Associated Forest Cover
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Hickories are consistently present in the broad eastern upland
climax forest association commonly called oak-hickory, but they
are not generally abundant (18). Locally, hickories may make up
to 20 to 30 percent of stand basal area, particularly in slope
and cove forests below the escarpment of the Cumberland Plateau
(23) and in second-growth forests in the Cumberland Mountains,
especially on benches (14). It has been hypothesized that hickory
will replace chestnut (Castanea dentata) killed by the
blight (Cryphonectria parasitica) in the Appalachian
Highlands (10,12). On Beanfield Mountain in Giles County, VA, the
former chestnut-oak complex has changed to an oak-hickory
association over a period of 50 years. This association is
dominated by pignut hickory with an importance value of 41.0
(maximum value = 300), northern red oak (36.0), and chestnut oak
(25.0). White oak, red maple (Acer rubrum), and sugar
maple are subdominant species.
Pignut hickory is an associated species in 20 of the 90 forest
cover types listed by the Society of American Foresters for the
eastern United States (6):
Northern Forest Region
53 White Pine-Chestnut Oak
Central Forest Region
40 Post Oak-Blackjack Oak
44 Chestnut Oak
45 Pitch Pine
46 Eastern Redcedar
52 White Oak-Black Oak-Northern Red Oak
53 White Oak
55 Northern Red Oak
57 Yellow-Poplar
59 Yellow-Poplar-White Oak-Northern Red Oak
64 Sassafras-Persimmon
110 Black Oak
Southern Forest Region
75 Shortleaf Pine
76 Shortleaf Pine-Oak
78 Virginia Pine-Oak
79 Virginia Pine
80 Loblolly Pine-Shortleaf Pine
81 Loblolly Pine
82 Loblolly Pine-Hardwood
83 Longleaf Pine-Slash Pine
Because the range of pignut hickory is so extensive, it is not
feasible to list the associated trees, shrubs, herbs, and
grasses, which vary according to elevation, topographic
conditions, edaphic features, and geographic locality.
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Climate
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Pignut hickory grows in a humid climate with an average annual
precipitation of 760 to 2030 mm (30 to 80 in) of which 510 to
1020 mm (20 to 40 in) is rain during the growing season. Average
snowfall varies from little to none in the South to 2540 mm (100
in) or more in the mountains of West Virginia, southeastern New
York, and southern Vermont (25).
Within the range of pignut hickory, average annual temperatures
vary from 7° C (45° F) in the north to 21° C (70°
F) in Florida. Average January temperature varies from -4°
to 16° C (25° to 60° F) and average July
temperature varies from 21° to 27° C (70° to 80°
F). Extremes of 46° and -30° C (115° and -22°
F) have been recorded within the range. The growing season varies
by latitude and elevation from 140 to 300 days.
Mean annual relative humidity ranges from 70 to 80 percent with
small monthly differences; daytime relative humidity often falls
below 50 percent while nighttime humidity approaches 100 percent.
Mean annual hours of sunshine range from 2,200 to 3,000. Average
January sunshine varies from 100 to 200 hours, and July sunshine
from 260 to 340 hours. Mean daily solar radiation ranges from
12.57 to 18.86 million J m± (300 to 450 langleys). In
January daily radiation varies from 6.28 to 12.57 million J m±
(150 to 300 langleys), and in July from 20.95 to 23.04 million J
m± (500 to 550 langleys).
According to one classification of climate (20), the range of
pignut hickory south of the Ohio River, except for a small area
in Florida, is designated as humid, mesothermal. That part of the
range lying north of the Ohio River is designated humid,
mesothermal. Part of the species range in peninsular Florida is
classed as subhumid, mesothermal. Mountains in Pennsylvania, West
Virginia, North Carolina, and Tennessee are classed as wet,
microthermal, and mountains in South Carolina and Georgia are
classed as wet, mesothermal. Throughout its range, precipitation
is rated adequate during all seasons.
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Damaging Agents
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Pignut hickory is easily damaged by fire,
which causes stem degrade or loss of volume, or both. Internal
discolorations called mineral streak are common and are one major
reason why so few standing hickories meet trade specifications.
Streaks result from yellow-bellied sapsucker pecking, pin knots,
worm holes, and mechanical injuries. Hickories strongly resist
ice damage and seldom develop epicormic branches.
The Index of Plant Diseases in the United States lists 133 fungi
and 10 other causes of diseases on Carya species (4,9).
Most of the fungi are saprophytes, but a few are damaging to
foliage, produce cankers, or cause trunk or root rots.
The most common disease of pignut hickory from Pennsylvania
southward is a trunk rot caused by Poria spiculosa. Cankers
vary in size and appearance depending on their age. A common form
develops around a branch wound and resembles a swollen, nearly
healed wound. On large trees these may become prominent burl-like
bodies having several vertical or irregular folds in the callus
covering. A single trunk canker near the base is a sign that the
butt log is badly infected, and multiple cankers are evidence
that the entire tree may be a cull.
Major leaf diseases are anthracnose (Gnomonia caryae) and
mildew (Microstroma juglandis). The former causes brown
spots with definite margins on the undersides of the leaf. These
may coalesce and cause widespread blotching. Mildew invades the
leaves and twigs and may form witches' brooms by stimulating bud
formation. Although locally prevalent, mildew offers no problem
in the management of hickory.
The stem canker (Nectria galligena) produces depressed
areas with concentric bark rings that develop on the trunk and
branches. Affected trees are sometimes eliminated through
breakage or competition and sometimes live to reach merchantable
size with cull section at the canker. No special control measures
are required, but cankered trees should be harvested in stand
improvement operations.
A gall-forming fungus species of Phomopsis can produce
warty excrescences ranging from small twig galls to very large
trunk burls on northern hickories and oaks. Little information is
available on root diseases of hickory.
More than 100 insects have been reported to infest hickory trees
and wood products, but only a few cause death or severe damage
(1). The hickory bark beetle (Scolytus quadrispinosus) is
the most important insect enemy of hickory, and also one of the
most important insect pests of hardwoods in the Eastern United
States. During drought periods in the Southeast, outbreaks often
develop and large tracts of timber are killed. At other times,
damage may be confined to the killing of a single tree or to
portions of the tops of trees. The foliage of heavily infested
trees turns red within a few weeks after attack, and the trees
soon die. There is one generation per year in northern areas and
normally two broods per year in the South. Control consists of
felling infested trees and destroying the bark during winter
months or storing infested logs in ponds.
Logs and dying trees of several hardwood species including pignut
hickory are attacked by the ambrosia beetle (Platypus
quadridentatus) throughout the South and north to West
Virginia and North Carolina. The false powderpost beetle (Xylobiops
basilaris) attacks recently felled or dying trees, logs, or
limbs with bark in the Eastern and
Southern States. Hickory, persimmon (Diospyros virginiana),
and pecan (C. illinoensis) are most frequently
infested, but other hardwoods also are attacked. Healthy trees
growing in proximity to heavily infested trees are occasionally
attacked but almost always without success. