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Willows may produce dense thickets, making them valuable cover and nesting habitat for birds and small mammals and providing shade for fish in streams and ponds [8,89]. Willows are also valuable for cavity nesting woodpeckers [89]. Scouler's willow provides both visual and thermal cover for deer and elk, and nesting habitat, cover, and food for nongame birds, upland game birds, and small mammals [40,41,95,103,107,140,256]. In riparian areas, it provides shade, cooling stream temperatures and providing escape cover for fish [79].
The degree to which Scouler's willow provides cover for wildlife species is as follows [52,130]:
ID UT WY Pronghorn ---- poor poor Elk good fair fair Mule deer ---- good good White-tailed deer ---- ---- good Small mammal ---- good good Small nongame birds ---- good good Upland game birds ---- good good Waterfowl ---- ---- fairScouler's willow is generally a shrub, reaching 6 to 35 feet (2-10.6 m) in height [8,41,72,109,178,179,204]. It occasionally grows as a tree and may reach 65 feet (20 m) in height [8,32,34,53,76,171,178]. It is nonrhizomatous [218] with a deep, extensive root system [23,24,213,234] and may be multistemmed [23,24,79,204] or have one main trunk with twigs spreading or ascending [53]. It occurs in thickets and forests [202] forming a tall shrub layer in young stands [7,40], but is intolerant of shade and can persist only under thin canopies [7]. Beneath a tree canopy, Scouler's willow exhibits a tall, upright growth form [72,218,234], but if top-killed by disturbance it sprouts from the root crown creating a round growth form up to 16 feet (4.8 m) in diameter [234].
Scouler's willow has slender stems and branches [72] reaching 4 to 8 inches (10-20 cm) in diameter at the base [9,33,80]; smooth to flaky bark; and glabrous twigs [72]. Leaves are glabrous to leathery, obovate to oblanceolate, and occasionally serrate [9,54,72,109,121,194]. Twigs and leaf undersides of Scouler's willow are densely to thinly pubescent, with appressed, reddish hairs [8,10,33,72,121]. Young twigs and vigorous shoots are often densely pubescent, while older stems are smooth [9,121,194]. Stripped bark of Scouler's willow has a skunky odor [33,54,121].
A dioecious plant, Scouler's willow has large, single-scaled, floral winter buds [10,32,89], and lacks a terminal bud [89]. Aments, expanding before or with leaves and quickly deciduous [32,121], are usually sessile or borne on a short spur shoot and flower profusely [9,10]. Fruiting catkins are 0.8 to 2.4 inches long (2-6 cm) by 0.4 to 0.6 inches thick (1-1.5 cm) with dark floral bracts 4-5 mm long, one gland, and a capsule 5-8 mm long with dense, short hairs and a somewhat long beak [32,33,54,121,194].
Willows are greatly favored by fire in most habitats [89,148,253]. As a survivor and off-site colonizer [117,237,241,242,243], Scouler's willow is abundant following fire [146] and has a moderate regeneration period [121]. It is adapted to fire by rapidly resprouting from the root crown [145,167,179,216], and establishes from seed on severely burned sites [179]. Wind dispersed seeds facilitate rapid recolonization of burned areas [216,217]. In a north-west Montana study Scouler's willow was found on 80% of burned sites with no previous Scouler's willow presence [237]. Stand replacing fires favor regeneration of Scouler's willow [167], and good response from Scouler's willow seedlings can be expected on sites where fire damage is thorough enough to expose mineral soil [87]. However, it is rarely present on sites where more than 50% of the prefire overstory remains [70].
Scouler's willow layer groups are distinct shrub layers that occur in various habitat types and are created by stand replacing fires [218,227,228,229,231]. Severe wildfires expose patches of bare mineral soil, encouraging the development of Scouler's willow shrub layers [229,231]. These layer groups may also develop in response to mechanical scarification in clearcuts and broadcast burns, especially where exposed soil was mounded to trap water behind the mounds, creating well-watered seedbeds of mineral soil [229,231,233].
Scouler's willow is frequently a dominant or codominant in the persistent seral brushfields of northern Idaho. These brushfields are likely the result of dry weather patterns after canopy removal and repeated severe fires, which remove most large woody material, litter, and herbaceous fuels. Standing shrubs comprise most of the biomass, and these brushfields can burn in almost any season. If surface fuels are continuous and dry, spring fires spread readily. In the summer, brushfields are often hot and dry, and conditions are exacerbated where nighttime inversions occur. Hot, dry winds during drought conditions can drive severe fires through the shrub layer, with Scouler's willow readily regenerating from seed and sprouting [222].
