BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Populus tremuloides | Quaking Aspen 



GENERAL BOTANICAL CHARACTERISTICS : 
Quaking aspen is a native deciduous tree.  It is small- to medium-sized,
typically less than 48 feet (15 m) in height and 16 inches (40 cm) dbh
[75].  It has spreading branches and a pyramidal or rounded crown
[60,75,88,166].  The bark is thin.  Leaves are orb- to ovately shaped,
with flattened petioles [90].  The fruit is a tufted capsule bearing six
to eight seeds.  A single female catkin usually bears 70 to 100 capsules
[88,166].  The root system is relatively shallow, with widespreading
lateral roots and vertical sinker roots descending from the laterals.
Laterals may extend over 100 feet (30 m) into open areas [88].  Gifford
[59] found that vertical roots of quaking aspen in Utah extended more
than 9 feet (2.7 m) down, branching into fine, dense roots at their
extremities [88].

Quaking aspen forms clones connected by a common parent root system.  It
is typically dieocious, with a given clone being either male or female.
Some clones produce both stamens and pistils, however [88].  Quaking
aspen stands may consist of a single clone or aggregates of clones
[166].  Clones can be distinguished by differences in phenology, leaf
size and shape, braching habit, bark character, and by electrophoresis
[123].  In the West, quaking aspen stands are often even-aged,
originating after a single top-killing event.  Some stands, resulting
from sprouting of a gradually deteriorating stand, may be only broadly
even-aged [88].  Clones east of the Rocky Mountains tend to encompass a
few acres at most [125], and aboveground stems are short lived.  Maximum
age of stems in the Great Lakes States is 50 to 60 years.  Clones in the
West tend to occupy more area, and aboveground stems may live up to 150
years [86].  A male clone in the Wasatch Mountains of Utah occupies 17.2
acres (43 ha) and has more than 47,000 stems.  To date, it is the
world's most massive known organism.  Clone age can be great; the large
Utah clone is estimated to be 1 million years old [107].

Seedling morphology:  Quaking aspen seedlings can easily be
misidentified as cottonwood (Populus spp.) or willow (Salix spp.)
seedlings because quaking aspen seedlings bear only a slight resemblance
to the adult form.  Leaves of quaking aspen seedlings are nearly
lanceolate.  During the first growing season, vertical flattening of the
leaf petioles is not obvious, and there is no lateral branching.  By the
second growing season, leaves are characterisitically orbicular to
ovate, and there is vertical branching.  Renkin and others [133] have
published photographs of excavated quaking aspen seedlings.

Quaking aspen seedlings can be differentiated from root sprouts by leaf
morphology, lack of woody tissue, lack of vertical shoots, and presence
of a taproot [90,133].  There are a few visual clues that can
distinguish seedlings from sprouts without excavation.  Seedlings have
paired cotyledons or cotyledon scars a few millimeters above the soil
surface.  The first pair of true leaves is nearly opposite, at right
angles to, and directly above the cotyledons.  Leaf pattern of sprouts
is strongly alternate [133].

Physiology:  Quaking aspen is not shade tolerant [123,130]; neither does
it tolerate long-term flooding nor waterlogged soils [123].  Even if
quaking aspen survives flooding in the short term, stems subjected to
prolonged flooding usually develop a fungus infection that greatly
reduces stem life (and renders the wood commerically useless) [37,118,126].

Sprouting is hormonally controlled in quaking aspen.  Sprouting is
suppressed by auxin, which is transported from the stem to the roots.
Auxin therefore maintains apical dominace.  When stems are killed and
apical dominance is removed, cytokinins in the roots initiate root
sprouting.  Clones with a strong tendency to sprout probably have high
cytokinin:auxin ratios [145].


RAUNKIAER LIFE FORM : 
   Phanerophyte
   Geophyte


REGENERATION PROCESSES : 
Quaking aspen regenerates from seed and by sprouting from the roots
[146].  Stump and root crown sprouting is rare in older trees, but
saplings sometimes sprout from the stump and root crown as well as the
roots [123,145].

