Physiognomy and Structure: No Data Available

Floristics: The moderately open to closed overstory is highly complex and has high variability. Abies concolor dominates or codominates, if not in the overstory, then as regeneration. Pseudotsuga menziesii is a successional dominant and remains a codominant in late-successional stands, becoming minor in very old stands. Picea pungens, Pinus longaeva, Pinus flexilis, and Juniperus scopulorum may be important, as well as Pinus strobiformis at lower latitudes. Abies lasiocarpa and Picea engelmannii may occur in frost pockets as regeneration or occasional mature trees, but they are minor and almost always are under severe competition from dense regeneration and canopy dominance of Abies concolor and Pseudotsuga menziesii (Moir and Ludwig 1979). Pinus ponderosa is an accidental or minor species, since neither regeneration nor mature trees are important in late-successional stands (Moir and Ludwig 1979). Seral stands are often codominated by Populus tremuloides (Stuever and Hayden 1997a). The tall-shrub layer dominates the undergrowth typically dominated or codominated by Acer glabrum or less frequently Amelanchier alnifolia. Quercus gambelii, if present, has relatively low cover (<5%). Other short and dwarf-shrubs may be present with low cover, such as Holodiscus dumosus and Jamesia americana, which occur on cobbly substrates, along with Acer grandidentatum, Juniperus communis, Mahonia repens (= Berberis repens), Paxistima myrsinites, Physocarpus monogynus, Rosa woodsii, Shepherdia canadensis, and Symphoricarpos oreophilus; however, some stands have an open shrub of layer of Acer glabrum and little else. Herb layer species are typically low in cover. Common species are Artemisia franserioides, Bromus anomalus, Bromus ciliatus, Carex spp., Erigeron spp., Lathyrus spp., Maianthemum racemosum, Penstemon spp., Pyrola chlorantha, Solidago velutina, Stellaria longifolia, Thalictrum fendleri, and Viola canadensis.

Alexander et al. (1987) describe a Holodiscus dumosus phase for this type. This phase is the only form in which this association occurs on the Lincoln National Forest, and is characterized by the presence of Holodiscus dumosus. Alexander et al. (1987) describe three phases for this type in the Cibola National Forest of New Mexico. The first, an Acer glabrum typic phase found from 2440-2895 m (8000-9500 feet) on north ridges at high elevations and in cool drainages at lower elevations, shows the expected characteristics for the type. The second phase, a riparian phase found from 2290-2500 m (7500-8200 feet), is distinguished by the absence of conifers, other than Abies concolor, in the overstory, as well as high shrub cover and restriction to streamside canyon bottoms. This phase can occur in canyons bisecting warm, south exposures, if conditions are moist and shady. The third phase, Mahonia repens phase found from 2470-3140 m (8100-10,300 feet) on north ridge sideslopes as well as cool moist microsites at lower elevations, has less Acer glabrum and more Quercus gambelii than that of the typic phase.

Moir and Ludwig (1979) describe two phases. The first, a Mahonia repens phase found between 2710-2925 m (8900-9600) feet on east-, south-, or west-facing canyon sideslopes, on gentle north-facing mesatops at 2800-2865 m (9200-9400 feet), and extending down streamsides to 2350 m (7700 feet), features Mahonia repens in the low-shrub layer, as well as Paxistima myrsinites. The Mahonia repens phase is found in the northern forests of the region, in the mountains of northern New Mexico, with an outlier in the White Mountains of Arizona. The second phase, a Holodiscus dumosus phase occurring between 2400-2895 m (7900-9500 feet) on moderate to steep canyon sideslopes of east and northwest exposures, at higher elevations when found on south-facing slopes between 2865-3050 m (9400-10,000 feet), and extending along streamsides down to about 2070 m (6800 feet), lacks Mahonia, and is found in the southern forests, in the Sacramento, Mogollon, Chiricahua, and Pinaleno mountains.

At upper elevations, this type forms ecotones with spruce-fir types. The Abies concolor / Acer glabrum forms a lower elevation limit of many subalpine species (DeVelice et al. 1986). In more mesic areas, this type intergrades to the Abies concolor / Erigeron eximius type, and into the Abies lasiocarpa / Rubus parviflorus and Abies concolor / Acer grandidentatum types in cool wet drainages where conditions are especially moist (Fitzhugh et al. 1987). In more xeric locations, this type grades to the Abies concolor / Mahonia repens and Abies concolor / Quercus gambelii types, and the Pseudotsuga menziesii / Holodiscus dumosus (Scree) type on drier south-facing slopes. Lower slopes support Picea pungens - Pseudotsuga menziesii stands.

Dynamics:  Because of the cool, moist conditions characteristic of this habitat type, fires are mostly light, erratic, and infrequent (Moir and Ludwig 1979). The erratic nature of fires has resulted in mosaics in forest structure within this type (DeVelice et al. 1986). Logging and stand deterioration with increasing age have also contributed to the irregular structure of this type (Alexander et al. 1987).

Crane (1982) places this type in fire group eleven, which are the white fir habitat types. According to Crane, many mixed conifer stands originated following severe wildfires. Fires following lightning strikes during the wet season are variable and frequently remain as surface fires, burning relatively small areas due to the moisture content of the fuels. Such fires kill smaller understory trees, reduce fuels and occasionally flare up to open the canopy. All of these effects help to retain the seral species and thus to maintain the mixed composition and structure of the forests. The most frequent result of fire suppression has been development of dense understories of more shade-tolerant species. Dieterich (1980) gave a composite fire interval of 36 years for a mixed forest in Arizona.

In a generalized forest succession, there will be an initial grass and forb community. Any fire would help maintain a grassland by eliminating regeneration. If Quercus gambelii or Populus tremuloides were in the previous forest stand, they will resprout normally within one to two years. Periodic moderate to severe fires will tend to rejuvenate these stands while excluding conifer regeneration. A single light surface fire could prepare a seedbed for Pinus ponderosa and other conifers. A later surface fire could thin sapling regeneration. A surface fire in the pole stage would preferentially thin the stand, favoring Pinus ponderosa and Pseudotsuga menziesii over the slower-growing white fir. It might also thin any aspen and Quercus gambelii. A severe fire would return the stand to the initial grass and forb state. Repeated low surface fires during the development of the stand from the pole stage could result in a mature, open Pinus ponderosa or Pinus ponderosa and Pseudotsuga menziesii stand with some understory Abies concolor establishment between fires. Without fire, Abies concolor and Pseudotsuga menziesii will dominate regeneration (Crane 1982).

Other processes which may affect this type are as follows: Pseudotsuga menziesii is subject to windthrow in this association. There may be some spruce budworm activity on the Pseudotsuga menziesii, as well as infection with dwarf mistletoe which can also infect the Abies concolor. Severe fires eliminate dwarf mistletoe when infected stands are destroyed.