AGE: -- The lifespan of SALIX BEBBIANA is not known and will be difficult to determine. It is known that they do live to at least 60 years of age (Atchley 1989). SALIX BEBBIANA populations appear to occur in a 'decadent' form in many areas (Atchley 1989, Waring 1991a). In these populations, individuals possess multiple, woody stems with some to many of these stems dead or dying, and open, sparsely-leaved canopies; there is no evidence of recent reproduction including juvenile and sapling individuals. One population in southeastern Montana has been protected from grazing pressure for eight years and shows no evidence of recruitment (Atchley 1989).
SEXUAL -- SALIX BEBBIANA is among a group of willow species that reproduces early in the growing season, flowering as early as April in Canada (Mosseler and Papadopol 1988). The length of its flowering period in Canada ranges from 9 to 12 days (Mosseler and Papadopol 1988). This species is dioecious and the flowers, like other willows, are thought to be largely insect-pollinated (Sacchi 1987, Argus 1974). Seed dispersal occurs in late May and early June in southeastern Montana (Atchley 1989) and in late June and early July at Fern Mt. in northern Arizona (Waring, pers. observ.).
A great deal is known about germination of SALIX BEBBIANA seeds (Atchley 1989, Densmore and Zasada 1983, Moss 1938). SALIX BEBBIANA seeds are small and short-lived, and require abundant light, soil moisture, and space (i.e., the absence of competitors) for survival (Atchley 1989, Densmore and Zasada 1983, Moss 1938, Waring 1991b). Brinkman determined that Bebb's willow seeds weigh about 0.20 mg (Brinkman 1974). Moss (1938) found that seed viability dropped from nearly 100% initially to 50% within 42 days in seeds collected near Edmonton, Ontario. Atchley (1989) found that seeds from southern Montana were inviable after 30 days. She also determined that seeds collected towards the end of the seed dispersal period were significantly more viable than those collected earlier.
Latitude and elevation have exerted an evolutionary influence on the timing of seed germination in Bebb's willows. Seeds on plants from Paxson, Alaska (elev. 850m), germinated at a lower temperature than those from Fairbanks (elev. 120m) (Densmore and Zasada 1983). While more than 90% of Bebb's willow seeds from Alaska germinated at 5o C, less than 5% of Bebb's willow seeds from southeastern Montana germinated at 5o C, with most germinating at 20o C (Atchley 1989). Reproduction in willows is strongly tied to cumulative growing degree-days and atypical weather patterns can lead to overlap in the timing of reproduction of species that normally do not overlap (Mosseler and Papadopol 1988).
According to Argus (1986) the primary ecological determinants for the establishment and growth of most willows are a moist substrate for seed germination and ample sunlight for subsequent growth. Seed germination and survival in some willow species has been shown to be positively affected by increased sunlight (Stevens 1989, Sacchi 1987), Hosner and Minckler 1960). Atchley (1989) determined that Bebb's willow is tolerant of moderate shade, although increased levels of light resulted in increased survivorship and growth. She tested seedling survivorship and growth at four light levels: 1% full sun (fs), 3% fs, 10% fs and 20% fs. Regrettably, she did not use higher 'fs' levels in her experiments. Seedling survivorship and growth were strongly tied to available light, with no seedlings surviving at 1% fs. Plants grown at 3% fs were susceptible to diseases, while plants grown at 10% fs were tall, but spindly. Plants exposed to 20% full sunlight had the highest levels of survivorship and grew the most, producing tall and thick stems. Larcher (1975) characterizes shade plants as tolerant of 20% full sun or less. While Bebb's willow does appear tolerant of 20% full sun, it would be useful to compare its performance at this level with that in higher 'full sun' levels. Bebb's willow may be a full sun plant that is able to tolerate low levels of shade. This should broaden its ability to invade or persist in communities in different successional stages.
Bebb's willow seeds undergo epigeal germination, which is characteristic of small seeds and shade intolerant plants (Atchley 1989). During germination, epigeal seeds extend the cotelydons (firsts sets of leaves) above the ground surface, so that they can capture light and photosynthesize (Ng 1978). After ten days in complete darkness, imbibed Bebb's willow seeds extended cotelydons up to 20 cm (Brinkman 1974).
