|Figure 1. Western sword fern in the forest understory. Photo courtesy of Hansen's Northwest Native Plant Database (http://www.nwplants.com).|
Western sword fern is closely related to Polystichum imbricans based on morphologic  and electrophoretic  analysis.
Hybrids: Hybridization and polyploidy have contributed to morphological diversity and taxonomic complexity in the widespread genus Polystichum . At least 1 or 2 hybrid plants are expected in large, mixed Polystichum populations, and several sterile interspecific hybrids have been reported. Hybrids between western sword fern and P. andersonii, P. californicum, P. scopulinum, P. braunii, P. kruckebergii, P. dudleyi, and P. lemmonii have been reported (review by ). According to Hickman , hybrids with P. dudleyi are called P. californicum. Polystichum munitum × P. andersonii and P. munitum × P. lemmonii hybrids are described by Soltis and others .SYNONYMS:
|Figure 2. Western sword fern distribution in the United States and Canada. Map courtesy of USDA, NRCS. 2012. The PLANTS Database. National Plant Data Team, Greensboro, NC. (2012, 29 February).|
Western sword fern occurs along the Pacific Coast from southeastern Alaska south to Baja, California [81,100,126,183], with disjunct populations in northeastern Washington, northern Idaho, northwestern Montana, northeastern Oregon [130,189], and the west Kootenays in British Columbia . Western sword fern is most abundant and widespread in coastal areas from central Vancouver Island and the adjacent mainland southward , throughout the western Coast and Cascade ranges [81,100], and into northwestern California . Its latitudinal distribution is described as low-subarctic, high-temperate .
Western sword fern occurs in the following states and provinces as of 2014:
United States: AK, CA, ID, MT, OR, PA, SD, WA
Canada: BC , YT 
In Mexico, western sword fern occurs on Guadalupe Island [58,98]. According to NatureServe , it occurs in Yukon Territory, is nonnative in Pennsylvania, and may be extirpated in South Dakota. It is naturalized in Europe .
SITE CHARACTERISTICS AND PLANT COMMUNITIES:
Western sword fern is widespread in the understory of mesic coniferous and moist mixed-evergreen forests at low to middle elevations (sea level to 7,200 feet (0-2,200 m)) along the northern Pacific Coast [36,58,100,121,126,170,189]. It is among the most common herbaceous species and is often dominant on nitrogen-rich or moist sites in forest habitats where western hemlock and/or coast Douglas-fir (hereafter, Douglas-fir) are characteristic species. These forests occur throughout low-elevation western Washington and on the western slopes of the Cascade Range, around the margins of the Willamette Valley, and in the Coast Ranges . Western swordfern sometimes codominates the understory of oak and dry Douglas-fir forests and woodlands west of the Cascade Range in the Puget Lowlands, Willamette Valley, and Klamath Mountains. It is especially common on sites that formerly supported grasslands and savannas in these areas . Western sword fern may dominate the herb layer in lowland riparian areas west of the Cascade Crest from British Columbia south into northwestern California. These habitats are typically comprised of tall (6-30 feet (2-9 m)), deciduous shrublands, woodlands or forests, or some mosaic of these. Red alder is the most widespread tree species .
Climate: Western sword fern thrives in humid coastal climates with mild winters. It also occurs in areas of moist, relatively mild continental climate in northern Idaho, northwestern Montana, and adjacent areas of British Columbia, but in these areas it tends to be restricted to shaded, moist, northern aspects [74,154]. Its frequency and abundance decrease with increasing elevation and continentality [41,121,175].
Western sword fern is among the most typical and abundant understory plants in Sitka spruce  and redwood  forests along the Pacific Coast. Sitka spruce forests are limited to a relatively narrow oceanside strip characterized by mild winters, cool summers, and abundant moisture throughout the growing season . Extremes in moisture and temperature are rare; the climate is uniformly wet and mild. Precipitation averages about 80 to 120 inches (2,000-3,000 mm) , but may reach 220 inches (5,600 mm) in some areas of Alaska and British Columbia . Frequent fog and low clouds add moisture during the relatively drier summer months . Climate in the redwood forest zone is similarly characterized as moist and temperate , with high rainfall and dense, dripping fog . Annual precipitation averages nearly 60 inches (1,500 mm), of which 10% to 25% falls in the warmest months and 60% to 70% from December to March . Plants in redwood forests depend on fog absorbed through foliar uptake to stay hydrated during the relatively dry summer . The high foliar uptake capacity of western sword fern may allow it to rely completely on fog water during the summer, when fog exposure is frequent (, but see Other Management Considerations). Western sword fern's foliar uptake capacity and morphological characteristics vary with latitude in the redwood forest zone (Table 1), with its greatest foliar uptake capacity in the central part of the zone and no foliar uptake capacity at the southern end .
|Table 1. Mean (SD) size and abundance of western sword fern crown and fronds measured inside plots at each of 7 old-growth redwood forest sites in California where western sword fern was the understory dominant. Sites are listed from northernmost to southernmost .|
|Frond length (cm)|
|Prairie Creek||5.4 (1.3)ab*||22.8 (20.6)a||123.0 (42.8)a||86.6 (19.7)a|
|Humboldt Redwoods||6.5 (3.0)a||14.5 (11.4)bc||94.2 (27.0)a||74.3 (21.1)b|
|Angelo Reserve||6.1 (1.9)a||18.9 (13.7)ab||115.3 (44.8)a||60.2 (11.9) cd|
|Hendy Woods||7.8 (2.6)a||13.9 (9.3) bc||108.1 (27.0)a||62.1 (12.9)c|
|Grove of Old Trees||6.8 (1.5)a||15.2 (12.6)bc||103.2 (26.0)a||56.7 (15.0) d|
|Roy's Redwoods||5.5 (2.0)ab||16.2 (10.8)bc||88.9 (32.5)a||51.4 (16.9) e|
|Big Basin||2.9 (1.9)b||11.5 (8.8)c||33.5 (29.7)b||48.1 (15.7)e|
|*Values followed by different letters within a column are significantly different (P<0.05).|
The climate in the western hemlock forest zone is wet and mild but varies with latitude, elevation, and location in relation to mountain massifs. Moisture and temperature extremes are greater farther inland . Precipitation occurs mainly during the winter, with brief summer droughts [64,173], and averages about 30 to 170 inches (800-4,400 mm) annually. Precipitation falls mostly as rain or cloud drip, with minimal snow accumulations [169,173,181]. While western sword fern's evergreen fronds are presumably somewhat resistant to frost, they may limit its ability to survive in areas with cold winters . It is not common on sites where snow pack is deep [81,131], and western sword fern associations typically occur where little snow falls (e.g., ).
Site characteristics: Western sword fern tolerates a range of site characteristics, from the coastal fog belt to montane and subalpine sites . It is an indicator of mesic and moist, nutrient-rich environments in western hemlock and Douglas-fir forests in western Oregon and Washington [4,39,81,94,131], and it is an indicator of moist, nutrient-rich, humus-rich forests and shaded slopes in British Columbia [121,175]. Plant associations where western sword fern is an understory dominant typically occur at low elevations on productive soils, especially those enriched by surface flow of fine organic materials . Soils typically remain saturated throughout the year due to subsurface water from upslope [81,131], flat topography with impeded soil drainage [118,169], near proximity to surface water, high precipitation, high humidity, or frequent fog (see Moisture relationships for more information).
Western sword fern associations often occur on sites with high timber productivity; these sites are frequently logged [64,161]. The western hemlock zone includes some of the most productive forest land in the Pacific Northwest [64,82,208], and communities where western sword fern dominates the understory are some of the most productive among these [64,65,71,95,97,198,208]. Impressive stands of Douglas-fir, western hemlock, and western redcedar are possible , with sparse shrub layers and well-developed herb layers that are often described as lush or luxuriant, because of abundant western sword ferns [53,62,64]. High moisture availability and nutrient-rich soils contribute to the high productivity of these sites [64,119]. In British Columbia, for example, these sites typically occur on valley bottoms and relatively flat alluvial terraces close to rivers. Soils are from base-rich parent materials and have loamy textures with high soil moisture holding capacity, few coarse fragments, and good drainage .
Western sword fern may reach its best growth and greatest abundance in undisturbed, old-growth stands, especially those dominated by western hemlock or Sitka spruce (Table 2); lush growth of western sword fern may indicate late-successional or relatively undisturbed sites. Western sword fern accounted for almost all understory cover in the central, protected valleys of Sucia Island, Washington, where vegetation is undisturbed, wind is minimal, summer temperatures are low, and soil moisture is high. Its fronds had a spread of nearly 7 feet (2 m) .
|Table 2. Western sword fern cover and site characteristics in remnant old-growth forests in the North Coast Range of Oregon .|
|Plant association||Western sword fern cover (%)||Elevation (feet)||Precipitation (inches)||Moisture group*||Slope (%)||Aspect (degrees)||Slope position|
|Sitka spruce/redwood-sorrel||40.0||1,200||110||41||73||slope, upper 3rd|
|Sitka spruce/redwood-sorrel||16.7||1,500||112||30||297||slope, upper 3rd|
|western hemlock/redwood-sorrel||34.0||1,180||118||wet||59||150||slope, lower 3rd|
|western hemlock/vine maple/western sword fern||40.0||1,320||110||wet||71||181||slope, lower 3rd|
|western hemlock/vine maple/western sword fern||28.3||1,400||107||mesic||41||200||bench|
|western hemlock/salal||13.7||480||105||mesic||52||215||slope, lower 3rd|
|western hemlock/Pacific rhododendron-salal||1.4||1,300||88||mesic||54||215||slope, middle 3rd|
|western hemlock/dwarf Oregon-grape-salal||6.0||1,570||97||mesic||59||133||slope, middle 3rd|
|western hemlock/vine maple-California hazelnut||4.3||2,030||90||dry||49||154||slope, upper 3rd|
|grand fir/California hazelnut/white insideout flower||18.3||1,100||61||dry||41||329||slope, upper 3rd|
|grand fir/California hazelnut/white insideout flower||3.0||1,200||65||dry||31||29||rounded ridge top|
|*Identified only for Douglas-fir-dominated stands >200 years old.|
Western sword fern tolerates disturbance and is common in second-growth forests after logging and other disturbances (see Successional Status), and sometimes occurs even in highly disturbed areas. For example, it was common along electrical transmission rights-of-way in western Washington and Oregon, where it ranged in cover from 2% to 55% . It was noted as particularly common along roadside clearings in southern coastal British Columbia .
The following discussion describes common site characteristics of western sword fern communities and gives examples showing its range of site tolerances. This information comes almost exclusively from vegetation classifications, although a few studies were available (as of 2015) that examined western sword fern site affinities. For example:
See the following vegetation classifications for details about western sword fern plant communities: [6,53,64,65,94,95,97,208]. See the following vegetation classifications for descriptions of western sword fern communities in specific locations: [16,53,94,95,99,143,208].
Elevation, landform, topography: Western sword fern is most common at low elevations, but it occurs from sea level to about 7,200 feet (0-2,200 m) . It is widespread in the western hemlock zone in the Pacific Northwest, which occurs from about 490 to 1,800 feet (150-550 m) in wet environments, such as the western slopes of the Olympic Mountains , and from sea level up to about 4,000 feet (1,200 m) in relatively drier environments [64,97,181]. Western sword fern is most common up to about 2,500 feet (760 m) on the Olympic National Forest . In California, it occurs below 5,200 feet (1,600 m) . It rarely occurs above 3,000 feet (900 m) in southwestern British Columbia, or above 300 feet (100 m) on the northern coast of British Columbia . Plants at high elevations near the coast may be stunted or sparse (e.g., at 6,300 feet (1,920 m) in the northern Cascade Range [49,50]). Inland populations occur at relatively higher elevations; for example, western sword fern's range in Montana is from 3,500 to 5,000 feet (1,070-1,500 m) .
Western sword fern communities in the western hemlock zone are most common on lower slopes and flats such as toe slopes, benches, and stream terraces, although occurrence on steeper slopes at any aspect is possible. For example, the western hemlock-Port-Orford-cedar/western sword fern-redwood-sorrel community occurs mostly on flats but sometimes on slopes up to 35%, mostly with northern aspects . In the western hemlock zone of the central western Cascade Range in Oregon, western sword fern communities tended to occur on slopes with north aspects, ranging from 18 to 45 degrees . The western hemlock/western sword fern association in Mt Rainier National Park occurs mostly on low slopes or sloping benches with southerly aspects, but it can be found on slopes of 70% or more with any aspect . Western sword fern was strongly correlated with aspect, favoring north- and east-facing slopes, in successional red alder stands in the Alsea River drainage of the Central Coast Ranges in western Oregon .
