Vegetation Transition Model
Mountain Grassland
Description.–Mountain grassland (also referred to as meadow) is characterized by an absence or very low cover of trees and shrubs, and dominance by grasses and forbs. Species composition changes substantially with soil conditions, and grazing history (Redders 2003a). Dominant species on relatively dry sites at low to mid elevations (ca. 2286 to 2743 m) may include Arizone fescue (Festuca arizonica), Kentucky bluegrass (Poa pratensis), fringed sage (Artemisia frigida), Wyethia amplexicaulis, strawberry (Fragaria vesca), Fragaria virginiana, pussytoes (Antennaria rosea), geranium (Geranium caespitosum), and Achillea millefolium. Dominant species on relatively dry sites at mid to high elevations ( > ca. 2591 meters) may include Therber fescue (Festuca thurberi), Kentucky bluegrass (Poa pratensis), harebell (Campanula rotundifolia), mountain parsley (Pseudocymopterus montanus), meadowrue (Thalictrum fendleri), and sneezeweed (Dugaldia hoopesii). In many places, mountain grasslands have been seeded with non-native species to increase livestock forage production. In these areas, common species include smooth brome (Bromopsis inermis), intermediate wheatgrass (Thinopyrum intermedium), timothy (Phleum pratense), crested wheatgrass (Agropyron cristatum), and orchardgrass (Dactylis glomrata).
Distribution.–Mountain grassland within the UPL is found at middle to higher elevations, above 2250 m (~7500 feet) (Figure-distribution map; Figure-elevation chart; Table-areal coverage). It commonly exists as openings within forests of ponderosa pine, mixed conifers, aspen, or spruce-fir. The lack of trees may be due to a variety of mechanisms, including competition from herbaceous plants, temperatures, grazing, soil heaving, and fire (Paulsen 1975). Low-elevation mountain grasslands generally are associated with valley bottoms where natural springs or human irrigation provide supplemental moisture. High-elevation mountain grasslands (sometimes called parks) are found in all topographic settings, but are somewhat more common on relatively dry settings such as ridgetops or south-facing slopes. Physiognomy of mountain grasslands is similar to that of semi-desert grasslands, but in our classification, mountain grasslands generally are on more mesic sites (and at higher elevations) than semi-desert grasslands, often in association with forests of various composition. Similar communities are found throughout the Colorado Plateau and southern Rocky Mountain regions (Johnston and Huckaby 2001).
Stand conditions.–Mountain grassland 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.–Although mountain grasslands do undergo succession, the dynamic changes in grassland communities following disturbances were considered to be below the resolution of our modeling work (i.e., too fine-grained for our chosen spatial resolution [25 m cells] and too rapid a temporal dynamic for our chosen temporal resolution [10 year time steps]). Thus, for the purposes of our modeling work we chose to treat mountain grasslands as static.
Disturbance Transitions.–Mountain grasslands are subject to disturbance by fire and animals (e.g., prairie dogs and pocket gophers). We chose not to model animal disturbances. In addition, many of the mountain grasslands in the region have been dramatically altered during the last 100 years by heavy grazing, plowing, and planting of non-native species, but these disturbances were not considered relevant for the HRV scenario. The effects of fire on meadows and grasslands usually are transient; when considered over a 10 year period fire does not alter vegetation structure or composition. This is because the fire may 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, but the prevalence of post-fire sprouting 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, fire in grasslands was allowed but it did not initiate a successional sequence; instead, it simply recycled the stand through the single stand condition.
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.