Both the texture and nutrient content of soil influence seedling establishment and growth in Bebb's willow. Atchley (1989) reported that plants in soil with higher silt and clay content tended to grow more than plants in soils with higher sand content. This may relate to the greater water-holding capacity of clays compared to sand. Atchley (1989) tested responses of Bebb's willow to soils with varying nutrient levels, including four field soils (including sandy loam from a bank cut and from silt bars) and commercial soils (inert sand, unfertilized mix of silt loam, sand and peat moss, and fertilized mix of peat moss, vermiculite and perlite). The only soils that produced a significant increase in growth were fertilized soils (Atchley 1989), suggesting that natural soils are often nutrient-poor. Pregent et al. (1987) found that Bebb's willow seedlings grew only 34 cm in 6 years following introduction into nitrogen-poor borrow pits in eastern Canada. They performed more poorly than seedlings of jack pine or nitrogen-fixing alders.
In an experiment evaluating the effect of different intensities of fire on plant colonization, Bebb's willow along with other willow species had highest levels of germination and survivorship on plots that had been severely burned (Zasada et al. 1983). This result may be due to reduced plant competition and to improved nutrient status of such soils following fire. Levels of inorganic or available nitrogen in soil are typically higher following fire (Covington and Sackett 1990, Covington and de Bano 1990).
While pre- and post-zygotic forms of reproductive isolation exist amongst many willow species, SALIX BEBBIANA produces vigorous F1 hybrids when crossed with S. PETIOLARIS (Mosseler 1990, Mosseler and Zsuffa 1989). Some of these hybrid progeny were more vigorous than parentals. Crosses with S. ERIOCEPHALA (Subgenus VETRIX, Section Cordatae) and S. EXIGUA (Subgenus SALIX, Section Vetrix) were variable, producing large proportions of inviable and 'distinctly' inferior progeny (Mosseler 1990).
SALIX BEBBIANA has a chromosome number of 38 (2n count), while some members of the section Vetrix are tetraploid or hexaploid (Mosseler 1990).
REPRODUCTION: ASEXUAL -- Cuttings of most riparian willow species produce roots from primordial buds or adventitious roots from other plant parts (e.g. Chmelar 1974), and this vegetative form of reproduction is more important than sexual reproduction for some species (Stevens 1989, Krasny 1988). The ability of stem tissue to propagate itself through rooting is highly adaptive for riparian species that experience major episodes of flooding likely to remove plant parts and for colonization of sites unavailable to seedlings, such as dry or nutrient-poor sites (Stevens 1989, Krasny 1988, Densmore and Zasada 1983).
SALIX BEBBIANA may rely more on sexual reproduction than asexual reproduction. In one experiment cuttings of S. BEBBIANA did not produce roots or become vegetatively propagated (Densmore and Zasada 1978). Densmore and Zasada concluded that S. BEBBIANA is not a truly riparian species compared with other willow species that rapidly produced roots and stem growth in the same experiment. Mosseler (1990) also found S. BEBBIANA to have `a poor rooting ability' and suggested that this species may lack primordial buds that are capable of producing roots. Forty percent of S. BEBBIANA cuttings produced roots and shoots in several studies (Atchley 1989, Holloway and Zasada 1979). Atchley found that stems younger than three years did not produce roots. She also found that stems grown in soils higher in organic matter and water holding capacity produced more root and shoot biomass than stems grown in soils devoid of organic matter and with poor water retention. (Atchley 1989). Overall, it appears that vegetative propagation does not play a major role in S. BEBBIANA reproduction. The occurrence of Bebb's willow along and near smaller 1st and 2nd order streams suggests that it does not experience major flooding events.
ROLE IN RIPARIAN COMMUNITIES: The role that SALIX BEBBIANA plays in riparian communities is difficult to assess due to a paucity of studies focused on this subject. With available information, I have tried to piece together a sense of its position in such communities and the factors determining this. Aside from a recently conducted survey of Bebb's willow populations in the West (Waring 1991a), most information on this subject comes from studies and observations made of Bebb's willow populations in Alaska. The information on Alaskan populations provides many insights into the natural history of Bebb's willow, although some of it may not apply to Bebb's willow as it occurs at lower latitudes in the western U.S.
