Biodiversity Guide - The Intertwine

egional conservation strategybiodiversity guideChapter 3 Major Habitat Types of the RegionImportance to Native AmericansThe history of oak habitat in the region includesthe indigenous people that managed this areafor thousands of years before approximately1840. The Cowlitz and Upper Chehalis Indiansof the Puget lowlands and the Kalapuya tribes ofthe Willamette Valley regularly set fires to favorplants on which they depended for food andmedicine. Beyond oak, important savanna plantswere camas (Camassia sp.), wild onion (Alliumsp.), and tarweed (Madia sp.). Some woodlandswere deliberately left unburned to provide areaswhere deer, elk, grouse, and other game wouldconcentrate. The imprint left by that history continuestoday.the situation is likely worse within the Portland-Vancouver region than in the more rural upperWillamette Valley—especially within the Oregonportion of the region, where urban and agriculturaldevelopment has replaced nearly all theformer oak areas. Few large known examples ofthe habitat remain.Distribution of Oak Habitats in the RegionBecause oak habitat has not been well mapped,its distribution is not known with precision.Remnant habitats within the more urbanizedportion of the greater Portland-Vancouver regionprovide connectivity to areas with more extensivehabitat. Patchy but mostly contiguous—althoughdegraded—areas of oak stretch along the WillametteRiver, east and south through Milwaukie,Oregon City, and Wilsonville to the WillametteValley. Oak habitats also are found as remnants ofthe historical floodplain forests of the ColumbiaRiver. These habitats extend upslope to rockyoutcrops in the Coast Range foothills and into theColumbia River Gorge National Scenic Area tothe east. The farmlands of the Tualatin Valley andin Clackamas, Clark, Marion, and Yamhill countiesare populated by the remnants of oak habitats.Large single oaks and patches of oak oftenare located on the hillsides and along streamsand wetlands in the rolling hills of the greaterPortland-Vancouver region, perhaps because ofthe difficulties in developing or planting fieldcrops in these areas.Large, often isolated oaks found along roads inurban or rural areas, in rural residential settings,and in agricultural fields are clues to the formerregional extent of oak habitat. These ecologicallyvaluable reminders of our region’s natural historyare decreasing as they decline with age, areharvested, or are cut down for development. Evenon good soils where Oregon white oak grows at arelatively fast rate, the replacement of large-diameteroak trees that are favored by wildlifecan take more than a century.conversion, and invasive species. These factorshave led to the decline or loss of many speciesof native plants and wildlife populations thatdepend on large, open-grown oak trees or nativebunchgrass prairie such as western bluebirds,white-breasted nuthatches, acorn woodpeckers,and western gray squirrel.Lack of fire over the past 150 years has allowedconifers and big-leaf maple to overtop and shadeout oaks. Evidence for this can be seen in the narrowcanopies or skeletons of formerly large oaks(and madrones) in existing Douglas fir and maplestands. In some areas, oaks have increased in density,with dense stands of narrow-crowned oaktrees replacing the open-grown oak so valuableto wildlife. Small, shaded, or crowded oak treesproduce fewer acorns, make fewer and smallercavities, are more vulnerable to fire and mayeventually succumb to other forest types. Firesuppression may also be a cause for reduced oakreproduction in the region.Conversion of oak habitats to farms, productionforest, or residential areas has led to smallerpatch size and increased isolation. This not onlylimits the use of oak habitats to species with smallhome ranges, but decreases the viability of plantand wildlife populations within the patch, leadingto loss of local biodiversity.The understory of many remaining oak habitatsis degraded by non-native invasive speciessuch as English and Irish ivy (Hedera sp), nonnativeblackberries (Rubus armeniacus andlaciniatus), Scot’s broom (Cytisis scoparius),and various non-native grasses. As a result, moststands have low diversity and cover of nativegrasses and forbs (i.e., wildflowers) and theanimals that depend on them.Entities Working on the IssueRestoration of oak habitats is under way bynonprofit institutions such as The Nature Conservancy,Tualatin Riverkeepers, Columbia LandTrust, and the University of Portland, and by governmentagencies such as Clean Water Services,the Natural Resources Conservation Service,Metro, the City of Portland, Tualatin Hills Parksand Recreation District, and the U.S. Fish andWildlife Service. The Oregon Oak CommunitiesWorking Group meets occasionally to shareinformation and projects.Threats and ChallengesMeaningful conservation of oak habitats is difficultfor several reasons. The habitat is poorlymapped, and protected areas generally are smalland isolated. Much of the original oak landscapein its various forms has been developed, andwhat remains generally is degraded. Finally, oakand prairie habitat need ongoing active managementthat requires some degree of staff expertiseand resources (although several useful guidesdo exist). The following are issues specific to themanagement of oak habitats:n Valley woodlands once dominated by widelyspaced oaks are becoming forests crowded withconifers and shade-tolerant trees. White oakssurvive only a few decades in such conditions.n Those legacy oaks that persist in residentialareas or on pastures and woodlots are being cutdown as agricultural practices intensify, or theyare aging and not being replaced.Historical and Current OccurrenceExplorers and settlers arriving in the WillametteValley in the 1800s found vast areas of prairieand oak habitat. In 1841, explorer Charles Wilkesn Vineyard development on land once unsuitablefor farming threatens some remaining oakdescribed the landscape as being “destitute oftrees, except oaks.” Oak woodland and savannahabitat.once covered about 400,000 acres in the WillametteValley; this was in addition to 1 millionExamples within the Regionn The lack of a strong market for oak creates littleacres of prairie. Today less than 7 percent of theMuch of the remaining oak habitat in the regioneconomic motivation to maintain oak stands andoriginal habitat remains in the Willamette Valley.is in private ownership. Oak is found throughout favors conversion to conifers.Approximately 460,000 acres of oak and prairiemost of the region at elevations below 2,000 feet,were present in the greater Portland-Vancouverbut especially in the southernmost and westernmostn Invasive, non-native plants such as Scot’sregion in 1850. Lack of accurate current habitatareas of our region. Table 3-1 lists known broom, Himalayan blackberry, and non-nativemapping makes accurate estimates of the degree Condition of Existing Oak Habitatsexamples of oak habitat within the region. grasses reduce the survival and growth rate ofof loss for the region impossible to determine, but The structure and composition of remaining oakoak seedlings and compete against wildflowershabitats often are degraded by lack of fire, habitat50 51