Vegetative reproduction:  Root sprouting is the most common method of
regeneration.  Root suckers originate from meristems in the root's cork
cambium and can develop anytime during secondary growth [140].  Saplings
may begin producing root sprouts at 1 year of age [123].  There are
thousands of suppressed shoot primoridia on the roots of most mature
quaking aspen clones.  Recently initiated meristems or primordia usually
sprout and elongate more vigorously than older primorida or suppressed
root buds [145].  Root suckering is affected by depth and diameter of
parent roots.  In Utah and Wyoming, Schier and Campbell [144] found that
25 percent of sprouts came from roots within 1.6 inches (4 cm) of the
surface, 70 percent from within 3.2 inches (8 cm), and 92 percent within
4.7 inches (28 cm).  Compared with parent roots of quaking aspen in the
Great Lakes States, those of quaking aspen in the West were deeper.  On
a Utah burn site, high-severity fires increased the depth of the parent
roots from which sprouts originated.  Range in diameter of roots
producing sprouts was 0.04 to 3.7 inches (0.1-9 cm).  Sixty percent of
suckers grew from roots smaller than 0.4 inch (1 cm) in diameter, 88
percent from roots smaller than 0.8 inch (2 cm), and 93 percent from
roots smaller than 1.2 inches (3 cm) in diameter.  On a Wyoming site,
the percentages were 38 percent, 68 percent, and 86 percent,
respectively.

Sprout development is largely suppressed by apical dominance [145].
Closed stands produce a few inconspicuous sprouts each growing season;
the sprouts usually die unless they occur in a canopy gap.  When stems
are removed by cutting, burning, girdling, or defoliation, suppressed
primoridia, buds, and shoots resume growth.  Best sucker production
follows either a fire that kills all parent trees and brush or other
complete clearing [23].  The number of suckers produced can vary
markedly among clones [7,159], but the potential for suckering is
enormous.  Jones [87] indicated that 20,000 to 30,000 sprouts per acre
is typical the first year following top-kill.  Natural thinning is heavy
and effective.  The least vigourous suckers die within 1 to 2 years.
After 5 to 10 years, most sucker clumps reduce to a single stem [87].
Most stems are overtopped by more vigorous neighbors.  Diseases,
insects, browsing mammals, and snow damage also reduce sprout density
[35,87,108].  Bella and De Franceschi [17] reported that in Alberta and
Saskatchewan, stem density averaged 280,000 per hectare at age 2;
190,000 per hectare at age 3; and 80,000 per hectare at age 5.

Seedling establishment:  Quaking aspen commonly establishes from seed in
Alaska, northern Canada, and eastern North America.  Seedling
establishment is less common in the West, where rainfall is often
followed by dry periods that kill newly germinated seedlings [90].  Even
in the West, however, quaking aspen may establish from seed more
frequently than previously thought.  Studies on frequency of seedling
establishment in the West are conflicting.  Some researchers found
absolutely no quaking aspen seedling establishment despite diligent
searching [4,5,16]; others reported the presence of only one [51] or a
few [52] seedlings, while still other researchers documented the
presence of hundreds of seedlings [7,90,97,167].  Only since the
stand-replacement fires of the late 1980's have researchers used
permanent plots to monitor quaking aspen seedling establishment and
survival in the West.  Data from one such study are summarized after the
following discussion of sexual reproduction in quaking aspen.

Sexual reproduction:  The staminate-pistillate ratio of adult clones is
1:1 in most localities, although it may be as high as 3:1 or more [117].
Some clones alternate between staminate and pistillate forms in
different years, or produce various combinations of perfect, staminate,
and pistillate flowers [50].  Quaking aspen first flowers at 2 to 3
years.  Minimum tree age for production of large seed crops is 10 to 20
years, and maximum seed production occurs at about 50 years of age.  In
Utah, one 23-year-old tree produced an estimated 1.6 million seeds in
one spring [123].  There are 3- to 5-year intervals between heavy seed
crops [55,102,110,148].  Seeds disperse a few days after they ripen.
Dispersal lasts 2 to 3 weeks [123].  The plumose seeds are dispersed by
wind for distances of 1,600 feet (500 m) to several miles with heavy
winds.  Seeds also disperse by water, and can germinate while floating
or submerged [54].  Viability of fresh seed is good; germination of 80
to 95 percent is reported under laboratory conditions [103,109,142].
Viability lasts 2 to 4 weeks under favorable conditions of low
temperature and humidity [123], but seed loses viability rapidly under
less than optimum conditions [54,171].

