BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Quercus macrocarpa | Bur Oak 



GENERAL BOTANICAL CHARACTERISTICS : 
Bur oak is a spreading, deciduous, large shrub to large tree [20,40].
On eastern forested sites, it is typically a large tree and reaches up
to 100 feet (31 m) in height and 3 to 4 feet (0.9 to 1.2 m) or more in
diameter [40].  Bur oak grows best on the rich bottomlands of southern
Illinois and Indiana, where it may reach 170 feet (52 m) with a diameter
of 6 to 7 feet (1.8 to 2.1 m) [34].  On rocky bluffs with thin soil and
at the north and west ends of its range, bur oak more commonly grows as
a small tree or large shrub [20].  On poor soil along the top of bluffs
at the western edge of its range, it often forms dense thickets of low
straggling shrubs which may reach only 1.7 to 3.3 feet (5 to 10 dm) in
height [34].

Twigs are thick and corky [40,43].  Branches are low and stout, and the
crown is generally open, broad, and round [40].  Leaves are variable,
although generally large (2-10 in [5-25 cm] long), and deeply divided
into five to nine rounded lobes [14]. 

Staminate flowers are borne in small, pendant catkins, and solitary to
few pistillate flowers are in the leaf axils.  Fruits are relatively
large acorns up to 2 inches (5.1 cm) in length.  They are partly or
entirely enclosed in a fringed or "mossy" cap [40].

Roots have great tensile strength [41].  The well-branched, widely
spreading roots allows growth in dry areas [41]. 

Bur oak is slow growing and long lived, sometimes reaching 200 or 300
years of age [34,41]. 


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Bur oak reproduces both sexually and vegetatively.  Trees are wind
pollinated [25].  Pollen from one tree germinates better on the stigmas
of a second tree, favoring cross-pollination [46].

Minimum seed-bearing age is 35, with optimum seed production occurring
between 75 and 150 years of age [46].  Acorns are produced in abundance
every 2 to 3 years with light crops in intervening years [23,45].  Seeds
mature within 1 year [46].  When conditions are favorable, acorns
germinate immediately after dropping from the tree [25].  Seed is
dispersed primarily by gravity and small mammals, with water playing a
lesser role [4].  Although most bur oak acorns germinate in the fall,
acorns of Q. m. var. olivaeformis, a northern variety, usually germinate
in spring [46].  Germination under laboratory conditions ranged from 45
to 99 percent, although much variation has been noted [3,23,46].  The
acorns do not have a dormancy period [40].

Acorns are extremely vulnerable to predation by insects, small birds and
mammals, and unless germination is rapid, few seeds survive [25].
Studies in Iowa uplands have shown that bur oak germination is best when
litter is removed.  Litter-covered acorns appear to be more
vulnerable to rodents, insects, and fungus [46].

Under laboratory conditions, acorns are generally pretreated by 1 to 2
months of stratification prior to planting and are subjected to
alternating temperatures of 68 and 86 degrees Fahrenheit (20-30 deg C)
[30,40].  Stratification is apparently not required for good
germination, however.  Seed is stored over winter in a cool, moist place
at 33 to 40 degrees F (0.6-4 degrees C) [45].

Under some conditions, bur oak produces plural-seeded acorns which
result in poor germination or weak and misshapen seedlings [7].  The
production of plural-seeded acorns is believed to be genetically
controlled, although modifications brought about by environmental
conditions may also contribute to their production [3].  High
production of plural-seeded acorns at a particular site may result in
decreased seedling production.

Bur oak taproots grow rapidly.  They reach approximately 9 inches (22.9
cm) before leaves unfold [41].

In many areas, relatively few seedlings are produced from acorns [34],
and vegetative regeneration is much more common.

Bur oak sprouts vigorously after fire or other disturbance.  It sprouts
prolifically from the root crown when the main stem is damaged [28];
however, stump sprouting has also been well documented [31,40].
Pole-sized or smaller trees sprout well after burning or cutting,
although the quality and form of sprouts is described as "relatively
poor" [46].  Larger trees also produce some sprouts.  The relationship
between the size, age, and vigor of the parent and its ability to sprout
is unknown [46].


