Ecological dynamics

The modal plant associations that define this site concept are the widespread Ponderosa pine / Idaho Fescue and Ponderosa pine / Bluebunch wheatgrass associations (PIPO/FEID and PIPO/PSSP as described in Johnson and Clausnitzer 1992, and Johnson and Simon 1987). These communities exist toward the warm and dry end of the ponderosa pine forests of MLRA 43c in Oregon. As such, productivity is lower than other ponderosa pine forests in these mountains. In its reference phase, these mature ponderosa forests would have included a perennial grass component of Idaho fescue and bluebunch wheatgrass, with bluebunch wheatgrass and cover of bare ground increasing on soils with greater surface coarse fragments. Other herbaceous species include Sandberg bluegrass (Poa secunda), squirreltail (Elymus elymoides), common yarrow (Achilea millefolium) and tailcup lupine (Lupinus caudatus). This site concept also includes less locally common ponderosa pine communities that include components of shrub and tree species adapted to droughty soils and exposures such as antelope bitterbrush, western juniper and big sagebrush. These species are especially common on the southernmost extent of this site and less common in locations further north where exposure to Columbia river weather patterns instead favors minor components of shrubs such as rose (Rosa sp.) and snowberry (Symphoricarpus sp.). Mountain mahogany (Cercocarpus ledifolius) may also occur in this plant community on upper slope positions and ridgetops with rocky soils.

Ponderosa pine forests were historically subject to frequent surface fires primarily ignited by lightning strikes and Native American cultural burning practices. These fires would have likely occurred at less than 35 year intervals and approximated Landfire fire regime group 1 (0-35 year frequency, surface severity). These low intensity fires would have decreased the density of young regenerating understory conifers which are less tolerant of fire when young and may otherwise act as ladder fuels to ignite crown fires and lead to stand replacing events. Overtime, frequent low intensity fires, as well as occasional mixed severity fires, would have favored the development of mature, uneven-aged ponderosa pine stands with open canopies. Mixed severity fires, where direct overstory mortality was patchy, were less common than surface fires, and true stand replacement fires were rare. Fire-resistant ponderosa is well-adapted to these conditions, developing increasing fire resistance with age by growing thick bark and self-thinning lower limbs. With longer time between fire, increased development of understory fuels such as young pine, juniper and down wood, along with the development of a closed canopy, can promote an increased frequency of stand replacing fires and insect outbreaks. In addition to direct impacts of wildfire, disturbance factors that resulted in the death of mature pine included injury from lightning strikes, wind events, weather extremes, and the collective influence of various damage agents such as bark beetles, pine engraver, mistletoe, and other adapted insects and diseases associated with injury or competitive stressors.

Understory species in these forests were adapted to respond to this fire driven disturbance regime. Many cool season perennial grasses, such as bluebunch wheatgrass, would resprout from crowns following fire. Understory forbs, such as common yarrow, may resprout from rhizomes while others, such as lupine, can effectively respond to post fire conditions that may delay the reestablishment of other species, such as low soil nutrient levels, by fixing their own nitrogen.

As a forested site with a grass dominated understory, much of these forests were historically subject to livestock grazing, especially cattle and sheep. Prolonged historical use by these ungulates may have altered the composition of understory herbaceous and shrub communities. A decrease in palatable shrubs such as bitterbrush, and perennial grasses such as Idaho fescue may have paralleled increases in forbs such as lupin and yarrow and exotic annual grasses such as cheatgrass (Bromus tectorum), Medusahead rye (Taeniatherum caput-medusae), and North Africa grass (Ventenata dubia). Increases in exotic grasses have been associated with other impacts as well, including off highway vehicle use and proximity to human development. At high levels, the impacts of these invasions on nutrient cycling, wildlife habitat and fire cycles may be severe.

Current stand conditions in this ecological site do not, in most instances, conform to the historic range of variability (as represented by the reference state below) in terms of the expected frequency and severity of future wildfire events. Since the arrival of Euro-American settlers to the region in the late 1880’s, the character and function of these forests have changed. Logging, grazing, conversion to other uses, and fire exclusion have impacted the natural processes of this fire-dependent ecosystem. Depending on the severity and degree of impact, alternative states (which function outside of the parameters of the reference state), have developed. Within this forest type across the Blue Mountains, fire regimes are considered to have experienced “high” departure from historical conditions (Natureserve 2020).

The state and transition model below represents a generalized and simplified version of forest change in response to major disturbance types in this ecological site. It does not attempt to model the potential effects of climate change on ecosystem function or process. 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, this may include the possibility of regeneration failure following wildfire disturbance (Halofsky et al. 2020). As evidence increases, this model will likely undergo alterations and updates to reflect our emerging understanding.