No. 23. Westside Riparian-Wetlands

Christopher B. Chappell and Jimmy Kagan

Geographic Distribution. This habitat is patchily distributed in the lowlands and low mountains throughout the area west of the Cascade Crest south into northwestern California and north into British Columbia. It also occurs less extensively at mid- to higher elevations in the Cascade and Olympic mountains, where it is limited to more specific environments.

Physical Setting. This habitat is characterized by wetland hydrology or soils, periodic riverine flooding, or perennial flowing freshwater. The climate varies from very wet to moderately dry and from mild to cold. Mean annual precipitation ranges from 20 to >150 inches (51 to >381 cm) per year. This habitat is found at elevations mostly below 3,000 ft (914 m), but it does extend up to 5,500 ft (1,676 m) in Washington and 6,500 ft (1,981 m) in Oregon in the form of Sitka alder communities. Wetlands above these elevations are generally considered part of the Subalpine Parkland habitat and are not included here. Topography is typically flat to gently sloping or undulating, but can include moderate to steep slopes in the mountains. Geology is extremely variable. Gleyed or mottled mineral soils, organic soils, or alluvial soils are typical. Flooding regimes include permanently flooded (aquatic portion of small streams), seasonally flooded, saturated and temporarily flooded. Nutrient-poor bogs are also included here.

Landscape Setting. This habitat typically occupies patches or linear strips within a matrix of forest or regrowing forest. The most frequent matrix habitat is Westside Lowlands Conifer- Hardwood Forest. If not forest, the matrix can be Agriculture, Urban, or Coastal Dunes and Beaches habitats, or rarely Westside Grasslands or Ceanothus-Manzanita Shrublands. This habitat also forms mosaics with or includes small patches of Herbaceous Wetlands. Open Water habitat is often adjacent to Westside Riparian-Wetlands. The major land use of the forested portions of this habitat is timber harvest. Livestock grazing occurs in some areas. Peat mining occurs in some bogs.

Structure. Most often this habitat is either a tall (6-30 ft [2-10m] ) deciduous broadleaf shrubland, woodland or forest, or some mosaic of these. Short to medium-tall evergreen shrubs or graminoids and mosses dominate portions of bogs. Trees are evergreen conifers or deciduous broadleaf or a mixture of both. Conifer-dominated wetlands in the lowlands are included here whereas mid-elevation conifer sites are part of Montane Coniferous Wetland habitat. Height of the dominant vegetation can be >200 ft (62 m). Canopy height and structure varies greatly. Typical understories are composed of shrubs, forbs, and/or graminoids. Water is sometimes present on the surface for a portion of the year. Large woody debris is abundant in late seral forests and adjacent stream channels. Small stream channels and small backwater channels on larger streams are included in this habitat.

Composition. Red alder (Alnus rubra) is the most widespread tree species, but is absent from sphagnum bogs. Other deciduous broadleaf trees that commonly dominate or co-dominate include black cottonwood (Populus balsamifera ssp. trichocarpa), bigleaf maple (Acer macrophyllum), Oregon ash (Fraxinus latifolia), and, locally, white alder (Alnus rhombifolia). Pacific willow (Salix lucida ssp. lasiandra) can form woodlands on major floodplains or co-dominate with other willows in tall shrublands. Oregon white oak (Quercus garryana) and California black oak (Q. kelloggii) can be important in the interior valleys of western Oregon. Conifers that frequently dominate or co-dominate include western redcedar (Thuja plicata), western hemlock (Tsuga heterophylla), and Sitka spruce (Picea sitchensis). Grand fir (Abies grandis) sometimes co-dominates, especially in drier climates and riverine floodplains. Douglas-fir (Pseudotsuga menziesii) is relatively uncommon. Shore pine (Pinus contorta var. contorta) is common in bogs and in deflation plain wetlands along the outer coast. Dominant species in tall shrublands include Sitka willow (Salix sitchensis), Hooker’s willow (S. hookeriana), Douglas’ spirea (Spirea douglasii), red-osier dogwood (Cornus sericea), western crabapple (Malus fusca), salmonberry (Rubus spectabilis), stink currant (Ribes bracteosum), devil’s-club (Oplopanax horridum), and sweet gale (Myrica gale). Labrador-tea (Ledum groenlandicum, L. glandulosum), western swamp-laurel (Kalmia microphylla), sweet gale, and salal (Gaultheria shallon) often dominate sphagnum bogs. Vine maple (Acer circinatum) or Sitka alder (Alnus viridis ssp. sinuata) dominate tall shrublands in the mountains that are located on moist talus or in snow avalanche tracks.

