Ecological dynamics

ECOLOGICAL DYNAMICS OF THE SITE
An open stand structure characterized the late seral development state, which was the most extensive expression of this ecological site. In this representative stage, widely spaced, older mature1 Ponderosa pine (Pinus ponderosa) dominated the upper canopy layer, with both mature Douglas-fir (Pseutotsuga menziesii) and limited western larch (Larix occidentalis) in the mid-and upper layer(s) of the forest. Low severity ground fires were frequent, occurring at typical intervals of 2-35 years, which maintained these mature forests in a “park-like” appearance. Pinegrass (Calamagrosis rubescens) dominated the understory vegetation layer along with limited levels of low growing shrub species, kept in check from the impacts of the repetitive surface fires.
Some grand fir (Abies grandis) could be found reproducing and growing in shaded pockets on the moister sites, especially where ground fire had been absent for a longer period of time, but it was historically limited in expression over most of this ESD due to climatic limitations and long- term vulnerability to mortality from fire.
The Douglas-fir potential vegetation “series” (a series is named for the dominant fine scale climax tree species2) occupy positions which are transitional from the warmer, dryer Ponderosa pine (Pinus ponderosa) series, progressing to the moister and cooler grand-fir (Abis grandis) series. At this scale, vegetative transitions are often gradual in the absence of abrupt changes in soil types, aspect, or other features that exert strong influence on the expression of the potential natural plant community (PNC).
This ecological site encompasses two potential vegetation types (PVTs) of the series; the Douglas-fir/pinegrass (PSME/CARU) plant association which is the modal representation of the ESD, and the less extensive Douglas-fir/elk sedge (PSME/CAGE) plant association. These group of plant association(s) are collectively part of the extensive and broad “Dry Mixed Conifer Forest” group of the northwestern United States (this group also includes the warmer and drier Grand fir plant associations(s) with similar understory PVT identifiers).
This ecological site was characteristically maintained in an early to mid-seral condition by the influence of relatively frequent surface fires. Mixed and stand replacement fires are less frequent than surface fires, but they both occurred under natural historic conditions in this ecological site.
Ponderosa pine is the predominant early seral forested conifer species of the ecological site under pre-European historic conditions. Ponderosa pine, a long-lived species (averaging 300 and up to a 750-year life span), occurs in the early and mid-seral successional stage, and typically persists in the overstory into the climax ecological phase. Western larch is similar Ponderosa pine in terms of fire adaptation, longevity and seral status; however, it would occur less often in this ecological site due to moisture limitations. Historically grand fir would have been a minor component as it is susceptible to fire mortality throughout much of its growth stages, and it requires more favorable moisture/temperature conditions to flourish. However, the limited grand fir trees that were able to persist into older age and size would gain resistance to surface fire as their root systems grew deeper into the soil, and as their bark thickened, especially near the surface of the forest floor.
The increase in moisture for this ecological site (compared to the dryer Ponderosa pine series) allows Douglas-fir to establish under partial shade given an adequate seed source, with the natural progression in age and size classes under favorable conditions. In the late pole size class, typically around 40 years of age, Douglas-fir trees begin to acquire fire resistance similar to that of Ponderosa pine, due to the thick, corky bark that protects individual Douglas-fir trees from low intensity surface fires. Western larch has similar bark protection, and with open growing (i.e. low foliage crown volumes) that is less vulnerable to crown fire damage.
In addition to direct impacts of wildfire, abiotic disturbance factors that impacted all trees included injury from lightning strikes, wind events and, weather extremes (e.g. snow, ice and drought damage). Ponderosa pine suffers from 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, 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 produce 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 wellbeing 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.

Depending on the disturbance type and frequency, a landscape level mixture of open stands of fire-resistant trees was intermixed with various stand mosaics across the broader landscape (which would result in “patchy” and “clumpy” features). Progression to the late seral edaphic climax would be rare within the parameters of the pre-European fire regime. (In this ecological site Douglas-fir is considered as the climax species of the site).
Fire exclusion within the past 100 years (the Post-European period) 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 serial tree species, and corresponding in increases in crown density and continuity. Fuel loads have increased at all levels of present day stands.