Vegetation Transition Model
Pure Aspen Forest
Description.--Pure (or stable) aspen forest consists of sparse to dense stands of aspen (Populus tremuloides) with no or almost no coniferous species present. Symphoricarpos oreophilus forms a conspicuous shrub understory in many stands, sometimes with other shrub species, including Gambel oak (Quercus gambelii), Sambucus racemosa, and common juniper (Juniperus communis). Forbs dominate the herbaceous stratum in most stands, but grasses and forbs are equally abundant in some stands. Some common understory herbaceous species include meadowrue (Thalictrum fendleri), Senecio serra, one-sided wintergreen (Orthilia secunda or Ligusticum porteri), Richardson geranium (Geranium richardsonii), Delphinum barbeyi, Osmorrhiza obtusa, American vetch (Vicia americana), peavine (Lathyrus leucanthus), elk sedge (Carex geyeri), Fragaria ovalis, Viola nuttallii, and Bromus ciliatus. brackenfern (Pteridium aquilinum) is also very common in some stands.
Distribution.–Pure aspen forest is widespread at middle to 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) - Dense ground cover consisting of grasses, forbs and low shrubs, and a moderate to dense cover of tree sprouts, seedlings and saplings with an open canopy. This condition is characterized by the recruitment of a new cohort of trees, principally sprouts, following a stand-replacing disturbance.
2. Stem exclusion (SE) - 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, 200 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 10, after which stands begin transitioning to the SE condition. Stands may persist in the SI condition until age 30, after which all stands will have transitioned to the SE.
2. SE - persists until age 60, 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 stands will succeed to UR if they are at least 50 years old.
3. UR - persists until age 100, after which stands begin transitioning to the SM condition. Stands may persist in the UR condition until age 200, after which all stands will have transitioned to SM. Following low mortality wildfire stands will succeed to SM if they are at least 100 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, even in low-intensity fires, but the root system usually survives even high-intensity 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 >50 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 >100 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.–We did not model any insect or pathogen disturbances in pure aspen forests.