Forests and willow, spirea, and dogwood shrublands within this habitat are limited to the area west of the Cascade Crest. Oregon ash communities occur primarily in the southern Puget Lowland (King County south), Willamette Valley, and Klamath Mountains ecoregions. White alder occurs only in the Willamette Valley and southwestern Oregon. Sitka spruce communities are mainly found in the Coast Range ecoregion in areas of coastal fog influence. Western hemlock and western redcedar riparian and wetland habitats are largely absent from the southern Oregon Cascades and the Klamath Mountains. Sitka alder and vine maple communities are located in the mountains, mainly in western Washington but to a lesser degree on the east slope of the Cascades and in the Oregon Cascades. Sweet gale communities are found primarily at low elevations on the western Olympic Peninsula. Lodgepole pine- dominated communities are found as bogs in western Washington and along the outer coast of Oregon. Most sphagnum bogs are found in low elevation western Washington.

Shrubs that commonly dominate underneath a tree layer include salmonberry, salal, vine maple, red-osier dogwood, stink currant, Labrador-tea, devil’s-club, thimbleberry (Rubus parviflorus), common snowberry (Symphoricarpos albus), beaked hazel (Corylus cornuta), and Pacific ninebark (Physocarpus capitatus). Understory dominant herbs include slough sedge (Carex obnupta), Dewey sedge (C. deweyana), Sitka sedge (C. aquatilis var. dives), skunk-cabbage (Lysichiton americanus), coltsfoot (Petasites frigidus), great hedge-nettle (Stachys ciliata), youth-on-age (Tolmiea menziesii), ladyfern (Athyrium filix-femina), oxalis (Oxalis oregana, O. trillifolia), stinging nettle (Urtica dioica), swordfern (Polystichum munitum), golden-saxifra (Chrysosplenium glechomifolium) great burnet (Sanguisorba officinalis), scouring-rush (Equisetum hyemale), blue wildrye (Elymus glaucus), and field horsetail (Equisetum arvense). Bogs often have areas dominated by ³1 species of sedge (Carex spp.) or beakrush (Rhynchospora alba) and sphagnum moss (Sphagnum spp.) that are included within this habitat, despite their lack of woody vegetation. Sphagnum moss is a major ground cover in most bogs.

Other Classifications and Key References. This habitat includes all palustrine, forested wetlands and scrub-shrub wetlands at lower elevations on the westside as well as a small subset of persistent emergent wetlands, those within sphagnum bogs ⁵³. However, drier portions of this habitat in riparian floodplains may not qualify as wetlands according to Cowardin’s definition⁵³. They are associated with both lentic and lotic systems. Much of this habitat is probably not mapped as distinct types by the Gap projects because of its relatively small scale on the landscape and the difficulty of distinguishing forested wetlands. A portion of this habitat is mapped as the Oregon Gap II Project¹²⁶ and Oregon Vegetation Landscape-Level Cover Types ¹²⁷ westside cottonwood riparian gallery, palustrine forest, palustrine shrubland, NWI palustrine emergent, NWI (National Wetland Inventory) estuarine emergent, and alder/cottonwood riparian gallery. In the Washington Gap project, this habitat occupies portions of open water/wetlands (especially riparian), hardwood forest, and mixed hardwood/conifer forest, and to a minor degree, conifer forest in the following zones: Western hemlock, Sitka spruce, Olympic Douglas-fir, Puget Sound Douglas-fir, Cowlitz River, Willamette Valley, and Woodland/prairie mosaic.³⁷ This habitat also occupies much of hardwood forest in the Silver fir, Mountain hemlock, portions of Subalpine fir, Interior western hemlock/redcedar, and Grand fir zones. Other references describe this habitat ^{41, 71, 85, 88, 90, 91,
104, 113, 114, 115, 138, 210, 220}.