In interior Alaska, Foote [69] identified six community developmental stages following fire in black spruce forests. These are: 1) newly burned, 2) moss-herb, 3) tall shrub-sapling, 4) dense tree, 5) hardwood or mixed hardwood-spruce, and 6) black spruce. Arising from sprouts, Scouler's willow can average up to a few thousand stems per hectare 1 year following wildfire depending on preburn density, and thus is an important part of the newly burned stage. It is then often dominant or co-dominant through the tall shrub-sapling stage of succession for about 30 years. It thereafter becomes less frequent, as larger trees outgrow and overtop it.
Sprouting occurs in response to overstory thinning [217] where Scouler's willow is well-distributed and in need of rejuvenation [87]. Generally, fast spreading fires produce more willow sprouts than slow fires that can damage root crowns [222]. Density and canopy coverage frequently increase after fire because root crowns produce multiple sprouts [179]. Four years postfire in Alaska, Scouler's willow presence was 4 times greater on burned sites than on adjacent unburned sites [253]. In northern Idaho, Scouler's willow cover was much higher on burned clearcuts than on unburned clearcuts, particularly where there had been repeated fires over a 30 year period [172]. Postfire immature stands (<90 years) in Montana have significantly more (p<0.05) presence and percent cover of Scouler's willow than old growth stands (>150 years) [5]. The increased presence of Scouler's willow in Douglas-fir/ponderosa pine stands following elimination of frequent fires is likely the result of stand opening associated with logging [19]. Sprouting Scouler's willow creates a round growth form up to 16 feet in diameter and may as a result promote reestablishment of shade tolerant species like Douglas-fir, in turn posing a greater risk of stand replacing fires and favoring growth of Scouler's willow [234].
Without fire, closing conifer canopies lead to the deterioration of Scouler's willow [84]. In dense second growth stands of sequoia in California, Scouler's willow debris creates a fuel hazard; formerly abundant stands of Scouler's willow grew in dense clones that were shaded out and killed, forming dense tangles of fuel for wildfire [28].
FIRE REGIMES:
Fire reutrn intervals for plant communities and ecosystems in which Scouler's willow occurs are summarized below. 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".
Prescribed fire is widely used as a wildlife management tool to rejuvenate decadent willow stands and stimulate sprouting. In areas where Scouler's willow is scattered through the vegetation and in low vigor due to overbrowsing, prescribed fires that kill aboveground plant parts and expose mineral soils are favorable. This allows existing shrubs to sprout and creates favorable seedbeds for establishment of Scouler's willow. In Douglas-fir/pinegrass habitat types of Montana, burning during late summer or fall exposes 30 to 50% of mineral soil [87]. A quick, hot fire maximizes sprouting while slower, longer burns cause more extensive damage and reduce browse [89]. The deep root system and multistemmed growth of Scouler's willow allows for higher tolerance to disturbance [19], and it establishes rapidly in clearcut and heavily burned areas [61,80]. Scouler's willow is favored by conditions on burned areas; it is scarce on areas not subjected to fire but very abundant on broadcast burns [171]. However, broadcast burns do not always burn hot enough to duplicate the effects of severe wildfire and create an adequate seedbed for Scouler's willow, which is favored by light soil scarification [150,218,226,227,228,229]. Competition may limit Scouler's willow establishment; the frequency and percent cover of Scouler's willow were significantly less (p<0.05 and p<0.01 respectively) on a burned, artificially seeded site than on a burned, unseeded site [208].
On sites in northern Idaho, Scouler's willow had substantially higher cover on a 30-year-old burn than after any intensive silvicultural treatment (ranging from thinning to clearcut), with no presence in the control [104]. However, after 30 to 40 years of fire exclusion in ponderosa pine forests, Scouler's willow presence increased [22]. Logging and fire suppression allowed vigorous development of Scouler's willow in a Douglas-fir/ponderosa pine community [87].
Prescribed fire enhances vigorous regrowth and germination of Scouler's willow, and it is effective in increasing biomass [24]. In the cedar-hemlock zone of Idaho, it generally produces the most cover of any shrub species on broadcast burned areas. Cover and frequency of Scouler's willow is substantially higher on broadcast burns than on areas not broadcast burned, as are the mean height values [171]. Total shrub volume of Scouler's willow in Douglas-fir stands increased from 15 to 80% 2 years postfire [139]. Mean canopy coverage of Scouler's willow increased significantly (p<0.01) following selective logging and low intensity broadcast burning (intensity ~ 127 kcal/m/s) [12]. In the 1st year following burning, Scouler's willow may produce up to 28 times as many sprouts as the previous year [136,162]. Scouler's willow may grow significantly wider and taller (p<0.05) after fall burning than after spring burning [136]. However, fall burning removes the following winter's food supply for animals, while spring burns have substantial regrowth by summer [130]. After stand mechanical treatment and understory burning, Scouler's willow was reduced by 9% from mechanical damage and an additional 16% from fire. The surviving plants were substantially more vigorous post-treatment [15,16]. While modest Scouler's willow mortality may result after overstory removal and prescribed fire, the percentage of high vigor plants increases; in one study the amount of vigorous plants increased from 15% at pretreatment to 70% post treatment [23]. In northern Idaho, burning at 5-year intervals did not result in decreased vigor [135]. Scouler's willow was not markedly affected by burning until it suffered deep charring of the root crown. The following table presents the change in Scouler's willow population characteristics in response to different treatments (% change is relative to pretreatment conditions) [23]:
Treatment Cover reduction Mortality High vigor plants Control 1 3 15 Harvest only (shelterwood cut) 33 14 60 Low consumption (shelterwood cut and low consumption burn) 62 22 71 High consumption (shelterwood cut and high consumption burn) 58 26 69Shelterwood cuts combined with prescribed burning in a ponderosa pine resulted in modest Scouler's willow mortality; plants remaining in the harvest only and burned treatments had higher vigor than those in the control [19,23].
