BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pteridium aquilinum | Bracken Fern 



GENERAL BOTANICAL CHARACTERISTICS : 
The leaves or fronds of bracken fern are normally from 1 to 10 feet
(3-30 dm) long including a stipe (leaf-stalk) that may be as long as 39
to 59 inches (10-15 dm) but is usually shorter than the leaf blade
[119].  The blades of the fronds are divided into pinnae, the bottom
pair of which are sometimes large enough to give the impression of a
three-part leaf.  Each pinna is in turn divided into pinnules.  Above
the first division of the stipe into a frond, it is called a rachis.  On
fertile fronds the spores are borne in sori beneath the outer margins of
the pinnules.  The sori are protected by the inrolled pinnule margins on
one side and a thin membrane called an indusium on the other [119].

Nectaries are found at the base of the pinnae during spring and early
summer [141,232].  The largest nectaries are found near the base of the
frond and the nectaries get progressively smaller going up the rachis
[141].  Ants are attracted by and feed on sugars produced by these
extra-floral nectaries [110,111,227].  It has been suggested but not
proven that an ant-plant mutualism may exist where the ants would attack
other insects feeding on the plants.  The ants do attack introduced
caterpillars and they tend an aphid species on bracken fern in Arizona
[110,111,144,227].

The fronds are killed by frost.  In northern climates they are killed
each winter and new fronds grow in spring; in mild areas individual
fronds persist for 2 to 3 years before being replaced [195].  Dead
fronds form a mat of highly flammable litter that insulates the
below-ground rhizomes from frost when there is no snow cover.  This
litter also delays the rise in soil temperature and emergence of
frost-sensitive fronds in the spring [237].

Rhizomes are the main carbohydrate storage organs [48,243].  Rhizomes
also store water and are consistently around 87 percent water [211].
Rhizomes can be up to 1 inch (2.5 cm) in diameter [79] and branching is
alternate [236,238,239].  The rhizome system has two components.  The
long shoots form the main axis or stem of the plant [239].  They
elongate rapidly, have few lateral buds, do not produce fronds, and
store carbohydrates [48,236,243].  Short shoots, or leaf-bearing lateral
branches, may be closer to the soil surface [33].  They arise from the
long shoots, are slow growing, and produce annual fronds and many
dormant frond buds.  Transition shoots start from both short and long
shoots and may develop into either [48].  Thin, black, brittle roots
extend from the rhizome and may extend over 20 inches (50 cm) deeper
into the soil [211,238,239].  Endotrophic mycorrhizae have been found on
the roots of bracken fern [41,126].

Fossil evidence suggests that bracken fern has had at least 55 million
years to evolve and perfect antidisease and antiherbivore chemicals
[192].  It produces bitter tasting sesquiterpenes and tannins,
phytosterols that are closely related to the insect moulting-hormone,
and cyanogenic glycosides that yield hydrogen cyanide (HCN) when
crushed.  It generates simple phenolic acids that reduce grazing, may
act as fungicides, and are implicated in bracken fern's allelopathic
activity [42].  Severe disease outbreaks are very rare in bracken fern
[126,192].

Most work describing bracken fern has been done on var. aquilinum which
is closely related to varieties latiusculum, pseudocaudatum, and
pubescens [232].  Some differences between the varieties are noted below
[90,106,198,205,232,239].

var. latiusculum - Growth of the long rhizomes is relatively slow with
rates of 4 to 7 inches (10-17 cm) versus 10 to 35 inches (25-90 cm)
annually so it is less weedy than other varities. The growing tip of the
rhizome has no hairs or a few whitish hairs.  The terminal segment of
the frond is not much longer than lateral segments; thus the frond
appears triangular or three-parted.  The only pubescence is along the
pinnule margins and midvein.

var. pseudocaudatum - The frond blade is usually completely glabrous and
rarely ternate.  The terminal segment of the frond is much longer than
the lateral segments and between six and fifteen times as long as broad.
The growing tip of the rhizome usually has a tuft of dark hairs.

var. pubescens - The frond blade is ovate-triangular but not ternate,
while the upper surface of the frond is frequently pubescent and the
lower surface is usually densely pubescent.  There is a tuft of dark
hairs on the growing tip of the rhizome.


RAUNKIAER LIFE FORM : 
   Undisturbed State:  Cryptophyte (geophyte)
   Burned or Clipped State:  Cryptophyte (geophyte)


REGENERATION PROCESSES : 
Most regeneration in bracken fern is vegetative.  Many investigators
have searched for young plants growing from spores [186, Stickney 1989,
personal communication], but few have found them.  However, spores do
germinate and grow readily in culture [7,33,37,40].

Young bracken fern plants can produce spores by the end of the second
growing season in cultivation but normally do not produce spores until
the third or fourth growing season [40,97].  A single, fertile frond can
produce 300,000,000 spores annually [38,40].  Spore production varies
from year to year depending on plant age, frond development, weather,
and light exposure [40].  Production decreases with increasing shade
[40,189].  The wind-borne spores are extremely small.  Dry spores are
very resistant to extreme physical conditions, although the germination
of bracken fern spores declines from 95 to 96 percent to around 30 to 35
percent after 3 years storage [190].  The spores germinate without any
dormancy requirement.  Under favorable conditions, young plants could be
found 6 to 7 weeks after the spores are shed [37,40].  Under normal
conditions the spores may not germinate until the spring after they are
shed [33,38].

Sufficient moisture and shelter from wind are important factors in fern
spore germination [167].  Bracken fern spore germination appears to
require soil sterilized by fire [37,186].  On unsterilized soils spores
may germinate, but the new plants are quickly overwhelmed by other
growth [37].  Temperatures between 59 and 86 degrees F (15-30 degrees C)
are generally best for germination, although bracken fern is capable of
germination at 33 to 36 degrees F (1-2 degrees C).  A pH range of 5.5 to
7.5 is optimal for germination [38,167].  Germination of bracken fern is
indifferent to light quality; it is one of the few ferns that can
germinate in the dark [189,240].  Despite limitations on spore
germination, genotype analysis in the Northeast indicates that many
stands of bracken fern represent multiple establishment of individuals
from spores [96,250].

When spores germinate, they produce bisexual, gamete-bearing plants
about 0.25 inch (0.6 cm) in diameter and one cell thick.  These tiny
plants (gametopytes or prothalli) have no vascular system and require
very moist conditions to survive.  The young spore-bearing plant
(sometimes called a sporling) which develops from the fertilized egg is
initially dependent on the gametopyte until it develops its first leaf
and roots.  The first fronds are simple and lobed.  They develop into
thin, delicate fronds divided into lobed pinnae.  They do not look like
adult plants and are frequently not recognized as bracken fern [37,189].
Cultivated plants of var. aquilinum begin to resemble adult bracken fern
after 18 weeks.  The rhizomes begin to develop after there are a number
(up to 10) of fronds and a well-developed root system or in the
fifteenth week of growth under optimal conditions.  In the first year
rhizomes may grow to 86 inches (217 cm) long [20].  By the end of a
second year the rhizome system may exceed 6 feet (18 dm) in diameter
[20,37].

Bracken fern's aggressive rhizome system gives it the ability to
reproduce vegetatively and reduces the plant's dependence on water for
reproduction [42].  The rhizomatous clones can be hundreds of years old,
and some clones alive today may be over 1,000 years old [186,192,250].
Rhizomes have a high proportion of dormant buds [236].  When disturbed
or broken off, all portions of the rhizome may sprout, and plants
growing from small rhizome fragments revert temporarily to a juvenile
morphology [48, 192].  A recent study of bracken fern genotypes using
isozyme patterns found individual clones in New England were up to 400
feet (120 m) in diameter, and clones often intermingled in an area
[250].


SITE CHARACTERISTICS : 
Bracken fern grows on a variety of soils with the exception of heavily
waterlogged soils [23].  Its efficient stomatal control allows it to
succeed on sites that would be too dry for most ferns, and its
distribution does not normally seem limited by moisture [230,235].
Bracken fern grows best on deep well-drained soils with good
water-holding capacity, and it may dominate other vegetation on such
sites [57,68].  Its productivity increases with increasing soil profile
development on Michigan entisols and spodosols [113].  In northern Idaho
the surface soil horizon under bracken fern is an acidic, dark mineral
layer, while under interspersed conifer stands the surface soil horizon
is an acidic, light mineral layer [59].

Bracken fern rhizomes are particularly effective at mobilizing
phosphorus from inorganic sources into an available form for plant use
[168].  Bracken fern contributes to potassium cycling on sites and is
associated with high levels of potassium [28,157,175].  Fertilization of
cultured plants increases frond dry weight; using both nitrogen (N) and
phosphate (P) increases rhizome length, while using N, P, and potassium
(K) increases both rhizome length and rhizome dry weight [49].  Bracken
fern is characteristically found on soils with medium to very rich
nutrients [91,105,235].  In southeastern Alaska bracken fern prefers a
pH of 5.0 to 6.0 [225].  It is absent from soils contaminated with zinc
[131].

In northern climates bracken fern is frequently found on uplands and
side slopes, since it is susceptible to spring frost damage [47,150].
Fronds growing in the open or without litter cover are often killed as
crosiers by spring frost damage, since the soil warms earlier and growth
begins sooner [237].  The result is that fronds appear earlier in shaded
habitats [113,204].  Cultivated and shaded plants produce fewer, thinner
but larger fronds than open-grown plants [49].  A New York study found
that fronds growing in the shade were twice as likely as fronds growing
in the open to be cyanogenic [204].  That was also true in Great Britain
[43], however, a New Jersey study found no cyanogenic plants [226].
Shaded plants produce fewer spores than plants in full sun [189].

Elevation:  Elevational ranges in some western regions are [56,142,179]:

                        Minimum                  Maximum
                   feet      meters         feet      meters

New Mexico         8,000      2,438         9,500     2,896
California         sea level               10,000     3,048
Utah               5,500     1,676          8,000     2,438
Colorado           5,300     1,615         10,000     3,048
Wyoming            4,800     1,463          8,500     2,591
Montana            4,300     1,311          5,000     1,524

Var. pubescens is generally found in open forests, pastures, and on open
slopes; it is common following fire [189,232].  In the Pacific Northwest
bracken fern is found along the coast on stabilized dune meadows and in
coastal prairies.  It is found in the forests of western Washington and
northwestern Oregon and it may be a dominant in grassy balds of the
Coast Mountains, subalpine meadows, and on avalanche tracks and
southerly slopes in the Cascades [57,78,169].  Bracken fern increases
from west to east across the central Washington Cascades [53].  Within
the rain shadow area of the eastern slope of the Olympic Mountains,
bracken fern is a dominant understory species in Oregon white oak
(Quercus garryana) savanna [50,228].  In the Columbia Basin of eastern
Oregon and Washington bracken fern grows in riparian communities with
Douglas hawthorn (Crataegus douglasii) [78].  It is more frequent on
south-facing slopes in northern Idaho [175] and north-central Washington
where its cover is greater below 3,800 feet (1,150 m) than at higher
elevations [229].  It grows well on snow chutes in subalpine fir (Abies
lasiocarpa) habitat types in northwestern Montana [248].  In British
Columbia it grows best in areas with a humid climate, mild winters, and
a relatively long growing season [97].  In southeastern Alaska bracken
fern is found in the ecotone between forest and bog [180] or in muskegs
[225].  Bracken fern is found in the coastal redwood region of
California and on flood plains and gentle slopes under the giant sequoia
(Sequoiadendron giganteum) in California's Sierra Nevada [108,235].  In
Arizona it is an understory species in deciduous, riparian forests [21].
In New Mexico and Arizona bracken fern is found in the mountains under
blue spruce (Picea pungens) and Douglas-fir, in pinyon-juniper or Gambel
oak (Quercus gambelii) and ponderosa pine (Pinus ponderosa) woodlands,
and in grassy meadows [19,134,142,170,194].  Bracken fern is found with
aspen in Colorado [15,121,122].

Var. latiusculum:  In Wisconsin, northern Michigan, and probably
Minnesota, bracken-grasslands, doubtless initially caused by fire, are
found on soils ranging from loam to fine sand [47].  Some of these
bracken-grasslands occupy depressions with bracken fern dominant on the
surrounding slopes.  Bracken fern is also a common understory species in
Wisconsin oak (Quercus spp.) openings and barrens [47].  In New England
var. latiusculum and var. pseudocaudatum prefer dry woods, clearings,
fields, and thickets.  Bracken fern is not found on limey soil [205].
In White Mountain forests it is most often found on dry areas of shallow
bedrock or outwash [147].

