Vegetation Transition Model

Mesic Sagebrush

      Description.–Mesic sagebrush consists of sparse to dense shrublands of big sagebrush (Seriphidium tridentatum ssp or Artemisia tridentata ssp) and silver sagebrush (Seriphidium canum spp viscidulum or Artemisia cana ssp viscidula), with bitterbrush (Purshia tridentata) and other shrubs present in places, plus sparse to dense grasses, e.g., Kentucky bluegrass (Poa pratensis) and forbs, e.g., American vetch (Vicia americana). Species composition probably was altered by heavy grazing in the late 1800s and early 1900s.

      Distribution.–Mesic sagebrush is found at a broad range of elevations, although its bimodal distribution may be reflective of two distinctive sagebrush communities that differ somewhat in species composition (Figure-distribution map; Figure-elevation chart; Table-areal coverage). However, because sagebrush is the dominant species across all these sites, we chose to model it as a single cover type. Similar communities are elsewhere in the Colorado Plateau and southern Rocky Mountain regions (Romme et al. 1993, Johnston and Huckaby 2001, Romme et al. 2003).

      Stand conditions.–We recognized two separate stand conditions based on the density and size of the shrubs (principally sagebrush).

      1.   Herbs - shrubs (HS) – Low density of young, small shrubs and herbs, with moderate cover of grasses & forbs.

      2.   Shrubs - herbs (SH) – high density of older, large shrubs and herbs, with some grasses & forbs.

      Succession Transitions.–In the absence of another disturbance, succession transitions occur as follows (Figure-model):

      1.   HS - persists from age 0 until age 40, after which stands begin transitioning to the SH condition. Stands may persist in the HS condition until age 100, after which all stands will have transitioned to the SH.

      2.   SH - persists until the next stand-replacing disturbance.

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

      Disturbance Transitions.–We chose to model wildfire disturbances only in this cover type. Wildfires tend to be high-mortality, stand-replacing fires that initiate a process of post-fire forest succession. High-mortality fire kills the dominant shrubs, most of which must re-establish from seed. Herbaceous plants lose above-ground biomass to fire, but most 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 the stand in this condition.

      2.   SH - high mortality wildfire returns the stand to the HS condition, while a low mortality wildfire maintains the stand in this condition.

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