Rocky Mountain Gambel Oak Shrubland

This ecological system occurs in the mountains, plateaus and foothills of the southern Rocky Mountains and Colorado Plateau, including the Uinta and Wasatch ranges and the Mogollon Rim. These shrublands are most commonly found along dry foothills, lower mountain slopes, and at the edge of the western Great Plains from approximately 2000 to 2900 m in elevation and are often situated above pinyon-juniper woodlands. Substrates are variable and include soil types ranging from calcareous, heavy, fine-grained loams to sandy loams, gravelly loams, clay loams, deep alluvial sand, or coarse gravel. The vegetation is typically dominated by Quercus gambelii alone or codominant with Amelanchier alnifolia, Amelanchier utahensis, Artemisia tridentata, Cercocarpus montanus, Prunus virginiana, Purshia stansburiana, Purshia tridentata, Robinia neomexicana, Symphoricarpos oreophilus, or Symphoricarpos rotundifolius. There may be inclusions of other mesic montane shrublands with Quercus gambelii absent or as a relatively minor component. This ecological system intergrades with the lower montane-foothills shrubland system and shares many of the same site characteristics. Density and cover of Quercus gambelii and Amelanchier spp. often increase after fire. In Texas, this system includes high mountain shrublands dominated by the deciduous oak species Quercus gambelii. This species often forms nearly monotypic shrublands, but other species present may include Cercocarpus montanus, Robinia neomexicana, Symphoricarpos oreophilus, and Rhus trilobata. These shrubland patches represent southern outliers of the extensive and diverse system further north.

Vegetation types in this system may occur as sparse to dense shrublands composed of moderate to tall shrubs. Occurrences may be multi-layered, with some short shrubby species occurring in the understory of the dominant overstory species. In many occurrences of this system, the canopy is dominated by the broad-leaved deciduous shrub Quercus gambelii, which occasionally reaches small tree size. Occurrences can range from dense thickets with little understory to relatively mesic mixed-shrublands with a rich understory of shrubs, grasses and forbs. These shrubs often have a patchy distribution with grass growing in between. Scattered trees are occasionally present in stands and typically include species of Pinus or Juniperus. Characteristic shrubs that may co-occur, or be singularly dominant, include Amelanchier alnifolia, Amelanchier utahensis, Arctostaphylos patula, Artemisia tridentata, Cercocarpus montanus, Ptelea trifoliata, Prunus virginiana, Purshia stansburiana, Robinia neomexicana, Rosa spp., Symphoricarpos oreophilus, and Symphoricarpos rotundifolius. The herbaceous layer is sparse to moderately dense, ranging from 1-40% cover. Perennial graminoids are the most abundant species, particularly Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua gracilis, Aristida spp., Carex inops, Carex geyeri, Elymus arizonicus, Eragrostis spp., Festuca spp., Koeleria macrantha, Muhlenbergia spp., and Stipa spp. Many forb and fern species can occur, but none have much cover. Commonly present forbs include Achillea millefolium, Artemisia spp., Geranium spp., Maianthemum stellatum, Thalictrum fendleri, and Vicia americana. Ferns include species of Cheilanthes and Woodsia. Annual grasses and forbs are seasonally present, and weedy annuals are often present, at least seasonally.

This ecological system typically occupies the lower slope positions of the foothill and lower montane zones. They may occur on level to steep slopes, cliffs, escarpments, rimrock slopes, rocky outcrops, and scree slopes. Climate is semi-arid and characterized by mostly hot-dry summers with mild to cold winters and annual precipitation of 25 to 70 cm. Precipitation mostly occurs as winter snows but may also consist of some late-summer rains. Soils are typically poorly developed, rocky to very rocky, and well-drained. Parent materials include alluvium, colluvium, and residuum derived from igneous, metamorphic, or sedimentary rocks such as granite, gneiss, limestone, quartz, monzonite, rhyolite, sandstone, schist, and shale. Although this is a shrub-dominated system, some trees may be present. In older occurrences, or occurrences on mesic sites, some of the shrubs may acquire tree-like sizes. Adjacent communities often include woodlands or forests of Abies concolor, Pinus ponderosa, Pseudotsuga menziesii, or Populus tremuloides at higher elevations, and Pinus edulis and Juniperus osteosperma on the lower and adjacent elevations. Shrublands of Artemisia tridentata or grasslands of Festuca sp., Stipa sp., or Pseudoroegneria sp. may also be present at the lower elevations. In Texas, this system primarily occurs on limestone formations on slopes and rolling landforms of the Trans-Pecos mountains, on Limestone Hill and Mountain and High Montane Conifer Ecological Sites.

Fire typically plays an important role in this system, causing die-back of the dominant shrub species in some areas, promoting stump sprouting of the dominant shrubs in other areas, and controlling the invasion of trees into the shrubland system. Natural fires typically result in a system with a mosaic of dense shrub clusters and openings dominated by herbaceous species. In some instances, these associations may be seral to the adjacent Pinus ponderosa, Abies concolor, and Pseudotsuga menziesii woodlands and forests. Ream (1964) noted that on many sites in Utah, Gambel oak may be successional and replaced by bigtooth maple (Acer grandidentatum).

LANDFIRE developed a state-and-transition vegetation dynamics VDDT model for this system which has four classes in total (LANDFIRE 2007a, BpS 2311070). These are summarized as:

A) Early Development 1 All Structures (shrub-dominated - 5% of type in this stage): Shrub cover is 0-20%. Post-replacement sprouts to approximately 2 feet high. Dense resprouting with high number of stems/acre. Abundant grass and forb cover.

B) Mid Development 1 Closed (tree-dominated - 50% of type in this stage): Tree cover is 21-70%. Oak 3-6 feet tall to 3 inches dbh. There will be some stem mortality due to competition and self-thinning, with slight decrease in understory species due to shading. Grasses and forbs declining.

C) Mid Development 1 Open (tree-dominated - 15% of type in this stage): Tree cover is 51-70%. This class has >6 feet tall and >3 inches dbh oak. Small stands <30 m across usually scattered throughout a grassland or shrub type (Brown 1958).

D) Late Development 1 Closed (shrub-dominated - 30% of type in this stage): Tree cover is 71-100%. This class has >6 feet tall and 3 inches dbh. Nearly continuous stand two or more hectares in size with only occasional openings (Brown 1958).

Fire regime group IV or III. The primary disturbance mechanism is replacement fire, resulting in >75% top-kill. Gambel oak responds to fire with vigorous sprouting from the root crown. Larger forms may survive low-intensity surface fire. Extended drought also contributes to disturbance (LANDFIRE 2007a, BpS 2311070).

Threats and stressors to this shrubland system include altered fire regime, fragmentation from roads and development near urban areas, mining, invasive species, livestock grazing disturbance or other human disturbances (CNHP 2010). These disturbances can cause significant soil loss/erosion and negatively impact the water quality within the immediate watershed. Invasive exotic species such as Bromus tectorum can become abundant in disturbed areas and alter floristic composition and provide fine fuels that many increase fire frequency and severity beyond the natural range of variation.

This system occurs in the mountains, plateaus and foothills of the southern Rocky Mountains and Colorado Plateau, including the Uinta and Wasatch ranges and the Mogollon Rim. It also extends into the high mountains of the Trans-Pecos of Texas.