BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Betula papyrifera | Paper Birch 



GENERAL BOTANICAL CHARACTERISTICS : 
Paper birch is a medium-sized, single- or multiple-stemmed, deciduous
tree.  In forests it typically has a slender trunk with a narrow crown,
but in openings it has a wider crown spreading out from near the base
[24].  Multiple-stemmed trees are relatively common as a result of
browsing by moose and snowhoe hares [21].  Throughout much of its range,
mature trees are 70 to 80 feet (21-24 m) tall and 10 to 12 inches (25-30
cm) in trunk diameter, but sometimes grow up to 30 inches (75 cm) in
diameter [31,57].  In Alaska, paper birch trees are commonly 20 to 60
feet (6-18 m) high and 4 to 12 inches (10-30 cm) in trunk diameter [66].

Paper birch is short-lived.  Height growth ceases at about 60 to 70
years of age; few trees live more than 140 years [24].  Paper birch is
shallow-rooted with few roots found deeper than 24 inches (60 cm) below
the soil surface [57].  The bark is reddish-brown on saplings.  On
mature trees bark is thin, white, and smooth, often separating into
papery strips, and is easily peeled off in sheets [24,66].

Male and female flowers occur in separate, pendulous catkins on the same
tree [24].  Fruits are winged-nutlets 0.06 inch (1.5 mm) long by 0.03
inch (0.8 mm) wide [57].


RAUNKIAER LIFE FORM : 
   Undisturbed State:  Phanerophyte (mesophanerophyte)
   Burned or Clipped State:  Chamaephyte
   Burned or Clipped State:  Hemicryptophyte
   Burned or Clipped State:  Therophyte


REGENERATION PROCESSES : 
Seed production:  Paper birch is a prolific seed producer.  Seed
production begins at about age 15, with optimum production at 40 to 70
years of age [11].  Trees produce good seed crops about every other
year.  Seed production over a 3-year period in Maine ranged from 2.4 to
9.8 million seeds/acre (5.9-24.2/ha) in a paper birch stand with 149
seed trees per acre (368/ha) [8].  In undisturbed paper birch stands in
Alaska, seed production varied between 2.2 and 294 million seeds/acre
(5.4-728 million/ha) [72].

Dispersal:  The small, double-winged seeds are dispersed primarily by
wind.  Most seeds fall 100 to 200 feet (30-61 m) from the parent tree
[43].  Seedfall at a clearcut edge was 60 percent of that within the
uncut stand, and at 328 feet (100 m) into the cut seedfall was 10
percent of that within the stand [57].  Seed may travel great distances
when blown across crusted snow [57].  Nearly all the seed (about 90 to
95 percent) is shed from September through November [51,57].

Seed quality and dormancy:  Discolored and empty seeds make up 14 to 47
percent of a crop [57].  Seed viability is highest during heavy seed
crop years and lowest during light seed crop years.  In Maine,
germination was 77 percent during a heavy seed year, but only 13 and 24
percent during 2 normal years [42].  Seeds dispersed early have lower
germination rates than those dispersed lateer [8].  A small percentage
of the seeds can remain viable on the forest floor for several years
[51].

Germination and seedling establishment:  Germination normally takes
place in the spring following dispersal.  Germination is generally best
on disturbed mineral or mixed mineral-organic soil seedbeds [24,57].
The small seeds are sensitive to soil moisture and temperature.  Thus
shade usually favors germination and initial establishment by preventing
seedbeds from drying out and reaching excessively high temperatures
[43].  South or southwest aspects, excessively drained soils,
insufficient rainfall, competing vegetation, and unshaded and
undisturbed seedbeds deter establishment [51].  Seedlings will not grow
on soils with a pH less than 5.0 [51].  Although germination and early
survival are often best on mineral soils, seedling growth is best on
humus seedbeds in moderate or full sunlight [42].  First year seedlings
are about 2 to 5 inches (5-12 cm) tall [51].

Vegetative reproduction:  Paper birch sprouts following cutting or fire.
Sprouts typically arise from the stump base or root collar [74].
Prolific sprouting is common in young trees, with some individuals
producing up to 100 sprouts [74].  Sprout growth is rapid, sometimes up
to 24 inches (60 cm) in the first growing season [24].  Sprouting vigor
decreases with age.  Forty to fifty percent of 100- to 125-year-old
trees produced stump sprouts within 1 year after cutting, while 80 to 90
percent of 40- to 50-year-old trees produced sprouts [74].


SITE CHARACTERISTICS : 
Paper birch grows in climates ranging from boreal to humid and tolerates
wide variations in the amount and pattern of precipitation [24].  It
grows at the northern limit of tree growth in arctic Canada and Alaska,
in boreal spruce woodlands and forests, in montane and subalpine forests
of the West, in wooded draws of the northern Great Plains, and in
coniferous, deciduous, and, mixed forests of the Northeast and Lake
States [18].  It is shade-intolerant, and abundant on burned-over and
cut-over lands where it often forms pure stands [31].  It is restricted
to openings in older forests.

