Vegetation Transition Model

Semi-Desert Grassland

      Description.-Semi-desert grasslands support a sparse cover of low shrubs, grasses, and forbs, with much bare ground. Dominant species include the low shrubs shadscale saltbush (Atriplex confertifolia), Eurotia lanata, and Chrysothamnus viscidiflorus, plus the cool-season grasses Stipa comata and Oryzopsis hymenoides, and the warm-season grasses galleta (Hilaria jamesii) and Bouteloua gracilis. Species composition varies from place to place with local differences in soil conditions and disturbance history. Chronically disturbed areas may be dominated by the native increasers plains prickly pear (Opuntia polyacantha) and snakeweed (Gutierrezia sarothrae), or by the invasive non-native annual grass Bromus tectorum.

      Terrestrial cryptogams composed of cyanobacteria, algae, lichen, moss, fungi, or liverwort are often significant components in semi-desert grasslands forming crusts on the ground surface that contribute to soil stability, nutrient supply (organic matter and nitrogen), and biodiversity.

      Distribution.–Semi-desert grasslands are found at the lowest elevations and most commonly on relatively fine-textured soils derived from Morrison shale and Dakota sandstone on the lower slopes of large 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.   Early grass - forb (EGF) -- herbaceous species dominant, with a mix of annuals and perennials.

      2.   Mid grass - shrub (MGS) -- herbaceous dominant, mostly perennials, with some shrubs becoming established.

      3.   Shrub dominated (SD) – mix of mature shrubs and perennial herbs.

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

      1.   EGF - persists from stand age 0 until age 20, after which stands begin transitioning to the MGS condition. Stands may persist in the EGF condition until age 30, after which all stands will have transitioned to MGS. Following low mortality prairie dog disturbance stands may either transition all the way to the SD condition or be maintained in the EGF condition if they are at least 20 years old.

      2.   MGS - persists until age 50, after which stands begin transitioning to the ST condition. Stands may persist in the MGS condition until age 70, after which all stands will have transitioned to SD. Following low mortality prairie dog disturbance stands may either transition to the SD condition or be maintained in the MGS condition if they are at least 20 years old.

      3.   SD - 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 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):

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

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

      3.   SD – high mortality wildfire returns the stand back to the EGF condition while a low mortality wildfire either returns the stand back to the MGS condition or maintains the stand in this condition.

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

      Prairie Dog Disturbance Transitions.--Prairie dogs inhabit many western grasslands, and may be a key disturbance agent in these ecosystems. These rodents not only feed on grass, but typically remove all vegetation from the vicinity of their extensive burrows or "towns" in order to detect predators. As long as a prairie dog town is active, the animals will intensively remove all vegetation and maintain the area at the beginning of the earliest successional stage (EFG), but when they abandon a site, the successional sequence proceeds eventually to the final shrub-dominated stage. Selective feeding by prairie dogs around the periphery of the town may remove grasses but not shrubs, thus hastening development of the final shrub-dominated successional stage (Figure-model):

      1.   EGF -- high mortality prairie dog activity recycles the stand through the EGF condition while a low mortality prairie dog activity can either accelerate the succession transition all the way up to SD, if grasses and herbs are disturbed enough leaving primarily shrubs as the dominant life form, or maintain the stand in this early-successional condition if the stand is at least 20 years old.

      2.   MGS – high mortality prairie dog activity will return the stand back to EGF while a low mortality prairie dog activity will either accelerate the succession transition to the SD condition or maintain the stand in this mid-successional condition if the stand is at least 20 years old.

      3.   SD – high or low mortality prairie dog activity will maintain the stand within this late-successional condition.

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