BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Alnus rubra | Red Alder 



GENERAL BOTANICAL CHARACTERISTICS : 
Red alder is the largest American alder.  It is a rapidly growing,
short-lived, medium-sized, deciduous tree, generally with one straight
distinct trunk.  Red alder reaches a maximum height of about 120 feet
(37 m) with a maximum trunk diameter of about 32 inches (80 cm).
However, mature trees are typically from 80 to 100 feet (24-30 m) tall
and 14 to 18 inches (36-46 cm) in diameter [14,25,34,59].  Maximum age is
one hundred years [58].  Trees growing in the Puget Sound area exibit
the following age-growth characteristics [14,34]:

     Age          Height         Diameter Breast Height
   (years)    (feet/meters)       (inches/centimeters)
 
      5         18 / 5.5                ------
     10         40 / 12.2               ------
     20         65 / 19.8               ------
     30         82 / 25.0              11 / 27.9                 
     40         90 / 27.4              13 / 33.0
     50         98 / 29.9              16 / 40.6
     60        105 / 32.0              18 / 45.7

Red alder has thin (less than 0.75 inch [1.9 cm]), smooth gray-whitish
mottled bark, which is often covered with green moss.  The root system
is shallow, but wind throw is seldom a problem, since the leaves are
absent during winter and early spring when winds are the strongest and
when soils are saturated with moisture [59]. Male and female flowers
occur on the same tree in catkins.  The drooping staminate catkins are
up to 5 inches (12 cm) long and clustered near the end of a twig.  The
pistillate catkins are erect, 0.6 to ).8 inch (1.5-2 cm) long, turning
woody and conelike at maturity [14,26].

Red alder is closely related to white alder (Alnus rhombifolia), and the
two species are difficult to differentiate when growing together.
However, their distribution and habitats do not overlap to any great
extent.  In the summer, leaf characteristics can be used to separate
these alders, but in the winter proper identification is based on many
subtle points.  Several keys exist for proper identification.


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Sexual reproduction:  Red alder regenerates primarily by seed.  Plants
are monoecious and are primarily wind pollinated.  Flowering generally
occurs from late February to early May depending on latitude and climate
[36].  After fertilization female catkins develop into woody cones about
0.5 to 1 inch (1.2-2.5 cm) long, containing 50 to 100 small, flattened,
winged, nutlike seeds [16,67].

Seed dispersal and production:  Seed dispersal begins soon after
ripening in late summer, but most seeds are shed during fall and winter
[14,66].  The seeds are very lightweight (about 666,000/pound
[1,465,000/kg]) and are normally carried up to several hundred yards in
the direction of the prevailing winds.  Seed production begins at about
10 years [58] (but sometimes sooner), and continues throughout maturity,
with optimum production at about 25 years of age [36].  A prolific
seeder, red alder produces peak crops about every 4 years, with moderate
to light crops produced in between [36].  Total seed crop failure is
very rare; however, a total crop failure did occur following a severe
freeze in November 1955 [16,36].

Germination:  Under natural conditions, germination occurs in the
spring.  Germination is best on moist mineral soil in full sunlight
[14,36].  Seed also germinates well on rotten wood and duff [45], and to
a lesser extent on soil organic horizons and on rock-surfaced logging
roads, but the roots must quickly penetrate to a moist nutritious
substrate if seedlings are to survive [14].  Sunlight is required for
germination [36].  Seeds under thick vegetation or buried deeply in the
soil, willnot germinate until the site is disturbed, exposing the seeds
to sunlight.  Germination percentages range from 59 to 84 percent
[52,58].  Germination rates of stratified and nonstratified seeds are
about equal.  Low germination percentages may be due to a high
proportion of empty seeds known to occur in red alder [52].  Seeds
remain viable in storage for about 3 years [25].

Seedling establishment:  Generally, exposed mineral soil is needed for
seedling establishment.  Seed production is normally so prolific that
dense stands quickly develop on exposed soils of logging roads,
clearcuts, and burned over areas [25].  These areas may have from
several hundred thousand to several million red alder seedlings per
hectare in the spring of the first year after the disturbance [69].
Regeneration and establishment in dense, thick, brush fields is
infrequent due to the lack of exposed sights, but any disturbance which
removes the brush and exposes the soil will favor red alder
establishment [67].  Plants often reach 6 to 18 inches (15-45 cm) in 1
year and may reach 18 feet (5.5 m) in 5 years [16].  This rapid juvenile
growth gives the shade-intolerant red alder a competitive edge over
conifers, as it quickly overtops them.