SITE CHARACTERISTICS : 
Bur oak grows in a range of habitats from upland woods to valley floors
[11].  It occurs throughout the southern hardwood forests and through
much of the northeastern hardwood forests [34].  Bur oak is a prominent
constituent of oak-basswood (Quercus spp.-Tilia americana) forests,
upland oak-hickory (Quercus macrocarpa, Q. borealis, Carya cordiformis,
and Ostrya virginia) forests, mixed-oak communities (Quercus spp.), and
is transitional to sugar maple-basswood (Acer saccharum-Tilia americana)
communities of the upper Midwest [6,28].  This species commonly forms
nearly pure savannas, occasionally interspersed with white oak (Quercus
alba).  Bur oak savannas were formerly much more abundant, but have
declined due to grazing and fire suppression [34].

Soils: Bur oak tolerates a wide range of soil types.  It commonly grows
on medium to somewhat coarsely-textured soils, and more rarely on clays.
It is well adapted to fairly acidic (>4.0 pH) to moderately basic,
nonsaline soils [40].  Optimum soil depth is greater than 20 inches (51
cm) [7].  It thrives on the moist flats and bottomlands along the Ohio
River and grows well on shallow uplands and bluffs on soils derived from
limestone [40].

Moisture regime: Bur oak is well adapted to growth on moist, rich
bottomlands with sufficient, but not excessive, amounts of water [34].
It is intolerant of flooding and may be killed by floods during the
growing season [40].  It grows in areas receiving 15 to 40 inches (38 to
102 cm) maximum annual precipitation but does best where maximum annual
precipitation averages 30 to 40 inches (76 to 102 cm) [43].

Associates: Species commonly growing in association with bur oak
include boxelder (Acer negundo), black ash (Fraxinus nigra), white ash
(F. americana), red maple (Acer rubrum), shellbark and bitternut
hickories (Carya laciniata, C. cordiformis), American elm (Ulmus
americana), hackberries (Celtis spp.), eastern cottonwood (Populus
deltoides), black locust (Robinia pseudoacacia), basswood (Tilia
americana), northern red oak (Q. aborealis), northern pin oak (Q.
ellipscidalis), white oak (Q. alba), and swamp white oak (Quercus
bicolor) [26,28,34,40].

Elevation: Elevational ranges of bur oak are as follows [6]:

         from    3,200 to 3,500 ft (976 to 1,068 m) in MT
                 3,200 to 3,200 ft (976 to 976 m) in WY


SUCCESSIONAL STATUS : 
Facultative Seral Species

Bur oak may have climax or seral status depending on the site.  In many
dry, exposed areas of Wisconsin, it is a climax species.  It is also a
climax species in most areas of the Black Hills [32].

Bur oak is an early seral species in many mesic forest habitats of
Wisconsin [32].  In many areas, this species, which is moderately
intolerant of shade, is replaced by more shade-tolerant hardwoods [40].

Bur oak is a pioneer at prairie margins [40], and is a pioneer or early
seral species after fire in the Black Hills [32].  In parts of Minnesota
where fire is suppressed, bur oak is replaced by maple (Acer spp.) or
basswood (Tilia americana) communities [6,31].


SEASONAL DEVELOPMENT : 
Bur oak flowers shortly after leaves develop, but prior to or early in
the period of major stem expansion [1,40].  Initiation of flowers and
leaves in the spring is not closely related to temperature [1].
Flowering usually begins in April in the South, but may not begin until
June in the North [40].  In North Dakota, trees flowers from May through
June [46].  Acorns ripen from August to November [40].  Most seed
dispersal takes place from August to September [45].  The following
sequence of phenological development was recorded during a 6-year
observation period in northeastern Minnesota [1]:

                    Phenology 1951 - 1956                   
                 Bud    Leaf    Flowering    Pollen    Seed fall
earliest date   4/24    5/11      5/12        5/23        ----
latest date     5/21    5/25      6/2         6/8         10/3

                    Leaf color      Leaf fall       Cambial activity
                    change                           Init.      End
earliest date         9/13            9/26           5/15       7/16     
latest date           9/19           10/14           5/29       8/23
      

Related categories for Species: Quercus macrocarpa | Bur Oak