WATER REQUIREMENTS: Bebb's willow has been described as a nonriparian willow species (Densmore and Zasada 1978), and it adult plants may have lower water requirements than other riparian willow species, although this has not been tested. In Alaska, Bebb's willow commonly occurs in drier upland terraces associated with older floodplains, according to three scientists who have studied this species (J. Bryant and L. Viereck, U of Alaska, Fairbanks Alaska; J. Zasada, N.S.F.S. Institute of Northern Forestry, Fairbanks, Alaska; and PNW Exp. Stat., Corvallis, Ore.). Another upland species, SALIX SCOUERIANA, is commonly found with it in these habitats. Bebb's willow rarely occurs in close proximity to stream channels where other willow species are found (Viereck, pers. comm.).
In the western U.S., Bebb's willow occurs along stream channels, on the edges of drainages, along seeps, and in perched sites that appear to be receiving little water (Atchley 1989, Waring 1991a). These patterns suggest that it's occurrence along stream margins may be a facultative rather than obligate phenomenon. These patterns also suggest that the absence of other willow species at some sites may be due to lower water availability, as well as to chance.
While adult Bebb's willow can persist in seemingly dry sites, such as Fern Mountain, AZ (Waring 1991a), there is no doubt that establishment of Bebb's willow seedlings is strongly dependent on ample water. This is borne out by recent experiments by Atchley (1989), Maschinski (1991), and Waring (1991b), and observations of seedling distributions in western populations (Waring 1991a). In a recent study, Waring (1991b) determined that seeds placed in plots near established plants would not germinate without supplemented water.
LIGHT REQUIREMENTS: Although most information suggests that Bebb's willow thrives in high light conditions, there is some indication that this species is tolerant of considerable shade. Experiments and field observations verify its positive responses to sunny conditions (Atchley 1989, Maschinski 1991, Waring 1991b; Bryant, Viereck, and Zasada, personal communications from these), while Atchley (1989) determined that it can germinate and grow in 10-20% full sun. Conflicting information does suggest a tolerance of low and high light conditions. Although high densities of seedlings and juvenile plants were found only in sunny microsites during a recent survey of western populations (Waring, pers. observ.), it has been reported that this species often colonizes communities with well-established vegetation that might limit available sunlight (White 1965, Bryant, Viereck, and Zasada, pers. comm.). In the shrub-carrs of Wisconsin, Bebb's willow colonizes peat soils only after they have been previously colonized by other shrub species including SALIX CANDIDA and BETULA PUMILA (White 1965), perhaps because colonizing plants may improve the nutrient status of such soils. Light conditions in colonization microsites were not given in this study. In the northern Rockies, it is abundant along first and second order streams where it is often dominant and may have colonized following disturbance events which opened up habitat. It has been suggested that adults are able to survive considerable and long-term shading by species such as spruce, although willow growth in such conditions appears to be nearly negligible and no one knows how long it can tolerate such conditions (Bryant, pers. comm.). It seems clear that Bebb's willow does well in full sun. However, its behavior in partial shade is not clear.
SUCCESSIONAL STATUS: Bebb's willow seedlings colonize habitats that are newly disturbed, and habitats that have been previously colonized by species such as grasses, CAREX and shrubs (White 1965, Waring 1991a; Bryant, Zasada and Viereck, pers. comm.). These patterns suggest that it may play an intermediate successional role in some plant communities. This successional scheme has been proposed for other willow species (Fig. 3; Walker et al. 1986, Walker and Chapin 1986). The poor performance of Bebb's willow seedlings in levels of sunlight below 10% (Atchley 1989) would suggest that this species would not be able to replace itself as larger tree species begin to colonize a community, although this relationship is not resolved (see above). Although the details of Bebb's willow colonization in Alaskan sites are not well-studied, Viereck (pers. comm.) suggests that this species may colonize wooded upland sites following large-scale or small-scale fires that open up otherwise heavily shaded habitat.
