Download COMMON SPECKLED ALDER (Alnus rugosa)

River-Lab 5 Guide Manual
COMMON SPECKLED ALDER (Alnus rugosa)
Alders contribute to
basin productivity by stabilizing
the stream banks where they
stand;
by enriching soil,
including
making
nitrogen
available to other plants; and by
providing food and habitat.
Alders are fast-growing
shrubs that branch out over the
ground. They spread rapidly as
new plants sprout from the base
of established plants and from
underground stems.
Even
branches can produce new plants. When a drooping branch touches the ground, it may
send down roots and send up new shoots. With many trunks clumped together, thickets
of alder plants can protect streambanks from the erosive attack of floodwater. The dense
mat of roots and underground stems helps hold soil to keep streambanks stable.
Soil enrichment is an alder specialty. Alders enrich soil by trapping mulch,
including their own dropped leaves and flowering parts, under the low branches. Even
more importantly, alders are one of a special group of plants that have bacteria in nodules
(swellings) on their roots that convert nitrogen into forms plants can use. Nitrogen is an
essential element for plant growth. Air pockets in soil contain nitrogen gas, but this gas
must be changed into ammonia or nitrates before plants can use it. The bacteria that live
on alder roots make this necessary conversion, which benefits all plants in the alder’s
immediate area.
Alders provide food and habitat for many animals. Seeds are eaten by birds,
especially chickadees and goldfinches. Goldfinches and redwing blackbirds nest in the
alder branches in spring. Other wildlife find cover in alder thickets. Deer and muskrats
browse on twigs. Muskrats and shrews tunnel among the roots underground. In spring,
insects such as beetles eat the leaves and lay eggs on them. In late summer and early
autumn, woolly aphids cover stems and eat the sap of the alder. A small, brown and
orange butterfly, the wanderer, feeds on the aphids.
The speckled alder may grow as a small tree, up to twenty feet tall, or as a large woody shrub six
to fifteen feet tall. It prefers wet, spongy areas such as the banks of streams. A relative of the birch, the
alder’s dark grayish-brown bark is marked with white speckles. The dull, dark green leaves, two to four
inches long, are egg-shaped, with the base broader than the tip.
© 1999 Mill River Wetland Committee, Inc.
5GM - 14
River-Lab 5 Guide Manual
COMMON SPECKLED ALDER, cont’d.
The alder’s flowering parts, its male and female catkins, develop on the same
plant, which makes alders monoecious. New male catkins look like miniature brown fox
tails, grouped near the tips of the alder branches. They develop into long, drooping, soft
greenish-yellow tassels of tiny blossoms. These tassels grow longer and longer as they
mature (to 1 1/2 – 1 5/8 inches), swelling and finally bursting with yellow grains of
pollen.
Female catkins resemble small pinecones. They grow in clusters above the male
catkins. In the spring these small cone-like catkins (1/2 – 5/8 inch long) have dark
“hairs” that stick out. These are the stigmas that catch wind-blown pollen. Like the male
catkins, these catkins will grow larger as they mature (to about 3/4 inch). In autumn they
will be filled with tiny seeds, which will be dispersed by winds and animals. In spring,
the old female catkins can still be seen on the same shrub with the new smaller ones.
Most of the old ones are empty, but some may still contain seeds. Whether they are
dropped to the ground in wastes of animals that have eaten them or dropped directly from
the catkins, fertilized alder seeds will give rise to new alder shrubs, ensuring that the
special benefits of this plant’s contributions to the basin system will continue.
5GM – 15
© 1999 Mill River Wetland Committee, Inc.