Ecological dynamics

In its historical reference condition, a mature forest with an open stand structure was the most common expression of this ecological site. In this representative stage, widely spaced, older mature Douglas-fir (Pseutotsuga menziesii) and ponderosa pine (Pinus ponderosa) dominate the upper canopy layer, with mature grand fir (Abies grandis) and limited western larch (Larix occidentalis) in the mid-and upper layer(s) of the forest. Lodgepole pine (Pinus contorta) may also be found on this site, especially during early development phases following fire. Pinegrass (Calamagrosis rubescens) and elk sedge (Carex geyeri) dominate the herbaceous vegetation layer along lesser amounts of forbs that commonly include strawberry (Fragaria spp.), heartleaf arnica (Arnica cordifolia), common yarrow (Achillea millefolium), white hawkweed (Hieracium albiflorum) and tailcup lupine (Lupinus caudatus). Shrub species found on this site may include common snowberry (Symphoricarpos albus), creeping Oregon grape (Berberis repens), baldhip rose (Rosa gymnocarpa), birchleaf spirea (Spirea betulifolia), service berry (Amalanchier alnifolia) Scouler’s willow (Salix scouleriana) and mallow ninebark (Physocarpus malvaceus).

The modal plant associations that define this ecological site group concept are the widespread Douglas fir/Common snowberry and Grand fir/Pinegrass associations (as described in Johnson and Clausnitzer 1992, and Johnson and Simon 1987). Additional closely related plant associations include Douglas-fir/Pinegrass and Douglas-fir/Ninebark. Within MLRA 43C in Oregon, these communities exist in areas with greater moisture than pure ponderosa pine and ponderosa pine Douglas-fir forests and the warm and dry end of the climate range supporting grand fir forests. This site group corresponds to the USFS potential vegetation group “Dry Upland Forest” and plant association group “Warm Dry Upland Forest”.

Dry mixed conifer 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). Landfire estimated average fire return intervals for this forest type are 135 years, 110 years and 30 years for replacement, mixed and surface severity fires respectively. Frequent, low intensity fires would have decreased the density of young regenerating understory conifers, such as Douglas-fir and grand fir which are less tolerant of fire when young. Overtime, these fires, would have favored the development of mature, uneven-aged stands of Douglas-fir and ponderosa pine with open canopies. Less frequent mixed and surface severity fires would have increased the spatial and structural heterogeneity of these forests by creating patches of regeneration and removing portions of overstory components.

Fire-resistant ponderosa is well-adapted to fire, often surviving low severity fire when very young and developing increasing resistance with age by growing thick bark and self-thinning lower limbs. Douglas-fir is intolerant of fire until later developmental stages (~40 years) at which time it’s resistance is bolstered by very thick bark and deepening root habit. Western larch exhibits similar protective bark attributes, along with an open growing crown (i.e. low foliage volumes) that makes this species less vulnerable to consumption or scorch damage in the event of a crown fire. Grand fir is very intolerant of fire when young yet with advanced maturity gains some moderate resistance due to thickened bark and deep roots. Lodgepole pine is susceptible to mortality from fire with thin bark even with maturity, yet is well adapted to colonize following fire with prolific seed production and cones sometimes exhibiting serotiny. With longer time between fire, increased development of understory fuels such as young conifers, shrubs and down wood, along with the development of a closed canopy, can promote an increased frequency of stand replacing fires and insect outbreaks. Much of this site currently exists within this condition, as long-term fire exclusion has allowed for understory infill of shade tolerant Douglas-fir and grand fir and increases in fuel loads.

Understory species in these forests were adapted to respond to this fire driven disturbance regime. The dominant graminoids, such as elk sedge and pinegrass, will resprout from rhizomes and increase in cover following fire. Many shrubs found on site, including spiraea and snowberry, would also resprout from crowns or rhizomes following fire. These and other shrubs including serviceberry, may respond to high severity fire by forming dense shrubfields on sites where they were present prior to disturbance. Understory forbs, such as common yarrow and heartleaf arnica, 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, and use by native ungulates such as elk and mule deer. Often adjacent to climax grand fir sites with more dense understories and higher shrub cover, these sites may attract increased use. Prolonged historical use by these ungulates may have altered the composition of understory herbaceous and shrub communities. A decrease in preferred perennial grasses such as pinegrass as well as trampling impacts, may parallel increases in forbs such as heartleaf arnica, strawberry, vetch (Vicia spp.) and lupine, elk sedge 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, yet this site may be less prone to widespread invasion than drier forest types.

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 biotic damage agents. Ponderosa pine is prone to injury or mortality from bark beetles, pine engraver, mistletoe, and other adapted insects and diseases. Douglas-fir is often killed or weakened by Douglas-fir beetle and other wood borers, tussock moth and western spruce budworm. These agents often cause “secondary effect” post-fire mortality when fire stresses, but does not outright kill, individual mature Douglas-fir. Endemic bark beetle results in patch mortality, whereas epidemic outbreaks typically cause larger scale mortality. Root disease, age, overstocking, and other biotic and abiotic stressors will also impact the health and well-being of individual Douglas-fir trees, often leading to mortality. Western larch is susceptible to damage from dwarf mistletoe, needlecast and various fungi. Larch casebearer, sawfly, spruce budworm and tussock moth are common defoliators of larch.

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.