Var. pseudocaudatum:  Southern bracken fern is most common on
well-drained sandy soils under open stands of longleaf pine (Pinus
palustris), shortleaf pine (P. echinata), and mixtures of pine (Pinus
spp.) and oak [35,88,92,135].  It is also associated with pocosin [135].
In West Virginia bracken fern was found on a high plateau growing among
other vegetation in a heath meadow with scattered small spruce [44].  On
the Alabama piedmont it is associated with upper slopes and ridges with
shallow soils [88].  Along the Atlantic Coastal Ridge of southern
Florida, bracken fern is found on low hammocks and disturbed sites
[200].  Var. caudatum may also be found in this area on low hammocks and
disturbed sites [200].  On low hammocks bracken fern is associated with
oaks and cabbage palmetto (Sabal palmetto) [200].  It is also found in
the margins of scrub vegetation where the sandy soil contains more clay
and silt and thus retains water better [178]


SUCCESSIONAL STATUS : 
Bracken fern is basically a shade-intolerant pioneer and seral species
that is sufficiently shade tolerant to survive in light-spots in
old-growth forests [127,192,216].  A study in southwestern Oregon
suggested that bracken fern is an indicator of light intensity.  In this
study bracken fern cover was 75 percent at 60 to 100 percent of full
sunlight, and dropped to 50 percent between 25 and 60 percent of full
sunlight.  When light intensity was under 25 percent of full sunlight,
bracken fern cover was less than 5 percent [61].

The light, windborne spores of bracken fern allow it to colonize newly
vacant areas.  Bracken fern has been documented as a pioneer on sterile,
cooled lava slopes [190].  After disturbance in western Washington and
northwestern Oregon forests, bracken fern often invades sites where it
was not previously present [78,100].  It enters the dry meadow stage of
succession on coastal sand dunes of the Pacific Northwest and was an
early seral species following the eruption of Mount St. Helens where
some plants were observed originating from rhizome fragments
[78,101,164].

In areas unaffected by coastal moisture bracken fern rarely establishes
from spores [68].  However, solitary plants may expand from rhizomes
following disturbance [220,221].  These plants may depend upon canopy
level removal or openings for establishment of a system of clonal
ramets.  Under a canopy of oak and pine in the New Jersey pine barrens,
bracken fern distribution resembles that of sexually reproducing herbs
rather than that of clones [161].

In western forests very small amounts of bracken fern persist under a
canopy for at least 200 to 400 years [94,133,169].  Following
disturbance, bracken fern is a common seral species that may be dominant
in coastal forests from Oregon to Southern Alaska and in New England
[50,94,114,133].  In the Pacific Northwest annuals may be followed
closely by bracken fern and other perennials [45,203].  It is seral in
Oregon's interior valleys [89], in California coastal redwoods, and in
valley oak (Quercus lobata), blue oak (Q. douglasii), and digger pine
(Pinus sabiniana) savannas [93,249].  It follows disturbance in grand
fir and cedar hemlock forests of the northern Rocky Mountains [153].  It
occurs in seral brush fields in northern Idaho and southwestern Oregon
[95,109].  In contrast, a study in white fir (Abies concolor) forests of
the Sierra Nevada found bracken fern predominantly in mature or late
seral stands with low light intensities [36].  Authors of a New Jersey
study with similar results suggested that bracken fern distribution in
their area was spotty and showed no real preference for low light [24].

In Southern longleaf pine plantations bracken fern is associated with
disturbance following thinning operations but is absent from patch or
clearcut areas [245].  Following fire in a Pennsylvania scrub oak
(Quercus ilicifolia) community, bracken fern increased rapidly
immediately after burning but declined sharply after the first year due
to competition from blueberry (Vaccinium spp.) and huckleberry
(Gaylussacia spp.) [99].

Where bracken fern invades grasslands and low shrublands, it may exhibit
a cyclic succession.  If undisturbed, the dense bracken fern cover
gradually deteriorates into sparse bracken fern with grass and shrubs.
Eventually dense bracken fern may reinvade [159,238].


SEASONAL DEVELOPMENT : 
In North America, fronds usually begin to emerge between March and early
May.  Frost-killed fronds are replaced through mid-July [33].  In a
northern Idaho study, bracken fern first appeared in early May and
continued growth through mid-July.  The fronds began to change color by
mid-August, probably because of limited soil moisture [58].  Spore
maturation and dispersal begins at the base of the frond and proceeds up
to the tip resulting in an extended period of spore dispersal [40].  In
New England and the Carolinas, bracken fern produces spores from early
July to late September [198,205].  Spore release in Michigan is between
the first of June and mid-August [115] and from July to September on the
Great Plains [90].  In Canada sporulating begins as early as June 24 in
Ontario, June 29 in Quebec, July 16 in Nova Scotia, July 22 in British
Columbia, July 29 in New Brunswick, August 1 on Prince Edward Island,
and August 5 in Manitoba [33].

Related categories for Species: Pteridium aquilinum | Bracken Fern