Other willow species such as SALIX NIGRA and S. INTERIOR are considered to be early colonizers because the success of their seedlings increases as overstory decreases (Hosner and Minckler 1960). More information is needed on the light requirements of Bebb's willow seedlings to determine conditions under which it enters riparian communities. As with other willows (e.g. SALIX NIGRA, McLeod and McPherson 1973), it can not be inferred that the conditions that adult Bebb's willow occur in are suitable for seedling establishment (Waring 1991a). Adult plants appear to be tolerant of less disturbance and less water than seedlings.
Bebb's willow is often replaced by alder and spruce in undisturbed habitats (White 1965, Padgett et al. 1989). Bebb's willow is one of the last willow species to die out as sites develop into forest communities in interior Alaska because it can grow taller than most other willows, which helps it to escape moose damage (Miquelle and Van Ballenberghe 1989). However, it is being outshaded by taller forest trees such as spruce (Miquelle and Van Ballenberghe 1989).
DISTURBANCE FACTORS AFFECTING BEBB'S WILLOW ESTABLISHMENT AND PERSISTENCE: Disturbance factors such as flooding, fire and herbivory are regarded as major organizing factors in plant communities. They can exert a strong influence on both successful recruitment and persistence in plant populations by affecting plants directly and indirectly by influencing processes such as succession.
A recent survey of western Bebb's willow populations revealed that most populations are comprised largely or entirely of large, old individuals, indicating that recruitment is a relatively rare event in most populations. The same appears to be true for Bebb's willow in Alaska, that is, young plants are rarely encountered (L. Viereck, pers. comm.). However, in Alaska, Bebb's willow is most commonly represented by older plants that occur in a shrubby juvenilized form due to severe and chronic fire and herbivory (Bryant, Viereck and Zasada, pers. comm.).
In the western U.S. Bebb's willow recruitment was found to be occurring in several types of habitats: at seeps where the ground was wet, sunny, open and colonized by CAREX; at open areas along streams; in wide floodplain sites and in wet disturbed sites, such as along roadways (Waring 1991a). These patterns suggest that disturbed sites, such as road margins, are readily colonized, and less disturbed sites, such as CAREX stands, are also colonized. This latter may be possible as long as water and sunlight are adequate. Juvenile plants, as well as seedlings, were most commonly found at the seep sites. Current year seedlings only were found growing in cobble and sand bars along streams, suggesting that these sites are unstable due to increased seasonal water flows, or low level flooding. Saplings were found at a stream site in the White Mt.'s, AZ, at the stream edge and up to 30 m away, on the edge of the floodplain.
In Alaska, recruitment occurs in upland terraces associated with old floodplains, and in highly disturbed sites including the sides of roadways and gardens (Bryant, Viereck and Zasada, pers. comm.) The upland sites are sediment terraces that have accumulated over considerable time, are removed from flowing stream channels and are being colonized by plants (Viereck, pers. comm.). At these sites, Bebb's willow appears to colonize sites that are in an intermediate stage of succession, such as sites that have been colonized by shrubs and spruce trees (Viereck, pers. comm.).
In a recent experiment conducted at the Fern Mountain site, it was determined that seedling germination and survivorship were greatest in plots that had been cleared of additional vegetation (Waring 1991b). Intermediate levels of germination occurred in plots with intermediate levels of vegetation. From this it appears that disturbance is likely to enhance recruitment in Bebb's willow.
One woody plant association that may be beneficial for Bebb's willow involves alder, ALNUS spp., a genus of nitrogen-fixing species. Bebb's willow often occurs with alders and it may benefit directly from this because alder increases ambient nitrogen levels in soil. In experiments, Walker and Chapin (1986) and Walker et al. (1986) showed that willows grew more when grown in 'alder' soil than in soil not previously occupied by alder. This may be a beneficial association if soil nutrient levels are low.
There is typically a layer of low shrubs and herbaceous plants associated with SALIX BEBBIANA communities including RIBES spp., ROSA spp., SYMPHORICARPOS OREOPHILUS, ACTAEA spp., CAREX spp., IRIS MISSOURIENSIS, JUNCUS spp., and GERANIUM spp. (Padgett et al. 1989, Szaro 1989). Many of these species are consistently found under Bebb's willow canopies, implying that this species provides valuable habitat. This suggests that the presence of Bebb's willow in riparian communities strongly enhances plant diversity.
