Green-scaled willow (Salix chlorolepis) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

Scientific name:

Salix chlorolepisFernald in Rhodora 7:186.1905.

Relevant synonyms:

none

French name:

saule à bractées vertes

English name:

green-scaled willow

Family name:

Salicaceae (Willow Family)

Green-scaled willow is a member of the genus Salix, which includes approximately 450 species, 103 of which occur in North America (Argus, 1997). A number of taxonomists have divided the genus into several subgenera and sections. The green-scaled willow is classified in the subgenus Chamaetia (Dumortier) Nasarov and, more specifically, in the section Myrtilloides (Borrer) Andersson (Argus 1997, 1999, 2004). The taxonomic status of green-scaled willow is controversial: Dorn (1976) believes that it is a hybrid between Salix brachycarpa and Salix pedicellaris, whereas Argus (1965, 1997) believes that it may be a glabrous form of Salix brachycarpa. Nevertheless, Argus (1965, 1997, 1999, 2004) continues to recognize the green-scaled willow as a distinct species.

Morphological description

Green-scaled willow is a branched dwarf shrub with erect branchlets (Figures 1 and figure2). It typically reaches 10 to 30 cm in height, but sometimes up to 100 cm (Fernald, 1905). The leaves are short-stalked and initially glaucous (covered with a whitish-waxy coating) on both surfaces, with the upper surface subsequently turning green. The leaves are entire, becoming hairless, lanceolate or elliptical-oblong, obovate at the apex and measure 10 to 25 mm in length and 4 to 14 mm in width. The herbaceous catkin bracts are glabrous (hairless), olive green in colour and retuse (Figure 3). They measure 2 to 3 mm in length and are persistent during maturation of the capsule. The catkins are short-stalked, ovoid or cylindrical measuring 5 to 13 mm in length. The staminate flowers have two glands near the base. The filament is pale, hairless and 5 mm long, with the two anthers each 0.7 mm long. The pistillate flowers have styles 0.5 to 1.3 mm long and bifid stigmata that are narrow, elongated and divergent. The fruit (Figure 1) is a short-stalked hairless capsule 4 mm long (Argus, 1965; Coursol, 2001). The chromosome number is 2n=38 (Gervais, 1995). A more detailed description of a number of the morphological characters of the green-scaled willow is available in Argus (1965, 2004).

Figure 1. Illustration of the green-scaled willow (drawing by Réjean Roy): growth form (X0.75) and single pistil (X10).

Illustration of the green-scaled willow (drawing by Réjean Roy): growth form (X0.75) and single pistil (X10)

Figure 2. Female green-scaled willow

Female green-scaled willow

Figure 3. Fruit of the green-scaled willow

 Fruit of the green-scaled willow

The green colour of the bract, which had been believed to be one of the two distinctive traits of the green-scaled willow, along with hairless capsules (Fernald, 1905, 1950), is instead correlated with the ontogenetic development stage and is not limited to this species. A comparative analysis of the short-fruit willow (Salix brachycarpa) and the green-scaled willow shows a striking correlation between flower development and the colour of the bracts (Argus, 1965). The catkins have primarily green bracts at anthesis (time when flowers open and pollen is shed) and yellow to buff bracts after anthesis. There is a continuous sequence of intermediate colours if the ontogenetic processes and the various differences between individuals are taken into account. The high frequency of green bracts during flowering of the short-fruit willow calls into question the validity of the use of bract colour to differentiate the green-scaled willow (Argus, 1965).

It is difficult to distinguish green-scaled willow from short-fruit willow. The main differences, according to Schneider (1918), are the glabrous filament on the green-scaled willow, which is more or less pilose in the short-fruit willow, and the presence of a large number of stomata on the upper leaf surface in the green-scaled willow, whereas there are no stomata on the upper leaf surface in the short-fruit willow. However, Argus (1965) reaches different conclusions and counts nine specimens with stomata on the upper surface and 10 specimens of short-fruit willow without stomata on the upper leaf surface. Here again, the presence of stomata on the upper leaf surface cannot be used to distinguish the green-scaled willow from the short-fruit willow.

Argus (1965) uses the following seven criteria to distinguish the green-scaled willow from the short-fruit willow:

Green-scaled willow (Salix chlorolepis)

Short-fruit willow (Salix brachycarpa)

branchlets and buds hairless or pruinose

branchlets and buds non-pruinose and densely hairy

leaves hairless except for a marginal fringe in some specimens

leaves densely hairy

catkin with few flowers

catkin with many flowers

bracts hairless and pruinose (with a surface bloom)

bracts not pruinose and hairy

ovaries and capsules hairless

ovaries and capsules densely hairy

style long and undivided

style short and divided

filament hairless

filament hairy

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