Natural Disturbance Regime. The primary natural disturbance is flooding. Flooding frequency and intensity vary greatly with hydro-geomorphic setting. Floods can create new surfaces for primary succession, erode existing streambank communities, deposit sediment and nutrients on existing communities, and selectively kill species not adapted to a particular duration or intensity of flood. Most plant communities are more or less adapted to a particular flooding regime ¹³⁸, or they occupy a specific time in a successional sequence after a major disturbance ⁸⁵. Debris flows/torrents are also an important, typically infrequent, and severe disturbance where topography is mountainous ²⁰⁰. Fires were probably infrequent or absent because of the combination of landscape position and site moisture, although fires within the watershed would usually have effects on the habitat through impacts on flooding, sedimentation, and large woody debris inputs. Windthrow of trees can also be significant, especially near the outer coast or on saturated soils. Beavers act as important disturbances by changing the hydrology of a stream system through dams. Grazing by native ungulates, e.g. elk, can have a major effect on vegetation.

Succession and Stand Dynamics. Riparian, i.e., streamside, habitats are extremely dynamic ¹⁶². Succession varies greatly depending on the hydro-geomorphic environment. A typical sequence on a riparian terrace on a large stream involves early dominance by Sitka willow, mid-seral dominance by red alder or cottonwood, with a gradual increase in conifers, and eventual late-seral dominance of spruce, redcedar, and/or hemlock. Such a sequence corresponds with increasing terrace height above the bankfull stream stage ⁸⁵. Some communities in bogs or depressional wetlands, as opposed to riverine, seem to be relatively stable given a particular flooding regime and environment. Successional sequences are not completely understood and can be complex. Beaver dams or other alterations of flood regime often result in vegetation changes.

Effects of Management and Anthropogenic Impacts. Intense logging disturbance in conifer or mixed riparian or wetland forests, except bogs, often results in establishment of red alder, and its ensuing long-term dominance. Salmonberry responds similarly to this disturbance and tends to dominate the understory. Logging activities reduce amounts of large woody debris in streams and remove sources of that debris²⁷. Timber harvest can also alter hydrology, most often resulting in post-harvest increases in peak flows¹⁰⁷. Mass wasting and related disturbances (stream sedimentation, debris torrents) in steep topography increase in frequency with road building and timber harvest ¹⁹⁸. Roads and other water diversion/retention structures change watershed hydrology with wide-ranging and diverse effects⁹³, including major vegetation changes. The most significant of these are the major flood controlling dams, which have greatly altered the frequency and intensity of bottomland flooding. Increases in nutrients and pollutants are other common anthropogenic impacts, the former with particularly acute effects in bogs. Reed canarygrass (Phalaris arundinacea) is an abundant non-native species in low-elevation, disturbed settings dominated by shrubs or deciduous trees. Many other exotic species also occur.

Status and Trends. This habitat occupies relatively small areas and has declined greatly in extent with conversion to urban development and agriculture. What remains is mostly in poor condition, having experienced any of various anthropogenic impacts that have degraded the functionality of these ecosystems: channeling, diking, dams, logging, road-building, invasion of exotic species, changes in hydrology and nutrients, and livestock grazing. Current threats include all of the above as well as development. Some protection has been afforded to this habitat through government regulations that vary in their scope and enforcement with jurisdiction. Of the 77 plant associations representing this habitat in the National Vegetation Classification, almost half are considered imperiled or critically imperiled ¹⁰.

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