In redwood forests, western sword fern associations occur on both slopes and flats, and western sword fern cover may be greater near the coast than farther inland. In old-growth redwood stands in the Coast Ranges of northern California, western sword fern cover was greater on coastal sites than inland sites. Its cover was similar on slopes and flats near the coast, but it occurred only on flats inland . Western sword fern occurred in all streamside redwood sites studied in northwestern California . In southern Monterey County, the redwood/western sword fern-Pacific trillium ecological type occurs on mesic, moderate to very steep (50-100%), lower slopes. Most stands face north to north-northwest and are 50 to 175 feet (15-53 m) from the nearest stream .
Soils: Western sword fern grows on a variety of soils and parent materials including exposed bedrock, but the most luxuriant growth is found on deep, loamy soils, especially those developed on fluvial parent materials (e.g., [53,64,97]). Western sword fern is characteristic of moist but strongly drained, deep soils in northwestern Washington  and Canada , although western sword fern associations may indicate relatively shallow soils in some sites (e.g., [43,94,97]). Many western sword fern associations occur on moderately deep to deep (effective rooting depths of 3 feet (1 m) or more), well-drained but moist soils with fine to moderately fine (silty clay loam to sandy loam) surface textures [16,53,64,91,97,99,169]. Soils range from relatively stone free [53,94] to moderately stony (5-30% coarse fragments) or stonier [53,65,94]. Parent materials include alluvium, colluvium, outwash, and glacial till [53,97] derived from andesite, basalt, sandstone, siltstone, tuffs, breccias, lahar, or tephra deposits [53,65,95]. Western sword fern occurs only occasionally on rock , such as shaded talus slopes , cliffs, outcrops [98,192], and terraces adjacent to the ocean . On Junipero Serra Peak in California, western sword fern occurred in Coulter pine and sugar pine stands on rock outcrops .
Western sword fern is nitrophytic  and grows best on nutrient-rich soils. It is a characteristic understory species on soils that are moderately to very rich in nutrients (e.g., [81,97,118,120,123,131,169,175]). Abundant and vigorous growth of western sword fern may indicate nutrient-rich sites . In Canadian forests, presence of western sword fern indicates base-rich parent materials  or nitrogen-rich sites. It has greater frequency and cover on soils with relatively high pH and mineral nitrogen and relatively low carbon:nitrogen ratio .
Soils in western sword fern communities are often rich in organic matter (e.g., ), and western sword fern can grow on decaying stumps . In redwood/western sword fern forests, soils are Ultisols; base saturation is low, but organic matter content is high . In cool mesothermal forests in southern British Columbia, western sword fern occurs on a variety of organic substrates, but it is an indicator of moder and mull humus forms [123,175], which are differentiated by a layer of organic materials with more than 17% organic carbon overlying mineral soil . A comparison of vegetation on stumps and on the ground in coniferous (Douglas-fir-western hemlock-western redcedar) and deciduous (red alder) forest sites in Olympia, Washington, showed that western sword fern had similar stem density in each forest type, and had higher stem density on ground microsites in both forests (P<0.005). It sometimes occurred on stumps in the deciduous forest, but not in the coniferous forest .
Moisture relationships: Western sword fern associations are characteristically found in moist or shaded locations, where drainage is good but little moisture stress is encountered. In western hemlock forests, western sword fern is most often dominant in moist and wet communities  (e.g., see Table 3). For example, the western hemlock/western sword fern-redwood-sorrel association typically occurs on very moist streamside slopes, and the western hemlock/western sword fern association occupies toe slopes and northern aspects . On the Olympic National Forest, the Sitka spruce/western sword fern-redwood-sorrel association occurs along toe slopes or in areas of high precipitation, high humidity, or fog . The western hemlock/Pacific rhododendron/western sword fern association on the Siuslaw National Forest is the moistest of the Pacific rhododendron associations .
|Table 3. Examples of western sword fern associations in the western hemlock forest zone on the Gifford Pinchot National Forest on a moisture gradient from wet to dry (Topik and others 1986 as cited by )|
|Wet group||Moist group||Mesic group||Dry group|
|devils club-western sword fern||
western sword fern-redwood-sorrel
western sword fern
|dwarf Oregon-grape-western sword fern||-none-|
The 2 most important understory or groundlayer species in western hemlock and Douglas-fir forests are western sword fern on relatively moist sites and salal on slightly drier sites [16,41,62,63]. In Douglas-fir/western hemlock forests in the eastern portion of the Olympic Peninsula, for example, forest understories are dominated by dense salal on the drier west- and south-facing slopes, and by western sword fern, twinflower, oneleaf foamflower, and northwestern twayblade on north- and east-facing slopes . Western sword fern and salal may codominate on intermediate, mesic sites [64,150]. On alluvial landforms of the McKenzie River valley, Oregon, 2 topoedaphic climaxes were identified. Understories were dominated by western sword fern and redwood-sorrel on terraces with fine, sandy loam to silt loam soils derived from silty river alluvium; and by dwarf Oregon-grape and salal on rockier soils with coarser alluvium or glacial outwash. Sites with finer-textured soils had greater late summer moisture availability . Western sword fern is an indicator of relatively cool, moist sites in Douglas-fir forests on the HJ Andrews Experimental Forest, Oregon , and in tanoak-redwood and tanoak-vine maple associations in southwestern Oregon [13,14].
In British Columbia, western sword fern is most abundant in ecosystem associations with moderately well-drained to poorly drained soils [118,140]. Western sword fern is sporadic to scattered on water-shedding sites, and plentiful to dominant on water-receiving and colluvial sites enriched by surface flow of fine organic materials . In the coastal Douglas-fir zone of southern Vancouver Island, where summer moisture deficits are common, western sword fern occurs only where seepage water augments soil moisture supply. In wetter climates it is less restricted by soil moisture .
In drier forest zones, western sword fern presence or abundance may indicate relatively moist sites. In the Willamette Valley, the Oregon white oak/California hazelnut/western sword fern forest community was the most mesic and occurred on moderate to steep, sheltered slopes. Western sword fern cover averaged 34% but reached 80% with fronds >3 feet (1 m) tall on the best sites. In progressively drier communities, frequency, cover, and height of western sword fern declined (Table 4), and in the Pacific poison-oak community it grew "only as a sporadically distributed, depauperate plant" .
|Table 4. Western sword fern cover and frequency (%) in Oregon white oak plant communities listed from mesic to xeric |
|California hazelnut/western sword fern||sweet cherry/common snowberry||Saskatoon serviceberry/common snowberry|
In the Selway-Bitterroot Wilderness in Idaho, western sword fern is a common understory species in "older-aged" grand fir-western redcedar forests on creek bottoms and smaller stream ravines .
Plant communities: Western sword fern is widespread, abundant, and often an understory dominant in the maritime forests along the northern Pacific coastline; it also occurs in some inland forest types. Maritime forests occur between the ocean shore and the crest of the Coast Ranges and are variously dominated by western hemlock, Sitka spruce, redwood, western redcedar, and Douglas-fir [62,136]. It is an understory dominant in some Port-Orford-cedar, Pacific silver fir, Oregon white oak, and tanoak communities, and it often occurs in the understory of successional red alder communities. Western sword fern also occurs in drier forest zones, and at higher elevations near the Pacific Coast but is less often an understory dominant .Information compiled from vegetation classification literature from Oregon and Washington shows the average cover and frequency of western sword fern in plant communities in each of 5 forest zones (Table 5). Western sword fern is most common in the western hemlock zone, where it is most prominent in communities on cold, wet sites. It is well suited to the moist and wet climates in the Pacific silver fir zone but uncommon in the drier sites of that zone. Western sword fern is present in the warmer-drier Douglas-fir zone, but cover is low. It is infrequent in the mountain hemlock zone and absent or rare in the subalpine fir zone .
|Table 5. Average cover and frequency of western sword fern by selected forest associations in 5 forest zones in the Pacific Northwest |
|Forest zone/association||western sword fern frequency (%)||western sword fern cover (%)|
|Subalpine fir zone|
|Mountain hemlock zone|
|big huckleberry-Alaska blueberry||---||---|
|Pacific silver fir zone|
|western sword fern||100||21|
|Western hemlock zone|
|vine maple/sweet after death||100||9|
|Pacific dogwood/vine maple||73||4|
|western sword fern||100||37|
|western sword fern-threeleaf foamflower||100||39|
|dwarf Oregon-grape-western sword fern||100||26|
|western sword fern-salal||100||20|
|Pacific rhododendron-western sword fern||82||22|
|Pacific rhododendron-dwarf Oregon-grape||70||10|
|creambush oceanspray-dwarf Oregon-grape||78||2|
|creambush oceanspray-baldhip rose||76||3|
Western hemlock zone forests: The regional climatic climax for the western hemlock zone in the Oregon Coast Ranges is thought to be the western hemlock/western sword fern association . Shrub layers are not well developed in these habitats, although vine maple or dwarf Oregon-grape may dominate. Lush herbaceous understories are common in western hemlock/western sword fern habitats. Foamflowers, twinflower, deer fern, lady fern, and several lilies are among the numerous succulent herbs that cover the forest floor in these rich habitats . See Appendix B for a list of western sword fern associations that occur in western hemlock forests and are described in regional vegetation classifications.
Sitka spruce zone forests: Western sword fern is a common understory species in the long, narrow Sitka spruce zone along the Pacific coastline from southeastern Alaska to southern Oregon. Coniferous forest stands in this zone are typically dense, tall, and among the most productive in the world. Western hemlock-Sitka spruce forests are mostly restricted to a narrow coastal band but may occur as much as 50 miles (80 km) inland on cool, moist slopes, along stream bottoms, and around lake margins where fog is prevalent during the growing season. Western swordfern, redwood-sorrel, and red huckleberry are common in these optimum conditions . Western sword fern and redwood-sorrel are the most important understory species in old-growth forests dominated by Sitka spruce and western hemlock in the very heavy rainfall area on the western slopes of the Olympic Mountains. These forests have distinctive characteristics including massive trees, numerous canopy levels, evergreen dominants, a profusion of epiphytes, an abundance of bigleaf maple and vine maple, abundant nurse logs, and "an intangible overall quality of growth" . See Appendix B for a listing of Sitka spruce/western sword fern associations described in regional vegetation classifications.
Western sword fern is less common in Alaska and was mentioned in only a few vegetation classification publications from there. Details were lacking in all but one of these. In the Stikine area of Southeast Alaska it occurred at low frequency and cover in closed-canopy forests (60-100% canopy cover) dominated by western hemlock, Sitka spruce and/or western redcedar, and in open-canopy (25-59% canopy cover) western hemlock forests. Its greatest frequency was in western hemlock/blueberry/spreading woodfern communities (22%), where its cover ranged from 1% to 6%; and in Sitka spruce-western hemlock/American skunkcabbage/sphagnum communities (33%), where its cover ranged from 3% to 10% .
Pacific silver fir forests: Western sword fern is an understory dominant in Pacific silver fir forests on the Olympic Peninsula (see Appendix B). In the Cascade Range of Oregon and southern Washington, western sword fern occurs on warm sites in the Pacific silver fir series [26,93] but typically has low cover (about 1-2%) [53,93].
|Redwood forests: Redwood forests occur in a narrow strip from southwestern Oregon to Santa Cruz and are characterized by pure stands of redwood on floodplains and with a mixed lower tree stratum on hillsides. Western sword fern is one of the most abundant plants in redwood forests [20,25,41,205], especially in old-growth stands [21,23] on moist sites  such as alluvial flats and lower slopes . See Appendix B for a list of redwood/western sword fern associations.|
|Figure 3. Western sword fern growing under towering redwoods on the Simpson Reed Grove Trail, Redwood National Park. Photo courtesy of the National Park Service, www.nps.gov|
Tanoak forest: Western sword fern is among the most common ferns in the tanoak series of the Siskiyou Region of southwestern Oregon. It is most abundant in coastal associations such as tanoak-coast redwood and tanoak-vine maple [13,14] (Appendix B).