A summary of Scouler's willow's response to different types of disturbance is presented below [228]:
Type of disturbance: Clearcut, no site prep Shelterwood cut, mechanical scarification Clearcut, mechanical scarification Clearcut, broadcast burn Stand destroying wildfire Response: major vegetative response minor vegetative response major vegetative response, minor response from seed major vegetative response, minor response from seed major vegetative response, minor response from seedScouler's willow has a wide range of adaptation [39]. It is found in drier habitats than most willows [72], occurring as scattered individuals on dry uplands as well as swamps, and mountain streams [8,33,107,108,121,129,179,200], and is capable of establishing in dry rocky conditions at high elevations [39]. Scouler's willow commonly grows on gentle to moderate slopes [40,41,128]. While it does occur in riparian areas, Scouler's willow is more common on upland sites above riparian areas [80,90,146,195], and is found primarily in forests, meadows, on slopes [54,193], and in transitional zones between riparian and upland areas [149].
Scouler's willow may be found rarely on wet areas, but more commonly on moist areas or intermittent watercourses, and establishes both within gullies and at their bases [20,63,81,120,129]. In riparian areas, Scouler's willow establishes on relatively stable banks and lower sideslopes in valleys, reflecting a preferred environment of transport rather than retention of materials and moisture [63,212].
Scouler's willow is found on shallow to moderately deep soils [19,23,40,166]. It exhibits tolerance to a range of soil moisture conditions [38], occurring on moderately- to well-drained soils [38,41,54,80,166,180,185,193]. Scouler's willow commonly occurs on stony and silty soil with upper soil horizons dry during most of the growing season [63], and Forsyth [70] found that the cover of Scouler's willow was greater at intermediate soil moisture levels than at either extreme. Soils on Scouler's willow sites ranges from fine textured to gravelly [29,38,90,155,185,194,204]. It is commonly found on a variety of depositional land forms, including gravel bars [67]; glacial till; morainal blankets; river terraces; pumice flows; and alluvial, colluvial, and lacustrine deposits [7,55,152,155,185,189]. Parent materials are derived from a broad range of materials [19,23,174], but appear to be of little significance in distribution [125]. Scarification of the soil surface provides mineral soil important for Scouler's willow establishment [87].
The elevational range of Scouler's willow includes sea level to over 10,000 feet (3000 m) [4,8,10,11,19,20,23,29,33,42,51,53,54,60,63,72,129,149,174,205,244,249,260]. Scouler's willow is found in the lower and upper montane elevational belts in the northwest U.S. [4,33,51,96,260] and higher elevations in the southwest [10,129,149,244,249]. Annual precipitation on these sites may range from 9.5 to 63 inches (240-1600 mm) [4,7,19,23,29,42].
In British Columbia, Scouler's willow prefers drier, low elevation sites; mid- to lower slopes with rolling terrain and level to moderately sloping [90,194]. In the western U.S., it is most common in upland forests, cut-over areas, and burned areas in drier locations [53]. The range of Scouler's willow stretches from valley bottoms to the lower subalpine forest zone in Idaho. At the low to mid-elevations it grows in moist riparian habitats and generally attains a small tree stature; at the higher elevations it becomes a medium to large shrub and tends to inhabit relatively drier sites [33]. It may occur more frequently on north- and east-facing slopes in the western U.S. [72,125,129,174]; Irwin and Peek [106] found that maximum height growth of Scouler's willow in Idaho shrubfields occurred on east facing slopes. However, Mueggler [171] found that frequency and cover were not substantially related to aspect. Quaking aspen/Scouler's willow communities in the Intermountain Region range from 5,800 to 7,400 feet (1,800-2,300 m) in elevation [174]. Distribution of Scouler's willow in the southern part of its range may be somewhat restricted to montane riparian zones or other moist sites at high elevations [101,112,149,204,205,249], though in the southwest, Scouler's willow occurs in mixed conifer forests on steep relatively dry slopes [10] and in low elevation canyons [113]. In California it may also grow from near sea level to 10,000 feet (3048 m) [204]. In northern Mexico, it has been found in the mountains on north-facing aspects [11].
