Biodiversity Guide - The Intertwine

egional conservation strategybiodiversity guideChapter 3 Major Habitat Types of the Regionand oak and madrone dominated areas. Deciduoustrees filled an important role as early occupiersafter disturbance, occupying openings (i.e.,gaps) or constituting the understory in establishedforest (especially red alder [Alnus rubra]and big leaf maple [Acer macrophyllum] but alsomany others). Stands of Douglas fir or ponderosapine mixed with Oregon white oak (Quercusgarryana) and Pacific madrone (Arbutus menziesii)occupied the drier and more fire-prone areasof the region, with the oak also being a typicalcomponent of some riparian areas.Historically, fire and wind (and to a lesserextent disease) were the disturbance typesresponsible for initiating new forests or shapingestablished ones. Fires, including those purposefullyset by Native Americans, covering manythousands of acres initiated or altered standdevelopment in much of the Pacific Northwest.Full canopy closure often developed over manydecades, frequently after stands were re-burned.Many features were retained from the burned forest,including large standing living and dead treesand large fallen trees. Long establishment periodsallowed for a diverse, mature shrub componentin developing forests. Wind generally producessmaller, localized disturbances. The resultingopenings create habitat for sun-dependent forband shrub species that support many insect andbird species.Current ConditionsCurrently, old-growth forest occupies a tinyfraction of the greater Portland-Vancouverregion. Most upland forests in the region haveoccupied areas of former prairie or oak habitat.Forests managed for timber production (i.e.,much of the foothills of the Coast Range andCascades) typically are less than 60 years old, aredensely planted with Douglas fir, and generallylack a significant shrub and tree layer beneath thecanopy. Although clear-cut harvest creates openconditions favorable for many species, it doesnot typically leave important elements of biologicallegacies from the previous stand. Large deadwood in most commercial stands is limited to thefew remaining old stumps of the original forest.Forests in conservation areas that have developedfollowing harvest and abandonment (e.g., SandyRiver, Gresham Buttes) or fire (Forest Park) varygreatly in species composition, depending on harvestmethod, fire intensity, and conditions immediatelyafter harvest. Such stands range fromalmost pure conifers (this is uncommon withoutactive stand management) through mixed coniferand broadleaf stands to almost pure deciduousstands dominated by big leaf maple, alder, anddiverse shrubs.BiodiversityPlant and animal use of forests follows thechanges in forests over time and environmentalconditions that influence forest composition andstand structure. At each stage of development,forests provide different conditions that, in turn,provide habitat for different types of living things.Very young natural stands with open conditionssupport a high diversity and productivityof shrubs and forbs and the wildlife species thatdepend on them (see Appendix D). As the forestcanopy closes, biodiversity drops dramatically.When a forest reaches the old-growth stage andhas more open canopy and extensive gaps, itbegins to once again provide habitat for manylight-dependent plant species while also providinghabitat for species that depend on large trees,snags, cavities, and large wood on the forest floor.Species that depend on older forests tend to behabitat specialists. For example, the northern flyingsquirrel (Glaucomus sabrinus) depends on thedecayed logs, dense canopy, and understory coverthat occur in old-growth forests. The endangeredspotted owl (Strix caurina) relies on the northernflying squirrel as a primary food source and alsouses old-growth forest as primary nesting areas.Significant population reductions in habitat-specialistspecies associated with old-growth forest,such as northern spotted owl, flying squirrel,pileated woodpecker, and many cavity- nestingspecies, reflect modern changes in overall foreststructure across the region.Landscape IssuesThe size, shape, and distribution of forest habitatpatches affect their value in terms of biodiversity(see Chapter 6 of this document and Chapter 7of the Regional Conservation Strategy for a moredetailed discussion of this issue). Patches of 30acres begin providing habitat for species thatrequire interior habitat, but true interior conditionsand population viability probably requirespatches of 300 acres or more. Before 1850, forestsin the greater Portland-Vancouver region werewell connected, with patches of thousands oreven hundreds of thousands of acres. Althoughlarge areas of contiguous forest still exist aroundthe edges of the region (mostly in commercialforestry and related riparian areas), there hasbeen a profound trend toward smaller patch sizeand increased isolation in more developed areasof the region.Threats and ChallengesThe major threats to biodiversity in establishedforest include lack of important habitat features,invasive species, fire suppression that can lead tocatastrophic wildfire, and habitat loss or fragmentationas a result of development and conversionto agricultural lands.Invasive Non-native SpeciesNon-native plants and animals represent a substantialthreat to forest health and are the primarythreat to protected forests, especially in the nearurbanarea. Climbing species such as Englishand Irish ivy and old man’s beard can kill ortopple mature trees. Shade-tolerant weeds such asEnglish and Irish ivy, garlic mustard, and spurgelaurel can smother and eliminate native plants onthe forest floor. These species and more lightlovingones such as Himalayan blackberry canprevent establishment of young trees in gaps createdby dying canopy-dominant trees. Non-nativemammals such as squirrels and Virginia opossumcompete for habitat resources and prey directlyon native species, including cavity-nesting birds.Non-native insects such as the Asian gypsy moththreaten entire stands of trees. Although no nonnativediseases currently threaten our primaryforest species, non-native diseases have beenresponsible for the loss of dominant species suchas American chestnut and American elm in otherparts of the country. Forests managed for singlespecies are particularly vulnerable to the impactsof non-native species.Habitat Loss and IsolationPoorly planned conversion of remaining forestlands to residential areas, agriculture, or roadscould compromise the existence of (1) forestpatches of a size sufficient to maintain biodiversity,and (2) the remaining biodiversity corridorsthat connect upland forests and other habitats.The issue is more severe in the near-urban area,but care should be taken to build and maintainconnections between patches of forest managedfor biodiversity values throughout the region.Lack of Important Habitat FeaturesAlthough commercial forests provide manybenefits, they have limited value for many plantand wildlife species because they are dominatedby small Douglas fir, lack the range of age classestypical of natural forests, have a poorly developed44 been harvested multiple times or have recently45