Vegetation Transition Model

Greasewood

      Description.–The shrub greasewood (Sarcobatus vermiculatus) dominates stands, with a sparse to well-developed herbaceous stratum that may include alkali sacaton (Sporobolus airoides) and other salt-tolerant grasses and forbs. Other shrubs, including shadscale saltbush (Atriplex confertifolia), fourwing saltbush (Atriplex canescens), and rubber rabbitbrush (Chrysothamnus nauseous) also may be present. Recently burned stands have a greater herbaceous component; long-unburned or heavily grazed stands may support a nearly continuous shrub layer with little herbaceous vegetation.

      Distribution.–Greasewood is scattered throughout the region at lower elevations (Figure-distribution map; Figure-elevation chart; Table-areal coverage), usually in flat or gently sloping sites where seasonal flooding or inherently saline substrates result in relatively high soil salt content. Soil moisture usually is available within 1 m of the soil surface (West 1988). Similar communities are found throughout the Colorado Plateau region (West 1983, 1988; Heil et al. 1993, Romme et al. 1993).

      Stand conditions.–Greasewood was treated as a static cover type (i.e., no succession) and therefore did not have separate stand conditions.

      No Conditions (NC) – we did not recognize separate stand conditions in this cover type.

      Succession Transitions.–We do not simulate disturbance and succession in this cover type in our modeling work (RMLANDS) because of its relatively small extent and the rapidity with which it usually recovers following disturbance (see below).

      Wildfire Disturbance Transitions.–Wildfires historically burned greasewood communities, but fire intervals during the reference period are poorly known. The effects of fire in this cover type appear to be relatively transient; when modeled over a 10 year time step (RMLANDS) fire does not alter vegetation structure or composition. This is because the fire may kill or consume above-ground plant parts, but below-ground structures survive and re-sprout promptly. Seedling establishment also occurs where the fire removes litter and plant cover. The combination of post-fire re-sprouting and seedling establishment usually leads to rapid (< 10 years) restoration of pre-fire structure with little change in composition. Thus, within the framework of the 10-year time step employed in RMLANDS, wildfire in greasewood communities occurs but does not initiate a successional sequence. Wildfires invoke the following potential transitions:

NC – wildfire (all treated as high mortality) recycles the stand through this condition.

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