Scouler's willow is common in open areas following disturbance [165,178] and readily sprouts in ravines and on roadsides [47]. Its limited shade tolerance, tall growth habit, and sprouting ability enable it to persist in small openings on timbered sites [171,218,226]. Scouler's willow occurrence increases with full sunlight [70,172]; Mueggler [171] found that the frequency and crown cover of Scouler's willow was substantially higher under tree cover less than 25% than under tree cover exceeding 41%. Hungerford [103] found that maximum height of Scouler's willow coincides with 40 to 50% of available light during canopy development. As a result of logging and slash burning in Douglas-fir/ponderosa pine forests in Montana, Scouler's willow became established in direct proportion to the amount of stand opening and ground disturbance [53].
Scouler's willow is an important browse species for domestic livestock and wildlife ungulates [6,8,17,36,60,68,124,131,132,150,164,179,204,218,219,226,227], providing critical winter and spring browse [19]. It is often the most preferred browse in ponderosa pine forests for mule deer, white-tailed deer, elk, bighorn sheep, moose, and domestic livestock [23,24]. Upland sites are heavily used by deer and elk; in riparian areas moose particularly prefer Scouler's willow [26,146,179,190,192,223,235,256,260,264], especially in winter months [87]. In disturbed areas, Scouler's willow may contribute more moose forage than any other species present [194]. Scouler's willow, occurring in younger stands, is more important in the moose diet than willows present in older stands [219]. Scouler's willow leaves, twigs, and bark are utilized as browse [209].
In British Columbia, it is of moderate to high importance for black-tail deer, is utilized from April through November, and is favored during spring and summer months [45]. Areas of high Scouler's willow cover have been associated with high elk use [82]. In Idaho, it is preferred elk forage, important in both summer and winter months [105,108,130]. Elk use tends to be higher in early rather than late summer [60]. In Montana, Scouler's willow is a large part of the regular winter diet for elk as well as a reservoir of surplus feed on which elk depend whenever climatic conditions are of unusual severity [73]. It is also heavily utilized by white-tailed deer and mule deer in Montana and Idaho, predominately as winter forage [115,116]. In Utah, it provides important summer browse for mule deer [221]. In California, Scouler's willow provides abundant browse of satisfactory quality for domestic livestock and deer. Domestic cattle feed on it in all habitats, while domestic sheep and goats feed on it on drier sites. It has been rated good to fair browse for domestic sheep and goats, fair for deer and domestic cattle, and poor for horses [204]. Small mammals also browse Scouler's willow [264], and it provides food for grizzly bears [48].
Upland game birds, ducks, and other birds feed on willow buds, leaves, twigs, and seeds [8,89], and Scouler's willow provides nesting and feeding habitat for small birds [226,227]. Scouler's willow buds provide an important winter food source for grouse, Clark's nutcracker, and the Rocky Mountain jay [95,179,218].
In the early spring, honey bees use willow pollen and nectar as a source of food for brood rearing [8].
In the southern part of its distribution, Scouler's
willow is a common understory component of spruce/fir [244], Douglas-fir/Engelmann spruce/ponderosa pine
[109], ponderosa pine,
mixed conifer [109,167], Douglas-fir/white fir (Abies
concolor) [95], high-altitude willow [125], montane chaparral [43], and scree forests
[129]. It also occurs occasionally in stands of giant sequoia (Sequoia
gigantea) [118,257].
Though Scouler's willow is also known as upland willow
and occupies drier habitats than most willows [54], it occurs in
riparian communities and floodplains in the northwest [9,147,258] and may dominate
early seral vegetation on gravel bars [67]. It is also a characteristic
species of riparian woodlands and scrub in the southwest [95,129,137,248], and occurs in wet meadow vegetation
[29].
