Vegetation Transition Model

Ponderosa Pine-Oak-Aspen Forest

      Description.--Ponderosa pine-oak-aspen forest consists of sparse to dense stands of ponderosa pine (Pinus ponderosa) and aspen (Populus tremuloides), with occasional Douglas fir (Pseudotsuga menziesii), Rocky Mountain juniper (Juniperus scopulorum), or pinyon pine (Pinus edulis). There usually is a well-developed understory of Gambel oak (Quercus gambelii) and occasional other shrubs, including mountain mahogany (Cercocarpus montanus), serviceberry (Amelanchier sp.), buckbrush (Ceanothus fendleri), bitterbrush (Purshia tridentata), Oregon grape (Mahonia repens), kinnikinnik (Arctostaphylos uva-ursi), and snowberry (Symphoricarpos rotundifolius). The ground layer vegetation is extremely variable, depending on aspect, soil type, overstory canopy cover, grazing and fire history. Common ground layer species include daisy (Erigeron formosissimus), cinquefoil (Potentilla hippiana), goldenrod (Solidago simplex), beardstongue (Penstemon barbatus), geranium (Geranium caespitosum), strawberry (Fragaria virginiana), pasqueflower (Pulsatilla patens), pussytoes (Antennaria rosea), mountain parsley (Pseudocymopterus montanus), peavine (Lathyrus leucanthus), yarrow (Achillea lanulosa), Parry oatgrass (Danthonia parryi), Arizona fescue (Festuca arizonica), Muhlenberia montana, Koelaria macrantha, bottlebrush squirreltail (Elymus elymoides), muttongrass (Poa fendleriana), Kentucky bluegrass (Poa pratensis), and elk sedge (Carex geyeri).

      Structure and composition of this cover type is highly variable among sites depending on the relative abundance and spatial configuration of aspen and oak, both of which function to suppress pine establishment. Aspen is generally restricted to the cooler, moister microsites within the stand, whereas oak is often patchily distributed throughout the stand on the warmer, drier microsites. It is important to note that the aspen is not well-distributed throughout these stands since they are generally too warm and dry for aspen. Consequently, following stand-replacing disturbances, the proliferation of aspen via re-sprouting is patchy, which allows ponderosa pine to get established in the interstices. Within the aspen patches, succession is probably very similar to that described elsewhere for pure aspen stands. Similarly, within the remainder of the stand, succession is probably very similar to that described elsewhere for ponderosa-pine-oak forest. Hence, ponderosa-pine-oak-aspen stands are actually a fine-scale mosaic of these two cover types, and succession in reality is a complex mosaic of two different models. Unfortunately, this spatial mosaic of cover types and succession models is below the resolution of our data and we must therefore integrate them into a single model.

      Distribution.–Ponderosa pine-oak forest is an extensive cover type found at middle elevations on Dakota sandstone (Figure-distribution map; Figure-elevation chart; Table-areal coverage). Ponderosa pine–oak forest and ponderosa pine–oak–aspen forest overlap in distribution; it is not known why some stands contain aspen while others do not. 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 Pinus ponderosa) 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) – Moderate ground cover of grasses, forbs, and shrubs; moderate to dense cover of trees (primarily Populus tremuloides with Pinus ponderosa 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 (primarily Quercus gambelii); varying density of trees (primarily Pinus ponderosa, with 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 (primarily Quercus gambelii); variable density of trees (Pinus ponderosa is dominant with 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 (primarily Quercus gambelii); low density of large trees (Pinus ponderosa are still much 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 40, 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 low mortality 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 low mortality 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-severity 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-severity 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 can either maintain the SE condition or accelerate 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.–Ponderosa pine-oak-aspen forest is subject to a single insect disturbance process in RMLANDS: Pine beetle complex. This insect complex kills ponderosa pine trees, especially in the larger size classes (> ca. 8 inches dbh) and invokes the following potential transitions (Figure-model):

      1.   SI – high or low mortality outbreak maintains the stand in this condition. Note, high mortality does not recycle the stand through this condition (as with wildfire) because the aspen sprouts, which dominate during this stage, are not affected by pine beetles.

      2.   SE – high or low mortality outbreak can either maintain the SE condition or accelerate the succession transition to the UR condition if the stand age is >70 years. Note, high mortality 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 pine beetles.

      3.   UR – high or low mortality outbreak will cause the succession transition to the SM condition if the stand is >300 years old. Note, any outbreak will likely cause the loss of many or most of the dominant pine trees and accelerate the transition to the SM condition, but due to the presence of the declining aspen the stand will not return to the SI condition (as with ponderosa-pine-oak forest).

      4.   SM – high mortality outbreak returns the stand to the SI condition while a low mortality outbreak maintains the stand in the SM condition. Note, at this stage the stand is dominated by ponderosa pine - the aspen are but a remnant.

      5.   FMO – high mortality outbreak returns the stand to the SI condition while a low mortality outbreak maintains the stand in the FMO condition. Note, at this stage the stand is dominated by ponderosa pine - 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.