Biodiversity Guide - The Intertwine

egional conservation strategybiodiversity guideChapter 3 Major Habitat Types of the RegionRiparian and BottomlandHardwood ForestsElaine Stewart, MetroAs transitional areas between aquatic and terrestrialenvironments, riparian forests are exceptionalin their diversity and habitat value. Morethan 90 percent of the region’s wildlife speciesregularly use water-associated habitats, and nearly50 percent are closely associated with them. Thesedynamic areas experience frequent and patchydisturbance events such as floods, windstorms,and disease outbreaks that over time create amosaic of habitats with varying stand ages andcompositions. In the greater Portland-Vancouverregion, riparian forests include cottonwoodgallery forests, Douglas fir, western red cedar,Oregon ash/Pacific willow swamps, and variousmixes of Oregon ash, red alder, big leaf maple,Oregon white oak, and black cottonwood. Bottomlandhardwood forests (BLH) are wetlands,with associated hydric soils and regular flooding;they are dominated by Oregon ash and sometimescottonwood.Historical and Current OccurrenceHistorically, the region’s broad floodplainsincluded a diverse mix of riparian forests, bothupland and wetland. Before major settlementbegan in the mid-nineteenth century, floodplainforests along the Willamette River (especiallysouth of Albany) were as much as 3.2 kilometerswide; near major confluences, floodplainforests were up to 10.5 kilometers wide. Theseforests provided organic matter in the form ofleaf litter and downed wood. They stored carbon,recharged aquifers, and shaped streams. The vastfloodplains flooded frequently, trapping sedimentand nutrients from floodwaters. Stream channelstypically were braided and frequently changedcourse as trees fell and shifted on the floodplain.Sections of the Willamette River had more than500 snags per kilometer; snagging records indicatethat many snags were more than 50 meterslong and up to 2 meters in diameter. Althoughthe floodplains in the greater Portland-Vancouverregion are smaller than those along the upperWillamette, they probably had similar structuresand processes.Throughout the greater Portland-Vancouverregion, riparian zones have been significantlyaltered by harvest, development, clearing foragriculture, construction of dams, irrigation, andremoval of wood in streams to facilitate navigation.Loss of BLH wetland is estimated to bemore than 70 percent in the Willamette Valley.Remaining riparian areas often are reduced tothin strands, with frequent gaps and lack of connectivityto upland habitat. Loss of riparian andBLH forests and development within floodplainsdisrupted the wood cycle, resulting in decreasedthe structural complexity of rivers, streams andriparian habitats and reduction of the region’s riversto a single channel; this has decreased habitatcomplexity and the amount of active floodplainand shoreline in the region.Some of the best remaining examples of BLHwetlands are along Multnomah Channel, on thenorth end of Sauvie Island, at Smith and BybeeWetlands Natural Area, and in the Columbiafloodplain areas in Washington, including RidgefieldNational Wildlife Refuge Complex and thestate-owned Shillapoo Wildlife Area. Relativelyintact riparian habitat can be found in areas ofthe Clackamas and lower Molalla and Puddingrivers, the lower Tryon Creek watershed and oneof its tributaries, Arnold Creek, as well as at theheadwaters of Kelley Creek, which is a tributaryof Johnson Creek in Multnomah County.Other notable areas include West Hayden Island,Government Island, and Meldrum Bar Park inGladstone.Important Processes and SpeciesRiparian and BLH forests provide many ecosystemservices, including stream shading andassociated temperature regulation and provisionof large wood to streams. These vegetated zonesfilter sediments and other pollutants in stormwaterand stabilize streambanks, thus preventingerosion. Trees and shrubs store carbon and helpmoderate air temperatures. Forest leaves interceptrainfall, while root systems help soils hold waterand release it more slowly to streams, thus reducingthe flashiness of urban streams.The beaver is a keystone species of riparianand BLH habitats. Its tree-felling and dam-buildingactivities create openings and ponds that providefish habitat, trap sediments, provide refugiafor aquatic species during droughts, and kill treesthat then become snags for wildlife. Other keystonespecies include Chinook and coho salmonand steelhead trout, whose spawning migrationsreturn nutrients to tributaries and headwaters.Black cottonwood trees provide food and habitatfor migrating and nesting songbirds, nest sitesfor bald eagles and great blue heron, and downedwood for terrestrial and aquatic species.Myriad species use the region’s riparianhabitats. Neotropical migratory birds travelthrough riparian zones, and species such as theSwainson’s thrush, Wilson’s warbler, and westernwood-pewee nest there. Resident birds such asbrown creeper and black-capped chickadee mayuse riparian forests year-round, while others (e.g.,Steller’s jay and ruby-crowned kinglet) spendwinters there and return to higher elevationsfor breeding. Mammals in the region’s riparianzones range from black-tailed deer and riverotter to Pacific shrew. Native amphibians spendmuch time foraging in riparian areas, and manyamphibians and reptiles use riparian habitats foroverwintering.The Oregon and Washington state conservationstrategies list a number of species that relyon healthy riparian and BLH habitats. The willowflycatcher and yellow-breasted chat require shrubhabitats near streams and wetlands. The westernpond turtle needs basking logs in streams andwetlands, open areas in the riparian zone fornesting, and a suitable duff layer for hibernationand summer dormancy. The Oregon spotted frogdepends on riparian habitats and spends most ofits life in riparian areas. Aquatic species that areidentified in the state conservation strategies asbenefitting from high-quality riparian and BLHhabitat include the Willamette floater (a freshwatermussel); brook and Pacific lamprey; Chinookand coho salmon; and steelhead and cutthroattrout. BLH wetlands provide refugia and rearinghabitat for juvenile salmon.Threats and ChallengesWetland and riparian protection programs areslowing and reducing the loss of remaining riparianand BLH forests, and a number of initiativesand grant programs are rehabilitating degradedriparian zones and previously converted floodplainsthroughout the region. However, manychanges on the landscape are irreversible, andmany threats remain. Dam operations on majorrivers have reduced the historical disturbanceregime (i.e., flooding) to a fraction of its formerextent and amplitude. The region’s iconic cottonwoodforests depend on this disturbance; withreduced flooding, new forests are no longer beingcreated. Development in floodplains has permanentlydisconnected many of them from theirrivers.Threats in remaining riparian and BLH forestsinclude invasive species such as reed canarygrassand English and Irish ivy that (1) prevent nativeplants from becoming established, or (2) in thecase of ivy, can topple trees from the weight oflarge vines. The invasion of the emerald ash32 33