Plant Associations:
Scouler's willow is commonly
associated with quaking aspen (Populus tremuloides), paper birch, Rocky
Mountain
maple (Acer glabrum), white spruce, common juniper (Juniperus communis),
greyleaf willow (Salix glauca), and American green alder (Alnus
viridis ssp. crispa) [7,73,92,189,245,253]. In Idaho, it is frequently associated with quaking aspen,
water birch (Betula occidentalis), thinleaf alder (Alnus tenuifolia), and black cottonwood
(Populus balsamifera ssp. trichocarpa) overstories at the low to mid-elevations; with aspen, Engelmann spruce, Douglas-fir, or lodgepole pine open
overstories at the higher elevations; and in upland forest openings created by
disturbance [33]. In the cedar-hemlock zone of northern Idaho, Scouler's willow is negatively
associated with redstem ceanothus (Ceanothus sanguineus), Utah
honeysuckle (Lonicera utahensis), and elderberry (Sambucus spp.),
but positively associated with thimbleberry (Rubus parviflorus) [171]. It has
specifically been identified in
the following plant associations: white spruce-gray leaved willow-scrub birch (Betula
glandulosa) association, lodgepole pine-scrub birch-lichen association
[154,155], aspen/birch/red-osier dogwood (Cornus sericea)
association [90],
Habitat and Community Types:
Scouler's willow is a seral species common in the
following habitat types: Douglas-fir/pinegrass (Calamagrostis rubescens)
[86],
Pacific silver fir (Abies amabilis)/devil's club (Oplopanax horridus),
Alaska-cedar (Chamaecyparis nootkatensis)/ovalleaf huckleberry (Vaccinium ovalifolium)
[71], grand fir/blue huckleberry (Vaccinium globulare) [75],
subalpine fir/beargrass (Xerophyllum tenax) [218], white fir/prince's
pine (Chimaphila umbellata)
[152],
Douglas-fir/white spiraea (Spiraea
betulifolia) [76], grand fir/Rocky mountain maple [78], Douglas-fir/ninebark
(Physocarpus malvaceus) [12,41,247,251,268], Engelmann spruce/myrtle
huckleberry, subalpine fir/forest fleabane (Erigeron eximius), subalpine fir/thimbleberry,
blue spruce (Picea pungens)/red-osier dogwood, white fir/Rocky mountain
maple, white fir/forest fleabane [129], and Engelmann spruce/forest
fleabane [166]. In Montana and Idaho, it is common to many habitat types
in the Douglas-fir, grand fir, and subalpine fir series [46,66,80].
Scouler's willow is dominant in the mid-seral, Scouler's
willow shrub layer group of the
following habitat types: grand fir (Abies grandis)/blue huckleberry [226],
Douglas-fir/pinegrass [229], Douglas-fir/blue huckleberry [17], Douglas-fir/ninebark
[17,228], Douglas-fir/Rocky mountain maple [227], Douglas-fir/white spirea
[233], subalpine
fir/fool's huckleberry (Menziesia
ferruginea) [17], and grand
fir/mountain maple (Acer glabrum) [228,231]. It may
also dominate seral stands in warm, moderate to dry habitat types of Douglas-fir, grand fir, and
ponderosa pine [222].
Scouler's willow occurs in a variety of community
types, which include the lodgepole pine-subalpine fir, quaking aspen-white
spruce, quaking aspen-lodgepole pine-white spruce [185], and
lodgepole pine-white spruce-subalpine fir community types in British Columbia
[186]; and white spruce-russet buffaloberry (Shepherdia canadensis),
white spruce-quaking aspen-russet buffaloberry-twinflower (Linnaea borealis), and
quaking aspen-bearberry (Arctostaphylos uva-ursi) community types in the Yukon Territory
[55]. Scouler's willow occurs as a dominant in the tall mountain shrub
component of the north Idaho seral brushfields
[21,42,56,92,96,97,141,172,181,242,251,262,266]. It is
dominant in the Scouler's willow community type in the Yukon, with russet buffaloberry as the most prominent understory shrub [55], and may
dominate other deciduous forest communities in the montane zone of British Columbia
and the Yukon Territory [55,94]. Scouler's willow occurs as a dominant
shrub
in aspen stands and in the Salix spp. community type of Wyoming [35,81]. In Nevada, Scouler's willow is a dominant
in the Scouler's willow/tall forb community type and may dominate the tall forb
undergrowth vegetation type that is transitional from riparian to upland sites
[149]. Scouler's willow dominates the Scouler's willow
riparian community type in the southwestern United States [248]. It is also a
codominant tree in the
quaking aspen/Scouler's
willow community type [63,174].
Classifications describing plant communities in which
Scouler's willow is a dominant species are as follows:
Arizona [248,250]
British Columbia [119]
California [99]
Idaho [174,226,227,228,229,231,232,233]
Nevada [149,174]
New Mexico [248,250]
Utah [174]
The deep root system, multiple stems, and ability to sprout from a
subterranean root crown rather than surface sprouting may increase Scouler's
willow's tolerance to disturbance [24].
Fire can be used to maintain shrub productivity on
seral brushfields and encourage shrub dominance after harvest. Tree establishment
can be encouraged by excluding wildfire, using site preparation methods that do
not encourage grass or shrub species, and planting and seeding pioneer species
under the shade of killed shrubs [222].
Prescribed fire has been used to stimulate sprouting
of Scouler's willow that has grown out of reach of big game browsers
[130,131,135]. In burned areas, the current annual growth of willows
is considerably higher than in control areas, and fire increases both browse
production and availability [18]. Prescribed fire was used in the 1960's to rejuvenate northern Idaho brushfields for elk winter range [136]. When prescribed burning causes greater than 50% canopy mortality,
substantially higher current annual growth (CAG) of Scouler's willow is produced.