Other community types: Western sword fern may be an understory dominant or indicator species in Oregon white oak, canyon live oak, and coyotebrush communities. The Oregon white oak/California hazelnut/western swordfern association occurs on moist sites in the Willamette Valley, Oregon [194,203], and western sword fern was characteristic of the Oregon white oak/common snowberry woodland type in the Bald Hills oak woodlands in Redwood National Park, California . A coyotebrush/western sword fern association is described in California northern coastal sage scrub . A Douglas-fir/canyon live oak/western sword fern association was described in the northern Sierra Nevada , and western sword fern occurs with low cover in mature canyon live oak forest on the San Bernardino National Forest in southern California .
Riparian: Western sword fern is a common to dominant species in many riparian communities throughout its range. It is a characteristic species in widespread and highly productive black cottonwood-willow riparian forests occurring on bottomlands, riverbars, streambanks, and meadows along the Pacific coast ; and in black cottonwood riparian forests in bottomlands, floodplains, gravelbars, and banks of perennial streams in northern coastal California . In northwestern Oregon, western sword fern occurs in several plant communities on cobble bars and low floodplains, and may reach 17% to 25% cover in some, though it is not a named community dominant. Western sword fern also occurs in swamp communities, such as the red alder/slough sedge-American skunk cabbage community, where it may be abundant on logs and stumps and reach 45% cover . In California, western sword fern is common in riparian forests dominated by Port-Orford-cedar in ravines and draws, along streams, and on mesic, protected slopes with a strong maritime influence .
Eastern Washington and Oregon: Western sword fern is rare in eastern Washington and Oregon, though it occurs as an understory dominant in the relatively warm, moist grand fir series in the Blue Mountains  with wild ginger and oak fern . A grand fir/western sword fern-wild ginger potential vegetation type occurs on cool, very moist site types . These communities typically occur on lower slopes or riparian areas and may exist as residual stringers that do not burn as often as associated uplands .
Western sword fern is classified as a facultative wetland species in eastern Washington, where it occurs in several riparian and wetland communities with low cover, including those in the Pacific silver fir, western hemlock, and black cottonwood series . It also occurs in small amounts in old-growth western redcedar-western hemlock forest , Pacific ponderosa pine (hereafter, ponderosa pine) communities, the grand fir-Oregon boxwood association, the western hemlock-Oregon boxwood association, and the western redcedar-devil's-club association .
Idaho: Western sword fern is not widespread in Idaho but is most abundant in western redcedar [37,163], grand fir [37,56,163], and western hemlock communities . It may also occur in riparian habitats . In northern Idaho, western sword fern dominated the diverse understory in a western redcedar/northern maidenhair habitat type along Nylon Creek north of Dworshak Reservoir. Western sword fern commonly occurs in the western redcedar/wild ginger habitat type and may occasionally dominate the forb layer in the grand fir/wild ginger habitat type . Western sword fern occurred with low cover at all stages of succession in the Rocky Mountain juniper/red-osier dogwood habitat type in eastern and southern Idaho .
Montana: Western sword fern is rare in Montana. It was recorded in late-seral to climax stands of the western hemlock/oak fern habitat type with an average cover of 1% . Populations have been recorded in the Bitterroot and Sapphire ranges; a population at Charles Waters Memorial Campground was extirpated by collectors .
South Dakota: According to a flora published in 1977 , western sword fern occurred in moist places in the Black Hills. However, it was listed among the rare plants of South Dakota in 1985, with its status "undetermined" and this comment: "Recently reported but population has apparently been extirpated" .See Appendix B for a list of western sword fern association described in vegetation classifications, or the Fire Regime Table for a list of plant communities in which western sword fern may occur and information on the fire regimes associated with those communities.
|Figure 4. Western sword fern frond. Photo courtesy of Hansen's Northwest Native Plant Database, http://www.nwplants.com.|
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [10,58,98,100,106,126,130,170,183]). Morphological similarity among Polystichum species may make identification difficult .
Western sword fern populations consist mostly of sporophytes; gametophytes are extremely rare , and no description of western sword fern gametophytes was found in the literature. The following description applies to sporophytes.
Western sword fern is a medium-sized, perennial, long-lived, evergreen fern [170,189] with many leaves (fronds), sometimes 75 to 100 , clustered on a short, vertical rhizome . Stems are erect or ascending and fronds arch outward [58,126], forming a crown [36,155,170,175]. Fronds are leathery and stay evergreen for several years; withered fronds remain attached to the rhizome . Frond size varies depending on location and site conditions and may range from about 8 to 98 inches (20-250 cm) long [36,58,74]. Fronds are usually at the smaller end of this range in northern [7,106,170] and inland  parts of western sword fern's distribution, and in sunny places . Plants with fronds >3 feet (1 m) long typify western sword fern where it is an understory dominant [100,170,216]. Petioles are one-eighth to one-fourth the length of the frond, densely scaly, and gradually diminishing in size distally. Leaflets are 0.4 to 6 inches (1-15 cm) long. Those of shade-growing plants occur in 1 plane; those of sun-growing plants are twisted or contorted . Sori are large and circular  and occur in 1 to several rows  on leaflets (Figure 6).
Western sword fern is closely related to narrowleaf sword fern based on morphologic  and electrophoretic analysis ; it is possible that some of the older literature misidentified these species. When western sword fern plants are subjected to increased insolation (e.g., after canopy removal, in rocky outcrops, or at high elevations), the fronds become dwarfed and more erect, and the leaflets become crisped and more crowded, such that it more closely resembles narrowleaf sword fern .
Western sword fern rhizomes are stout, woody, and scaly [36,170], often covered with old stipe bases and ending in a crown of new fronds . Rhizomes are short , and plants grow in separate, individual clumps  but often form extensive populations . Populations can cover very large areas and consist of thousands of individuals. Disjunct populations east of the Cascade Range are typically smaller, consisting of a few hundred individuals .
Characteristics of western sword plants (clonal fragments) were described on 3 sites on the Olympic Peninsula occupied by densely stocked, second-growth forests, 40 to 60 years after clearcutting (Table 6). Clonal fragments consisted of all live plant parts (rhizomes, stolons, roots, buried apical buds, and aerial shoots) connected to an initially selected, undamaged aerial shoot. Most western sword fern clonal fragments consisted of a single rhizome segment that thickened with the production of new fronds, although 5% had more than one ramet emerging from a branched rhizome. Perennating buds were shallow, ranging in depth from about 0.2 to 1.8 inches (0.5-4.5 cm). Roots were wiry and densely branched, mostly occurring in the top 8 to 12 inches (20-30 cm) of soil and spreading laterally in the upper organic horizon; maximum root depth was about 28 inches (72 cm). Soils were silt loams and very gravelly sandy loams, and the thickness of the organic horizon ranged from about 0.4 to 2 inches (1-5 cm) .
|Table 6. Characteristics* of western sword fern rhizomes, roots, and shoots in young coniferous forests on the Olympic Peninsula, Washington (n=20 western sword fern clonal fragments) |
|Total length (cm)||Max. diam (mm)||Lateral spread (cm)||Max. length (cm)||Max. spread (cm)**||No. of ramets||Plant height (cm)***|
|*Values are means with ranges in parentheses.
**Maximum root spread is the greatest horizontal distance from the junction of a root and rhizome to the farthest root tip.
***Maximum height to stem or leaf.
At the start of the growing season western sword fern rosettes of the previous year are flattened by rain or snow, but new fronds grow fast, reaching about 3 feet (1 m) by mid-May on sites in western Washington . In Oregon, western sword fern fronds were partially unfurled on 26 May, and by 28 July the fronds were mature and spores were nearing maturity (Stewart 1976 as cited by ). Western sword fern maintained relatively constant cover throughout the growing season in western Oregon . Western sword fern spores mature in August , and sporangia dehisce in early to midsummer, releasing thousands of spores . The plant's evergreen fronds persist for several years. Evergreen ferns like western sword fern retain some spores over winter and release them the following spring .
|Figure 5. Early spring fiddleheads. Photo courtesy of Hansen's Northwest Native Plant Database, http://www.nwplants.com.||Figure 6. Western sword fern sori. Photo courtesy of Hansen's Northwest Native Plant Database, http://www.nwplants.com.|
Western sword fern populations consist mostly of mature plants, with small, juvenile plants present in low numbers; gametophytes are extremely rare. Reproduction in western sword fern is mostly by sexual means. High levels of genetic variation in western sword fern suggest that spores disperse away from parent plants (sporophytes) and that gametophytes usually cross with other gametophytes rather than self-fertilize. Vegetative reproduction is limited ; it can regenerate from rhizomes after top-kill but does not spread vegetatively .
Pollination and breeding system: The mating systems of Polystichum species are outcrossing, and western sword fern is highly outcrossing [189,190]. Rates of intragametophytic selfing in western sword fern range from 0% to 3% .
Hybrids are frequent where 2 or more Polystichum species occur. Sterile hybrids are recognized by their misshapen sporangia .
Spore production: Most medium-sized forest ferns such as western sword fern begin to produce spores between 1 and 5 years of age, and spore production takes place very regularly from year to year. Most ferns similar in size to western sword fern produce tens of millions of spores on each frond .
Spore dispersal: Western sword fern spores are primarily wind-dispersed  but are also dispersed by other means [35,74]. The tiny spores are ejected into the air when mature and are carried away by gravity, wind, water, or animals. Fern spores can travel thousands of miles, but in forests there may be a lack of air current to carry them aloft. Airborne spores are often brought down during rainstorms .
Spore banking: No information was available in the literature regarding western sword fern spore persistence under field conditions. Under ideal conditions (e.g., dry storage), fern spores may remain viable for 2 to 4 years, but viability and speed of germination decrease with age. Fern spores in a dry, resting state are resistant to physical extremes. They can withstand intense radiation and very low temperatures but are reportedly very sensitive to temperatures above 131 °F (55 °C) (review by ).
Germination: Fern spores usually germinate only after being soaked in water. Spores of western sword fern germinate best when exposed to light, but some germination occurs in darkness (review by ). In the laboratory, western sword fern spore germination rate was >20% in light and 2% to 10% in the dark .
Gametophyte establishment and plant growth: Very little information was available regarding western sword fern gametophyte establishment (as of 2015). One source says that gametophytes are extremely rare in western sword fern populations . Another source notes that western sword fern and other ferns establish from spores on fresh alluvium following deep sediment deposition in redwood forests on alluvial terraces; the new alluvium is rich in nutrients and is an excellent seedbed .
Vegetative regeneration: Vegetative reproduction in western sword fern is limited ; it can regenerate from rhizomes after top-kill but does not spread vegetatively. Fronds have determinant growth and do not grow back. New fronds emerge from rhizomes early each growing season  and after top-kill or removal. Heavy cropping of western sword fern does not reduce cover for long; new fronds quickly regain cover after old ones have been removed .
Western sword fern rhizomes may become highly branched with age but are not creeping or spreading [74,189]. Most western sword fern plants studied on the Olympic Peninsula consisted of a single rhizome segment supporting a single ramet; 5% of the plants had more than one ramet emerging from a branched rhizome  (see Botanical Description). Although the species often occurs in pure, uniform stands, these stands probably represent populations of individual sporophytes rather than colonies .
Western sword fern is typically present during most stages of stand development . It can establish in primary succession, such as after deglaciation or volcanic eruption, and in early secondary succession, but typically reaches its best growth and greatest abundance in mid- to late-successional forests .
|Figure 7. Western sword fern reflecting lightspots in the forest understory. Photo courtesy of Hansen's Northwest Native Plant Database, http://www.nwplants.com.|
Shade tolerance: Western sword fern has a high tolerance for shade but can also grow in the open [35,95,100]. Western sword fern communities often occur in the understory of closed-canopy forests, but western sword fern plants can persist following canopy-opening disturbances . Some authors have noted that western sword fern may have lower cover in dense shade and grows best in "lightspots" under dense canopies [16,192]. Carey  notes that western sword fern grows rapidly when forest canopies are even moderately open and forms dense carpets that can shade out other plants.
Although it tolerates a wide range of light conditions, western sword fern seems to have optimal growth under relatively low light conditions, and its response to increased light depends a lot on site moisture availability. In the coastal redwood region, western sword fern's optimum light requirement (where it reached its highest cover at about 2%) was less than 3%, but it grew in areas having up to 40% of full sunlight . Many examples in the literature demonstrate that western sword fern persists after canopy-opening disturbances (see Secondary succession); however, its abundance and vigor may depend on site conditions and disturbance severity (i.e., moisture availability, size and pattern of canopy openings, and degree of soil disturbance). In clearcut areas, for example, western sword fern often prefers protected, shaded locations. This probably reflects sensitivity to high evoptranspirative losses rather than intolerance of light itself , and it may be more pronounced in clearcuts on south-facing slopes (e.g., ). When plants of western sword fern are subjected to increased insolation through removal of overstory shade, the fronds can become dwarfed and more erect, and the pinnae become crisped [74,181] (see Botanical Description).
