Vegetation Transition Model

Mountain Shrubland

      Description.–Mountain shrubland is a diverse, shrub-dominated cover type. This tall (up to 3 m) shrubland type occurs as relatively pure stands of Gambel oak (Quercus gambelii) or as a mix of Gambel oak and other deciduous shrubs. It also includes stands dominated by mountain mahogany (Cercocarpus montanus), serviceberry (Amelanchier alnifolia), or other shrubs where Gambel oak is absent or a minor component. Stands tend to have moderate to high canopy cover with low to moderate herbaceous cover. Other shrubs associated with this type include Woods rose (Rosa woodsii), chokecherry (Prunus virginiana), big sagebrush (Seriphidium tridentatum sp or Artemisia tridentata sp), fendlerbush (Fendlera rupicola), snowberry (Symphoricarpos rotundifolius), skunkbrush (Rhus trilobata), and squawapple (Peraphyllum ramosissimun). Pinyon pine (Pinus edulis), Utah juniper (Juniperus osteosperma), Rocky mountain juniper (Juniperus scopulorum), ponderosa pine (Pinus ponderosa), white fir (Abies concolor), or Douglas-fir (Pseudotsuga menziesii) trees may be present in the mountain shrubland type, usually occurring as scattered individuals with < 25% total canopy cover. Common herbs associated with this type include goldenrod (Solidago simplex), locoweed (Oxytropis), milkvetch (Astragalus), lupine (Lupinus spp.), geranium (Geranium caespitosum), yarrow (Achillea lanulosa), dandelion (Taraxacum officinale), meadowrue (Thalictrum fendleri), American vetch (Vicia americana), mulesears (Wyethia arizonica), daisy (Erigeron formosissimus), trailing fleabane (Erigeron flagellaris), cinquefoil (Potentilla hippiana), muttongrass (Poa fendleriana), junegrass (Koeleria macrantha), blue grama (Chondrosum gracile), bottlebrush squirreltail (Elymus elymoides), needle-and-thread (Hesperostipa comata), Indian ricegrass (Achnatherum hymenoides), Kentucky bluegrass (Poa pratensis), elk sedge (Carex geyeri), and mountain muhly (Muhlenbergia montana).

      Distribution.–Mountain Shrubland is an extensive cover type found at middle to high elevations on ridges of Dakota sandstone and especially on unstable soils in concave basins of Morrison shale (Figure-distribution map; Figure-elevation chart; Table-areal coverage). It occurs on upland sites with well-drained soils, and is often found on steep slopes with southerly aspects. It occurs as small patches in forest-dominated landscapes, but sometimes occupies extensive areas. It is found in association with pinyon-juniper, ponderosa pine, and warm-dry mixed conifer vegetation types in the semi-arid, lower montane, and montane climate zones. Mean annual precipitation ranges from about 14-25 inches. Similar communities are found in many parts of western Colorado (Floyd et al. 2000, Spencer et al. 1996, Johnston and Huckaby 2001).

      Stand conditions.–We recognized three separate stand conditions:

      1.   Herb -Shrubs (HS) – During the first decade after a severe disturbance, stands are dominated by re-sprouting stems of the shrub species that were present at the time of the fire, plus re-sprouting graminoids and forbs.

      2.   Early Shrub Dominated (ESD) – After about a decade, the re-sprouting shrubs consist of dense clumps <2 m tall. Herbs are present between clumps, but not within the dense clumps.

      3.   Late Shrub Dominated (LSD) – After several decades, the shrubs develop a taller (2-3 m), more open canopy. Herbs grow between clumps of shrubs and also beneath the shrub canopies.

      Succession Transitions.–Succession transitions occur as follows (Figure-model):

      1.   HS - persists from stand age 0 until age 20, after which stands begin transitioning to the ESD condition. Stands may persist in the HS condition until age 30, after which all stands will have transitioned to ESD.

      2.   ESD - persists until age 50, after which stands begin transitioning to the LSD condition. Stands may persist in the ESD condition until age 70, after which all stands will have transitioned to LSD.

      3.   LSD - persists until the next stand-replacing disturbance.

*Note, see Succession Rules for the formal implementation of these rules in RMLANDS.

      Wildfire Disturbance Transitions.--Fire kills the above-ground portions of the shrubs and herbs, but most shrubs and herbs promptly re-sprout from surviving below-ground organs. Wildfires invoke the following potential transitions (Figure-model):

      1.   HS – high mortality wildfire recycles the stand through the HS condition while a low mortality wildfire maintains this condition.

      2.   ESD – high mortality wildfire returns the stand to the HS condition while a low mortality wildfire maintains the stand in this condition.

      3.   LSD – high mortality wildfire returns the stand back to the HD condition while a low mortality wildfire maintains the stand in this condition.

*Note, see Disturbance Rules for the formal implementation of these rules in RMLANDS.