CAG of vigorously growing plants has a higher nutritive value and nutrient
concentration than mature, slower growing plants [183]. Big game prefer browsing on the
current annual growth of burned willows than that of unburned willows [18]. Utilization by elk demonstrated an obvious preference for the sprouts of
burned Scouler's willow following mechanical treatment and understory burning; the
surviving plants were substantially more vigorous, with greater live biomass and
better palatability than the unburned plants [15,16,23,131]. Scouler's willow experiences a dramatic
increase in available browse production immediately following burning, the
result of shrub removal and sprouting that reduces the height and increases
the availability of browse [130,134,139,140,263]. In one study, Scouler's willow reached 10 feet in height 4 years
after burning, with 80% of twig production available for elk browse. However,
repeated treatment may be necessary to maintain browse availability [132]. Scouler's willow browse production increased from 6.5 to 44.1 kg/ha 3 to 7
years after wildfire in Alaska. The increase in production was due to an
increase in the number of shrubs during the 1st 5 years and an increase in
the number of twigs per shrub during all 7 years. Stems were available after the
first 2 years; 4 years postfire, browse production in the burned area was twice
that in the unburned area; and at 7 years postfire browse was 5 times greater.
Browse production is likely to peak at 10-15 years and decrease by 20 years
postfire [264].
Scouler's willow responds well to both fall and spring
burning; however, fall burning eliminates the following winter's food supply for
animals. Spring burns regrow rapidly, with 5 foot (1.5 m) sprouts common by
summer on burns in Idaho [130]. It has shown no decreased vigor from
burning at 5 year intervals [135]. Brushfield burning to improve elk
winter range in Idaho has often occurred during the spring or fall. However,
another very important browse species, redstem ceanothus, is not always replaced
by seedlings under these treatments because the seedcoat required high soil
temperatures to crack and allow germination. Therefore, for winter range
improvement, summer prescribed fires may need to be considered [259].
Willows produce, within limits, denser growth when
they are browsed [8], with browsed
stems branching laterally [253]. Browsing stimulates production of Scouler's willow,
though continuous browsing over several years may eventually deplete plant or
soil reserves resulting in an eventual decline in productivity [263]. Following wildfires in Alaska, Scouler's willow
sprouts have suffered intense browsing by snowshoe hares, which often migrate
to burned areas to feed. In some locations, 100% of the current annual growth of
sprouts was removed for 2 succeeding winters [253,263].
This impact normally lasts for only a few years. Flowering and fruit production of Scouler's willow are affected by heavy browsing,
possibly resulting in low seedling success and recruitment, and browsing may
suppress sprouting [24,34]. In burned and
unburned clearcuts, grazed sites had no Scouler's willow present, while ungrazed
sites had 5 to 7% Scouler's willow cover 11 years after treatment [61].
Where it is subject to overbrowsing, the loss of Scouler's willow may result in
substantial losses to elk herds [73].
Mechanical thinning may prevent suppression of
Scouler's willow in Douglas-fir/ponderosa pine forests [87].
Scouler's willow increased in biomass and vigor in response to thinning in a
Douglas-fir/ponderosa pine stand in Montana, indicating a positive response to
overstory reduction [24]. Winter carrying capacity for mule deer
may be substantially increased after mechanical treatment due to the
establishment and growth of Scouler's willow [87]. It is difficult
to achieve control of Scouler's willow using mechanical treatment due to the
sprouting of damaged shrubs, and clearcutting can result in major canopy
increases. Chemical treatments may be required if
managing for ponderosa pine establishment due to its inability to compete with
Scouler's willow [227,228].
Sprouting: Willows sprout quickly after fire if depth of the burn in the soil is low to moderate [93]. When fire is intense enough to kill live foliage but does not kill the vascular cambium, Scouler's willow experiences vigorous epicormic sprouting from the root crown [2,3,19,24,30,31,47,76,103,131,134,139,140,142,153,171,172,191,222,226,230,239,247,261]. Root crowns of Scouler's willow are often so large that some buds always survive, except when disturbance is really severe [169]. New shoots have been observed growing within days of a fire [69], and rapid sprouting after burning results in low overall plant mortality [19]. Multiple sprouts result in increased Scouler's willow density following fire [116]. Scouler's willow has a high postfire response rating; the species population will regain its preburn frequency and cover in 5 years or less [195].
Scouler's willow increases dramatically following a variety of burn intensities, especially on relatively moist sites [195]. Basal area after 3 years may exceed that on unburned sites [253]. Dramatic increases in volume occur over the first 15 years postfire [239]. Sprout height growth may be dramatic, reaching up to 10 feet (3 m) after the first growing season [46,131,259]. Within 3 years after burning, plant crowns can average over 11 feet (3.4 m) in height [131]. Following a prescribed summer burn in Idaho, nearly 80% of height growth of Scouler's willow over a 7-year period was attained within 2 growing seasons [140]. Following a summer wildfire in northern Idaho, Scouler's willow reached peak cover values within 8 years [240].