Vegetative reproduction:  Red alder will sprout following an injury to a
stem, but in the absence of disturbance, sprouting is infrequent.  Red
alder's sprouting ability following cutting is summarized as follows
[25,27]:

(1)  Sprouting vigor is greatest on trees 1 to 3 years old.  Vigor falls
     off dramatically after 15 years.  Trees 15 years or older
     rarely have live sprouts 2 years after cutting.

(2)  The average number of sprouts per cut stem increases significantly
     as stump height is increased. 

(3)  Stumps with the cut surface facing south or west have the least
     mortality and are most likely to sprout.    

(4)  Level cuts have higher mortality and fewer sprouts than angled
     stumps. 

(5)  Stems cut in January have the lowest mortality; stems cut during the
     growing season, especially July or August, have the highest
     mortality.

(6)  The number of sprouts per cut stem is not affected by the season of
     cutting.

Propagation:  Cuttings of red alder do not root easily [66].
  


SITE CHARACTERISTICS : 
Historical evidence suggests that the distribution of red alder was much
more restricted than it is today; it occurred chiefly along streams and
in other wet areas [18].  Continual disturbance over the past 100 years,
primarily from logging, has created an abundance of open areas with bare
mineral soil (both are required for seedling establishment) which red
alder has colonized, thus increasing its acreage dramatically. This is
especially true of uplands, where it was previously infrequent.

Red alder is found primarily within Douglas-fir, western hemlock,
western redcedar, sitka spruce, and grand fir forests in the Pacific
Northwest [16,66,67].  Individual trees or clumps of trees may occur to
varying degrees of mixture within these coniferous forests, but stand
development is best along streams, moist bottomlands, and moist lower
slopes [14,35,66,67].  In these mesic locations, pure stands are nearly
always even-aged.  Along the southern portion of its range in California
and in the dry interior valleys of Washington and Oregon, red alder is
restricted to riparian deciduous forests [8,19].  In these areas it may
mix with white alder.

Soils:  Red alder occurs on a wide variety of soil types ranging from
well-drained gravels and sands to poorly drained clay or organic soils
[25].  The best stands are found on deep, well-drained loams or sandy
loams of alluvial origin [14,16].  Stands also grow well on residual or
colluvial soils of volcanic origin [14].  Soils under red alder stands
develop higher available and total nitrogen contents than soils under
adjacent coniferous stands because of red alder's ability to fix
nitrogen.  Nitrogen accretion rates vary with stand location, vigor,
age, and density, with rates varying from 40 to 300 pounds of nitrogen
per acre (45-355 kg/ha) per year [14,25].  As soil nitrogen increases,
soil pH under red alder stands drops.  In coastal Oregon, pure alder
stands had soil pH values averaging 4.3 to 4.4, while adjacent conifer
stands had pH values averaging 5.3 [20].  Red alder leaves also contain
significant amounts of nitrogen.  Leaves decompose rapidly, forming a
deep humus and thus improving soil structure [16].

Climate:  Red alder grows in humid coastal climates characterized by
cool wet winters and warm dry summers.  Mean annual precipitation ranges
from 16 to 220 inches (40-560 cm) [25]; precipitation occurs chiefly as
rain during the winter.  Trees need more than 25 inches (64 cm) of
precipitation annually, and most stands are located on sites receiving
in excess of 40 inches (102 cm) [16].