FLOODING: Bryant, Viereck and Zasada (pers. comm.) believe that major flooding has probably not been an important selective factor in SALIX BEBBIANA's evolutionary history. The evidence for this includes the following patterns: 1) the occurrence of this species along small 1st and 2nd order streams that are unlikely to experience severe flooding (V. Baker, U of Ariz., pers. comm.), 2) the occurrence of this species farther away from stream edges than other willow species (Waring, pers. obs.), 3) the poor rooting ability of its stem tissue (See Asexual Reproduction section), 4) its low leaf water-vapor conductance levels and 5) apparently low photosynthesis rates. The presence of Bebb's willow, including saplings and adults, on terraces and slopes above stream channels as well as along stream margins suggests that it may be capable of occurring across a greater moisture gradient than many willow species. This appears to be true of willow species such as SALIX LASIOLEPIS and may relate to the fact that many willow species are thought to have deep root systems (P. Price, per. comm.).
Lafleur (1988) found that SALIX BEBBIANA had the lowest level of leaf stomatal conductance of four Alaskan wetland species including CAREX PALEACEA, ALNUS RUGOSA and SALIX DISCOLOR. CAREX PALEACEA had the highest conductance, while SALIX DISCOLOR had the highest conductance of the woody species. Maximum leaf conductance is an important ecological and physiological plant characteristic since it determines the upper limit for transpiration losses and CO2 uptake (LaFleur 1988). This physiological `conservatism' may contribute to Bebb's willow's ability to occur farther away from streamsides, because lower conductance involves reduced evapotranspirational loss of water. Xerophytic plants have lower conductance levels than mesic plants (Noble 1983). Similarly, John Bryant and students (pers. comm.) have found that Bebb's willow has a lower photosynthetic rate than many other Alaskan willow species.
These patterns suggest that Bebb's willow typically does not occur in areas that are likely to be subjected to major flooding events, and consequently, major flooding probably does not influence establishment or persistence in this species.
While major flooding events seem unimportant to Bebb's willow germination and establishment, seasonal runoff from snowmelt in small streams may be responsible for the mortality seedling a year-old plants in these habitats. Colonizable sites--sunny, open sites--along streams are rare and concentrated close to stream banks where they are susceptible to increased flows from seasonal runoff. Runoff from snow melt and seasonally increased flows are also thought to be an important source of mortality for SALIX LASIOLEPIS seedlings (P. Price, pers. comm.).
FIRE: In Alaska, fire is regarded as a major selective factor for Bebb's willow, and the consensus is that this species is highly tolerant of it (Zasada and Viereck, pers. comm). Zasada et al. (1983) have shown that seedling success is enhanced in severely burned soils (See Sexual Reproduction section), and Viereck (pers. comm.) has speculated that seedling germination in upland sites in Alaskan drainages may be tied to local burns that clear away vegetation including spruce. This aspect of Bebb's willow biology is not well-documented, however. Bryant suggests (pers. comm.) that fire may be a more prominent factor for Bebb's willow in Alaska than in the lower 48 states. Alaska has a 'let-burn' policy about forest fires, and fires of the recent Yellowstone Park magnitude are relatively common. The average frequency of major fires in interior Alaska is thought to be about every 50 years.
Apparently mature Bebb's willow are highly tolerant of fire in Alaska. Individual plants are commonly reduced to stumps, which then resprout in a juvenilized form (Bryant, Zasada, Viereck, pers. comm.). It has been suggested that this species can tolerate hotter fires than can associated plants such as black or white spruce (PICEA spp.) (Viereck, pers. comm.). According to Zasada (pers. comm.), although the fate of individual plants following fire has not been studied, it is thought that fires must be hot enough to 'burn to mineral soil' in order to kill Bebb's willow. Therefore, fire appears to be a highly beneficial factor by enabling Bebb's willow to persist in regions where it might otherwise be replaced by later successional species.