Paper birch is most abundant on rolling upland terrain and alluvial
sites but grows on almost any soil and topographic situation, including
rugged mountain slopes, open slopes, rock slides, muskegs, and borders
of bogs and swamps [21,24,57].  In interior Alaska, paper birch tends to
dominate cool, moist, north and east aspects, while aspen dominates
warmer and drier, south and west aspects [57].  In the mountains of New
England and New York, paper birch is one of the few hardwoods found near
timberline [76].  In North Dakota, it is mostly restricted to moist
draws on north-facing slopes [9].

Soils:  Paper birch grows best on deep, well-drained to moderately
well-drained, sandy or silty Spodosols, Inceptisols, and Entisols common
on glacial deposits [24,57].  It grows on a wide range of soil textures
from gravels to silts, and grows on organic bog and peat soils [24].

Associated trees:  In addition to those species listed under Habitat
Types and Plant Communities, common associates include bigtooth aspen
(Populus grandidentata), pin cherry (Prunus pensylvanica), yellow birch,
gray birch (Betula populifolia), sugar maple (Acer saccharum), red maple
(A. rubrum), and white ash (Fraxinus americana) in the southern and
eastern part of its range, and red spruce (Picea rubens) and jack pine
in boreal regions [76].

Understory:  Conifer seedlings and saplings are typical under mature
paper birch stands.  Associated shrubs include American green alder
(Alnus crispa), beaked hazel (Corylus cornuta), raspberries and
blackberries (Rubus spp.), common bearberry (Arctostaphylos uva-ursi),
blueberries (Vaccinium spp.), Bebb willow (Salix bebbiana), Scouler
willow (S. scouleriana), highbush cranberry (Viburnum edule),
Labrador-tea (Ledum groenlandicum), elder (Sambucus spp.), gooseberry
(Ribes spp.), and dwarf bush-honeysuckle (Diervilla lonicera)
[20,57,66].  In Alaska, Canada reedgrass (Calamagrostis canadensis) is
prominent in paper birch stands, but other grasslike plants, sedges
(Carex spp.), and lichens are principally absent [39].  In Labrador,
lush herbs create a nearly continuous ground layer under paper birch
stands [20].


SUCCESSIONAL STATUS : 
Paper birch is a short-lived, shade-intolerant, pioneer species.  It
rapidly colonizes open disturbed sites created by wildfire, windthrow,
or avalanche but lasts only one generation before it is replaced by
shade-tolerant conifers or northern hardwoods.  Paper birch seeds-in
aggressively after wildfire, often forming large, essentially pure
stands [19,21,39].  Depending on the recovery of other species following
fire, paper birch may also occur in mixed postfire stands with spruces,
aspen, and other hardwoods [15,27,39].

Seral paper birch stands resulting from wildfire in Alaska commonly have
3,000 to 6,000 trees per acre (7,470-14,820/ha) 20 years after
establishment [39].  By 60 to 90 years, stands have thinned to 500 to
800 trees per acre (1,235-1,976/ha) [19,39].  Seedbeds under these paper
birch stand are unfavorable for germination of birch seed, but spruce
seedlings are common.  By 120 to 150 years after fire, black or white
spruce dominate [39,66].

In southeastern Labrador, paper birch seedling establishment begins
promptly after fire and is restricted to the first 15 postfire years.
At 40 to 50 years after fire conifer seedlings appear in the paper birch
understory.  At 75 to 100 years, paper birch stands begin to deteriorate
and are eventually replaced by conifers unless another fire initiates
paper birch establishment [21].

In boreal mixed woods, paper birch begin dying by 75 years after fire.
At this time jack pine, black spruce, and white spruce begin to dominate
or codominate.  By 125 years most paper birch are dead [17].

In contrast to other boreal regions, paper birch persists in forests for
more than 200 years in eastern Quebec.  This is probably due to spruce
budworm outbreaks which cause white spruce to decline after about 200
years [5].

In the East, paper birch is commonly replaced by northern hardwoods on
well-drained mineral soils, and by spruces and balsam fir on shallow or
poorly drained soils [58].  In Minnesota, paper birch is often replaced
by communities dominated by shrubs, particularly beaked hazel [58].  


SEASONAL DEVELOPMENT : 
Paper birch male catkins are partially formed in the fall, remain
dormant in the winter, and expand to a length of about 4 inches (10 cm)
before flowering in the spring [43].  Female catkins appear in the
spring before the leaves are fully expanded.  In the southern portion of
its range flowering begins in April [24].  In Alaska flowering occurs in
May and June [66].  Seed dispersal may begin as early as August, but
most seed is dispersed from September throughout November [8].

Paper birch phenological events proceed as follows in northeastern
Minnesota [2]:  

Phenological event    Time

bud burst               April
leafing out             late April - early May
flowering begins        April
pollen shed             late April - May
seedfall begins         August
leaf color change       September
leaf fall               late September - October

Related categories for Species: Betula papyrifera | Paper Birch