Ecological dynamics

The modal plant associations that define this ecological site group concept are the widespread Grand fir/Twinflower and Grand fir/ Big huckleberry associations (as described in Johnson and Clausnitzer 1992, and Johnson and Simon 1987). Additional closely related plant associations include Grand fir/grouse huckleberry and Grand fir/queencup beadlily. Within MLRA 43c in Oregon, these communities exist in areas receiving greater moisture than ponderosa pine - Douglas-fir forests and at lower elevations and warmer conditions than subalpine fir dominated forests. This site group corresponds to the USFS potential vegetation group “Moist Upland Forest” and plant association group “Cool Moist Upland Forest”.

In its historical reference condition, the most widespread expression of this ecological site was a mature forest with a closed canopy. In this representative stage, mature grand fir (Abies grandis) dominates with Douglas-fir (Pseutotsuga menziesii) very common and Engelmann spruce (Picea engelmannii), western larch (Larix occidentalis) and scattered ponderosa pine (Pinus ponderosa) also commonly found. Limited amounts of western white pine (Pinus monticola) and lodgepole pine (Pinus contorta var. lat.) may also occur. Understory regeneration of grand fir is common. A diverse number of understory plants may be found, yet the presence of thinleaf huckleberry (Vaccinium membranaceum), especially in openings, and twinflower (Linnaea borealis), in shaded understories, characterizes the site. In this mature forest, shade tolerant forbs such as sidebells wintergreen (Orthilia secunda), miterwort (Mitella spp), bride's bonnet (Clintonia uniflora) and western rattlesnake plantain (Goodyera oblongifolia) are commonly found. Other herbaceous and shrub species may include Columbia brome (Bromus vulgaris), northwestern sedge (Carex concinnoides), pipsissewa (Chimaphila umbellata), pinegrass (Calamagrosis rubescens), elk sedge (Carex geyeri), strawberry (Fragaria spp.), heartleaf arnica (Arnica cordifolia), white hawkweed (Hieracium albiflorum), baldhip rose (Rosa gymnocarpa), birchleaf spirea (Spirea betulifolia) and violet (Viola spp.). Within the range of this site, dominance of either bride’s bonnet or grouse huckleberry (Vaccinium scoparium) can represent soil moisture microsites with higher or lower relative moisture respectively.

In comparison to adjacent and associated mixed conifer forests occupying drier aspects, lower elevations or droughty exposures, this forest type historically was subject to less frequent fire. Fire return interval estimates suggest that surface fires were rare, mixed fires occurred at a roughly 150 year cycle and replacement fires on a 200 year cycle approximating Landfire fire regime 3, 35-100+ year frequency, mixed severity (Landfire 2017). These conditions would have allowed the otherwise fire intolerant grand fir to attain canopy dominance overtime and reduced the relative cover of more fire tolerant species such as Douglas-fir and western larch. When fire does occur, lodgepole pine often responds and may dominate early regeneration phases. This may be especially common on gentle topography where cold air pooling favors this cold tolerant conifer. Western larch may also be highly successful during early forest development stages and is often more competitive on steeper slope positions than lodgepole pine. As a shade intolerant species, ponderosa pine will often be more common during early forest development stages, yet may also withstand high severity fire and persist as mature trees in the canopy. Engelman spruce is common in mature stands and increases in areas with higher soil moisture.

Understory species in these forests were adapted to respond to the progression of forest changes from early to late successional phases. Many shrubs found on site including thinleaf huckleberry, spiraea and common snowberry (Symphoricarpus albus), would also resprout from crowns or rhizomes following fire. These and other shrub species may respond to high severity fire by forming dense shrubfields on sites where they were present prior to disturbance. Thinleaf huckleberry will often dominate following fire then decline as canopy closure progresses. Thinleaf huckleberry may provide solar cover for shade tolerant subshrubs and forbs such as twinflower to establish. Grazing by both livestock and native ungulates will decrease the cover of these two common species with subsequent increases in some forbs such as violets and longstalk clover (Trifolium longipes).

Douglas-fir is weakened or killed from Douglas-fir beetle (sometimes as secondary fire mortality), wood borers, tussock moth, and western spruce budworm. Western larch is susceptible to damage from dwarf mistletoe, needlecast and a host of fungi. Larch casebearer, sawfly, spruce budworm and tussock moth are common larch defoliators. Western white pine is greatly reduced from its historic expression by the impacts of white pine blister rust. Lodgepole pine is commonly impacted by western gull rust, mistletoe, and bark beetles, among other agents. Both Douglas-fir and grand-fir are very susceptible to root diseases (i.e. annosus and armillaria) and subsequent insect mortality, especially where stands become overstocked in combination with lower quality soils, and when seasonal climatic or weather conditions are poor. Common causes of insect damage to grand fir is from spruce budworm, tussock moth and the fir engraver beetle. Grand fir is also susceptible to a variety of pathogens and other injurious agents. Engelmann spruce is commonly impacted by wood-rotting fungi and broom rusts. Spruce beetle can cause serious mortality during outbreaks. The impact from diseases or subsequent insect infestation can be small and endemic, or larger scaled over time when these root disease and damage agents actually become epidemic.

Fire exclusion within the past 100 years has drastically changed the structural expression of the present forest in this ecological site, resulting in an increase in the proportion of mid to late seral tree species, and corresponding in increases in crown density and continuity. Fuel loads have increased at all levels on the majority of present day stands.

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.