Ecological dynamics

Reference plant community:

The Reference Native Plant Community under the natural disturbance regime, is characterized by an open stand of western juniper and ponderosa pine, with an understory dominated by bitterbrush, big sagebrush and cool season perennial grasses. Wax currant (Ribes cereum) may also be common in the shrub layer. The herbaceous layer includes an abundance of Idaho fescue (Festuca idahoensis) and bluebunch wheatgrass (Pseudoroegneria spicata), as well as lesser amounts of bottlebrush squirreltail (Elymus elymoides), rosy pussytoes (Antennaria rosea), and common yarrow (Achillea millefolium). Vegetative composition in this state is approximately 55 to 60 percent grasses, 3 percent forbs, and 20 percent shrubs and trees.

Potential plant communities will differ across the range of this site due to naturally occurring variability in soil, aspect and climate. An increase in stoniness results in an increase in drought tolerant species such as juniper, sagebrush, Sandberg bluegrass (Poa secunda), and bluebunch wheatgrass. Bluebunch wheatgrass increases on south exposures, Idaho fescue on more northerly aspects and Sandberg bluegrass on shallower soils. Bitterbrush is scarce where soils are saturated during the spring. Western juniper increases on shallow, well drained soils and at the lower end of the precipitation range. Ponderosa pine increases as precipitation approaches 14 inches or more.

Disturbance:

Dry ponderosa pine forests were often maintained by frequent, low severity surface fire, whereas juniper woodlands often had longer fire return intervals. As a transitional site between these ecological types, mean fire return interval estimates range between 10 to 50 years, somewhat longer than adjacent ponderosa pine forests with higher productivity and shorter than that of bordering western juniper forests and sagebrush steppe with lower productivity (Miller et al. 2019, Miller and Heyerdahl 2008, Landfire 2007). Historically, low severity fire would have decreased juniper, sagebrush and mature bitterbrush, while favoring high herbaceous cover and mature ponderosa pine development with open stand conditions. With longer time between fire, increased stocking of juniper and ponderosa pine and the development of a closed canopy, can promote an increased frequency of stand replacing fires and insect outbreaks. This condition characterizes much of the historically open canopy, dry ponderosa forests due to a history of selective logging and fire suppression (Ritchie et al 2005). Prolonged anthropogenic fire suppression may lead to cycles of overstocking and high severity fires, yet evidence is insufficient for the characterization of this pattern as an alternative state. Since this site occurs toward the lower precipitation range suitable for ponderosa pine reproduction and growth, ponderosa pine may struggle to reestablish post fire except for during climate cycles of increased moisture. In fact, across the Western US, many dry ponderosa pine forests existing toward the fringe of their climate suitability may be transitioning into grass shrub communities due to warming and drying growing season conditions (Davis et al. 2018). Ponderosa pine recovery following stand replacing fire will also be dependent on seed sources on site or in nearby forests.

Pine beetle outbreaks are another important disturbance agent in this system helping thin overstocked, closed canopy conditions (Ritchie et al 2005). Livestock grazing is a common land use of this site. Overgrazing may reduce Idaho fescue and Ross’ sedge (Carex rossii), while allowing Thurber’s needlegrass (Achnatherum thurberanium), bottlebrush squirreltail and rabbitbrush (Ericameria and Chrysothamnus sp.) cover to increase. Invasive annual grass may also occur on this site, with coarse textured soils likely favoring cheatgrass (Bromus tectorum) over medusahead (Taeniatherum caput-medusae) or ventenata (Ventenata dubia). Low forest productivity of these sites often makes them impractical for commercial timber harvest, however, given the favorable climate of this site, areas with suitable soils may also be converted to agricultural use.

The state and transition model below represents a generalized and simplified version of plant community change in response to major disturbance types in this ecological site. It does not attempt to model all of the complex interacting effects of grazing, fire and invasive species on ecosystem change and the potential restoration pathways emerging from these dynamics. Emerging evidence is suggesting that climate change is leading to hotter and drier conditions in western forests that will increase fire frequency and extent and lengthen fire seasons (Halofsky et al. 2020). When combined with the interacting impacts of fire suppression, drought, and insect outbreaks, it is possible that this ecological system will experience unpredictable ecosystem shifts and additional alternative states. For warm and dry sites, these impacts may include the possibility of regeneration failure following wildfire disturbance (Halofsky et al. 2020). As this site is updated in future iterations, and further research informs our understanding of East Cascades oak woodlands, descriptions will include more thorough treatments of disturbance and ecological change. The Reference State is largely based on Landfire biophysical settings model 710532 (Landfire 2007).