Ecological dynamics

Open, park-like stands characterized the late seral development state of this ecological site. In this representative plant community phase, widely spaced, older mature 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 20 to 35 years, which maintained these mature forests in a “park-like” appearance. Fires were started by natural lightning (mainly in summer), and by Native Americans (mainly in the spring and fall). Mixed and stand replacement fires are less frequent than surface fires, but they both occurred under natural, historic conditions.

Pinegrass (Calamagrosis rubescens) and elk sedge (Carex geyeri) dominate the understory vegetation of this ecological site. These grasses were present along with lesser amounts of low and moderate growing shrub species. Shrub growth was kept in check from the impacts of the repetitive surface and mixed severity fires. Oregon grape (Berberis repens), baldhip rose (Rosa gymnocarpa), birchleaf spirea (Spirea betulifolia), and an occasional common snowberry (Symphoricarpos albus) and pinemat manzanita (Arcotostiaphylos nevadensis) were some of the shrubs encountered in pre-European forests operating under natural conditions.

Very limited amounts of grand fir (Abies grandis) could be found reproducing and growing in shaded pockets on more moist micro-sites, especially where ground fire had been absent for a longer period of time, but it was historically restricted in expression over most of this ESD due to climatic restrictions and by the species vulnerability to fire kill. Low levels of early seral lodgepole pine (Pinus contorta) could also occur at times.

The Douglas-fir potential vegetation “series” (a series is named for the dominant fine scale climax tree species) occupy positions which are transitional from the warmer and dryer moisture gradient from the Ponderosa pine (Pinus ponderosa) series, progressing to the moister and cooler grand-fir (Abies grandis) series. 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).

The Douglas-fir/pinegrass (PSME/CARU) plant association is the only potential vegetation type (PVT) in this particular ecological site. This plant association is collectively a part of the extensive and broad “Dry Mixed Conifer Forest” group of the northwestern United States (this group also includes other Douglas-fir plant association(s), and the warmer and drier Grand fir associations).

Ponderosa pine is the dominant early seral forested conifer species of the ecological site under pre-European historic conditions. Ponderosa pine, a long-lived species (averaging 300 years, but capable of up to a 750-year life span), establishes in the early successional stage following favorable disturbance episodes, and typically persists in the overstory into the climax ecological plant community phase. Western larch (also an early seral species) is similar to Ponderosa pine in terms of fire adaptation, longevity and seral status; however, it would occur less frequently in this ecological site due to climatic moisture limitations.
The increase in moisture for this ecological site (compared to the dryer Ponderosa pine series) allows Douglas-fir (a mid-seral species) 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, developing a thick, corky bark that protects individual Douglas-fir trees from low intensity surface fires. 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. The very limited number of grand fir (late seral) trees that were able to germinate and persist into the older age and size classes would gain resistance to surface fire as their root systems grew deeper into the soil, and as their bark thickened, especially near the ground surface.

In addition to the direct impacts of wildfire, other abiotic disturbance factors that can affect all tree species include injury from direct lightning strikes, wind events, and weather extremes (e.g. snow, ice and drought damage).

Ponderosa pine is prone to damage or outright 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 lower severity fire stresses, but does not outright kill, individual mature Douglas-fir. The stress is often in the form of foliar reduction or cambial scorch. This is also referred to as delayed mortality.

Endemic bark beetle often results in patchy mortality, whereas epidemic outbreaks cause larger impact 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.

Human caused 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 and limited late serial tree species, a reduction of Ponderosa pine recruitment in the understory, and corresponding increases in crown density and in fuel loading and continuity.