Optimum conditions for germination and seedling survival include a moist
mineral seedbed with adequate drainage, moderate temperature, and
freedom from competition [104].  In various collections, seeds have
germinated at temperatures from 32 to 102 degrees Fahrenheit (0-39 deg
C), with germination sharply reduced from 35 to 41 degrees Fahrenheit
(2-5 deg C) and progressively curtailed above 77 degrees Fahrenheit (25
deg C) [54,172].  Quaking aspen seed from northern Utah showed optimal
germination between 59 and 68 degrees Fahrenheit (15-20 deg C), and had
no light requirement.  Seeds germinated best on the soil surface, with
emergence decreased by shallow burial [104].  Burned or scarified soil
is an excellect seedbed [61]; litter provides the poorest seedbed.  The
primary root grows slowly the first few days following germination, and
during this critical period the seedling depends upon a brush of hairs
to absorb water and anchor the plant [123].  Minor disturbances can
uproot surface-germinated seedlings, and a drying seedbed can rapidly
desiccate seedlings [104].

Seedlings may reach 6 to 24 inches (15-61 cm) in height by the end of
their first year, and roots may extend 6 to 10 inches (15-25 cm) in
depth and up to 16 inches (41 cm) laterally.  Roots grow more rapidly
than shoots; some seedlings show little top-growth until about their
third year [23].  During the first several years, natural seedlings grow
faster than planted seedlings but not as fast as sprouts.  High
mortality characterizes young quaking aspen stands regardless of origin.
In both seedling and sprout stands natural thinning is rapid.  Stems
that occur below a canopy die within a few years [123].

Seedling study:  Kay [90] documented postfire quaking aspen seedling
establishment following 1986 and 1988 fires in Grand Teton and
Yellowstone National Parks, respectively.  He found seedlings were
concentrated in kettles and other topographic depressions, seeps,
springs, lake margins, and burnt-out riparian zones.  A few seedlings
were widely scattered throughout the burns.  In Grand Teton National
Park, establishment was greatest (950-2,700 seedlings/ha) in 1989, a wet
year, but hundreds to thousands of seedlings established each year
despite drought conditions in 1986-1988 and 1990-1991.  Seedlings
surviving past one season occurred almost exclusively on severely burned
surfaces.  In Grand Teton National Park, where seedlings were monitored
for several years, surviving seedlings were associated with bare mineral
soil, ash, and the absence of competing vegetation.  In both Parks, 100
percent of seedlings were browsed, and mean heights of seedlings at
postfire year 5 (Grand Teton) and postfire year 3 (Yellowstone) were
nearly equal to mean heights at postfire year 1.  During the same
period, 0 percent of lodgepole pine seedlings were browsed.  Kay
predicted that long-term survival of quaking aspen seedlings will be
low.  Most seedlings established on depressions that are subject to
spring flooding.  Since quaking aspen does not tolerate standing water,
seedlings on depressions such as kettles and lake margins will probably
die in the first prolonged flood.  At postfire year 5, quaking aspen
seedlings in Grand Teton National Park attained only 5 percent more
height growth than attained in the first postfire year.  In contrast,
lodgepole pine seedlings had increased in height by an average of 176
percent.


SITE CHARACTERISTICS : 
Quaking aspen occurs on a wide variety of sites [40,111].  It grows on
moist upland woods, dry mountainsides, high plateaus, mesas, avalanche
chutes, talus, parklands, gentle slopes near valley bottoms, alluvial
terraces, and along watercourses [40,109,158,166].  

Climate:  Climatic conditions vary widely over the range of quaking
aspen, especially minimum winter temperatures and annual precipitation.
Generally, quaking aspen occurs where annual precipitation exceeds
evapotranspiration.  In Alaska and northwestern Canada, quaking aspen is
common in the boreal zone and extends into the warmest, frost-free sites
of the permafrost zone.  At the eastern edge of quaking aspen's range,
climate is humid, with snowfall exceeding 120 inches (3,050 mm) per
year.  The southern limit of quaking aspen distribution in the East is
roughly delinated by the 75 degree Fahrenheit (24 deg C) mean July
temperature isotherm.  In the central Rocky Mountains, altitude plays an
important role in quaking aspen distribution.  The lower limit of its
range coincides with a mean annual temperature of 45 degrees Fahrenheit
(7 deg C) [123].

Soils:  Quaking aspen grows on soils ranging from shallow and rocky to
deep loamy sands and heavy clays.  Good quaking aspen sites are usually
well drained, loamy, and high in organic matter and nutrients [123].
Cryer and Murray [36] stated that stable quaking aspen stands are found
on only one soil order - mollisols - and a few soil subgroups of which
Agric Pachic Cryoborolls and Pachic Cryoborolls are dominant.  The best
stands in the Rocky Mountains and Great Basin are on soils derived from
basic igneous rock such as basalt, and from neutral or calcareous shales
and limestones.  The poorest stands are on soils derived from granite.
In the Great Lakes States, the best stands occur in lime-rich, gray
glacial drift [123].

