Ecological dynamics

Occurring in the high elevations of the Blue and Wallowa mountains, the reference plant community of this site at maturity is represented by a subalpine fir dominated forest. Engelman spruce (Picea Englemanii) may be subdominant or codominant at maturity, with other conifers such as lodgepole pine (Pinus contorta), western larch (Larix occidentalis) and Douglas-fir (Pseudotsuga menzisii) more common in younger forests. Douglas-fir and grand fir (Abies grandis) may be more common on warmer microsites within this concept, while whitebark pine (Pinus albicaulis) may be present toward the coldest, upper elevation limits of the site. The modal concept for this site is the USFS plant association, subalpine fir/grouse huckleberry (ABLA/VASC) yet the breadth of the concept also includes plant associations such as subalpine fir/grouse huckleberry/Jacob's ladder, subalpine fir/big huckleberry, subalpine fir/twinflower and subalpine fir/pinegrass. The understory of mid development and mature forests is often characterized by shrubs and herbaceous species such as grouse whortleberry (Vaccinium scoparium), thinleaf huckleberry (Vaccinium membranaceum), prince’s pine (Chimaphila umbellata), twinflower (Linnea borealis), heartleaf arnica (Arnica cordifolia), pinegrass (Calamagrostis rubescens), elk sedge (Carex geyeri), Ross’ sedge (Carex rossii), Columbia brome (Bromus vulgaris) and Oregon boxwood (Pachistima myrsinites).

These forest sites were historically subject to relatively infrequent, stand replacing and mixed severity wildfires approximating Landfire fire regime 3,35-100+ year frequency, mixed severity (Landfire 2007). Both mixed and stand replacement fires occurred at roughly equal frequencies, with an average estimate of 125 for any fire event. Surface fires were largely absent from this forest type. Fire frequency is influenced by the cold, wet climate and intensity is influenced by the low fire resistance of the dominant tree species. The two most common conifers on this site at maturity, subalpine fir and Engelman spruce, share very low resistance to fire with high mortality occurring even with low intensity fires. This is the result of shallow rooting, thin bark and low hanging branches that characterize both species. Following fire, lodgepole pine will often dominate young tree regeneration due to its prolific seed production and establishment on newly burned sites. Engelmann spruce also produces high amounts of viable seed and may be common following disturbance, whereas subalpine fir often regenerates later in the subcanopy. The superior shade tolerance of Engelmann spruce and subalpine fir in comparison to lodgepole pine, will help to allow them to dominate the canopy overtime in the absence of fire. This is common within the reference conditions, yet sites experiencing more frequent fire will tend toward forest compositions with higher proportions of lodgepole pine, sometimes persisting for 100s of years.

Western larch is also common following disturbance and given adequate light will grow quickly. With thick bark at maturity, western larch is highly resistant to fire and large individuals may persist in mature stands. Douglas-fir is common on warmer microsites such as southern aspects or the lower elevations of this site. Like western larch, older individuals are highly fire resistant and may survive in mature stands. Grand fir may be present on lower elevation sites and is especially common during early regeneration and young forest phases. As a shade tolerant conifer, it may also regenerate in mature stands where sites are favorable.

Many of the common understory shrubs and graminoids species of this site are adapted to resprout following fire. Both big huckleberry and grouse huckleberry will resprout, with big huckleberry sometimes following after grouse huckleberry. Pinegrass and elk sedge will resprout from rhizomes in addition to some forbs such as heartleaf arnica which may increase following fire. Understory composition will change overtime with canopy closure and subsequent alteration of surface microclimates. Relative cover of grouse huckleberry will decrease with canopy closure as big huckleberry increases somewhat.

In addition to fire disturbance, this ecologic site is also impacted by periodic windthrow, and by significant insect and disease disturbances that affect major tree species. Of particular interest are spruce beetles which can kill both mature subalpine fir and Engelman spruce trees, and mountain pine beetle which targets lodgepole pine, especially mature trees. The interactions of the disturbances are described in the Landfire BPS model 0710550, Rocky Mountain Subalpine Dry-Mesic Spruce-Fir Forest and Woodland. The state and transition model below is largely based on the dynamics described in this BPS.

More protected sites with less droughty or windswept aspects may include dynamics that include those described in the Landfire BPS model 0710560 Rocky Mountain Subalpine Mesic-Wet Spruce-Fir Forest and Woodland, which has a somewhat longer replacement fire return interval and includes root diseases as a disturbance agent. Note that Balsam wooly adelgid is not present in this historical reference model since it is an introduced insect, despite currently contributing to high mortality of true firs.

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). While these changing conditions are likely to impact lower elevation forests first, catastrophic fire may carry through to higher elevation forests. However, when combined with the interacting impacts of drought and insect outbreaks, it is possible that this ecological system will experience unpredictable ecosystem shifts and additional alternative states. Further, while the effects of climate change may contribute to transitions of this forest type to species adapted to warmer and drier climates, further research is required to confidently model these impacts. For this reason, this scenario has not yet been included as a potential alternative state.