Ecological dynamics

The Reference Plant Community is dominated by Idaho fescue (Festuca idahoensis), bluebunch wheatgrass (Pseudoroegneria spicata), and antelope bitterbrush (Purshia tridentata). Sandberg bluegrass (Poa secunda) and Mountain big sagebrush (Artemisia tridentata ssp. vaseyana) are common along with multiple buckwheat species (Eriogonum spp.). Western juniper (Juniperus occidentalis) may be present in trace amounts, but under normal a disturbance regime is controlled by periodic fire.

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 was strongly influence by drought cycles and insect or disease attacks on the plant community. Introduction of exotic annual grasses compromises the resistance and resiliency of the site, putting it at higher risk of crossing a threshold into another state.

Periodic drought regularly influences sagebrush 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). Plant communities within this MLRA have high spatial and temporal variability in precipitation both among years and within growing seasons.

Bluebunch wheatgrass is considered to be a highly fire-adapted grass species with low buds often protected from fire (Zlatnik 1999). Recovery following fire is rapid and it often increases relative to other plants post-fire, especially after spring burning. While burning may improve the nutritional quality of bluebunch, defoliation during the regeneration period can be very detrimental to the stand and should be avoided. Idaho fescue is also adapted to regenerate following light severity fires by resprouting from root crowns and increased tillering (Zouhar 2000). However, high severity fires, especially if they occur during the growing season, may kill the plants. As a highly palatable grass species, Idaho fescue is often preferred by livestock and is highly sensitive to heavy grazing.

Grazing disturbance outside the natural range of variability may cause a decrease in deep-rooted perennial bunchgrass, primarily Idaho fescue and bluebunch wheatgrass. Woody species such as bitterbrush and juniper may increase and the percentage of bottlebrush squirreltail (Elymus elymoides) and Sandberg bluegrass may also increase. Further disturbance will often decrease cover of these species as well. As grass cover declines the potential for weed invasion and expansion of juniper increases.

Western juniper (Juniperus occidentalis) is a native conifer species in western North America, but its density and range have dramatically increased since the late 1800s (Miller et al. 2008). This is likely due to a combination of factors, principally: reductions in fire frequency; heavy livestock grazing; and increased atmospheric carbon dioxide (Fryer and Tirmenstein 2019). Juniper is sensitive to fire and most young trees are killed by even low severity fire. Historically, fire return intervals of approximately 20 years would have minimized encroachment of western juniper into this site (Landfire 2007). As juniper trees mature and bark thickens, however, they become resistant to low severity fire rendering the site more resistant to community change following light severity fire. An increase in juniper crown density causes a decrease in understory perennial vegetation and an increase in bare ground. This allows for the invasion of non-native annual species such as cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae). At high levels of invasion, exotic annual grasses may increase the frequency of fires and extend the season when fires are likely by augmenting early season fine fuel loads and fuel continuity. Sites may be particularly prone to fire following years of above average precipitation during which invasive annual grass production can increase dramatically (Pilliod et al. 2017). With frequent wildfires these plant communities can convert to annual species with a sprouting shrub and juvenile tree overstory. While advanced disturbance may greatly decrease the cover of understory plant species and increase susceptibility to wind and water erosion, the low slope of this site makes a transition to an eroded state unlikely.

An understanding of the site specific ecological dynamics for this site are incomplete. Thresholds between states and phases have yet to be quantified and restoration pathways and outcomes are poorly understood. Current and anticipated effects of climate change are not included in this model, yet this site may experience significant impacts as climate continues to change. The description above and model below draws from the ecological dynamics described in disturbance response group 2B of Stringham et al. 2017, with modifications.