Pacific Northwest Lowland Streamside Forest

Lowland riparian systems occur throughout the Pacific Northwest. They are the low-elevation, alluvial floodplains that are confined by valleys and inlets and are more abundant in the central and southern portions of the Pacific Northwest Coast. These forests and tall shrublands are linear in character, occurring on floodplains or lower terraces of rivers and streams. Major broadleaf dominant species are Acer macrophyllum, Alnus rubra, Populus balsamifera ssp. trichocarpa, Salix sitchensis, Salix lucida ssp. lasiandra, Cornus sericea, and Fraxinus latifolia. Conifers tend to increase with succession in the absence of major disturbance. Conifer-dominated types are relatively uncommon and not well-described; Abies grandis, Picea sitchensis, and Thuja plicata are important. Riverine flooding and the succession that occurs after major flooding events are the major natural processes that drive this system. Very early-successional stages can be sparsely vegetated or dominated by herbaceous vegetation.

Stands occur on low-elevation, alluvial floodplains on alluvial soils in valleys and inlets, on riverbanks, outer floodplains or low terraces of rivers and streams.

Beaver activity is an important driver of hydrological change and subsequent development of a diversity of habitat patches. The contribution of large woody debris (LWD) from riparian or adjacent upland trees is important to maintaining the hydrological and sediment regimes. LWD has a significant impact on the evolution of channel morphology and also contributes to the spatial distribution and diversity of habitat patches within this system (Naiman and Bilby 1998). Major flood events and consequent flood scour, overbank deposition of water and sediments, and stream meandering are the key fluvial processes that provide new substrates, remove old banks and stimulate renewed growth of cottonwood and willow species (Sawyer et al. 2009). Natural fire-return interval was long or moderate with low-intensity surface fires.

Conversion of this type has commonly come from agricultural conversion and development of urban areas, loss of the floodplain through levee development, and complete inundation by creation of reservoirs. The greatest threat is the change to the natural hydrologic cycle through dams and water management and use. Other threats are more from direct impact and use. Reservoirs, water diversions, ditches, roads, and human land uses in the contributing watershed can have a substantial impact on the hydrologic and sediment regimes. Alterations to both processes can affect the establishment of new and maintenance of existing riparian vegetation. Management effects on woody riparian vegetation can be obvious, e.g., removal of vegetation by dam construction, roads, logging, or they can be subtle, e.g., removing beavers from a watershed, removing large woody debris, or construction of a weir dam for fish habitat. Logging activities tend to reduce the amounts of large woody debris in streams and remove future sources of that debris. Timber harvest can also alter hydrology, most often resulting in post-harvest increases in peak flows. Mass wasting and related disturbances (stream sedimentation, debris torrents) in steep topography increase in frequency with road building and timber harvest. Roads and other water diversion/retention structures change watershed hydrology with wide-ranging and diverse effects, 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. 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. This system has also decreased in extent due to agricultural development, roads, dams and other flood-control activities (WNHP 2011).

In the Pacific Northwest Regionally downscaled climate models project increases in annual temperature of, on average, 3.2°F by the 2040s. Projected changes in annual precipitation, averaged over all models, are small (+1 to +2%), but some models project wetter autumns and winters and drier summers. Increases in extreme high precipitation (falling as rain) in the western Cascades and reductions in snowpack are key projections from high-resolution regional climate models (Littell et al. 2009). Warmer temperatures will result in more winter precipitation falling as rain rather than snow throughout much of the Pacific Northwest, particularly in mid-elevation basins where average winter temperatures are near freezing. This change will result in: less winter snow accumulation, higher winter streamflows, earlier spring snowmelt, earlier peak spring streamflow and lower summer streamflows in rivers that depend on snowmelt (most rivers in the Pacific Northwest) (Littell et al. 2009).

Potential climate change effects could include: further reduction in summer flows (Littell et al. 2009); earlier high flow pluses may negatively affect cottonwood species dominance as their seed production is timed for June-July (Boes and Strauss 1994, Merritt and Wohl 2002), so cottonwood-dominated streams may shift to other deciduous trees; however, regional climate model simulations generally predict increases in extreme high precipitation over the next half-century, particularly around Puget Sound (Littell et al. 2009); drop in groundwater table; and increased fire frequency due to warmer temperatures resulting in drier fuels the area burned by fire regionally is projected to double by the 2040s and triple by the 2080s (Littell et al. 2009).

This system occurs throughout the Pacific Northwest below the Silver Fir Zone in elevation.