In the western U.S., Bebb's willow may occur in habitats that are less likely to experience intense, hot fires. Bebb's willow typically occurs at a higher elevation than ponderosa pine (PINUS PONDEROSA), which is a major fuel producing species. Another willow, SALIX LASIOLEPIS, occurs at lower elevations along streams that transect P. PONDEROSA forests and used to be exposed more often to frequent and hot fires that could open up habitat (P. Price, pers. comm.). While fire clearly enhances seedling establishment and stands to remove later successional species such as spruce, it may not be as important a factor in Bebb's willow natural history in the western U.S. as it appears to be in Alaska. It has been suggested that the greater fire frequency in Alaska has resulted in the creation of more habitat that is suitable for higher mammal densities (Bryant, pers. comm).
HERBIVORY: There is considerable evidence that herbivore pressure on established Bebb's willow's can be intense. Although the effects of herbivory on establishing plants in Alaska has not been well-studied, it seems likely to be intense, based on severe levels of herbivory on older Bebb's willows (See below).
More work is needed on the impact of herbivory on seedling establishment. However, a recently completed experiment at Fern Mountain, AZ, showed that herbivory had significant effects on Bebb's willow seedling survivorship (Waring 1991b). Densities of seedlings were significantly higher in plots that were covered with cages that excluded vertebrate and invertebrate herbivores. High densities of grasshoppers, ants and snails were seen at this Fern Mountain site, as well as numerous elk tracks, indicating that many herbivores were present.
Heavy mortality of SALIX LASIOLEPIS seedlings due to grasshopper herbivory is known to occur in northern Arizona (Sacchi 1987). Herbivory by grasshoppers accounted for over 70% seedling mortality in some experimental plots and no mortality in others. Seedling mortality due to herbivory was variable between years and tended to be greater following dry winters. In another experiment, over 20% of seedlings in plots were grazed by grasshoppers, but only 1-2% died as a result. This study suggests that levels of seedling mortality from invertebrate herbivores can be high although extremely variable.
In a recent survey of western Bebb's willow populations, seedling densities appeared to be lowest at sites that were heavily-grazed by cattle (Waring 1991a). It has been suggested that intense grazing in Utah may limit Bebb's willow community growth because seedlings are unable to establish (Padgett et al. 1989).
Conversely, in southeastern Montana, a Bebb's willow stand that had been protected from grazing pressure for 8 years showed no evidence of recruitment (Atchley 1989). This demonstrates that ultimately grazing has second order effects on Bebb's willow populations, while factors such as water availability and sunlight may be critical for germination and establishment.
Large vertebrates including moose, deer and cattle and small ones including rabbits and hares feed extensively on willows (Padgett et al. 1989, Cannon and Knopf 1984, Chapin et al. 1985). Grazing by large mammals is a major cause of willow mortality in the boreal forest (Chapin et al. 1985, Fox and Bryant 1984, Bryant and Kuropat 1980). Bark stripping of selective plant species by moose can alter relative densities of deciduous and coniferous plants in Alaskan forests (Miquelle and Van Ballenberghe 1989). Over 75% of the aspen and Bebb's willow canopy in an aspen-spruce community in Alaska was debarked by moose, while spruce was not used (Miquelle and Van Ballenberghe 1989). In one study, Bebb's willow was debarked more than any other willow species (Miquelle and Van Ballenberghe 1989). While moose can slow the rate of succession in forests (e.g. Risenhoover and Maas 1987), it appears to be increasing the rate of succession to spruce forest in some parts of Alaska (Miquelle and van Bellenberghe 1989). In the western U.S., cattle grazing has been found to strongly restrict population size in some willow species (Rickard and Cushing 1982).
In Alaska, snowshoe hare feeding on willows during population outbreaks is severe (Chapin 1985). Grazing by snowshoe hares may be the most significant form of herbivory for Bebb's willow (Bryant, pres. comm.), and plants are commonly reduced to stumps as a result.
Juvenilization of Bebb's willow plants, due to intense herbivory by vertebrates, such as the snowshoe hare, leads to the production of young shoots that are less palatable to herbivores (Chapin et al. 1985). There is no evidence, however, that the chemistry of unbrowsed branches is altered in response to herbivory, a phenomenon known for some plants (Chapin et al. 1985).
In a recent survey of western Bebb's willow, heavily grazed plants of less than 0.5 m in height were found to be up to 15 years old, indicating that vertebrate herbivory can be intense and the plants at this latitude are also tolerant of chronic herbivory, sometimes exhibiting the 'juvenilized form' described for Alaskan plants.