Western sword fern communities commonly occur in the understory of old-growth and late-successional forests, indicating shade tolerance of the species. Stands typical of the western hemlock/western sword fern association in the western Cascade Range, Oregon, have a relatively dense overstory averaging 70% to 80% canopy cover [53,64]. In the western hemlock zone of the central western Cascade Range, western sword fern communities tended to occur on sites with >100% cover of mature trees . In a Douglas-fir/vine maple/western sword fern community at Monument Peak, Oregon, "only the most shade-tolerant species", such as western sword fern or mosses, grew under the dense vine maple canopy . In 4 redwood stands studied at Humboldt Redwoods State Park, California, western sword fern and redwood-sorrel dominated the forest floor on 2 well shaded lowland sites that had only occasional sunlight breaks .
On the southeastern Olympic Peninsula, the western hemlock/salal/western sword fern association occurs in matrix forests among anthropogenically fire-maintained beargrass savannas that have grown into forests with fire exclusion. Former savanna plots have some western sword fern, but western sword fern is less abundant than in the forest matrix , suggesting that it spreads into the savanna slowly, after trees establish.
Primary succession: During primary succession on the foreland of Coleman Glacier on Mt Baker in northwestern Washington, western sword fern was uncommon but was most prevalent about 95 to 180 years after deglaciation. Vegetation in this age class had the highest tree canopy cover and was dominated by Pacific silver fir and western hemlock. Soils were volcanic ash and colluvium over glacial till; under forest canopies, the mineral soil was covered by an organic horizon up to 12 inches (30 cm) thick .
|Table 7. Average cover of western sword fern during primary succession on the foreland of Colman Glacier in northwestern Washington .|
|Approximate time since deglaciation (years)||0-24||70-95||95-180||300|
|Mean western sword fern cover (%)||0||0.32||2.93||0.54|
In a study comparing primary successional uplands to refugia (habitats where plants survived as rootstock) after the 1981 eruption of Mt Saint Helens, plants in the blast zone were inventoried in the summers of 1993 and 1994. Before the eruption vegetation in the area was a patchwork of forested and clearcut areas, typical of the Pacific silver fir zone in Washington. After the eruption, wind-dispersed invasive species dominated. Western sword fern was infrequent to common throughout the area in both primary successional habitat and refugia .
Secondary succession: Western sword fern associations occur in late-successional and old-growth forests with natural disturbance regimes typified by very infrequent large-scale and infrequent small-scale disturbances. The most common contemporary disturbances in many western sword fern habitats are logging and associated slash disposal and site preparation. The impact of these and other disturbances on western sword fern populations depends on light and moisture conditions in the postdisturbance understory environment. Reviews suggest its response to logging is variable, although western sword fern generally persists, and no studies report dramatic postlogging increases or decreases of its abundance or cover. Western sword fern probably maintains or increases its cover and vigor only on microsites without severe soil disturbance or significant summer moisture stress (reviews by [35,74]). In the absence of severe soil disturbance, western sword fern usually persists or reestablishes from surviving rhizomes following overstory mortality at most scales. Exceptions may be found on exposed and/or dry sites where moisture stress is severe  and at the extremes of its range, where sites may already be marginal for western sword fern and become less habitable with soil and canopy disturbance.
The following summary of the literature describes western sword fern's role in secondary succession following canopy opening disturbances and is organized by forest type. It includes several studies where vegetation response was monitored after canopy removal, almost entirely after logging, especially clearcutting, as well as general descriptions of secondary succession, such as those found in vegetation classifications that do not usually specify disturbance type, stating only, "after fire or logging". Specific examples of western sword fern response to fire and examples of successional changes in western sword fern communities following wildfires or prescribed fires other than slash burning are included in the discussion on Fire Effects and Management.
Western hemlock and Douglas-fir forests: Western sword fern is a common dominant in old-growth forests in the western hemlock zone. Prior to European settlement, these forests were mostly late successional and old growth with very large trees, dense canopies, infrequent, small-scale disturbances from windfall, insects, or disease; and less frequent, larger disturbances from landslides, blowdown, fluvial action, and fire. Although changes of ground cover in old-growth stands are relatively slow, they are constantly taking place. Understory and ground layer plants respond to canopy gaps in various ways; western sword fern response depends largely on moisture conditions .
Lush growth of western sword fern may indicate late-successional, old-growth, or relatively undisturbed sites. The regional "climatic climax" for the western hemlock zone in the Oregon Coast Ranges is thought to be the western hemlock/western sword fern association . Stands typical of the western hemlock/western sword fern association in the western Cascade Range, Oregon, are generally made up of old-growth Douglas-fir and western hemlock; western sword fern is the only constant herbaceous species, with an average cover of 25% [53,64]. In the western hemlock zone of the Gifford Pinchot National Forest, Washington, old-growth stands of the western hemlock/western sword fern-redwood-sorrel association have huge, widely-spaced Douglas-firs, western hemlocks, and western redcedars towering over a lush carpet of redwood-sorrel interspersed with western sword fern . Other old-growth types include the western hemlock/western sword fern-threeleaf foamflower association in the northern Cascade Range  and Olympic Peninsula; and the western hemlock/salal-western sword fern association on the Olympic Peninsula [159,166]. The western redcedar-Douglas-fir-western hemlock/western sword fern-spreading woodfern community is a late-successional community that develops in forests with infrequent stand-replacing fires (~150-200 years) and occasional mortality from windthrow, insects, or disease (leading to gap dynamics and multiage structure) in coastal British Columbia . The Oregon white oak/California hazelnut/western sword fern community occurs on the least disturbed sites in the Willamette Valley , and the western redcedar-grand fir/western sword fern community occurs in undisturbed locations on the moistest sites in interior valleys of the San Juan Islands, whereas areas disturbed by logging commonly support red alder communities . See Table 2 for western sword fern cover and site characteristics in remnant old-growth forests in the northern Coast Range of Oregon , and Appendix B for more examples of late-successional western hemlock communities in which western sword fern is a dominant species.
Western sword fern occurs in Douglas-fir forests on Mount St Helens . Following the 1980 eruption, it was associated with scorched forest but not with clearcut or blowdown forest . It occurred in the habitats with the greatest plant cover, usually in organic soil such as the area around a stump base . The maximum tephra depth from which western sword fern was excavated was 3.5 inches (9 cm), 13 to 15 months after the eruption. The authors concluded that western sword fern rarely produces buds in tephra . Most of the area was salvage logged between 1982 and 1984, then planted with noble fir. Western sword fern presence and abundance corresponded with litter depth and downed woody debris, which were greater on unsalvaged plots. In July of 2003 western sword fern cover and frequency averaged 0.05% and 28%, respectively, on salvage logged sites, compared with 0.4% and 80% on unlogged sites .
Postlogging succession: Because much of the western hemlock zone in the Pacific Northwest  and the majority of forests in the coastal Douglas-fir zone in British Columbia  have been logged during the last 2 centuries, much of the information available on succession in these communities comes from studies of plant community changes after logging and site preparation. The remainder of this subsection includes information from such studies as it may pertain to postfire succession. Timber harvesting operations in the western hemlock zone historically involved clearcut logging followed by slash burning under prescribed conditions to reduce fire hazard, prepare the seedbed for planting, and reduce shrub cover . According to Garrison and Smith , western sword fern is among the species in Douglas-fir forests of Oregon and Washington that "suffer the most loss in crown cover" from the accumulation of heavy slash and soil disturbance after clearcutting. Burning of logging waste causes additional changes . Although its abundance may be reduced initially, on sites where moisture is not limiting western sword fern recovers in early succession, and it may be an understory dominant or codominant in seral stands that develop after logging disturbances . For example, Douglas-fir/western sword fern and red alder/western sword fern community types are considered relatively young stands in the western hemlock/western sword fern association in Mt Rainier National Park .
Western sword fern occurs throughout postharvest succession in western hemlock and Douglas-fir forests, but it is generally most abundant in middle to late succession [17,18,46,122,160], with the exception of dense, closed-canopy stands, where its abundance drops off and it most frequently occurs in light spots (see Shade tolerance).
Post-harvest succession and the abundance of western sword fern follow some general patterns in the western hemlock forest zone, but vary depending largely on predisturbance community composition, site characteristics (especially available moisture), and disturbance type, timing, and severity, including postharvest site preparation. The general sequence begins with an herbaceous stage dominated by nonresident annual and perennial herbs, which typically lasts 4 or 5 years; residual herbs, such as western sword fern, may contribute substantial cover during this stage. A shrub stage follows, as residual shrubs develop in canopy openings, and continues until the shrubs are overtopped by tree saplings, after about 10 to 25 years .
On sites with greater available moisture, western sword fern recovers more rapidly [39,52], and red alder is likely to dominate early forest succession [39,62]. In the Oregon Coast Ranges, herbaceous species surviving from the predisturbance stand, especially western sword fern, often constitute an important component of the early-seral vegetation following logging; however, residual species such as western sword fern, broadleaf starflower, and redwood-sorrel are reported to be of only minor importance during the first 10 to 25 years of succession in the relatively drier Cascade Range (review by ). On relatively xeric sites in the HJ Andrews Experimental Forest in the western Cascade Range of Oregon, western sword fern cover was reduced by at least half in the first year  and maintained very low cover for at least 10 years after clearcutting and slashburning. The author suggested that western sword fern "apparently found survival difficult or impossible under clearcut conditions" on the steep, south-facing slopes in the study area . Following the period of shrub dominance, stands may go through a red alder stage on moist, productive sites such as western sword fern habitats . Red alder is among the first trees to establish on moist sites, and pure stands are common in the Oregon and Washington coastal mountains and parts of the Puget Sound region. Red alder is gradually replaced by Douglas-fir and western hemlock but may dominate for up to 90 years .
Western sword fern communities tended to have greater resilience than other community types in the HJ Andrews Experimental Forest. A series of studies used long-term sampling of permanent plots before and after clearcutting and slash burning in 100- to 500-year-old Douglas-fir forest [52,77,78]. Initial understory in the western sword fern community was richer and more structurally complex than in most other communities, and surviving perennial herbs and shrubs were abundant, resulting in relatively rapid community recovery. The resilience of the western sword fern community was attributed to the ability of dominant herb and shrub species to sprout from subterranean structures following slash burning, and western sword fern "regenerated continuously from surviving rhizomes or stem bases" [77,78]. Abundance of understory dominants, including western sword fern, was reduced during the first 2 years after slash burning, when wind-dispersed annuals such as tall annual willowherb and nonnative woodland ragwort dominated. This was followed by increases of initially subordinate (e.g., sweetscented bedstraw) and dominant (e.g., western sword fern) forest herbs. Shrubs began to dominate 14 years after disturbance. Trees did not dominate until after 20 years [52,78]. See the Research Project Summary of these studies for more information.
Western sword fern response varies with timing and severity of disturbance during logging and especially site preparation [52,78]. Techniques that remove the plant from the ground can nearly eliminate the species from the site. Brush bale scarification totally eliminated western sword fern from a study site in Oregon; scarification of several sites near Vancouver left western sword fern only on undisturbed microsites . In a comparison of the effects of chemical, fire, and mechanical site preparation in an area that was repeatedly logged, western sword fern frequency was similar among all but scarified sites, where soil was severely disturbed, and western sword fern was reduced from about 25% to 0% frequency .
Slash burning may alter succession and decrease western sword fern abundance compared to unburned, logged sites (e.g., [16,152,182]). Some authors note that western sword fern was initially reduced or eliminated on slash-burned sites compared to unburned sites [107,152,199]. Although western sword fern can survive severe fire, plants may be restricted to unburned patches and aboveground structures may be absent for several years following slash burning (reviews [35,74]). Western sword fern abundance may remain low and even decline as succession proceeds on slash-burned sites exposed to intense insolation, low humidity, and low soil moisture (e.g., [69,107,116]). If left unburned, residual species such as western sword fern can dominate early succession; if slash is burned, native and nonnative annual herbaceous species that establish from windborne or soil-banked seed, tend to dominate. In areas where mineral soil is exposed by logging activities, successional patterns may resemble those on burned sites .
