No. 24. Montane Coniferous Wetlands

Christopher B. Chappell

Geographic Distribution. This habitat occurs in mountains throughout much of Washington and Oregon, except the Basin and Range of southeastern Oregon, the Klamath Mountains of southwestern Oregon, and the Coast Range of Oregon. This includes the Cascade Range, Olympic Mountains, Okanogan Highlands, Blue and Wallowa mountains.

Physical Setting. This habitat is typified as forested wetlands or floodplains with a persistent winter snow pack, ranging from moderately to very deep. The climate varies from moderately cool and wet to moderately dry and very cold. Mean annual precipitation ranges from about 35 to >200 inches (89 to >508 cm). Elevation is mid- to upper montane, as low as 2,000 ft (610 m) in northern Washington, to as high as 9,500 ft (2,896 m) in eastern Oregon. Topography is generally mountainous and includes everything from steep mountain slopes to nearly flat valley bottoms. Gleyed or mottled mineral soils, organic soils, or alluvial soils are typical. Subsurface water flow within the rooting zone is common on slopes with impermeable soil layers. Flooding regimes include saturated, seasonally flooded, and temporarily flooded.  Seeps and springs are common in this habitat.

Landscape Setting. This habitat occurs along stream courses or as patches, typically small, within a matrix of Montane Mixed Conifer Forest, or less commonly, Eastside Mixed Conifer Forest or Lodgepole Pine Forest and Woodlands. It also can occur adjacent to other wetland habitats: Eastside Riparian-Wetlands, Westside Riparian-Wetlands, or Herbaceous Wetlands. The primary land uses are forestry and watershed protection.

Structure. This is a forest or woodland (>30% tree canopy cover) dominated by evergreen conifer trees. Deciduous broadleaf trees are occasionally co-dominant. The understory is dominated by shrubs (most often deciduous and relatively tall), forbs, or graminoids. The forb layer is usually well developed even where a shrub layer is dominant. Canopy structure includes single-storied canopies and complex multi-layered ones. Typical tree sizes range from small to very large. Large woody debris is often a prominent feature, although it can be lacking on less productive sites.

Composition. Indicator tree species for this habitat, any of which can be dominant or co-dominant, are Pacific silver fir (Abies amabilis), mountain hemlock (Tsuga mertensiana), and Alaska yellow-cedar (Chamaecyparis nootkatensis) on the westside, and Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), lodgepole pine (Pinus contorta), western hemlock (T. heterophylla), or western redcedar (Thuja plicata) on the eastside. Lodgepole pine is prevalent only in wetlands of eastern Oregon. Western hemlock and redcedar are common associates with silver fir on the westside. They are diagnostic of this habitat on the east slope of the central Washington Cascades, and in the Okanogan Highlands, but are not diagnostic there. Douglas-fir (Pseudotsuga menziesii) and grand fir (Abies grandis) are sometimes prominent on the eastside. Quaking aspen (Populus tremuloides) and black cottonwood (P. balsamifera ssp. trichocarpa) are in certain instances important to co-dominant, mainly on the eastside.

Dominant or co-dominant shrubs include devil’s-club (Oplopanax horridus), stink currant (Ribes bracteosum), black currant (R. hudsonianum), swamp gooseberry (R. lacustre), salmonberry (Rubus spectabilis), red-osier dogwood (Cornus sericea), Douglas’ spirea (Spirea douglasii), common snowberry (Symphoricarpos albus), mountain alder (Alnus incana), Sitka alder (Alnus viridis ssp. sinuata), Cascade azalea (Rhododendron albiflorum), and glandular Labrador-tea (Ledum glandulosum). The dwarf shrub bog blueberry (Vaccinium uliginosum) is an occasional understory dominant. Shrubs more typical of adjacent uplands are sometimes co-dominant, especially big huckleberry (V. membranaceum), oval-leaf huckleberry (V. ovalifolium), grouseberry (V. scoparium), and fools huckleberry (Menziesia ferruginea).

