Betula pumila
 
Swamp Birch

Previous Year's Pistillate Catkin

Greenbush Bog, 20 km West of Hudson Bay, SK on Highway #3
29-May-2017

Note the catkin is of the previous year.  Occasionally remains of the previous year's pistillate catkins can still be found on the plant (as is the case here).  However, the scales are not hard, woody, and persistent like they are for genus Alnus.

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Pumila: Answers to key questions in Conifers & Catkin-Bearing Trees and Shrubs of Saskatchewan, Fascicle 5, Flora of Saskatchewan by Anna Leighton leading to this species. The answers are in the order you would normally work through the key
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Shrubs or small trees to 10 m tall; bark of mature main stems darker, usually reddish brown, not readily peeling off (tight); lenticels on older bark inconspicuous or conspicuous and expanded horizontally up to 5 (10) mm.  NOT [Trees, 10 30 m tall; bark of mature trunks and branches usually creamy, chalky white, pinkish white or pale reddish, readily peeling off in sheets (strongly exfoliating); lenticels on older bark conspicuous, expanded horizontally up to 15 (20) mm]

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Shrubs to 4 m tall; leaf blades broadly ovate, obovate, orbicular or elliptic; blade apices broadly acute, obtuse, rounded or truncate; blade margins usually crenate, sometimes crenate-serrate; petioles usually <= 5 mm; samara wings narrower than the body; scales 2 4 mm, central lobe rounded to truncate at tip; lenticels on older bark inconspicuous.  NOT [Tall shrubs or small trees to 10 m tall; leaf blades broadly ovate to rhombic-ovate; blade apices acute or occasionally short-acuminate; blade margins serrate to irregularly double-serrate; petioles >= 5mm; samara wings (at least one of the wings) equal to or wider than the body; scales 4 5 mm, central lobe acute at tip; lenticels on older bark conspicuous, distinctly expanded horizontally]

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Leaf blades 2.5 5 (7) cm, obovate, nearly orbicular or elliptic; branchlets with small resinous glands; fruiting catkins short and wide, 0.8 1.5 (2) cm long, 0.8 1 cm wide; scales often with 3 light-colored lines on the inner (adaxial) surface. NOT [Leaf blades 0.5 2 (3) cm, broadly ovate, obovate to broadly orbicular; branchlets with small or large resinous glands; fruiting catkins may be wider (up to 2.5 long) or as narrow as 0.5 cm wide; scales usually uniform in color on the inner (adaxial) surface]

 

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Betula: Answers to key questions in Conifers & Catkin-Bearing Trees and Shrubs of Saskatchewan, Fascicle 5, Flora of Saskatchewan by Anna Leighton leading to this genus. The answers are in the order you would normally work through the key
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Fruit a samara, much shorter than 1 cm and subtended by a flat scale; bud scales and bracts of staminate catkins not coarsely hairy; pistillate catkins many-flowered, erect or pendulous, often borne on short shoots proximal to the staminate catkins. NOT [Fruit a nut over 1 cm long, enclosed in a tube-like involucre; bud scales and bracts of staminate catkins with coarse, straight, appressed hairs; pistillate catkins few-flowered and borne on short bud-like branches that are distal to the staminate catkins and visible primarily as reddish styles projecting from the bud at anthesis]

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Branchlets and young branches sparsely to densely covered with raised, round, translucent resinous glands (resinous glands may be absent in B. papyrifera, a tree with white bark exfoliating in sheets); woody cone-like fruiting catkins not present on branches at any season; scales of pistillate catkins deciduous, 3-lobed, not greatly thickened toward a truncate apex. NOT [Branchlets and young branches lacking resinous glands; woody cone-like fruiting catkins persisting on branches even in winter; scales of pistillate catkins persisting on the catkin after samaras have fallen, 5-lobed, greatly thickened towards a +/- truncate apex]

 

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Betulaceae: Answers to key questions in Budd's Flora  leading to this family. 
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shrubs or trees; NOT [herbs]

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male flowers, at least, in catkins or aments; NOT [flowers not in catkins or aments]

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flowers with only one floral ring, with sepals but not petals

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styles 2; NOT [styles 3 or more]

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fruit NOT an acorn

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seeds WITHOUT a tuft of hairs