Sitka spruce forests: Western sword fern is a common dominant in old-growth forests in the Sitka spruce zone of North America (for examples, see Table 2 and Appendix B). Old-growth Sitka spruce associations include the Sitka spruce/western sword fern association [159,166] and the Sitka spruce/western sword fern-redwood-sorrel association  on the Olympic Peninsula, and the Sitka spruce/western sword fern association in coastal British Columbia . Areas typical of the Sitka spruce/western sword fern-redwood-sorrel association on the Olympic Peninsula have burned very seldom in the last 500 years . Western sword fern was present in late-successional forest dominated by western hemlock, Sitka spruce, and western redcedar in Southeast Alaska, but was not mentioned as an important species in second-growth forests .
Early successional trends following fire or logging in the Sitka spruce zone are similar to those in the western hemlock zone, although dense shrub communities are more common in the Sitka spruce zone because of more favorable growing conditions [62,64], and young stands in the Sitka spruce zone are more likely to be dominated by western hemlock and Sitka spruce than by Douglas-fir. Red alder is also more abundant after logging in the Sitka spruce zone due to the preponderance of moist and wet sites [29,62]. Shrub communities can develop quickly after canopy-opening disturbances in these forests [62,64]. Several of these seral communities have been described and mapped along the northern Oregon coast, including the salmonberry-western sword fern, vine maple-western sword fern, and salal-red huckleberry types . In many cases, western sword fern is the major early-seral understory species after clearcutting in the Sitka spruce/western sword fern association on the Siuslaw National Forest; these sites are generally resilient to fire effects (review by ).
Western sword fern is usually present even in young stands of the Sitka spruce/western sword fern-redwood-sorrel association on the Olympic Peninsula, and it is often conspicuous by the time of crown closure (about 15 years). Western sword fern abundance becomes relatively stable by about 100 years . A chronosequence study on the Olympic Peninsula in the Sitka spruce-western hemlock zone showed that western sword fern had high frequency (50%-67%) but low cover (0.7%-6.25%) in all age classes from 0 to >199 years. Percent cover of western sword fern increased steadily with forest age, but differences were not significant among age classes (P>0.05) .
Redwood forests: Western sword fern occurs throughout succession in redwood forests but may be more abundant in second-growth stands. In Redwoods State Park, the redwood-Sitka spruce/evergreen huckleberry-western sword fern and Sitka spruce/western sword fern-salmonberry associations occur in cutover forests . Riparian forests in adjacent watersheds at Redwood National Park were studied to understand structural and compositional differences between old-growth and unmanaged second-growth forests about 40 to 50 years old. Old growth was dominated by redwood, and second growth was dominated by red alder, Douglas-fir, and redwood. Western sword fern, deer fern, and lady fern dominated the understory in both old-growth and second-growth forests. Western sword fern had the highest importance value and the highest relative cover of all understory plants for both old-growth and second-growth stands, 52.3% and 71.2%, respectively . Loya and Jules  identified 4 stages of postlogging development in redwood forests: initiation, closure, mature, and old growth; western sword fern was an indicator of the closure stage . Similarly, western sword fern was associated more with recently harvested sites than sites with longer times since harvest (P<0.05) in riparian redwood forests ranging from 10 to 110 years since harvest. In 0- to 30-year-old forests, overstory canopy cover was about 35%, and in >100-year-old forests, canopy cover was about 70% . Ten years after logging in redwood forests in the Santa Cruz Mountains, western sword fern was commonly found in canopy gaps created by logging .
Red alder forests: Seral stands dominated by red alder, particularly those with western sword fern dominating the herb layer, are most common in wetter parts of the western hemlock zone, (i.e., the northern Washington Cascade Range, the west side of the Olympic Mountains, and in the Coast Ranges) and in the Sitka spruce zone. Understory vegetation in red alder stands is most frequently dominated by salmonberry in the shrub layer and western sword fern in the herb layer . As the short-lived red alder dies, many of these stands may become dominated by a dense cover of salmonberry and western sword fern that can persist and slow establishment of conifers (review by ).
Western sword fern occurs throughout succession in red alder stands but tends to increase with stand age and reach its greatest abundance in middle to late succession. In the Alsea River drainage of the central Coast Ranges in western Oregon, Carlton  studied 44 red alder stands ranging in age from 7 to 87 years, and originating from different types of disturbance: mostly logging, but also fire, slides, debris flows, and unstable ravel soils. Western sword fern was most abundant in middle age classes and still a prominent component of older stands, along with salmonberry and vine maple. Five understory communities were identified in these red alder stands: 1) a western sword fern community was most characteristic of young red alder stands but also occurred under dense, middle-aged stands on moderate to steep slopes with low light levels in the understory; 2) a western sword fern-mixed shrub community occurred under middle-aged red alder stands; 3) a western sword fern-salmonberry community occurred in many middle-aged red alder stands; 4) a salmonberry community occurred on flat terraces or stream bottoms under middle-aged stands where western sword fern was present but much less abundant; and 5) a vine maple community where western sword fern was a codominant under most stands of old, senescing and several middle-aged red alder stands. None of the communities was restricted to only one age class of red alder . Fifteen red alder stands on river-bottom, alluvial soils in the Oregon Coast Ranges, ranging in age from 2 to 64 years, were evaluated between 1 August and 7 September 1969. Succession in the understory proceeded from a grass-herb to shrub-fern understory over a 60- to 70-year period. Western sword fern crown cover and frequency generally increased with stand age and were highest in stands aged 39 to 64 years, with cover ranging from 4% to 37% and frequency ranging from 18% to 92%. In stands younger than 35 years, cover ranged from trace to 4% and frequency from 4% to 22% . On steep terrain in the western hemlock zone of the central Oregon Coast Range, a debris flow deposited material containing western sword fern rhizomes onto the valley floor of a second order stream watershed. Western sword fern reproduced from rhizomes and increased rapidly. Its abundance increased steadily over time, from 4% frequency the first growing season after the deposition to 30% 9 years later .
Floodplains and river terraces: Western sword fern communities often indicate later stages of succession on floodplains and river terraces in the Pacific Northwest. The California bay-Douglas-fir/vine maple/western sword fern community in the Umpqua River valley in the Oregon Coast Ranges is an old-growth association on the highest floodplain terrace . A riparian chronosequence spanning more than 3 centuries along the Queets River, Olympic National Park, showed that the western sword fern/redwood-sorrel plant community occurred on the oldest sites in the study area (60-330 years old), on soils with relatively finer surface textures (38%-57% silt and clay), and on the highest terraces (6.2-8.5 feet (1.9-2.6 m)) above the low flow channel, mostly above the reach of annual floods . In 29 plots on floodplains of 2 rivers on the west coast of Vancouver Island, western sword fern was among the species "typical of rich climax ecosystems". Western sword fern was absent from the pioneer seral stage, averaged 12% cover in the young seral stage, and 23.5% cover in the mature climax stage of floodplain succession. The climax stage was dominated by Sitka spruce with an understory of salmonberry, coastal hedgenettle, threeleaf foamflower, and western sword fern .
On alluvial landforms of the McKenzie River Valley, Oregon, 2 topoedaphic climaxes were identified: the western hemlock/western sword fern-redwood-sorrel association on terraces with fine, sandy loam to silt loam soils derived from silty river alluvium; and the western hemlock/dwarf Oregon-grape-salal association occurring on rockier soils with coarser alluvium or glacial outwash. Three early seral communities (willow, red alder, and black cottonwood) were identified (Table 8). Western sword fern established during the red alder seral stage on relatively fine-textured soils. Dominants in the herb layer of the black cottonwood association include western sword fern and several other herbaceous species common to the grand fir association. Western sword fern reached its highest cover in the Douglas-fir/beaked hazelnut/western sword fern association; however, individuals were chlorotic and thin-leaved compared to those growing in stands of the western hemlock/western sword fern-redwood-sorrel association. The western hemlock/western sword fern-redwood-sorrel association was the most extensive in the area .
|Table 8. Western sword fern cover and frequency in successional communities on alluvial landforms in the McKenzie River Valley, Oregon .|
|Plant community dominants||Site characteristics||Cover (%)||Frequency (%)|
|willow||recent alluvium||no data||no data|
|red alder||Establishes following deposition of 20-30 cm gravelly sand, which increases terrace elevation; red alder is typically replaced within 30 to 50 years||1||40|
|black cottonwood||If little elevation of the floodplain occurs (through either sedimentation or downcutting of the river), black cottonwood replaces red alder||no data||no data|
|grand fir||Replaces red alder in areas with deeper fine sediment deposits, usually within 30 to 70 years||15||100|
|western hemlock/western sword fern-redwood-sorrel||Stands were 200-500 years old; most areas of this association had probably been through one or more fires||22||100|
|Douglas-fir/California hazelnut/western sword fern||Develops on sites that have burned within the last 100 to 200 years||26||100|
|Western hemlock/dwarf Oregon grape-salal||Develops on shallow to moderate cobbly loams to cobbly sandy loams on terraces and glacial outwash plains, at about 450-500 years||no data||no data|
|Douglas-fir/dwarf Oregon-grape-salal||Develops on shallow to moderate cobbly loams to cobbly sandy loams on terraces and glacial outwash plains||1||36|
Fonda  described 5 plant communities along the Hoh River in Olympic National Park, (Table 9). Western sword fern occurred in all of these communities and was most abundant in 400- to 750-year-old communities on 1st and 2nd terraces and rockslides. This study also describes soil characteristics for each community, noting a strong relationship between terrace age, soil moisture, and soil profile development. The younger land surfaces are substantially drier than the older terrace, which may affect western sword fern abundance .
|Table 9. Western sword fern cover and frequency in 5 plant communities representing 5 successional stages along the Hoh River in Olympic National Park |
|Plant community||Location||Stand age (years)||Cover (%)||Frequency (%)|
|red alder-Scouler willow||gravel bars||80-100||1.5||40|
|Sitka spruce-bigleaf maple-black cottonwood||1st terraces||400||22.3||88|
|bigleaf maple||rockslides where 2nd terrace meets valley wall||<750||19.0||100|
|Sitka spruce-western hemlock||2nd terraces||750||22.4||76|
|western hemlock||3rd terraces||>750||2.6||30|
Immediate fire effect on plant: Western sword fern is top-killed by fire , and rhizomes likely survive where soil heating is not severe and soil remains intact. No observations of wildfire effects on western sword fern were found in the current literature (2015), but observations after clearcutting and slash burning in Oregon revealed that western sword fern regenerated "continuously" from surviving rhizomes or stem bases [77,78].
Survival of rhizomes likely depends on fire severity (depth of burn) and postfire site conditions. Although this is not explicitly documented in the current literature (as of 2015), western sword fern rhizomes are less likely to survive where severe fire consumes the organic surface horizons. Only one study describing the burial depth of western sword fern roots and rhizomes was found. Rhizomes ranged in depth from about 0.2 to 1.8 inches (0.5-4.5 cm) in the upper organic horizon , where they would likely be killed by fire that consumes all or part of the organic horizon. Most western sword fern roots occurred in the top 8 to 12 inches (20-30 cm) of soil, and maximum root depth was about 28 inches (72 cm) ; roots are unlikely to be damaged by wildfire at these depths.
Postfire reductions in western sword fern abundance may indicate that fire killed western sword fern individuals. Greater reductions in severely burned areas compared to lightly burned and unburned areas  may also indicate that mortality is related to fire severity. For example, western sword fern was more frequent on unburned than slash-burned plots after clearcutting on sites throughout Washington and Oregon [16,152]. Immediately after logging and slash burning at HJ Andrews Experimental Forest, western sword fern occurred to a limited extent in disturbed areas; some individuals occurred on lightly burned areas, but none occurred on severely burned areas [51,52].
No information was available regarding the effects of fire on western sword fern spores; however, Haeussler and Coates  report that fern spores are generally very sensitive to temperatures above 131 °F (55 °C).
Postfire regeneration strategy (adapted from ):
Surface rhizome and/or a chamaephytic root crown in organic soil or on soil surface
Rhizomatous herb, rhizome in soil
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site spore sources)
Fire adaptations and plant response to fire:
Fire adaptations: Few direct references to western sword fern's fire adaptations were found in the literature (as of 2015); however, many observations of western sword fern sprouting after top-kill suggest that it is adapted to survive fires of low to moderate severity and is likely to increase in abundance during postfire succession under favorable conditions. Under less favorable conditions, such as sites at the extremes of its range or droughty, exposed, south-facing slopes, recovery of western sword fern after fire may proceed more slowly or not at all. High-severity fire may kill western sword fern (see Immediate fire effect on plant).
