Vegetation Transition Model

Mountain Shrubland

      Description.–Mountain shrubland consists of moderately dense to dense stand of tall shrubs (up to 3 m), including Quercus gambelii, Amelanchier utahensis, and often Symphoricarpos spp., with an understory of graminoids such as Poa fendleriana and Carex geyeri, plus a variety of forbs, including Wyethia amplexicaulis, Lupinus spp., Lathyrus pauciflorus, Lathyrus leucanthus, and Delphinium nelsoni.

      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). Similar communities are found in many parts of western Colorado (Floyd et al. 2000, Spencer et al. 1996, Johnston and Huckaby 2001). The strong bimodal elevation distribution suggests that this cover type may actually be comprised of two distinct communities. However, for purposes of RMLANDS simulations, we treated all shrublands as a single cover type.

      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.