Green-scaled willow (Salix chlorolepis) COSEWIC assessment and status report: chapter 6

Biology

Green-scaled willow is a perennial dioecious (having male and female plants) shrub that flowers, after leaf development, from early July to mid-August. Fruiting begins in the last week of July and continues until the first frosts (early September). Few studies have been conducted on the green-scaled willow and most observations come from Argus (1965) and the 2004 summer survey by the author. The lifespan of this shrub is unknown.

Life cycle and reproduction

The general biology of willows is relatively well documented. The green-scaled willow is a dioecious shrub. The number of males seems to be much lower than the number of females. Studies of the genus Salix show that the sex ratio can be as high as 4:1 (USDA, 2004).

The species generally depends on insect-pollination, and the presence of nectaries (nectar-producing organs) suggests non-specific pollination by insects (Argus, 1965), including bees. However, wind also plays a role in pollination because considerable quantities of pollen can be transported by the wind (Argus, 1965), especially on bare parts of Mount Albert. This would explain the relatively high frequency of hybridization between willow species.

The hybrid between green-scaled willow and short-fruit willow, named Salix x gaspeensis by Schneider (1922), is frequent on Mount Albert. An experimental cross between the short-fruit willow (Salix brachycarpa) (Argus 106-59, staminate individual) and the green-scaled willow (Argus 84-59, pistillate individual) resulted in the production of a few seeds, although the ovary was somewhat abnormal (Argus 1965). The ovaries of the green-scaled willow developed very slowly after fertilization, did not open at maturity and remained closed and twisted at the apex. The seeds were removed towards the end of the summer and, although some had begun to dry out, they germinated. Three young plants developed very slowly and died quickly. Their mortality is not necessarily an indication of their vitality because they had begun to dry out prior to extraction from the capsule, and the possibility that the growing conditions may not have been ideal must be considered. However, their mortality suggests that the pistillate flowers of the green-scaled willow are not highly effective and that a partial reproductive barrier may prevent rapid introgression of the rarer species (Argus, 1965). A number of hybrids were observed during the 2004 inventory, but it is difficult to determine whether the green-scaled willow is affected by hybridization (Lavoie and Fleurbec 1995). Since the species is a local endemic, its relative rarity is not surprising and is not in itself an indication of introgression (Argus, 1965). Further studies on this phenomenon will certainly provide answers, because the risk of introgression into short-fruit willow may not be as high as believed (Argus, 1965).

Physiology

The green-scaled willow, like other plant species associated with serpentine, is adapted to ultrabasic soils with high heavy metal concentrations, particularly magnesium. In serpentine rock formations, the heavy metal concentrations reach toxic levels for most plants. In such conditions, sparse non-diverse vegetation becomes established, composed of a specific mix of species. This loss of diversity is exacerbated at higher altitudes by the freeze-thaw cycle (congeliturbation). Congeliturbation stirs and churns the soil, limiting the establishment of vegetation by means of a strictly mechanical effect (Sirois, 1984). This effect combines with the increased toxicity caused by the rise of heavy metals to the surface. Plants associated with serpentine are poor competitors that take advantage of regularly disturbed, recently evolved substrates in which other plants do not succeed in establishing (Sirois, 1984).

The small leaf area and the distribution of stomata on the green-scaled willow are probably physiological adaptations, because these characters are modified by environmental factors. Argus (1965) indicates that, in specimens collected at Mount Albert, stomata are distributed either over the entire underside and on the upper side at the apex or on the entire surface, whereas in cultivated specimens, stomata occur only at the apex on the upper side. The differences observed are not well understood and are of little or no taxonomic value (Argus, 1965). The leaf area is multiplied fivefold in cultivated species, from an average of 128 mm² to 612 mm². There is no significant change in the length of the stomata, which measured 21.5 microns (±3.2 microns) on specimens from Mount Albert, and an average of 18.5 microns (±2.4 microns) on cultivated specimens (Argus, 1965).

Dispersal/migration

The seeds have a tuft of silky hairs that facilitates wind dispersal. They are believed to germinate shortly after dispersal, i.e., 12 to 24 hours (Argus 1997), and remain viable for 9 to 10 months (Argus 1997). According to Toepffer(1915) and Sugaya (1956), the seeds of the genus Salix can overwinter and germinate the following spring.

Adaptability

Since 1994, a specimen of the green-scaled willow has been under authorized cultivation in a serpentine-based soil mixture in a private garden. This specimen is vigorous, maintains its short size, and shows no signs of disease, despite a more favourable climate (Gaudreau, 2005).

Cuttings of male and female plants were taken during the 2004 inventory and provided to an amateur gardener, for propagation. A coarse mixture of peat and crushed serpentine was used to propagate the cuttings. The individuals quickly lost their leaves, as is the case in cutting propagation in willows, but the buds were still alive.

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