According to a review by Smith and Fischer , western sword fern sprouts from woody rhizomes and colonizes from off-site spores after fire; the source of this information is not cited. In their study of vegetation succession after wildfire in managed forests, Kayes and others  classified western sword fern as a "sprouting residual species", and a fire "endurer" that dominated "higher severity" sites.
Although western sword fern can survive severe fire, plants may be restricted to unburned patches, and aboveground structures may be absent for several years following slash burning (reviews [35,74]).
Plant response to fire: Information on response of western sword fern to fire was limited. Response depends on fire severity and site conditions, especially soil moisture availability. Severe fires on dry or shallow soils are more likely to eliminate or dramatically reduce sword fern cover and frequency (review by ). Plants may survive and sprout soon after fire (e.g., [60,77,124]), although cover may be reduced or lacking for several years (reviews [35,74,187]). Because western sword fern occurs in plant communities characterized by infrequent wildfire, most information about its response to fire comes from studies on the effects of slash burning following logging. Slash burning effects on western sword fern depend on fire severity, soil moisture , and severity of logging and related soil disturbance (e.g., [35,114]).
The few studies that record observations on response of western sword fern to wildfires suggest that it can sprout soon after fire, but the rate and magnitude of recovery vary among sites. Western sword fern sprouted within the first year after a wildfire in a red alder-black cottonwood-bigleaf maple riparian forest on Whatcom Creek in Bellingham, Washington . After 2 overlapping wildfires (the 1987 Silver Fire and the 2002 Biscuit Fire ) in mixed-evergreen forest in southwestern Oregon, western sword fern had 0.7% cover on unburned stands, was absent from once burned stands, and had 0.4% cover on twice burned stands 2 growing seasons after the 2nd fire . A chronosequence study in western hemlock-Douglas-fir/western sword fern communities in the Olympic Mountains examined different stages of postfire succession on sites burned by wildfires 2 to 515 years prior. Stands had similar aspect, elevation, and slope, and all were over 300 years old (old-growth stage) at the time of fire. Fires were of mixed severity, with extensive overstory mortality. Annuals tended to dominate early postfire years, but they were replaced by perennial herbs including ferns by year 19, when western sword fern reached its peak frequency (Table 10). As the canopy closed, the herb and shrub components, including western sword fern, were less prevalent. Western sword fern was 1 of 2 species observed in all study areas . See Agee and Huff  for a description of fuel characteristics and potential fire behavior at each stage of postfire succession in these stands.
|Table 10. Western sword fern frequency on forest plots of increasing postfire age |
|Fire name||Hoh||Hoh||Queets||North Fork||Mineral Creek||Olympus Guard|
|Stem exclusion||Understory reinitiation||Old
|Time since fire (years)||2||3||19||110||118||515|
|Western sword fern frequency (%)||5||21||37||19||19||10|
In the western hemlock zone on the Willamette National Forest in central western Oregon, fourteen 2-storied stands of remnant trees (>300 years old) over younger trees (65 to 125 years old) that established after wildfire were compared to 65- to 125-year-old stands without remnant trees. Mean species richness and cover of shrubs and herbs did not differ between stands with and without remnant trees. Western sword fern cover was not correlated to remnant tree density, basal area, volume or crown area, but it was negatively correlated with Douglas-fir abundance and positively correlated western hemlock abundance (P≤0.05)). The positive correlation with western hemlock may indicate greater available moisture. Western sword fern tended to be more abundant on plots with more coarse woody debris, which helps retain soil moisture and stand humidity and also helps maintain mycorrhizal associates through succession . Wildfire burned an area of dense conifer forest, the Tillamook Burn area, in 1933, 1939, and again in 1945. Sixteen years after the 3rd fire, western sword fern was frequent in all plant communities but had lower average cover on drier sites (Table 11) .
|Table 11. Western sword fern frequency and cover in 6 plant communities in the Tillamook Burn area in the winter of 1961-1962 (16 years after the last fire) . No data were given from before the fire or from unburned sites.|
|Plant community||Site characteristics||Western sword fern frequency (%)||Western sword fern average cover (%)|
|red alder/western sword fern||Steep, north-facing lower slope positions, <1,200 feet elevation, stony soils with silt loam or loam surface textures; dense, closed canopy||100||64|
|red alder/thimbleberry||Steep, lower and middle, north-facing slopes at 800-1,200 feet elevation; soils typically extremely gravelly||100||40|
|vine maple/western sword fern||Steep lower and middle slopes, all aspects but mostly south and east facing, 700-2,100 feet, high water availability; stony colluvial soils||100||24|
|thimbleberry/broadleaf starflower||1,200 to >1,800 feet, all aspects, slopes generally >60%, predominantly upper slopes and ridgetops; extremely gravelly, shallow, poorly developed soils||100||6|
|red huckleberry/salal||Upper, convex, 40%-60% slopes and exposed ridgetops from 800-1,900 feet||66||1|
|western bracken fern/big deervetch||Upper or middle slope positions, 60% slopes, all aspects, generally <1,200 feet, deeply weathered soils||100||<1|
In unburned alpine krummholz at about 6,200 feet (1,900 m) in North Cascades National Park, Washington, western sword fern had 1% frequency and only trace cover. It was absent from burned krummholz and from both burned and unburned heath in an area burned by wildfire 29 years earlier .
A comparison of logged areas with and without slash burning may provide additional insights into the postfire response of western sword fern, although the effects of wildfires without precedent logging are likely to be quite different with regard to the degree of canopy opening, soil disturbance, and a greater likelihood of postfire establishment of nonnative plants . Studies from the western hemlock forest zone indicate that western sword fern may recover more rapidly after logging when slash is left unburned (see Secondary succession for details of particular studies). On unburned sites, residual species such as western sword fern tend to dominate early succession; if slash is burned, species absent from the preharvest understory, such as nonnative annual herbaceous species that establish from windborne or seed banked seed, tend to dominate (review ). Western sword fern was one of the first plants to sprout after an herbicide and burn operation in a red alder brushfield in the Sitka spruce zone of the Oregon Coast Ranges. Sprouts began appearing within 2 to 3 weeks after the fire. Western sword fern codominated the herbaceous layer before burning. Its prefire frequency was 82% in June, burning was conducted in August, and its postfire frequency was 78% in September and 64% the following August .FUELS AND FIRE REGIMES:
Fire regimes: Western sword fern is a common understory dominant in Douglas-fir, western hemlock, and Sitka spruce forests that have fire regimes characterized by long-interval, stand-replacement and mixed-severity fires. However, it is also a dominant in communities that historically had more frequent surface and mixed-severity fires, such as redwood forests and Oregon white oak woodlands (Appendix B, Appendix C).
Western sword fern is a dominant or indicator species in several associations in coastal forests of the Pacific Northwest. Due to low lightning incidence and high fuel moisture content year-round, conditions favorable for large wildfires develop only briefly every few years during severe drought. Strong, dry, east (foehn) winds occasionally blow in western Washington and Oregon, causing severe fire conditions. Thunderstorms are uncommon but concentrated in some years over mountainous terrain . British Columbia's coastal temperate rainforests have a wet climate and are rarely impacted by stand-replacing fire. This results in a structurally complex, multiage, multicanopy, old-growth forest with large volumes of living and dead wood . Large lightning fires in Olympic National Park are usually associated with low relative humidity, prolonged drought, short-term dry spells before and after ignition, and locally strong winds .
A fire history of the Olympic Mountains indicates that lightning fires were responsible for most of the ignitions prior to 1850, and human-caused fires were likely prevalent during the period of European settlement, 1880 to 1910. Forests on the east side of the Olympics are generally younger than those on the west side. Most of the east side was thought to have burned about 300 years ago, while the west side consists of vast expanses of forest greater than 300 years old. The west side is less prone to burning due to its wetter climate . Western sword fern is a common understory dominant in plant communities on the Olympic National Forest (Table 12), and it is usually present in all ages of stands with average cover often exceeding 20%, although it may be inconspicuous or absent in second growth .
|Table 12. Fire and other disturbances in forested plant associations of the Olympic National Forest where western sword fern is a named understory dominant .|
|Plant association||Fire and other disturbance history|
|western hemlock/dwarf Oregon-grape/western sword fern||The typical area of this type has burned once or twice in the last 500 years, although most old stands of this type have been logged. Most old growth that remains is either about 280 or 450 years old.|
|western hemlock/Pacific rhododendron/western sword fern||The typical area of this type has burned once or twice in the last 300 years. Most areas have been logged or burned, so little old growth remains, and most stands are less than 80 years old.|
|western hemlock/western sword fern-redwood-sorrel||Little of this type has burned in the last 500 years, and most stands are very old. Some younger stands have originated from windstorms or small fires.|
|western hemlock/western sword fern-foamflower||The typical area of this type has burned once or twice in the last 700 years. Western sword fern is usually present in all ages of stands, although it may be inconspicuous or absent in densely stocked second growth.|
|Pacific silver fir/western sword fern||The typical area of this type has burned once or twice in the last 500 years.|
|Pacific silver fir/western sword fern-redwood-sorrel||The typical area of this type has burned very seldom in the last 500 years, and most stands are very old. Some younger stands have originated from windstorms or small fires.|
The following examples of fire regime information from other areas where western sword fern occurs demonstrate the range of fire regime characteristics with which this species may be associated. The western hemlock/western sword fern-redwood-sorrel association was the most extensive successional community on alluvial landforms in the McKenzie River Valley, Oregon. Stands were 200 to 500 years old, and evidence suggested that most areas of this association had been through 1 or more fires. The Douglas-fir/California hazelnut/western sword fern associations occurred on sites that had burned within the last 100 to 200 years . On Yellow Island, San Juan County, Washington, which had not been logged or grazed, western sword fern was a dominant understory species in Douglas-fir forest. The fire rotation for the island was estimated at 83 years; this frequency was probably influenced by humans . Western sword fern is a typical understory dominant in Oregon white oak forests in the Willamette Valley, where wildfires were common prior to European settlement. After European settlement in the mid-1800s, fires became less frequent and Oregon white oak savannas developed into forests . In the Blue Mountains of northeastern Oregon, western sword fern occurs in cool, moist grand fir stands, where long fire-return intervals and moderate-severity fires may be typical. Most of these locations are lower slopes or near riparian areas and may exist as residual stringers that do not burn as often as associated uplands .
At the extremes its range, soil and canopy disturbance may make sites uninhabitable for western sword fern, so it is more common in communities with long intervals between disturbances. For example, in western redcedar-western hemlock forests of northern Idaho, western sword fern had low frequency (1%-2%) on plots in seral shrub communities (2 to 60 years old) regardless of disturbance history (logging with and without pile or broadcast burning), but it occurred in only trace amounts on plots burned multiple times . In northern Idaho, western sword fern seems to be most common in communities with long fire-return intervals. It is an "important forb" in warm, dry to moderate interior Douglas-fir, grand fir, and ponderosa pine habitat types with presettlement fire regimes of frequent, low- or mixed-severity fire and occasional stand-replacing fire. It "occurs widely" in moderate and moist grand fir habitat types with variable presettlement fire regimes ranging from high-severity fires followed by persistent shrub fields, to areas where evidence of historic fire is difficult to find. It is "common" in mature and late-successional stands in moderate and moist western redcedar and western hemlock habitat types, where stand-replacing fires probably occurred at intervals of 200 to 250 years (review by ).
See the Fire Regime Table for additional information on fire regimes of vegetation communities in which western sword fern may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".
FIRE MANAGEMENT CONSIDERATIONS:
Western sword fern is generally resilient to canopy-opening disturbance and fire effects (see Secondary succession and Plant response to fire), and it is present in most stages of succession in the forests where it occurs as an understory dominant. However, it may be reduced or eliminated after severe soil disturbance . Therefore, while it is likely to persist following wild or prescribed fire in most of its main habitats, it may be sensitive to fire suppression activities that disturb the soil. This is especially true at the extremes of its range where it occurs in lower abundance or is more sensitive to environmental extremes and is more likely to be stressed.
