BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Alnus incana ssp. tenuifolia | Thinleaf Alder 



GENERAL BOTANICAL CHARACTERISTICS : 
Thinleaf alder is a deciduous multistemmed shrub or small tree which
tends to form thickets and may grow up to 40 feet (12 m) tall [2,42].
More typically, mature plants are 6 to 15 feet (2-5 m) tall, with 4 to 8
inch (10-20 cm) diameter trunks [45,53,55].  The bark is thin, smooth,
and green-gray, grayish-brown, or reddish-brown [22,42,45].  The leaves
are broadly elliptic or ovate-oblong, mostly 1 to 3 inches (3-7 cm)
long, dull green on both sides, with doubly dentate margins [22,42].
Male and female flowers occur on the same plant in catkins.  The
drooping staminate catkins are clustered at the end of a twig, each
about 1 to 4 inches (3-10 cm) long [22].  Clusters of three to nine
pistillate catkins (which develop into cones) are on short stout stalks.
Each catkin is about 0.4 to 0.6 inch (9-13 mm) long [22,42].  The cones
remain on plants for about a year after seeds are shed, aiding in
identification during winter.


RAUNKIAER LIFE FORM : 
   Undisturbed State:  Phanerophyte (microphanerophyte)
   Burned or Clipped State:  Hemicryptophyte


REGENERATION PROCESSES : 
Thinleaf alder reproduces both sexually and vegetatively.  

Sexual Reproduction:  Male and female flowers of thinleaf alder occur in
catkins on the same plant.  The female catkins are small, 0.4 to 0.6
inch (9-13 mm) long, semiwoody, conelike, and wind pollinated.  The
fruit is a small, single-seeded nutlet, with narrow lateral wings which
aid in dispersal by wind and water [15,39,50].  Thinleaf alder produces
abundant seed which is dispersed during fall and winter [15,59].  There
are about 675,000 cleaned air-dried seeds per pound (1,488,000/kg) [50].
Annual seedfall observations in Alaska showed that 3,305 seeds/m sq were
found in soil under thinleaf alder stands, but of these, only 745 were
viable [59].  In fact, seed viability can be quite low; as many as 95
percent of thinleaf alder seeds have been found empty [15,50].  In Oregon
and Washington, female cones are often disfigured with disease, but the
effects on seed viability have not been reported [20].  Seeds require no
treatment or prechilling to break dormancy and can be expected to
germinate immediately after dispersal when conditions are favorable [15].
Seed of the closely related speckled alder has remained viable in
storage for up to 10 years when stored in sealed containers at 34 to 38
degrees F (1-3 C) [50].  Germination and seedling establishment seems
better on exposed mineral soils than on organic substances [59].

Vegetative Reproduction:  Thinleaf alder often occurs in dense thickets,
which reportedly result from underground rhizomes or suckers [6].
However thinleaf alder is a prolific seeder, and thickets could be
produced by natural seeding alone [11].  More recent studies suggest that
plants found in thickets are not clones [51].  However, sprouting of
exposed thinleaf alder roots in streams has been noted, and submerged
branches sometimes form adventitious roots [11,15].  If plants are
damaged, sprouting can occur from the root collar or stump [15].  After
top-removal by beavers, plants have been observed to sprout heavily from
the cut [28].  Plants can also sprout from the root crown following fire
[59].


SITE CHARACTERISTICS : 
Thinleaf alder seldom grows away from water and is typically confined
to rivers, moist stream borders, overflow channels, or moist mountain
springs or seeps and only occasionally occurs in broad floodplains
[2,17,30].  Most sites are seasonally flooded, and water tables normally
remain within 3 feet (1 m) of the ground surface [17,28,31].  Restricted
to high water tables, thinleaf alder communities often form continuous
narrow stringers immediately adjacent to perennial streams [28,58].
Thinleaf alder has a high flood tolerance and thus stabilizes
streambanks.  Quite shade tolerant, it is frequently found growing in
the understory of coniferous forests on moist sites [2,15].

