• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

Scouler 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 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 willow has a skunky odor [33,54,121].

A dioecious plant, Scouler 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].

RAUNKIAER [197] LIFE FORM:

REGENERATION PROCESSES:

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 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 willow regenerates asexually by vigorously resprouting 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 willow sprouts typically have a tall, fast growth response [169].

Scouler 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 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].

SITE CHARACTERISTICS:

Scouler 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 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 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 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 willow was greater at intermediate soil moisture levels than at either extreme. Soils on Scouler 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 willow establishment [87].

The elevational range of Scouler 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 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 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 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 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 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 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 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 willow is common in open areas following disturbance [165,178] and readily resprouts 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 willow occurrence increases with full sunlight [70,172]; Mueggler [171] found that the frequency and crown cover of Scouler willow was substantially higher under tree cover less than 25% than under tree cover exceeding 41%. Hungerford [103] found that maximum height of Scouler 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 willow became established in direct proportion to the amount of stand opening and ground disturbance [53].

SUCCESSIONAL STATUS:

Scouler 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 resprouts 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 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 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 willow may persist in late seral and climax stands, but the layer groups fade as succession progresses [226,229].

Scouler 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 resprout allow Scouler 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 willow appeared only in the clearcut stands, 7 to 16 years after disturbance [6,7]. Following a stand replacing fire, Scouler 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 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 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 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].

SEASONAL DEVELOPMENT:

Related categories for SPECIES: Salix scouleriana | Scouler Willow