Slash burning has variable effects on western sword fern, depending mostly on site conditions, soil disturbance, and fire severity (see Secondary succession). A series of studies on the HJ Andrews Experimental Forest found that recovery of western sword fern declined as disturbance severity increased, whether by burning or mechanical damage, and cover of western sword fern tended to increase gradually with time since disturbance [52,78]. Disturbance severity was evaluated and sites were classified according to degree of soil disturbance: undisturbed; disturbed-unburned (litter removed or mixed with mineral soil, but little or no evidence of fire); lightly burned (surface litter was charred by fire but not completely removed); and heavily burned (surface litter completely consumed) . Immediately after logging and slash burning, western sword fern was most abundant on undisturbed sites and occurred to a limited extent in disturbed areas. Some plants occurred on lightly burned areas, but none on severely burned areas [51,52]. On undisturbed sites, fireweed dominated early succession, followed by dramatic but temporary release of the residual trailing subshrub common whipplea, which was followed by recovery of the initially dominant western sword fern. In contrast, heavily burned sites were dominated initially by annuals such as nonnative woodland ragwort, tall annual willowherb, and Siberian springbeauty; secondarily by biennials and small-statured perennials; and subsequently by tall woody perennials such as snowbrush ceanothus, redstem ceanothus, and thimbleberry. Western sword fern was not abundant on these sites . See the Research Project Summary of these studies for additional details. For detailed planning and operational considerations for slash burning, including determining site and/or community sensitivity and advantages and disadvantages to burning in different seasons, see Hawkes and others .
On the Clearwater National Forest, northern Idaho, western sword fern is among several plant species that have been identified as indicators of sites with high risk of mass movement because they are also indicators of high levels of soil moisture. Mass movement is more likely to occur on these sites following canopy-opening disturbances such as fire (review by ).Western sword fern is an important plant in mountain beaver habitat, and postfire changes in plant community composition may have negative impacts on mountain beavers in some areas. In October 1995, a wildland fire burned about 12,400 acres (5,000 ha) on the Point Reyes Peninsula, California. Postfire surveys (2 to 6 months after the fire) in coastal sage scrub indicated that mountain beaver burrow openings were fully exposed, and only 0.4% to 1.7% of mountain beavers within the burned area survived the fire and immediate postfire period. The fire initially reduced vegetation to a few charred skeletons of the larger shrubs and the charred bases of western sword ferns, and in some areas vegetation changed from dense western sword fern before the fire to stands of thimbleberry and blackberry. Even where mountain beavers survived the fire, little or no recovery of mountain beaver populations was detected 5 years later .
|Figure 8. Mountain beaver snacking on western sword fern near Seattle. Photo courtesy of https://shawncita.wordpress.com/.|
FEDERAL LEGAL STATUS:
Information on state- and province-level protection status of plants in the United States and Canada is available at NatureServe.
IMPORTANCE TO WILDLIFE AND LIVESTOCK:
Several mammals eat western sword fern, including elk, black-tailed deer, mountain beavers, American black bears (review by ), and mountain goats . Western sword fern communities also provide habitat for several bird species (e.g., [159,180,220], Table 13). Western sword fern was apparently not eaten by domestic sheep in a Douglas-fir plantation in the Oregon Coast Ranges . However, another study in coastal Douglas-fir plantations found that domestic sheep consumed a lot of ferns, although it was unclear how much of their forage consisted of western sword fern .
A review suggests that American black bears sometimes forage on western sword fern , and a study from Olympic National Park identified western sword fern in mountain goat fecal pellets collected in late winter or spring . However, most information regarding wildlife use of western sword fern concerns elk, black-tailed deer, and mountain beaver.
Western sword fern is an important elk food in Douglas-fir forests  and other habitats in Oregon and Washington [87,97,102,109,148]. Jenkins and Starkey  provide a summary of Roosevelt elk use of western sword fern throughout the Pacific Northwest. The elk consumed an abundance of ferns, including western sword fern, during winter and spring . On the Olympic Peninsula, western sword fern is seasonally important for elk [97,102]. Western sword fern was one of the 10 most frequently used elk foods in the southern Oregon Coast Ranges, and the western sword fern-redwood-sorrel habitat type provided much of the food for elk in both clearcuts and undisturbed forests .
Western sword fern is a moderately important food for black-tailed deer year-round [38,74]. Western sword fern made up 13% of the annual diet of black-tailed deer at one site (review ), and was found in 27 of the 178 stomach samples from black-tailed deer in western Washington . Western sword fern was among the most common species browsed by mule deer in the western hemlock/salal/western sword fern and the Pacific silver fir/western sword fern-redwood-sorrel associations on the Olympic National Forest. Deer sign was frequently observed, and recent activity was noted in late summer . Klein  observed heavy spring use of western sword fern by mule deer on Coronation Island in southeast Alaska, despite its apparent low palatability.
Western sword fern is an important plant in mountain beaver habitat from British Columbia to California, providing both food and cover [55,74,81,94,137,176]. Mountain beavers as young as 7 weeks old eat western sword fern . Mountain beaver may be abundant in western sword fern sites in western Oregon , where western sword fern is an important winter food . Western sword fern is a favorite mountain beaver food on the Siuslaw National Forest, and mountain beaver populations may be especially large in the moist stands of the western hemlock/Pacific rhododendron/western sword fern association .
Western sword fern communities provide habitat for several species of birds. In the Sitka spruce and western hemlock zones on the Olympic Peninsula and in the North Cascade Range, western sword fern is an understory dominant in old-growth forests providing northern spotted owl habitat . In western Washington, it is among the dominant understory species in second-growth Douglas-fir forests (after logging and fire) that serve as drumming sites for Pacific ruffed grouse. The 2 dominant species of ground vegetation at drumming sites are western sword fern and salmonberry (the abundance of one correlates with the scarcity of the other). Ruffed grouse select drumming logs that are higher than surrounding growth of western sword fern . A census in coniferous forests of the central Oregon Cascade Range showed that several species of birds used the 3 habitats shown in Table 13, which all contain substantial western sword fern cover.
|Table 13. Population density (individuals/km²) of breeding bird species recorded in a June 1972 census in the central Oregon Cascade Range |
Plant community (western sword fern cover)
|Douglas-fir/creambush oceanspray (8%)||western hemlock/Pacific rhododendron/dwarf Oregon-grape (3%)||western hemlock/vine maple/western sword fern-redwood-sorrel (22%)|
Palatability and nutritional value: Information on palatability and nutritional value of western sword fern was very limited in the available literature (2015). One source rated western sword fern as moderately palatable to big game  and another characterized it as having low palatability to slugs .
Klein  provides information on nutritional content of western sword fern collected on 2 dates from Coronation Island, Alaska. Western sword fern had the lowest protein content among deer forage species analyzed . In samples collected between 15 January and 15 February in the White River drainage of western Washington, it averaged 29% in vitro dry-matter digestibility and 11% crude protein .
Palatability and nutritional value of western sword fern may differ between logged and unlogged forests. On southern Vancouver Island, western sword fern growing in a recently logged area was described as "still struggling for survival in the newly logged and burned area" and was seldom eaten until winter (when preferred browse was less available). Conversely, western sword fern growing on an older burn site was preferentially selected and eaten by black-tailed deer in summer . Happe and others  provide information on nutrient content and astringency in current annual growth of western sword fern and other browse species collected in old-growth forest and 5- to 15-year-old clearcuts on the Olympic Peninsula during summer, fall, winter, and spring.
Cover value: Western sword fern habitats may provide cover for large and small animals. Mule deer and elk use of the western hemlock/vine maple/western sword fern association on the Willamette National Forest is moderately high, mostly for thermal cover. The dense shrub layer also provides good hiding cover . In the western hemlock zone of the Gifford Pinchot National Forest, the western hemlock/devil's-club/western sword fern association provides exceptional hiding cover for big game . On one plot in the western hemlock/western sword fern-foamflower association on the Olympic National Forest, a newborn elk was observed, indicating that the site was used as a calving area .
Western sword fern was used as bedding material in winter dens by American black bears in coastal British Columbia. One American black bear habitat was dominated by large Douglas-fir and western redcedar with an understory characterized by western sword fern, foamflower, sweet after death, and lady fern .
Mountain beaver and vagrant shrews may prefer western sword fern habitats. Western sword fern provides both food and cover for mountain beaver from British Columbia to California (see Importance to Wildlife). The western hemlock/vine maple/western sword fern association on the Siuslaw National Forest is prime mountain beaver habitat. Mountain beaver were present in over 40% of the plots in that type . In 40-year-old western redcedar and western hemlock stands in British Columbia, the vagrant shrew favored mesic habitats under western redcedar, where western sword fern provided understory cover (review by ).
VALUE FOR REHABILITATION OF DISTURBED SITES:
Western sword fern was recommended as a good ground cover to control erosion for conservation planting in the Northeast . Vegetative reproduction of western sword fern can occur through rhizome division .
Commercial harvesting of floral greens, including western sword fern , began in 1915 and remains one of the most enduring and stable year-round nontimber forest products industries in the Pacific Northwest [54,211]. Enormous quantities of western sword fern leaves are gathered for backgrounds in floral displays; the evergreen leaves keep well in cold storage and are even exported to Europe . Western sword fern requires partial shade to grow in forms acceptable for commercial harvest (with deep green, broad, spreading leaves). See Table 5 for information on which forest zones have the greatest potential for western sword fern production .
Western sword fern is used extensively for landscaping .
American Indians used western sword fern for a variety of purposes, including household tasks and applications, food, and medicine. Northwest coastal peoples used western sword fern leaves to line pits for cooking [54,81,170,210], to layer between food in baskets, drying racks, and storage boxes [170,210,221], and to cover floors and beds [81,170,210]. Western sword fern leaves were used in a traditional game known as pala-pala . The species also played a part in Kwakiut'l mythology and so was used in rituals . The large, basal clump of leaves and the rhizomes of western sword fern were used as food by several tribes. They were roasted or steamed, peeled, and eaten [81,170,210,221]
Traditional medicinal uses of western sword fern include rhizomes eaten to cure diarrhea , young leaves chewed and swallowed for sore throat or tonsillitis, an infusion of boiled rhizome used on sores and to ease pain, and a tea from boiled stems used in labor . Aerial parts of western sword fern are used to stimulate digestion in ruminants .
OTHER MANAGEMENT CONSIDERATIONS:
Western sword fern is among several forest products in the Pacific Northwest that may require special management considerations to be sustainable. For example, repeated harvesting of western sword fern fronds reduces frond length and plant survival considerably (review, ).
The presence of seeps or super-saturated soils typical in western sword fern habitats indicates that these areas are susceptible to soil compaction or erosion .
Because western sword fern may interfere with post-logging regeneration, it has been the object of herbicide applications. Information on studies pertaining to that topic can be found in these references: [35,74,149,195].
Climate change: Fog frequency has declined over the past 50+ years along the Pacific Coast of the western United States (Johnstone 2008, as cited by ), which has reduced the frequency of summertime leaf-wetting events. Considering the demonstrated importance of fog water for redwood forest plants, including western sword fern (see Climate in the Site Characteristics section), it seems likely that levels of plant water stress will increase in coastal communities as this important water subsidy is lost .