Graminoids that may dominate the understory include bluejoint reedgrass (Calamagrostis canadensis), Holm’s Rocky Mountain sedge (Carex scopulorum), widefruit sedge (C. angustata), and fewflower spikerush (Eleocharis quinquiflora). Some of the most abundant forbs and ferns are ladyfern (Athyrium filix-femina), western oakfern (Gymnocarpium dryopteris), field horsetail (Equisetum arvense), arrowleaf groundsel (Senecio triangularis), two-flowered marshmarigold (Caltha leptosepala ssp. howellii), false bugbane (Trautvetteria carolinensis), skunk-cabbage (Lysichiton americanus), twinflower (Linnaea borealis), western bunchberry (Cornus unalaschkensis), clasping-leaved twisted-stalk (Streptopus amplexifolius), singleleaf foamflower (Tiarella trifoliata var. unifoliata), and five-leaved bramble (Rubus pedatus).

Other Classifications and Key References. This habitat includes nearly all of the wettest forests within the Abies amabilis and Tsuga mertensiana zones of western Washington and northwestern Oregon and most of the wet forests in the Tsuga heterophylla and Abies lasiocarpa zones of eastern Oregon and Washington ⁸⁸. On the eastside, they may extend down into the Abies grandis zone also. This habitat is not well represented by the Gap projects because of its relatively limited acreage and the difficulty of identification from satellite images. But in the Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Types ¹²⁷ the vegetation types that include this type would be higher elevation palustrine forest, palustrine shrubland, and NWI palustrine emergent. These are primarily palustrine forested wetlands with a seasonally flooded, temporarily flooded, or saturated flooding regime ⁵⁴. They occur in both lotic and lentic systems. Other references describe this habitat ^{36, 57, 90, 101, 108, 111, 114, 115, 118, 123, 132, 221}.

Natural Disturbance Regime. Flooding, debris flow, fire, and wind are the major natural disturbances. Many of these sites are seasonally or temporarily flooded. Floods vary greatly in frequency depending on fluvial position. Floods can deposit new sediments or create new surfaces for primary succession. Debris flows/torrents are major scouring events that reshape stream channels and riparian surfaces, and create opportunities for primary succession and redistribution of woody debris. Fire is more prevalent east of the Cascade Crest. Fires are typically high in severity and can replace entire stands, as these tree species have low fire resistance. Although fires have not been studied specifically in these wetlands, fire frequency is probably low. These wetland areas are less likely to burn than surrounding uplands, and so may sometimes escape extensive burns as old forest refugia ¹. Shallow rooting and wet soils are conducive to windthrow, which is a common small-scale disturbance that influences forest patterns. Snow avalanches probably disturb portions of this habitat in the northwestern Cascades and Olympic Mountains. Fungal pathogens and insects also act as important small-scale natural disturbances.

Succession and Stand Dynamics. Succession has not been well studied in this habitat. Following disturbance, tall shrubs may dominate for some time, especially mountain alder, stink currant, salmonberry, willows (Salix spp.), or Sitka alder. Quaking aspen and black cottonwood in these habitats probably regenerate primarily after floods or fires, and decrease in importance as succession progresses. Lodgepole pine is often associated with post-fire conditions in eastern Oregon ¹³¹, although in some wetlands it can be an edaphic climax species. Pacific silver fir, subalpine fir, or Engelmann spruce would be expected to increase in importance with time since the last major disturbance. Western hemlock, western redcedar, and Alaska yellow-cedar typically maintain co-dominance as stand development progresses because of the frequency of small-scale disturbances and the longevity of these species. Tree size, large woody debris, and canopy layer complexity all increase for at least a few hundred years after fire or other major disturbance.

Effects of Management and Anthropogenic Impacts. Roads and clearcut logging practices can increase the frequency of landslides and resultant debris flows/torrents, as well as sediment loads in streams ^{198, 199, 229}. This in turn alters hydrologic patterns and the composition and structure of montane riparian habitats. Logging typically reduces large woody debris and canopy structural complexity. Timber harvest on some sites can cause the water table to rise and subsequently prevent trees from establishing ²²¹. Wind disturbance can be greatly increased by timber harvest in or adjacent to this habitat.

Status and Trends. This habitat is naturally limited in its extent and has probably declined little in area over time. Portions of this habitat have been degraded by the effects of logging, either directly on site or through geohydrologic modifications. This type is probably relatively stable in extent and condition, although it may be locally declining in condition because of logging and road building. Five of 32 plant associations representing this habitat listed in the National Vegetation Classification are considered imperiled or critically imperiled ¹⁰.