Ecological dynamics

The Reference Plant Community of this site is characterized by an open, savannah-like woodland dominated by white oak and western juniper. Ponderosa pine is common to a lesser extent in the overstory and pine reproduction may be sporadic in the openings. The understory composition is dominated by Idaho fescue and bluebunch wheatgrass. Canby bluegrass (Poa secunda, formerly Poa canbyi) is prominent in the understory. A variety of perennial forbs occur throughout the stand such as lupine (Lupinus spp.), lomatium (Lomatium spp.), balsamroot (Balsamorhiza spp.), common yarrow (Achillea millefolium), groundsel (Senecio spp.) and agoseris (Agoseris spp.). Bitterbrush, serviceberry (Amalanchier alnifolia), snowberry (Symphorocarpus spp.) and buckwheat (Erioganum spp.) are prominent shrubs and subshrubs on the site. Vegetative composition is approximately 60 percent grasses, 10 percent forbs, and 30 percent shrubs and trees. The southern range for this site is largely outside of the influence of the Columbia gorge climate influence and therefore has little oak, leaving juniper and pine as the principal overstory. Southerly and droughty aspects have higher proportions of bluebunch wheatgrass and less Idaho fescue.

Disturbance:

As one of the driest woodland types in Oregon, white oak – ponderosa pine communities on the eastside of the Cascades this site was historically subject to frequent, low severity fires. This 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). Western juniper woodlands, by contrast, often had longer fire return intervals. As a transitional site between these ecological types, mean fire return intervals likely approximate those of very dry ponderosa pine – western juniper forests which range from 10 to 50 years, (Miller et al. 2019, Miller and Heyerdahl 2008, Landfire 2007). Oregon white oak is adapted to fire by resprouting from bases following topkill as well as developing thick, fire-resistant bark with maturity (Devine et al 2013). Ponderosa pine is also fire-adapted, developing increasing fire resistance with age by growing thick bark and self-thinning lower limbs (Fryer 2008). While bitterbrush may re-sprout following fire, repeated fire may reduce its cover over time (Busse and Riegel 2009). Western juniper has thin bark and is readily killed by low to moderate severity fires, yet develops increasing resistance with age (Fryer and Tirmenstein 2019). It is well documented that expansion of western juniper in the Western US is in part associated with the suppression of fire (Miller et al. 2019). All three tree species are susceptible to mortality by low intensity fires when young and if fires are frequent enough, they can remove many of the young trees, 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).

Since this site occurs toward the lower precipitation range suitable for ponderosa pine reproduction and growth, ponderosa may struggle to reestablish post fire except for during climate cycles of increased moisture. In fact, across the Western US, many dry ponderosa 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).

As warm, dry, open, woodland communities, these sites may be highly susceptible to invasive plant species introductions. This site is may be used for livestock grazing yet overgrazing may reduce Idaho fescue, bluebunch wheatgrass and bitterbrush, depending on season of use. Plants likely to increase and invade with overgrazing include cheatgrass (Bromus tectorum), collinsia (Collinsia spp.), cleavers bedstraw (Galium aparine), bulbous bluegrass (Poa bulbosa), Pacific fescue (Vulpia microstachys) and medusahead (Taeniatherum caput-medusae). Clayey soils of this site are also likely to promote invasions of medusahead, a highly detrimental exotic annual grass.

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 the complex interacting effects of grazing, fire and invasive species on ecosystem change and the potential alternative states and pathways emerging from these dynamics. As this site is updated in future iterations, and further research informs our understanding of these woodland types, descriptions will include more thorough treatments of disturbance and ecological change. The reference state is largely based on Landfire biophysical settings models 0710532: Northern Rocky Mountain Ponderosa Pine Woodland and Savanna – Xeric and 0710600: East Cascades Oak-Ponderosa Pine Forest and Woodland (Landfire 2007).