Ecological dynamics

Reference Plant Community:

The reference plant community of this site is characterized by an open woodland of Oregon white oak and ponderosa pine, maintained by relatively frequent, low-intensity fires. The herbaceous component of the understory is primarily composed of Idaho fescue and bluebunch wheatgrass. Bitterbrush and deerbrush along with many, minor forbs and shrubs are common in the stand. Vegetative composition of the community is approximately 75 percent grasses, 5 percent forbs and 20 percent shrubs/trees. Variability in plant composition and yield is dependent on soil depth and coarse fragments, rather than precipitation and elevation ranges within the site. Idaho fescue is the predominant grass as a result of available moisture. Bluebunch wheatgrass will be in higher proportion on more coarse surface textures.

Disturbance:

As one of the driest woodland types in Oregon, white oak - ponderosa pine communities on the eastside of the Cascades were historically subject to frequent, low severity firefires that maintained the open, savannah-like appearance and higher cover of herbs in the understory, both due to natural and cultural ignitions (Landfire 2007). Mixed and stand replacement fires also occurred occasionally in these stands but were rare due to limited fuels and fire tolerant canopies (Landfire 2007). Oregon white oak is adapted to these conditions by resprouting from bases following topkill as well as developing thick, fire-resistant bark with maturity (Devine et al 2013). Ponderosa pine is also well adapted to these conditions, developing increasing fire resistance with age by growing thick bark and self-thinning lower limbs (Fryer 2008). Both tree species are susceptible to mortality by low intensity fires when young, yet gain resistance with age.

If fires are frequent enough, they can remove many of the pines and infill seedlings, leaving only those trees that managed to survive and continue growing (Landfire 2007). With a disruption of this fire regime, the canopy becomes more closed and the understory declines in cover and production (Devine et al. 2013). While bitterbrush may resprout following fire, repeated fire may reduce its cover over time (Busse and Riegel 2009). As a shade tolerant species when young, white oak can be a pioneer species following fire, and will attain dominance on sites not suitable for ponderosa pine or Douglas-fir. These sites may include those with rocky, shallow soils and droughty aspects, where white oak may be particularly competitive (Devine et al 2013). With age, white oak loses much of its shade tolerance and will often be outcompeted on favorable sites by conifers, especially if fire is suppressed (Gucker 2007).

This site represents conditions where ponderosa and white oak may attain codominance, yet information about long term stand dynamics is lacking. As warm, dry, open, woodland communities, these white oak sites may be highly susceptible to invasive plant species introductions (Lillybridge et al 1995). Common invasive plant species include cheatgrass (Bromus tectorum), diffuse knapweed (Centaurea diffusa), and bulbous bluegrass (Poa bulbosa). This site may be used for livestock grazing, with Idaho fescue as the preferred species during spring and early summer. If the condition of the site deteriorates as a result of overgrazing, Idaho fescue and bluebunch wheatgrass decrease while Canby bluegrass (Poa secunda formerly P. canbyi) and deerbrush increase. Idaho fescue is the preferred species during spring and summer. With further deterioration, bluebunch wheatgrass decreases and minor forbs increase and annual grasses invade. Under deteriorated conditions cheatgrass and a variety of forbs will nearly replace the stand of perennial bunchgrasses, possibly altering the fire regime (Gucker 2007). Restoration activities for East Cascades White Oak sites should be informed by the document “Oregon White Oak Restoration Strategy for National Forest System Lands East of the Cascade Range” by Devine et al 2013.

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 710600: East Cascades Oak-Ponderosa Pine Forest and Woodland (Landfire 2007).