Ecological dynamics

The modal plant association that defines this ecological site group concept is the widespread Ponderosa pine/Common snowberry association (PIPO/SYAL as described in Johnson and Clausnitzer 1992, and Johnson and Simon 1987). Additional closely related plant associations include Ponderosa pine/Elk sedge and Douglas-fir/Elk sedge. Within MLRA 43c in Oregon, these communities exist toward the moist end of the climate range supporting ponderosa pine forests and the warm and dry end of the climate range supporting Douglas-fir forests. As such, productivity is higher than other ponderosa pine forests in these mountains. This site group corresponds to the USFS potential vegetation group “Dry Upland Forest” and plant association group “Warm Dry Upland Forest”.

In its reference phase, these mature forests would been dominated by an overstory of large ponderosa pine, occasionally codominant with Douglas-fir, and a shrub/grass understory. Common shrubs include common snowberry, rose (Rosa spp.) birchleaf spiraea (Spiraea betulifolia), and creeping Oregon grape. Common members of the herbaceous component include pinegrass, elk sedge, Idaho fescue, heartleaf arnica, strawberry, common yarrow (Achillea millefolium) and lupine (Lupinus sp.). Douglas-fir is often found codominant with ponderosa pine on steeper slopes across varied aspects whereas ponderosa pine dominates across flat to steep slopes on primarily east to south facing aspects. Grand fir (Abies grandis) may be found occasionally in the understory of these forests in protected microsites where cooler temperatures and greater soil moisture are found.

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 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. Douglas-fir is also tolerant of fire at maturity, with very thick bark and a deep rooting habit. With longer time between fire, increased development of understory fuels such as young pine, Douglas-fir, occasional grand fir 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 trees 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 plants, such as elk sedge, pinegrass and snowberry, 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 productive herbaceous component in the 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 perennial grasses such as Idaho fescue and pinegrass may have paralleled increases in forbs such as lupine 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. In many cases, significant infill of young trees (especially those with greater shade tolerance such as Douglas-fir and grand fir) has occurred due to the factors stated above. In this forest type across the western US, fire regimes are considered to have experienced “moderate” to “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. As evidence increases, this model will likely undergo alterations and updates to reflect our emerging understanding.