Know Your Natives – Alder

Alder (Alnus serrulata) of the Birch (Betulaceae) family is a deciduous shrub found in the U.S. from Texas to Kansas to Maine and thence east and south to the borders.  This shrub, the only species in the Alnus genus in Arkansas, is found throughout most of the state except for portions of the Mississippi Alluvial Plain.  It grows in a wide variety of moist to wet soils in light shade to partial sun along banks of streams and ponds and bordering sloughs and wetlands.  The genus name is the Latin name for the alder.  The specific epithet refers to the toothed leaf margins (serrulate).  Our Arkansas species is also known as river alder, smooth alder, common alder and tag alder.

Alder is a multi-stemmed, suckering plant that may reach 15 feet tall, with stems up to 5 inches in diameter.  The suckering characteristic of the plant stabilizes stream banks from floods, and plants can be totally immersed during flooding without being uprooted.  Branching of lower stems and trunks is limited whereas upper branches and twigs are numerous.  Mature twigs and branches are terete (round), glabrous, waxy-smooth and brownish gray.  Main trunks of a mature plant have thin bark, but may become knobby and slightly roughened.  A symbiotic relationship exists between the alder and a bacterium, whereby (in root nodules) the shrub provides food and the bacterium provides nitrogen.  Alder is consumed by many insects and birds as well as deer and beaver.

Alder - Alnus serrulataPhoto 1:  Stems, trunks and green leaves of alder in flood zone of a perennial stream.  Trunk seen on right was taken by beaver.

Buds, appearing in late summer on short stalks (stipes) along current year’s growth, become the twigs of the next year.  These twig buds, covered by reddish valvate (without overlap) scales, have a match-head shape with a rounded to pointed tip.  In spring, light green pubescent slender twigs grow from these twig buds to produce simple leaves and, at ends of twigs, peduncles with catkins to bloom the next spring.  The peduncles grow from leaf axils protected by a leafy green stipule.  Twigs and peduncles become gray, with raised light gray lenticels (pores).

Alder has alternate, elliptic to obovate leaves with an upper surface that is medium green and glabrous and a lower surface that is slightly lighter green with light pubescence, especially along main veins.  Leaves, on pubescent 1-inch long petioles, have flat blades that are 2 to 4 inches long and 1 to 3 inches wide.  Margins of the simple leaves are crowded with small teeth, especially on the upper half, that are angled toward the tip (serrulate).  Leaf outline may be jagged to wavy, but still with teeth.  Side margins of leaves taper equally to base and apex with the base  being rounded to cuneate (wedge-shape) while the apex is rounded to obtuse and may terminate with a point.  Pinnate veins are prominent, with parallel lateral veins that are widely spaced and offset across main vein.  Veins of upper (adaxial) surface are sunken while on lower (abaxial) surface, veins are raised.  Blades are rugose (slightly elevated between veins) and appear somewhat leathery (coriaceous).  Leaves gradually drop from the plant into winter, with leaves on younger stems persisting longer.

Alder - Alnus serrulataPhoto 2:  Serrulate simple leaves with distinctive veins.  In late fall, male catkins (yellow arrow), female catkins (red arrow) and stalked buds (white arrow) for next year’s twigs are present.

Alder is monoecious (separate male and female flowers on same plant).  The developing inflorescences first begin to appear in late summer as dense spikes (catkins) composed of many inconspicuous flowers covered by floral scales.  Both sexes grow side-by-side in panicles of two to five catkins on separate peduncles at ends of twigs.  From fall into winter, male catkins are about 1 inch long while female catkins are about one-eighth inch long.  Male catkins are slender cylinders while female catkins are stubby erect cylinders which may appear similar to twig buds.  From fall into winter, the greenish male catkins are reddish on the sun-side while female catkins are reddish overall.

Actual blooming occurs in late winter to early spring before twig buds open.  With bloom, male catkins dangle, elongating to 4 inches, as floral scales that cover flowers loosen and catkins become yellow with pollen.  Female catkins expand to ½ inch long as flowers reach anthesis and reddish styles become exserted, causing the catkin to look prickly.  Pollination is by wind (anemophily).

Alder - Alnus serrulataPhoto 3:   In late winter, elongate dangling male catkins release pollen.  Erect female catkins can be seen at top of photo above the male catkins.  Stalked twig buds, still tightly closed, can be seen lower on stem.

After blooming, male catkins are shed while female catkins enlarge to form compound fruits (infructescences).  As seeds develop, female catkins become hard, shiny, green, ovoid, short-stemmed, cone-like structures with densely overlapping spiraling bracts along a central stem and a bumpy surface as scales thicken.

Alder - Alnus serrulataPhoto 4:  Upright female catkins in mid-fall.  Appearance dominated by floral scales protecting a spike of immature seeds.  Seeds occur above each scale.

Alder - Alnus serrulataPhoto 5:  Dangling male catkins in mid-fall.  Floral scales protect a spike of immature flowers.   Dangling position retained throughout bloom.

In late summer, the female “cones” become dark brown and woody, resembling small pine cones.  Bracts of the cones spread open and seeds (actually tiny 1-seeded fruits called achenes) are dispersed.  Empty cones remain on the plant into the following spring.  Achenes are flattened and obovate, with narrow wings around the edge with an indented base.  Achenes are dispersed by wind and flowing water.

Alder - Alnus serrulataPhoto 6:  In mid-February, the previous year’s empty dark brown seed cones remain while the current year’s dangling male catkins, stubby female catkins with exserted styles (red arrow), and stalked twig buds (white arrow) can be seen.

In summer, alder may be confused with Ozark witch hazel (Hamamelis vernalis) which has somewhat similar structure, height, bark and leaves, and grows in similar habitat in the Interior Highlands.  Read more about Ozark witch hazel here.

Article and photographs by ANPS member Sid Vogelpohl

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