Vegetation Transition Model

Semi-Desert Savannah

      Description.-Semi-desert savannah consists of scattered trees within a grassland matrix. This cover type is found on very dry sites that lack sufficient moisture to support a true woodland or shrubland vegetation type. Stands are composed of scattered small pinyon pine (Pinus edulis) and Utah juniper (Juniperus osteosperma), and a moderately dense cover of shrubs, including shadscale saltbush (Atriplex confertifolia), Bigelow sagebrush (Seriphidium bigelovii or Artemisia bigelovii), Eurotia lanata, Chrysothamnus viscidiflorus and grasses, including galleta (Hilaria jamesii), Bouteloua gracilis, and Stipa comata.

      Distribution.–Semi-desert savannah is found on very dry sites at the lowest elevations, commonly on gently sloping ridges of Dakota sandstone lying between major canyons (Figure-distribution map; Figure-elevation chart; Table-areal coverage). Similar communities are found throughout the Colorado Plateau region (Loope 1977, West 1983, 1988; Heil et al. 1993, Romme et al. 1993).

      Stand conditions.–We recognized three separate stand conditions:

      1.   Herb - Annual (HA) – herbaceous annuals dominant.

      2.   Herb - Perennial (HP) – herbaceous perennials dominant.

      3.   Herb - Shrub - Trees (HST) – herbaceous perennials dominant; sparse density of older Pinus edulis and Juniperus osteosperma with moderate to dense shrubs, grasses, and forbs.

      Succession Transitions.–Recovery of the tree component after intense disturbance is very slow, because of the dry conditions. The herbaceous component responds to disturbance much like the semi-desert grassland cover type, described elsewhere. Succession transitions occur as follows (Figure-model):

      1.   HA - persists from stand age 0 until age 10, at which time all stands will transition to HP.

      2.   HP - persists until age 20, after which stands begin transitioning to the HST condition. Stands may persist in the HP condition until age 30, after which all stands will have transitioned to HST.

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

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

      Wildfire Disturbance Transitions.-Wildfire is rare because of low fuel mass and continuity. When fire does occur, it usually kills the trees and 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.   HA – high mortality wildfire recycles the stand through the HA condition while a low mortality wildfire maintains this condition.

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

      3.   HST – high mortality wildfire returns the stand to the HA 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.