Scouler's willow plants that experience severe canopy mortality apparently concentrate their nutrients into vigorous new growth more than plants which experience only light canopy mortality. Analysis of aboveground plant part mortality classes from controlled burns in Idaho revealed that Scouler's willow plants which experience top-kill exhibit the most vigorous regrowth. Twig growth (length and weight) of Scouler's willow following fire was 3 to 4 times greater on plants with greater than 50% canopy mortality than on plants with less than 50% canopy mortality [183].
Seeding: Scouler's willow also has the potential to regenerate from off-site seed sources [30,31,46,86,91,103,172,222,226,239,247,258], and can establish in moist mineral soil postfire [30,31,66,76,222,226]. Sowing Scouler's willow seeds on different severity burns in upland black spruce sites in Alaska showed that germination occurred only on moderately (organic layers partially consumed) and severely (ash layer present, organic material in soil consumed or nearly so to mineral soil) burned seedbeds. Severely burned sites had the best germination percentages and represented the only burn severity class where Scouler's willow seedlings survived past 3 years [267]. Scouler's willow establishes quickly, but the rate of cover development or increase is slow [237,238,240].
Germinating seed originating from off-site plants often raises Scouler's willow frequency far above what would be expected from on-site surviving plants [140,145,240]. Stickney [240] observed that after a stand-replacing wildfire in northern Idaho, Scouler's willow seedlings made up the majority of the shrub component of the vegetation. The importance of seedlings in the postfire community was similarly observed by Lyon [140]. He recorded the postfire density of Scouler's willow plants for 7 years, summarized below (density = # of plants >18 inches (46 cm) tall per 1,000 ft2):
Postfire year Prefire - 1963 1 - 1964 2 - 1965 3 - 1966 4- 1967 5 - 1968 6 - 1969 7 - 1970 Density 0.3 0.1 0.2 0.2 0.6 2.4 3.6 4.4Scouler's willow regenerates from both from seed and vegetatively. Scouler's willow probably begins producing seed before 10 years of age [32]. Insects, especially bees, are important pollinators [88]. Seeds disperse in late spring, disseminated by wind and water [9,32,51,91,179,195,218]. These seeds have cottony hairs that allow them to travel long distances [32]. Seedlings may regenerate from windborne seed from as far as several miles away [87]. Scouler's willow seeds are nondormant and remain viable for only a few days without moisture [227]. Willow seeds are characterized by a short seed life and rapid germination [179,187], and Scouler's willow seeds usually germinate within 12 to 24 hours of dispersal [32]. The seeds are scarified by light burning [110] and moist mineral soil is required for germination and seedling establishment [187,218,227,267]. Zasada and others [267] found that on artificially seeded sites Scouler's willow had substantially more germinants, 1st year survivors, and 3rd year survivors on heavily burned sites than moderately burned sites.
In the laboratory, the germinative capacity of fresh seeds is high; normally 95 to 100% of seed germinates within 3 days [32,52]. The seeds contain substantial amounts of chlorophyll, and photosynthesis generally occurs as soon as the seed is moistened [32]. Field experiments found that Scouler's willow seeds were not viable under dry outdoor conditions; most seeds did not germinate and those that did produced abnormal seedlings [52]. Light is required for good germination. Seed may be stored up to 4 to 6 weeks if kept moist at 32 to 41 degrees Fahrenheit (0-5 oC) [32].
High seed to seedling ratios on seeded plots suggest that seeding is an inefficient way of using seed, and planting artificially regenerated plants may be a more successful method of establishment and a more efficient use of available seed [267].
Scouler's willow regenerates asexually by vigorously sprouting from a subterranean root crown [1,2,3,17,23,24,40,195]. This basal sprouting occurs in response to disturbance, including fire, flooding, and mechanical damage [1,2,47]. Scouler's willow sprouts typically have a tall, fast growth response [169].
Scouler's willow generally propagates readily from cuttings, with 40 to 80% rooting success [58,100,213] and roots developing within 4 weeks of planting in lab and field experiments [100]. Initial roots develop on the basal portion of the cutting [49] and continued rooting progresses along buried stem surfaces [62,100]. Root development is more rapid and successful with cuttings collected during the growing season [49,89]. Densmore and Zasada [49] found that cuttings planted in the field had a survival rate of 17% after two growing seasons. In general, willow cuttings are better able to establish if planted as rooted stock [168]. Softwood cuttings of Scouler's willow root as well or better than hardwood cuttings, which may offer alternatives for vegetative propagation and flexibility in producing stock for conservation planting [62].