Soils:  Thinleaf alder typically grows on poorly developed soils of
cobbles, gravels, or sands [17,31].  Soils usually remain moist
yearround due to high water tables.  As stands develop along moving
water, finer fluvial deposits are trapped, which eventually develop into
surface soil textures of loams to sandy loams overlying coarser
substrates [17,28,31].  Mountain springs and seeps in Oregon have
surface soils 6 to 20 inches (15-50 cm) deep, composed of organic mucks
or organic loams over a stony subsoil [31].

Soils under thinleaf alder are normally higher in available nitrogen
than adjacent communities, since thinleaf alder can fix between 41 and
349 pounds/acre (43-360 kg/ha) of nitrogen annually [15].  

Associates:  Thinleaf alder is commonly found with shrubs such as
re-dosier dogwood (Cornus serices), Hudson Bay currant (Ribes
hudsonianum), prickly currant (R. lacustre), woods rose (Rosa woodsii),
Douglas spirea (Spiraea douglasii), and common snowberry (Symphoricarpos
albus).  Common associated herbs include bullrush (Scirpus spp.), sedges
(Carex spp.), bluejoint reedgrass (Calamagrostis canadensis), timothy
(Phloem pratense), fowl bluegrass (Poa palustris), and Kentucky
bluegrass (Poa pratensis) [17,28,31,58].  In the Great Basin, thinleaf
alder is most commonly associated with water birch (Betula occidentalis)
and willows (Salix spp.) [33].

Elevation:  Thinleaf alder is typically a low to middle elevation
species.  Elevational ranges for the following western states are
presented below [8,25,31,55]: 

from 5,000 to 10,000 feet (1,524-3,048 m) in CO
     2,500 to 8,000 feet (762-2,438 m) in ID
     2,500 to 8,000 feet (762-2,438 m) in MT
     2,200 to 5,700 feet (671-1,737 m) in OR
     4,500 to 8,000 feet (1,372-2,438 m) in the Sierra Nevada Mtns.
     4,100 to 9,000 feet (1,250-2,745 m) in UT
     6,200 to 9,000 feet (1,890-2,745 m) in WY


SUCCESSIONAL STATUS : 
Thinleaf alder is an early seral species [28,59].  Seasonal disturbances
from flooding provide suitable seedbeds for establishment of new plants.
Many thinleaf alder communities appear to be seral to cottonwood
(Populus spp.) and willow (Salix spp.) [28,31].  


SEASONAL DEVELOPMENT : 
Both staminate and pistillate catkins of thinleaf alder are produced
during the gowing season prior to blooming and are exposed during the
winter.  Catkins then expand before the leaves emerge in the spring.
Flowering generally begins during March and April in the northwestern
United States [15].  Cones ripen in the fall, after which dispersal
occurs.  The empty cones remain on plants for about a year.  Leaves
remain green until they are dropped in the fall.  

The average dates of phenological events in Montana and Idaho are
presented below [49]:

                    East of Continental      West of Continental
                    Divide: Montana          Divide: Montana & Idaho

Leaf bud burst          May 19                     May 7
Leaves full green       June 18                    June 8
Flowering starts        April 15                   May 15
Flowering ends          May 6                      May 30
Fruit ripe              August 3                   August 24
Seed fall starts        June 6                     Sept 10
Leaves start to wither  Sept 11                    Sept 5
Leaves begin to fall    Sept 22                    Sept 20
Leaves fallen           Oct 5                      Oct 14

The flowering dates for several western states are presented below
[8,15,39,42,49,50]:

Flowering Begins        Flowering Ends        Location

  May                       ----                 AK 
  April                    August                CO
  March                    July                  ID
  March                    August                MT
  March                     ----                 OR
  February                  ----                 Great Basin
  March                     June                 WY         

Related categories for Species: Alnus incana ssp. tenuifolia | Thinleaf Alder