Elevation:  Quaking aspen spans an elevational range from sea level on
both coasts to 11,500 feet (3,505 m) in northern Colorado.  At its
northern limit, quaking aspen is found only up to 3,000 feet (910 m).  In
Baja California, it does not occur below 8,000 feet (2,440 m).  In
Arizona and New Mexico is is most abundant between 6,500 and 10,000 feet
(1,980-3,050 m); in Colorado and Utah, it occurs about 1,000 feet (300
m) higher.  At either either of its elevational limits, quaking aspen is
stunted.  At its lower limit, it grows as a scrubby tree along streambanks;
at high elevations, its stems are bent or prostrate [123].

Aspect:  In Alaska and western Canada, quaking aspen grows best on south
to southwesterly exposures.  It is common on all aspects in the West,
except in the Southwest, where it is most common on northern aspects.
In the prairie provinces of Canada, particularly on the prairie-woodland
interface, quaking aspen occurs on cooler north and east slopes, and in
depressions [123].


SUCCESSIONAL STATUS : 
Quaking aspen is shade intolerant and cannot reproduce beneath its own
canopy [23,40,98,123,126].  Beyond that, there is no single, generalized
pattern of succession in quaking aspen.  Quaking aspen is seral to
conifers in most of its range in the West, and in some portions of its
eastern range.  In the East, quaking aspen is also replaced by hardwoods
[23,98].  In the Great Lakes States, successional trends are toward
northern hardwoods, spruce-fir, ash-elm (Fraxinus-Ulmus spp.), oak
(Quercus spp.), swamp conifers, and pine (Pinus spp.) types, in
decreasing order of importance [23].  Where it is seral, quaking aspen
usually persists as a minor tree in late seral stages [98].

The canopy closes rapidly in young aspen stands [126].  A quaking aspen
stand in Ontario closed and reached maximum development (foliage/unit
area of soil surface) in 4 years [127,128].  If quaking aspen does not
remain stable, rate of succession to other species varies with with
soil, site, and invading species [71].  Mueggler [112] stated that
succession to conifers may occur in a single generation, or take longer
than 1,000 years.  Harper [72] found that in central Utah, quaking aspen
succeeded to conifers in 75 to 100 years on sandstone soils.  On
limestone or alluvial soils, succession to conifers took 140 years or
more.

Quaking aspen is apparently stable on some sites.  On some former pine
stands in the East, extensive clearcutting of the conifer overstory has
removed the pine seed sources.  Quaking aspen has formed an apparently
stable overstory on many of these sites [24].  Quaking aspen stands are
also considered stable in parts of Canada and the western United States
[71].  Some stands, however, remain stable for decades but eventually
deteriorate.  Deteriorating stands are often succeeded by conifers, but
shrubs, grasses, and/or forbs gain dominance on some sites [71].
Succession to grasses and forbs is more likely on dry sites and is more
common in the West than in the East [126].

Quaking aspen readily colonizes after fire, clearcutting, or other
disturbance [123].  In Emigrant Wilderness Area, California, red fir
(Abies magnifica) stands on north slopes have converted to quaking aspen
after fire [66].  In the Great Lakes States, quaking aspen has
regenerated on cut/burned sites through sprouting and seedling
establishment, becoming the dominant forest cover type [23].


SEASONAL DEVELOPMENT : 
Quaking aspen catkins elongate before the leaves expand.  In New
England, catkins appear in mid-March to April; in the central Rockies,
flowering occurs in May to June.  Sustained air temperatures above 54
degrees Fahrenheit (12 deg C) for about 6 days apparently trigger
flowering [55,123].  At high elevation, trees may flower before snow is
off the ground [5].  Female trees generally flower and leaf out before
male trees.  Local clonal variation produces early- and late-flowering
clones of either sex, however.  Catkins mature in 4 to 6 weeks (usually
in May or June).  Branches usually leaf out from early May to June
[123].  Seed dispersal in the Great Lakes States occurs from early May
to mid-June, beginning earliest on protected sites and in southern
portions of the region [23].

Related categories for Species: Populus tremuloides | Quaking Aspen