Stands may persist in the SE stage until age 250, 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 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. 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 will accelerate 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 will accelerate 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.

      Insects/Pathogen Disturbance Transitions.–Cool moist mixed-conifer forest is subject to three different insect disturbance process in RMLANDS: Douglas-fir beetle, spruce beetle, and spruce budworm. Douglas-fir beetle and spruce beetle kill Douglas-fir and Engelmann spruce trees, respectively, especially in the larger size classes (> ca. 8 inches dbh). Western spruce budworm affects Pseudotsuga menziesii and Abies lasiocarpa trees of all sizes, often weakening trees and making them more susceptible to beetle attack. Because cool moist mixed-conifer stands are assumed to contain a mixture of host species, no 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. Two or three of the insects working in concert, however, can result in near or complete overstory mortality and therefore invoke stand-replacement (although this is very uncommon). These insect disturbances invoke the following potential transitions (Figure-model):

      1.   SI – high mortality outbreak of Douglas-fir beetle, spruce beetle and spruce budworm, or of just spruce beetle and spruce budworm, recycles the stand through the SI condition while any other combination of insect outbreak maintains the condition.

      2.   SE – high mortality outbreak of Douglas-fir beetle, spruce beetle and spruce budworm, or of just spruce beetle and spruce budworm, returns the stand to the SI condition while any other combination of insect outbreak will accelerate the succession transition to the UR condition if the stand age is >100 years.

      3.   UR – high mortality outbreak of Douglas-fir beetle, spruce beetle and spruce budworm, or of just spruce beetle and spruce budworm, returns the stand to the SI condition while any other combination of insect outbreak will maintain the UR condition if the stand is <300 years old or cause the succession transition to the SM condition if the stand is >300 years old.

      4.   SM – high mortality outbreak of Douglas-fir beetle, spruce beetle and spruce budworm, or of just spruce beetle and spruce budworm, 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.