Ecological dynamics

Range in Characteristics:

Variability in plant composition and yield is dependent on aspect and soil depth rather than on precipitation and elevation ranges that occur within the site. Total annual production tends to increase on north slopes. There tends to be an increase of bluebunch wheatgrass and lower total production on south and southwesterly slopes, particularly at shallower depths.

Response to disturbance:

Following disturbance, Idaho fescue and other deep-rooted perennial bunchgrasses decrease, favoring shallow rooted species like Sandberg bluegrass and Kentucky bluegrass (Poa pratensis). With further deterioration, increaser forbs and non-native annual grasses invade. Under deteriorated conditions annuals and unpalatable forbs dominate the site.

Ecological Dynamics:

Ecological dynamics of this site are primarily driven by interactions between climatic patterns and disturbance regimes. Frequent, low intensity fires were the historical disturbance that maintained the reference state and drove plant community shifts within the state. Intensity and frequency of these fires is strongly influence by drought cycles. Introduction of exotic annual grasses and other non-native species compromises the resistance and resiliency of the site, putting it at higher risk of crossing a threshold into another state.

Periodic drought regularly influences these ecosystems and drought duration and severity has increased throughout the 20th century in much of the Intermountain West. Major shifts away from historical precipitation patterns have the greatest potential to alter ecosystem function and productivity. Species composition and productivity can be altered by the timing of precipitation and water availability with the soil profile (Bates et al. 2006).

The invasibility of plant communities is often linked to resource availability. Disturbance can decrease resource uptake due to damage or mortality of the native species and depressed competition or can increase resource pools by the decomposition of dead plant material following disturbance (Chambers et al. 2007). With disturbance outside the normal range of variability, deep rooted perennial grasses are often outcompeted by native and non-native shallow rooted perennial grasses or non-native invasive annual grasses.

With repeated severe disturbance deep rooted perennial grasses are commonly replaced by shallow rooted, sod forming grasses. Reduced bunchgrass vigor or density provides an opportunity for Sandberg bluegrass and/or Kentucky bluegrass to expand into the interspaces.

The annual grass species most likely to invade these sites are cheatgrass (Bromus tectorum), medusahead (Taeniatherum caput-medusae) and North Africa grass (Ventenata dubia); cool-season annual grasses that germinate in the fall, overwinter as seedlings, and initiate growth in the spring (Miller et al. 1999a). Expansion of these grasses creates seed reserves that can infest adjoining areas and cause changes to the fire regime. These grasses create fuel continuity, often resulting in increased fire frequency and more uniform burn patterns (less mosaic).

Fire Ecology:

Idaho fescue response to fire varies with condition and size of the plant, season and severity of fire, and ecological conditions. Mature Idaho fescue plants are commonly reported to be severely damaged by fire in all seasons (Wright et al. 1979). Initial mortality may be high (in excess of 75%) on severe burns, but usually varies from 20 to 50% (Barrington et al. 1989). Rapid burns have been found to leave little damage to root crowns, and new tillers are produced with onset of fall moisture (Johnson et al. 1994).

Fire will remove aboveground biomass from bluebunch wheatgrass but plant mortality is generally low (Robberecht and Defossé 1995) because the buds are underground (Conrad and Poulton 1966) or protected by foliage. Uresk et al. (1976) reported burning increased vegetative and reproductive vigor of bluebunch wheatgrass. Thus, bluebunch wheatgrass is considered to experience slight damage to fire but is more susceptible in drought years (Young 1983). Plant response will vary depending on season, fire severity, fire intensity and post-fire soil moisture availability.

Sandberg bluegrass (Poa secunda), has been found to increase following fire likely due to its low stature and productivity (Daubenmire 1975). Sandberg bluegrass may retard reestablishment of deeper-rooted bunchgrass.

(Adapted from: Stringham, T.K. et al., 2016)