Ecological dynamics

The grand-fir (Abies grandis) potential vegetation type (PVT) that typifies this Grand-fir/Moist Shrub (GF-MS) ecological site is the modal grand-fir/twinflower (Linnaea borealis) (ABGR/LIBO) plant association. ABGR/LIBO is the most extensive singular forested plant association in MLRA 43C. Less extensive plant associations that are included in this ecological site are grand-fir/big huckleberry (Vaccinium membranaceum) (ABGR/VAME), and grand-fir /Pacific yew (Taxus brevifolia)/twinflower (ABGR/TABR/LIBO).

The plant association(s) of the GF-MS ecological site are part of a broader “Moist-Mixed Conifer forest” plant association group (PAG) of the western United States.

This Grand fir/Moist Shrub ecological site is diverse and floristically rich. It occurs in the progression from the dry Ponderosa pine (Pinus ponderosa) ecological sites, which in turn give way to the warm/dry representations of Douglas-fir (Pesudotsuga menziesii) and similar grand fir (Abies grandis) ecological sites. Those specific Douglas-fir and grand-fir plant associations, which sit directly below this particular GF-MS site in terms of temperature/moisture climatic influences encompass the dryer and warmer end of the Douglas-fir and grand-fir plant associations (that is, the Douglas-fir and grand-fir series with elk sedge (Carex geyeri), pinegrass (Calamagrostis rubescens), snowberry (Symphoricarpos albus) and ninebark (Physocarpus malvaceus) phases. The broader collective name of this latter group of plant associations is referred to as the “Dry Mixed Conifer forest” group.

This Grand-fir/Moist Shrub ecological site is transitional to cold upland forests, typified by subalpine fir plant associations, that support understory species with higher moisture and cooler temperature thresholds.

Compared to the forest associations of the drier ecological sites, the occurrence and impacts of the natural historic fire disturbance(s) become more variable in the GF-MS ecological site. This site is described as a “mixed” fire regime . In this regime, wildfire disturbances occur at longer fire return interval periods and result in different levels of burn severity. The precise outcome of any given fire episode depends on a myriad of factors at the time of a particular event—fire is a predictable but uncertain dynamic process. The historic expression of these wildfires shaped the basic ecological function and forest structural attributes of the pre-European forest in a relatively constant manner over long periods of time.
It is possible that small representations of the lodgepole pine (Pinus contorta var. lat.)/grand-fir big huckleberry-twinflower (PICO(ABGR)/VAME-LIBO) plant association may be found within this provisional ESD. Single aged, dense lodgepole pine dominated stands typically occupy cold air drainages (i.e. “frost pockets”) on gentle slopes within the landscape, where early seral lodgepole pine establishes following replacement fires. Lesser amounts of grand fir will often be found intermingled with the lodgepole pine. Big huckleberry and twinflower are usually present, with other small shrubs such as grouse huckleberry (Vaccinium scoparium), pachistima (Pachistima myrsinites), and prince’s pine (Chimaphila umbellata), along with elk sedge and pinegrass herbaceous species in the understory.
The most common historic expression of the Grand-fir/Moist Shrub ecological site is represented as a late development stand structure stand largely comprised of very large mature and over-mature early and mid-seral tree species. Conifer species include Douglas-fir, scattered (late seral) overstory grand-fir , western larch (Larix occidentalis) and Ponderosa pine, with limited amounts of lodgepole pine and western white pine (Pinus monticola). Engelmann spruce (Picea Engelmannii) occurs as a late seral conifer. The older long-lived (over mature) and very large early/mid seral relict trees (those that survived the last major stand replacement disturbance) may be present, but at a much lower density within the stand. These relict species were predominately Ponderosa pine, larch and Douglas-fir. Depending on the stand history of various disturbance(s) events, these stands either developed as closed or open as they established and progressed to maturity.
Insect and disease impacts and disturbances become more widespread as stands mature, typically in the mid and late closed structural phases, where stocking stress can predispose stands to increased mortality.
Ponderosa pine is damaged by bark beetles, pine engraver, mistletoe and other agents. 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.
In the early stages of stand development, root diseases cause small scale patches to develop. 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 (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 on the majority of present day stands.