Ecological dynamics

This site is largely composed of the Douglas-fir (Pseudotsuga menziesii)/ninebark (Physocarpus malvaceus) plant association (PSME/PHMA) which is the largest representative plant association in this group. The less abundant Douglas-fir/Common Snowberry (Symphoricarpos albus) (PSME/SYAL) and the “Douglas-fir series” (PSME/unknown) are included in this provisional ESD.

This warm dry Douglas-fir forest type occurs above Ponderosa pine (Pinus ponderosa) forest types on a moisture gradient. The Douglas-fir Warm Dry Shrub ecological site has more favorable effective moisture utilization when compared to the Douglas-fir/pinegrass (Carex rubescens) plant association (PSME/CARU). Douglas-fir/pinegrass is within the same plant association series as the modal plant association for this ecological site.

The ecological site was characterized by a ponderosa pine dominated overstory in the historic, naturally occurring ecological setting. Mature stands typically contained large, widely spaced ponderosa pine along with similar age and size class Douglas-fir. Adapted shrubs and grasses were found in the understory.

Other conifers found in this ecologic site included very limited expressions of grand fir (most likely found on favorable micro-sites), and early seral western larch (Larix occidentalis) which typically persist in the mature stand as scattered remnants. Larch begins to occur in greater proportions compared to the dryer and warmer grass dominated Douglas-fir plant associations (the pinegrass and elk sedge associations) but is still a relatively minor component.

Pre-European frequent, low intensity surface fires (also called “under-burns”) maintained long-lived stands of fire-resistant late seral conifers in the older, more open stands. Less frequent mixed-severity and stand replacement fires resulted in mosaics of older and larger trees, intermingled with younger patches of regenerating forests. The early development (regeneration) plant community phase of this ecological site was dominated by Ponderosa pine seedling development with lesser amounts of larch regeneration, along with fire adapted understory brush species including snowbrush ceanothus (Ceanothus velutinus), Scouler’s willow (Salix scouleriana), and ninebark (found only in the PHMA phase). Understory grass species included pinegrass, elk sedge, western fescue (Festuca occidentalis) and various bromes (Bromus spp.).

Young, newly regenerated stands would normally progress to mid-development phases in time, given an absence of major disturbance impacts which could halt successional progression. The developing stands were dominated by large pole to small and intermediate sawtimber sized individual conifers. The stands expressed varying degrees of canopy closure depending on the initial seedling establishment success and succeeding stocking levels, coupled with the impacts of minor, small scale disturbance events that occurred over the course of development. Mid-development stands could in turn progress to mature stands with the passing of additional time and under favorable conditions. In the absence of significant surface fire, a disproportionate establishment of mid-seral Douglas-fir seedlings and saplings would begin to dominate the understory conifer layer as shade levels increased.

Understory infill of mid and late seral conifers (the occasional grand fir) is now a common and widespread condition across the west, a direct result of a policy of complete fire suppression. This policy of forest fire suppression was implemented in the Post-European era of settlement in the western United States and became progressively effective in extinguishing the majority of wildfires across dry western landscapes. This policy eliminated the beneficial aspects of naturally occurring wildfire (natural thinning, pruning and the reduction of overabundant late seral conifers), and lead to elevated fuel levels and increased present day catastrophic wildfires.

The expressions of the historical stands in this ecological site contained various tree species, ages and sizes. In the context of the large-scale landscape, these unique phases were found in a mosaic pattern across the landscape. Clumps, gaps and individual trees were common at the larger scale.

Although fire was the major disturbance factor in the historic context of this ecological site, insects (such as bark beetles and defoliators) and root diseases impacted and changed the forest over time. Less frequent abiotic disturbances such as high windstorms also occurred, locally altering structural and functional attributes of the stand.

Characteristics of the major tree species:
Ponderosa pine (an early seral species) is resistant to low and moderate severity fires, with resistance developing at a very young growth stage. Thick bark, high and open crowns, and the ability to develop self- and fire-induced pruning of lower branches are among the natural adaptations of this species. Ponderosa pine is a sporadic seed producer, and generally requires mineral soil under open, sunny conditions to germinate and establish. The natural longevity of the species is advantageous. Fire exclusion curtails the necessary recruitment of ponderosa pine seedlings, eliminating much of the younger age and size classes of the species over time.

Ponderosa pine is susceptible to mortality from western and mountain pine beetle at various age and stocking thresholds. Fire impacts that damage the cambial layer of this species will increase the likelihood of bark beetle infestation and mortality as a secondary fire impact.

Western larch (also an early seral species) is the most fire-resistant species in this ecologic site. Younger age and size classes are susceptible to fire kill, developing moderate fire resistance in the late pole size class, but older and larger trees typically survive the various impacts of wildfires of all types.

Western larch shares many of the natural fire adaptations of Ponderosa pine. The species thick bark and self-pruning adaptations are beneficial, and it is also long lived. Larch has an open foliar crown consisting of young, moist deciduous needles, allowing mature larch to survive crown fire events that would be lethal to many other conifer species.

Larch is also well adapted to regenerate under the favorable conditions that typically result from non-lethal fire: Light weight seeds are well dispersed, and establish best under open, sunny conditions on burned mineral soils. After establishment, larch experiences rapid juvenile growth.

Dwarf mistletoe and needlecast are common damaging agents. Insects such as the Western spruce budworm, larch sawfly(s) and occasional bark beetle infestation are biotic sources of disturbance in larch.

Rocky Mountain Douglas-fir (a mid-seral species) becomes resistant to low and moderately severe fires once it passes through the pole size class (which takes longer when compared to Ponderosa pine or Western larch—up to 40 years). Healthy individuals develop thick, corky bark which insulates against cambial scorch under surface fire conditions.

Douglas-fir is susceptible to dwarf mistletoe infestations which develop into dense brooms on the tree branches; these in turn increase the risk of crown and branch torching. Endemic Douglas-fir beetle and root disease impacts can decrease the health and vigor of individual trees and whole stands, increasing the likelihood of direct mortality, or of “secondary fire order” mortality in the period of time closely following the fire (example: where fire weakened trees fall prey to bark beetle attacks, leading to localized epidemic outbreaks).

All three of these conifer species are long-lived and have the potential to persist into the mature and over-mature plant community phases under favorable conditions.