BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Alnus rhombifolia | White Alder 



GENERAL BOTANICAL CHARACTERISTICS : 
White alder is a small to medium-sized deciduous tree.  It ranges from
16 to 115 feet (5-35 m) in height, but mature trees are typically 50 to
80 feet (15-24 m) tall [1,22,34].  In California, trees commonly reach
11 inches (28 cm) in d.b.h. and can reach up to 21 inches (53 cm) in
d.b.h. [5].  The largest tree on record is 112 feet (35 m) tall with a
38.6 inch (98 cm) diameter.  Mature trees typically have several trunks
arising from a single clump.  The bark is light gray, whitish, smooth or
slightly rough on young trees but becomes plated and reddish brown at
maturity [13].  The leaves are 2 to 4 inches (5-10 cm) long,
oblong-ovate, with coarsely double-toothed margins, dark green above and
light green beneath [32].  Male and female flowers occur on the same
tree in catkins.  The drooping staminate catkins are 1.25 to 4 inches
(3-10 cm) long, with two to several clustered near the end of a twig.
Pistillate catkins are erect, 0.4 to 0.6 inch (10-15 mm) long, turning
woody and conelike at maturity [13,20].

White alder is closely related to red alder (Alnus rubra).  Although the
two species are difficult to differentiate when growing together, their
distribution and habitats do not overlap to any great extent.  In the
summer, leaf characteristics can be used to separate these alders.
    


RAUNKIAER LIFE FORM : 
   Phanerophyte
   Hemicryptophyte
   Geophyte


REGENERATION PROCESSES : 
White alder regenerates well from both seeds and sprouts.  Seeds seem to
be important in the colonization of new areas, such as sand bars, but
established plants show a high degree of vegetative reproduction, mostly
from root or trunk sprouting [32,35].  In a California riparian study,
60 to 70 percent of mature or pole-sized white alder trees had either
root or trunk sprouts [35].  Another California riparian study found
that stands growing on river sediments reproduced mostly from layering
[25].  Layering did not occur after terrace buildup along the river
caused the ground surface to become higher and therefore drier.

White alder trees are monecious and primarily wind pollinated.  After
fertilization, female catkins develop into woody cones, which contain
numerous, winged, nutlike seeds [34].  There are approximately 650,000
air dried seeds per pound (1,430,000/kg), of which about 65 to 71
percent are viable [34].  The wind and water transported seeds germinate
rapidly on sunny, wet mineral sites exposed from receding flood waters.
Seedling establishment appears restricted to sites with a continuously
moist substrate.  Seedlings probably do not survive on sites that dry
out during the summer [6].


SITE CHARACTERISTICS : 
White alder occurs primarily in forest riparian areas but also extends
along major streams into nonforested bunchgrass, sagebrush-grass, and
chaparral types [22].  It is restricted to streams that run all year,
and in dry years is a better indicator of water than either cottonwoods
or willows [1,21].  Trees are mostly restricted to the flooding zone and
become infrequent farther away from streams [6,8,25,35].  Throughout
most of California, white alder is often a dominant or codominant in
riparian deciduous forests [21,31].  White alder tends to replace red
alder in the southerly valleys between the Cascades and the Coastal
Range in Washington and Oregon [12].

Associates:  Common associates are Fremont cottonwood, Oregon ash
(Fraxinus latifolia), velvet ash (F. velutina), arroyo willow (Salix
lasiolepis), red willow (S. laevigata), Pacific willow (S. lasiandra),
Hinds willow (S. hindsiana), boxelder (Acer negundo), valley oak
(Quercus lobata), and Oregon white oak (Q. garryana) [2,8,31].

In California, white alder occurs at elevations ranging from sea level
to over 8,000 feet (2,438 m) [18].


SUCCESSIONAL STATUS : 
White alder is a riparian pioneer species.  It produces large quantities
of wind- and water-transported seed, which often colonize fine-textured
soils of sandbars or other fresh alluvium exposed by receding flood
waters [6,25].  Seedling establishment appears restricted to these areas
which have continuously moist substrates.  With time, some stands form a
dominant canopy and may be self-perpetuating.


SEASONAL DEVELOPMENT : 
White alder is a deciduous tree.  In Oregon, white alder flowers in
March and the fruits ripen in late September to early October [34].  In
the southern portion of its range in southern California, it retains its
leaves most of the year, but farther north leaves are shed before the
onset of winter [34].

Related categories for Species: Alnus rhombifolia | White Alder