Coniferous forest associates:  Red alder grows both in pure stands and
in mixtures with native conifers [67].  Pure and mixed stands are mostly
even aged.  Trees generally become established in forest openings
created from a disturbance.  As stands develop and trees mature, they
prevent other red alder seedlings from becoming established, due to the
seedlings' shade intolerance.  Common coniferous associates include:
Douglas-fir, western redcedar, western hemlock, grand fir, and Sitka
spruce.  Red alder communities within coniferous forests contain a
number of deciduous trees and shrubs also.  Deciduous trees and shrubs
include bigleaf maple, vine maple (Acer circinatum), Pacific willow
(Salix lasiandra), and bitter cherry (Prunus emarginata).  Understory
shrubs and herbs include salmonberry, western thimbleberry, American
devilsclub (Oplopanax horridum), black elderberry (Sambucus racemosa),
trailing blackberry (Rhubus ursinus), Siberian minerslettuce (Montia
sibirica), and false lily-of-the-valley (Maianthemum dilatatum)
[19,21,67].

Deciduous riparian forest associates:  In California, red alder chiefly
occurs in riparian forests where it often codominates with Sitka spruce,
redwood (Sequoia sempervirens), black cottonwood, bigleaf maple, and
Pacific willow [54,56].  Along larger rivers in Oregon and Washington,
red alder typically codominates with bigleaf maple, Oregon ash (Fraxinus
latifolia), black cottonwood, and willows (Salix spp.) [18,19].  Other
deciduous forest associates include Pacific wax-myrtle (Myrica
californica), Pacific red elder (Sambucus callicarpa), and California
laurel (Umbellularia californica) [18,56].

Elevation:  Red alder is generally found no farther inland than 100
miles, at elevations below 2,500 feet.  Elevational ranges for several
western states are presented below [16,33,34,48,66]:

from sea level to 500 feet (0-152 m) in CA
     1,030-3,600 feet (314-1,097 m) in ID
     sea level to 2,500 feet (0-762 m) in OR
     sea level to 2,500 feet (0-762 m) in WA


SUCCESSIONAL STATUS : 
Red alder is common on many disturbed areas.  Many of the stands which
exist today were created because of the pioneering habit of red alder.
Red alder quickly invaded lands clearcut and burned during the 1920's to
1940's [3,39].  In 1920 there was an estimated 1/3 of a billion board
feet of red alder, but by the mid 1960's, red alder acreage increased
dramatically, containing an estimated 12 billion board feet [39].  By
1988, red alder covered an estimated 13 percent of commercial forest
lands along the Pacific Coast of Washington and Oregon [55].

Red alder is an early seral species.  It quickly invades forest
openings, such as those created from fires, logging, wind throws, or
road cuts, and it also pioneers volcanic mud flows [19,46].  Red alder
and Douglas-fir are reported as the principal pioneer tree species of
lower and middle elevation forests from southwestern British Columbia to
northwestern California [41]. Thus they often dominate the first
postfire community in the Pacific Northwest.  Disturbed areas are
naturally seeded by numerous wind-dispersed seeds, resulting in stands
that start out with several thousand alder trees per acre [3].  Due to
red alder's shade intolerance, stands are self-thinning; trees that do
not maintain their height in the canopy die, resulting in even-aged
stands [17,66].  Conifers such as Douglas-fir that become established at
the same time are quickly overtopped by this extremely fast growing
species.  These early seral red alder communities suppress competing
conifers, but after about 25 years, conifers equal red alder height and
begin to overtop them.  After about 40 years, Douglas-fir becomes
dominant.  Few red alder trees remain in stands past 60 years [16,67].


SEASONAL DEVELOPMENT : 
Red alder is a deciduous tree.  Flowers begin to form in the spring
before the leaves expand.  Flowering in Oregon and Washington generally
begins in late February and continues until early May [58].  Fruits
ripen in late summer and early fall; seeds are dispersed during fall and
winter [58] and can often be seen on fresh snow.  The following dates
have been recorded for phenological events in Oregon and Washington
[16]:

Phenological Event       17 locations throughout      Snohomish County 
                          Oregon and Washington          Washington

Leaf bud bursts           March 21 - April 23              April 3
Completely leafed out     April 9 - May 16                 May 2
Flowering begins          February 19 - May 4              March 29
Flowers all fallen        March 18 - June 5                April 12
Fruit ripe                August 5 - October 29            August 27
Leaves mostly fallen      September 18 - November 24       November 13

In the Cascade Mountains of Oregon, red alder is one of the first
deciduous trees to lose its leaves.  The majority of leaves fall during
October, with virtually all leaves lost by the first week in November
[10].

Related categories for Species: Alnus rubra | Red Alder