Ecological dynamics

In its reference state, this site is dominated by an overstory of Engelmann spruce (Picea englemanni) and a diverse understory of facultative and obligate wetland plant species. Shrub species often include gray alder (Alnus incana), prickly currant (Ribes lacustre), stinking swamp currant (Ribes hudsonianum), and red-osier dogwood (Cornus sericea spp. sericea). Forbs often include arrowleaf groundsel (Senecio triangularis), sweet-scented bedstraw (Galium triflorum), brook saxifrage (Saxifraga odontoloma), american speedwell (Veronica americana), heart-leaved miner’s lettuce (Claytonia cordifolia), sidebells pyrola (Orthilia secunda) and the fern ally common horsetail (Equisetum arvense). Graminoids may include tall mannagrass (Glyceria striata), drooping woodreed (Cinna latifolia), columbia brome (Bromus vulgaris), and soft-leaved sedge (Carex disperma). Understory trees such as grand fir (Abies grandis) and subalpine fir (Abies lasiocarpa) may be present, yet are less tolerant of saturated soils than Engelman spruce so will likely be found on raised hummocks or upper terrace positions. However, at the upper elevation limits of his site, subalpine fir may be a dominant overstory tree, especially on moist bench positions. This site also encompasses forested overstories of lodgepole pine (Pinus contorta) which are likely early seral communities that will be replaced by Engelman spruce overtime. Communities dominated by gray alder with subdominance of conifers may also occur within this site as early seral community types that will eventually succeed to conifer stands. Gray alder is an important species in these ecosystems, helping to enrich soils with nitrogen by hosting nitrogen-fixing actinomycetes in their roots.

Historically, the ecological dynamics of the site would have been influenced largely by climate cycles affecting seasonal snowpack, runoff, droughts, and flood. These processes would have been partly controlled by the type and cover of upslope forest vegetation throughout the watershed which would have modified water capture, storage and sediment supply. These upland dynamics would have been influenced by historical fire regimes and subsequently vegetation succession, erosion and runoff. In favorable habitats with lower channel gradients, beaver populations may have had impacts on water table depth and seasonality, frequency and duration of ponding and flooding, and stream channel structure. Natural transitions from alluvial bars to floodplains will decrease gray alder and favor the development of Engelmann spruce stands overtime. This occurs with the aggradation of fine-textured soil surface layers over coarse-textured materials thereby promoting the establishment of forested floodplains (Crowe et al. 2004).

These forests historically experienced infrequent fire (100+ year fire return interval, Landfire 2007) and when it did occur it was typically high severity and stand replacing. Fire frequency is influenced by the cold, wet climate and intensity is influenced by the low fire resistance of the dominant tree species. Engelman spruce has 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. Following fire, lodgepole pine may dominate young tree regeneration if a seed source is present due to its prolific seed production and establishment on newly burned sites. The superior shade tolerance of Engelmann spruce (as well as subalpine fir) will help to allow this species to outcompete lodgepole overtime in the absence of fire. Large increases in down woody debris would have likely occurred in the years following fire. Shrub species, especially gray alder, will dominate these communities following fire. Water tables may also rise following fire yet site transitions following this process are likely ephemeral (Kovalchik 1987).

In addition to fire disturbance, this ecologic site is also impacted by periodic windthrow, and by 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.

In comparison to lower elevation stream courses more commonly altered for agricultural purposes, the ecological dynamics of stream courses associated with this site have likely been less impacted since European settlement. However, removal of large woody debris sources and road construction may directly alter channel morphology. Additionally, contemporary changes in adjacent forest fire regimes may alter sediment loads and watershed hydrology, thereby impacting this site. Timber harvest is uncommon on these sites due to proximity to stream channels and wet soils. Where timber harvest does occur, removal of large diameter trees will have lasting impacts on stream channel morphology and valley water storage by limiting large woody debris inputs. Livestock grazing is sometimes present on this site. Care should be taken to avoid livestock grazing when soils are wet and subject to compaction. Additionally, deferment during late season periods when shrub species are more favored will help avoid loss of woody riparian vegetation overtime. Improperly managed grazing may promote the invasion of non-native facultative wetland grasses such as Kentucky bluegrass (Poa pratensis) as well as other invasive plant species.

This site may be vulnerable to alterations in surface water hydrology as a result of climate change. Snowpack is expected to decline across the mountains of Oregon with a warming climate (Mote et al. 2005) and shifts from snow to rain is expected to be most pronounced at middle elevations of the Cascade and Blue Mountains. Research suggests that expected shifts in precipitation timing and type will have far reaching effects on blue mountain riparian ecosystems (Dwire et al 2018). The state and transition model below does not take into account the potential impacts of a changing climate and instead represents an approximation of ecological dynamics resulting from a simplified model of this riparian system. Further work is needed to better understand community response to climate shifts as well as the existence of alternative states and plant communities that may exist within these states.