SPECIES: Polystichum munitum
|Appendix A. Scientific and common names of plant species associated with western sword fern and/or mentioned in this review|
|Common name||Scientific name|
|bigleaf maple||Acer macrophyllum|
|black cottonwood||Populus balsamifera subsp. trichocarpa|
|California bay||Umbellularia californica|
|canyon live oak||Quercus chrysolepis|
|coast Douglas-fir||Pseudotsuga menziesii var. menziesii|
|Coulter pine||Pinus coulteri|
|grand fir||Abies grandis|
|interior Douglas-fir||Pseudotsuga menziesii var.glauca|
|mountain hemlock||Tsuga mertensiana|
|noble fir||Abies procera|
|Oregon white oak||Quercus garryana|
|Pacific dogwood||Cornus nuttallii|
|Pacific ponderosa pine||Pinus ponderosa var. ponderosa|
|Pacific silver fir||Abies amabilis|
|red alder||Alnus rubra|
|Rocky Mountain juniper||Juniperus scopulorum|
|Sitka spruce||Picea sitchensis|
|subalpine fir||Abies lasiocarpa|
|sugar pine||Pinus lambertiana|
|sweet cherry||Prunus avium|
|western hemlock||Tsuga heterophylla|
|western redcedar||Thuja plicata|
|Alaska blueberry||Vaccinium alaskensis|
|baldhip rose||Rosa gymnocarpa|
|big huckleberry||Vaccinium membranaceum|
|California hazelnut||Corylus cornuta subsp. californica|
|common snowberry||Symphoricarpos albus|
|common whipplea||Whipplea modesta|
|creambush oceanspray||Holodiscus discolor|
|dwarf Oregon-grape||Berberis nervosa|
|evergreen huckleberry||Vaccinium ovatum|
|Oregon boxwood||Paxistima myrsinites|
|ovalleaf huckleberry||Vaccinium ovalifolium|
|Pacific poison-oak||Toxicodendron diversilobum|
|Pacific rhododendron||Rhododendron macrophyllum|
|red huckleberry||Vaccinium parvifolium|
|red-osier dogwood||Cornus sericea|
|redstem ceanothus||Ceanothus sanguineus|
|Saskatoon serviceberry||Amelanchier alnifolia|
|snowbrush ceanothus||Ceanothus velutinus|
|vine maple||Acer circinatum|
|American skunkcabbage||Lysichiton americanus|
|big deervetch||Lotus crassifolius|
|broadleaf starflower||Trientalis borealis subsp. latifolia|
|California manroot||Marah fabaceus|
|coastal hedgenettle||Stachys chamissonis var. cooleyae|
|garden vetch||Vicia sativa|
|northwestern twayblade||Listera caurina|
|oneleaf foamflower||Tiarella trifoliata var. unifoliata|
|Pacific trillium||Trillium ovatum subsp. ovatum|
|Siberian springbeauty||Claytonia sibirica var. sibirica|
|sweet after death||Achlys triphylla|
|sweetscented bedstraw||Galium triflorum|
|tall annual willowherb||Epilobium brachycarpum|
|threeleaf foamflower||Tiarella trifoliata|
|white insideout flower||Vancouveria hexandra|
|wild ginger||Asarum caudatum|
|woodland ragwort*||Senecio sylvaticus|
|youth on age||Tolmiea menziesii|
|Anderson's hollyfern||Polystichum andersonii|
|Braun's hollyfern||P. braunii|
|California swordfern||P. californicum|
|deer fern||Blechnum spicant|
|Dudley's swordfern||P. dudleyi|
|Kruckeberg's hollyfern||P. kruckebergii|
|lady fern||Athyrium filix-femina|
|Lemmon's hollyfern||P. lemmonii|
|mountain hollyfern||P. scopulinum|
|narrowleaf sword fern||P. imbricans|
|northern maidenhair||Adiantum pedatum|
|oak fern||Gymnocarpium dryopteris|
|spreading woodfern||Dryopteris expansa|
|western bracken fern||Pteridium aquilinum|
|slough sedge||Carex obnupta|
|feather moss||Hylocomium spp.|
|goose neck moss||Rhytidiadelphus loreus|
|Appendix B. Western sword fern associations described in vegetation classifications from areas where it is a community dominant. Associations are grouped by corresponding Potential Natural Vegetation Groups (PNVGs) and Biophysical Settings (BpSs).|
|Douglas-fir-western hemlock (wet mesic) PNVG
BpSs 0111780, 0211780, 0711780, 0110390, 0210390, 0310390, 0710390, 0110420, 0210420, 0710420
|Vegetation||Geographic area(s) (vegetation classification)|
|western hemlock/western sword fern association||Cascade Range, Oregon|
|Oregon Coast Ranges |
|southwestern Oregon |
|Mt Rainier National Park |
|Mt Hood National Forest |
|Gifford Pinchot National Forest |
|central portion of the western Cascade Range, Oregon |
|Siuslaw National Forest, Oregon |
|Willamette National Forest |
|western hemlock/western sword fern-redwood-sorrel association||Cascade Range, Oregon |
|Coast Ranges |
|Olympic National Forest |
|Gifford Pinchot National Forest |
|Mt Hood National Forest |
|central portion of the western Cascade Range in Oregon |
|western hemlock/vine maple/western sword fern association||Watershed 10, HJ Andrews EF |
|central portion of the western Cascade Range, Oregon |
|Siuslaw National Forest, Oregon |
|western hemlock/vine maple/western sword fern-redwood-sorrel association||western Cascade Range of Oregon and Washington |
|western hemlock/western sword fern-threeleaf foamflower association||Olympic National Forest |
|western hemlock/devil's club/western sword fern association||Gifford Pinchot National Forest |
|Mt Rainier National Park |
|western hemlock/dwarf Oregon-grape/western sword fern association||Mt Hood National Forest |
|Gifford Pinchot National Forest |
|Olympic National Forest |
|western hemlock/salal/western sword fern association||southwestern Oregon |
|western hemlock/evergreen huckleberry/western sword fern association||southwestern Oregon |
|western hemlock/ovalleaf huckleberry/western sword fern association||north coastal Oregon |
|western hemlock/western sword fern-northern maidenhair association|
|western redcedar/dwarf Oregon-grape/western sword fern association||southwestern Oregon |
|western redcedar/western sword fern||coastal British Columbia |
|western redcedar-western hemlock/western sword fern|
|western redcedar-Douglas-fir-western hemlock/western sword fern-spreading woodfern community||coastal British Columbia |
|western redcedar-grand fir/western sword fern community||interior valleys of the San Juan Islands |
|Sucia Island in Puget Sound |
|Port-Orford-cedar/western sword fern community||northern part of Port-Orford-cedar's range |
|Port-Orford-cedar-western hemlock/western sword fern||southwestern Oregon |
|western hemlock-Port-Orford-cedar/western sword fern-redwood-sorrel community||Oregon and northern California |
|Douglas-fir-western hemlock (dry mesic) PNVG
BpSs 0110350, 0210350, 0710350, 0111740, 0211740, 0711740, 0110370, 0210370, 0710370
|western hemlock/salal/western sword fern association||Olympic National Forest |
|southeastern Olympic Peninsula |
|western hemlock/Pacific rhododendron/western sword fern association||Siuslaw National Forest, Oregon |
|Olympic National Forest |
|coast Douglas-fir-western sword fern alliance||coastal British Columbia |
|coast Douglas-fir/western sword fern-feather moss association||southwestern British Columbia |
|coast Douglas-fir/western sword fern-foamflower association|
|Douglas-fir/salmonberry/western sword fern association||southwestern Oregon |
|Douglas-fir/dwarf Oregon-grape/western sword fern|
|western redcedar-grand fir-Douglas-fir/threeleaf foamflower-western sword fern||coastal British Columbia (Krajina 1969 as cited by )|
|western redcedar-grand fir-Douglas-fir/common snowberry/threeleaf foamflower-western sword fern|
|western redcedar-Douglas-fir/western sword fern-sweet after death|
|Douglas-fir/western sword fern||Cedar River Drainage in the Cascade Range of western Washington |
|Douglas-fir/western sword fern-twinflower|
|red alder/western sword fern|
|Douglas-fir/Pacific poison-oak/western sword fern association||interior valleys of the Umpqua River Basin, Oregon |
|western redcedar-Douglas-fir-Oregon white oak/common snowberry/western sword fern||coastal British Columbia (Krajina 1969 as cited by )|
|Sitka spruce-western hemlock PNVG
BpSs 0110360, 0210360, 0310360
|Sitka spruce-western sword fern association||Siuslaw National Forest |
|coastal British Columbia |
|Sitka spruce/western sword fern-redwood-sorrel association||Olympic National Forest |
|Sitka spruce-western hemlock/western sword fern association||coastal British Columbia |
|western redcedar-Sitka spruce/western sword fern association|
|western redcedar-Sitka spruce-Pacific silver fir-Douglas-fir/western sword fern||coastal British Columbia (Krajina 1969 as cited by )|
|western sword fern-thimbleberry||Oregon coastal prairie |
|Pacific silver fir (low elevation) PNVG
BpSs 0110420, 0210420, 0710420, 0111740, 0211740, 0711740
|Pacific silver fir/western sword fern association||Olympic National Forest |
|Pacific silver fir/western sword fern-redwood-sorrel association|
|Coast redwood PNVG
BpSs 0210150, 0310150, 0410150
|redwood/western sword fern alliance (Becking 1967 as cited by )||northern part of redwood's range|
|redwood/western sword fern association|
|redwood-western hemlock/western sword fern association [20,142]|
|redwood-western hemlock/salmonberry/western sword fern association|
|redwood-western redcedar/western sword fern association|
|redwood-Douglas-fir/western sword fern association|
|redwood-grand fir/salal/western sword fern association|
|redwood-tanoak/western sword fern association (Jimerson and Jones 2000 as cited by )|
|redwood-western hemlock-evergreen huckleberry-western sword fern coastal forest||Siskiyou Mountains, Oregon and California |
|redwood-Douglas-fir/tanoak-Pacific rhododendron/evergreen huckleberry-western sword fern association||Redwoods State Park (Mendonca, unpublished data, as cited by )|
|redwood-western hemlock-Sitka spruce/western sword fern association|
|redwood/western sword fern-evergreen huckleberry-redwood-sorrel association|
|red alder-Sitka spruce/salmonberry-western sword fern association|
|redwood/western sword fern-Pacific trillium ecological type||Southern Monterey County, California |
|redwood/California manroot-garden vetch ecological type|
|Oregon coastal tanoak PNVG
BpSs 0210430, 0310430
|tanoak-bigleaf maple-canyon live oak/western sword fern||southwestern Oregon |
|tanoak-western hemlock/evergreen huckleberry/western sword fern|
|Douglas-fir/tanoak/western sword fern-herb community type||Illinois River area, Oregon |
|California bay-Douglas-fir/vine maple/western sword fern community||Umqua River valley in the Oregon Coast Ranges |
|Oregon white oak and Oregon white oak-ponderosa pine PNVGs
BpSs 0110600, 0210600, 0710600, 0810600, 0210290, 0710290, 0110080, 0210080, 0710080, 0111200, 0211200, 0711200
|Oregon white oak/California hazelnut/western swordfern association||Willamette Valley, Oregon [194,203]|
|beaked hazelnut/western sword fern group||northwestern Oregon |
|salmonberry/youth on age-redwood-sorrel-western sword fern phase|
|salmonberry/western sword fern association|
|Fire regime information on vegetation communities in which western sword fern may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models , which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Pacific Northwest Grassland|
|Alpine and subalpine meadows and grasslands
|Pacific Northwest Woodland|
|Oregon white oak||Replacement||3%||275|
|Surface or low||78%||12.5|
|Oregon white oak-ponderosa pine||Replacement||16%||125||100||300|
|Surface or low||81%||25||5||30|
|Pacific Northwest Forested|
|California mixed evergreen (northern California and southern Oregon)||Replacement||6%||150||100||200|
|Surface or low||64%||15||5||30|
|Douglas-fir (Willamette Valley foothills)||Replacement||18%||150||100||400|
|Surface or low||53%||50||20||80|
|Douglas-fir-western hemlock (dry mesic)||Replacement||25%||300||250||500|
|Douglas-fir-western hemlock (wet mesic)||Replacement||71%||400|
|Lodgepole pine (pumice soils)||Replacement||78%||125||65||200|
|Mixed conifer (southwestern Oregon)||Replacement||4%||400|
|Surface or low||67%||22|
|Oregon coastal tanoak||Replacement||10%||250|
|Pacific silver fir (low elevation)||Replacement||46%||350||100||800|
|Pacific silver fir (high elevation)||Replacement||69%||500|
|Sitka spruce-western hemlock||Replacement||100%||700||300||>1,000|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Coastal sage scrub||Replacement||100%||50||20||150|
|Coastal sage scrub-coastal prairie||Replacement||8%||40||8||900|
|Surface or low||62%||5||1||6|
|California oak woodlands||Replacement||8%||120|
|Surface or low||91%||10|
|California mixed evergreen||Replacement||10%||140||65||700|
|Surface or low||32%||45||7|
|Surface or low||98%||20|
|Mixed conifer (north slopes)||Replacement||5%||250|
|Surface or low||88%||15||10||40|
|Mixed conifer (south slopes)||Replacement||4%||200|
|Surface or low||80%||10|
|Mixed evergreen-bigcone Douglas-fir (southern coastal)||Replacement||29%||250|
|Northern and Central Rockies|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Northern and Central Rockies Forested|
|Douglas-fir (warm mesic interior)||Replacement||28%||170||80||400|
|Mixed-conifer upland western redcedar-western hemlock||Replacement||67%||225||150||300|
Replacement: Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Mixed: Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects.
Surface or low: Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area [83,127].
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