Scouler's willow is a shade intolerant, persistent seral species [17,55,104,238]. It is often a minor understory component in a variety of forest types [5,43,44,45,59,68,72,76,77,85,95,110,163,185,258,260], occurring as scattered individuals in small openings. However, it increases after disturbance, including clearcutting, prescribed fire, soil disturbance, and wildfire [1,5,9,17,47,53,61,70,71,72,89,106,138,145,150,171,194,222].
Scouler's willow is an early to mid-seral species [43,44,73,84,85,94,102,103,127,132,145,184,185,198,218,236]. Where not already present, it rapidly invades disturbed sites [24,45,91,214,238] facilitated by its wind-dispersed seed [216,265], or sprouts following canopy removal [1,69,265]. It capitalizes on moderate to severely burned sites [110,177]. In clearcuts and young stands it forms a tall shrub layer, comprising a substantial percentage of the plant cover [7,41,104]. In some areas, Scouler's willow may dominate early seral plant communities following fire or clearcutting [28,55,150,210,211,265] and on river terraces and gravel bars [1,67]. It forms a mid-seral shrub layer, the Scouler's willow layer group, in several habitat types of the northern Rocky Mountains [17,226,227,228,229,231]. In the past, these layer groups formed in response to stand-replacing wildfires, but may also develop following mechanical scarification in clearcuts. Scouler's willow may persist in late seral and climax stands, but the layer groups fade as succession progresses [226,229].
Scouler's willow reaches its highest frequency and cover in stands at least 20 years old, with maximum frequency and cover reached between 30 and 40 years [171]. Slight shade tolerance, tall growth habit, and ability to sprout allow Scouler's willow to persist under moderately dense tree cover because small openings in the canopy can stimulate sprouting and rejuvenate individuals, making it somewhat less vulnerable to successional replacement [218,222,227]. It may remain present but substantially less abundant in climax cover types [17,41,55,68,76,95,185,260]. On paired stands of uncut and clearcut grand fir forest, Scouler's willow appeared only in the clearcut stands, 7 to 16 years after disturbance [6,7]. Following a stand replacing fire, Scouler's willow appeared in stands 30 to 90 years old but was not present in stands greater than 150 years old [7]. Presence and cover of Scouler's willow has been found to decrease with increasing stand maturity [17,172,219,240]; in one study, both values decreased significantly (p<0.05) as stands progressed from immature (<90 years old) to old growth (>150 years old) [5]. The disappearance of Scouler's willow from maturing stands is attributed to the increasing competition for light and moisture as the tree cover develops [34].
Successional trends in northern Idaho, following the removal of climax coniferous forests from wildfire or logging (occasionally accompanied by prescribed fire), show that the initial postfire vegetation is dominated by a grass-forb stage [42,72,92,115,133]. Within a few years this gives way to dense brushfields, of which Scouler's willow is often a dominant or co-dominant [42,56,70,72,92,97,106,108,115,133,240]. These brushfields eventually return to conifer-dominated sites; the time frame depends upon fire intensity, reburn history, seed sources, climatic conditions, and site characteristics [72].
The currently accepted scientific name of Scouler's willow is Salix scouleriana
Barratt ex Hook (Salicaceae) [98,112,114,175].
Currently recognized varieties are [112]:
Scouler's willow is effective in revegetating canyons disturbed by flooding water and debris. It is especially successful in establishing in riparian areas and at the base of dry slopes with sufficient moisture [38]. Willows contribute to streambank stability [79], and Scouler's willow is useful for stabilizing steep, erodible banks on drier sties above river courses [62,256] and is recommended for riparian revegetation projects [37]. It may also be useful for rehabilitating recreation areas; 90% rooting success has been achieved for planted cuttings [220].
Scouler's willow may provide site protection for conifer seedlings [226,227,228,256]. It provides light cover for the establishment of larch and Engelmann spruce after disturbance, encouraging revegetation [218,226,228]. Scouler's willow may also enhance Douglas-fir regeneration by providing a suitable microclimate for seedlings [111,218]. It provides "safe-site" cover to improve tree seedling establishment in revegetating grand fir habitat types; it is rated neutral to moderately efficient based on the ratio of "safe-site" cover to the percentage of seedlings occurring there [78]. However, Scouler's willow is a formidable competitor of ponderosa pine, which must outgrow Scouler's willow to survive. High densities of Scouler's willow may preclude ponderosa pine success following disturbance because it is shaded out by the height and lateral spread of Scouler's willow [227,228].
Scouler's willow has received the following ratings for reclamation suitability [256]:
Reclamation Suitability Criteria Suitability Rating Drought tolerance Very high High Medium Low Salt tolerance X X pH tolerance X (acid) Winter hardiness X Erosion control X Persistence X Palatability X Browse tolerance X Moisture preference Moist to wet, well to poorly drained waterlogged soils Soil preference Wide texture range, includes peat soils