Vegetation Transition Model
Warm Dry Mixed-Conifer-Aspen Forest
Description.-Warm dry mixed-conifer-aspen forest consists of sparse to moderately dense stands of Pinus ponderosa mixed with Pseudotsuga menziesii, Abies concolor, and Populus tremuloides. The understory usually is dominated by Quercus gambelii, with other shrubs often present also, including Ceanothus fendleri, Amelanchier alnifolia, Symphoricarpos rotundifolius, Mahonia repens, Chimaphila umbellatum, Arctostaphylos uva-ursi, and Arctostaphylos patula. Common grasses and forbs include Koeleria macranthra, Elymus elymoides, Poa fendleriana, Poa pratensis, Carex geyeri, Delphium nelsoni, Mertensia fusiformis, Erigeron formosissimus, Potentilla hippiana, Solidago simplex, Geranium caespitosum, Antennaria rosea, Pseudocymopterus montanus, Lathyrus leucanthus, and Achillea lanulosa (species list from J. Redders, San Juan National Forest, and personal observations). Species composition of this vegetation is very similar to that of the warm-dry mixed conifer forest, described elsewhere, except that Populus tremuloides is also an important component of this type. Young stands or frequently burned stands may be dominated by Populus tremuloides, but Populus tremuloides is gradually replaced by conifers over the course of succession in the absence of disturbance.
Distribution.–Warm dry mixed-conifer-aspen forest is an extensive cover type found at middle elevations (Figure-distribution map; Figure-elevation chart; Table-areal coverage). Its distribution overlaps the distribution of warm dry mixed-conifer forest; the reason why some mixed conifer stands also contain aspen and others do not is unknown. Similar communities are found in many portions of the Colorado Plateau and southern Rocky Mountain regions (Johnston and Huckaby 2001, Romme et al. 2003).
Stand conditions.–We recognized five separate stand conditions, largely 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 (Populus tremuloides are much more abundant than conifers) that are 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. 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; moderate to dense cover of trees (primarily Populus tremuloides with conifers secondary) that are 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. This condition was classified as aspen for purposes of FRAGSTATS analysis and wildlife habitat models.
3. Understory Reinitiation (UR) – Heterogeneous ground cover of grasses, forbs, and shrubs; varying density of trees (conifers more abundant than the declining Populus tremuloides) that are 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; conifers much more abundant that the scattered remnant Populus tremuloides) that are 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. 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.
5. Fire-Maintained Open Canopy (FMO) – Moderate to dense ground cover of grasses, forbs, and low shrubs; low density of large trees (Pinus ponderosa reproduction present; Pseudotsuga menziesii and Pinus ponderosa more abundant than the scattered dense patches of regenerating Populus tremuloides)--variable size classes with a patchy distribution and open canopy. This condition develops when low-severity fire burns a stand in the UR or SM condition; it persists as long as low-mortality fire continue to occur periodically.
Succession Transitions.–Succession transitions occur as follows (Figure-model):
1. SI - persists from stand age 0 until age 20 in the absence of disturbance, after which stands begin transitioning to the SE condition. Stands may persist in the SI condition until age 60, after which all stands will have transitioned to SE.
2. SE - persists until age 80 in the absence of disturbance, after which stands begin transitioning to the UR condition. Stands may persist in the SE condition 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 in the absence of disturbance, after which stands begin transitioning to the SM condition. Stands may persist in the UR condition until age 400, after which all stands will have transitioned to SM. Following low mortality wildfire stands will begin succession transition to FMO if they are at least 150 years old; all stands will succeed if they are older than 200 years. Following any non-stand-replacing insect outbreak stands will succeed to SM if they are at least 300 years old.
4. SM - transitions to the FMO condition following low mortality wildfire; otherwise, persists in the SM condition until the next stand-replacing disturbance.
5. FMO - persists in this condition for at least 50 years in the absence of wildfire, after which stands begin transitioning back to the SM condition. Stands may persist in the FMO condition in the absence of wildfire for a maximum of 100 years, after which all stands will have transitioned back to the SM condition.
*Note, see Succession Rules for the formal implementation of these rules in RMLANDS.
Wildfire Disturbance Transitions.–Wildfires are common and frequent, mortality depends on vegetation vulnerability and wildfire intensity. Low-mortality fires kill small trees and consume above-ground portions of shrubs and herbs, but do not kill large trees or below-ground organs of most shrubs and herbs which promptly re-sprout. High-mortality fires kill large as well as small trees, and may kill many of the shrubs and herbs as well. Fire kills the above-ground portions of the shrubs and herbs, but most shrubs and herbs promptly re-sprout from surviving below-ground organs. 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 can either maintain the UR condition or accelerate the succession transition to the FMO condition if the stand age is >150 years; stands >200 years old will always succeed to the FMO condition following low mortality wildfire.
4. SM – high mortality wildfire returns the stand to the SI condition while a low mortality wildfire will cause the succession transition to the FMO condition.
5. FMO – high mortality wildfire returns the stand to the SI condition while a low mortality wildfire maintains the stand in the FMO condition.
*Note, see Succession Rules and Disturbance Rules for the formal implementation of these rules in RMLANDS.
Insects/Pathogen Disturbance Transitions.–Warm dry mixed-conifer-aspen forest is subject to four different insect disturbance process in RMLANDS: Pine beetle, Douglas-fir beetle, spruce beetle, and spruce budworm. Spruce beetle outbreaks are of minor concern in this cover type due to the paucity of spruce trees, so while they are allowed to occur, they have no affect on transitions. Pine beetle and Douglas-fir beetle kill Pinus ponderosa and Pseudotsuga menziesii trees, respectively, especially in the larger size classes (> ca. 8 inches dbh). Western spruce budworm affects Pseudotsuga menziesii and Abies concolor trees of all sizes, often weakening trees and making them more susceptible to beetle attack. Because warm dry mixed-conifer-aspen 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). 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. These insect disturbances invoke the following potential transitions (Figure-model):
1. SI – Any combination of insect outbreak (i.e., high or low mortality of any or all insects) maintains the stand in this condition. Note, high mortality outbreaks of any combination of 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 any or all insects) accelerates the succession transition to the UR condition if the stand age is >70 years. Note, high mortality outbreak of any combination of 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 any or all 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 all 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 pine beetle, Douglas-fir beetle, and spruce budworm, or of just pine 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, the remnant aspen stems in this stage are no longer sufficient to maintain a tree canopy if the conifers are lost.
5. FMO – high mortality outbreak of pine beetle, Douglas-fir beetle, and spruce budworm, or of just pine beetle and spruce budworm, returns the stand to the SI condition while any other combination of insect outbreak maintains the stand in the FMO condition. Note, at this stage the stand is dominated by conifers - the aspen are restricted to scattered patches of regeneration.
*Note, see Succession Rules and Disturbance Rules for the formal implementation of these rules in RMLANDS.
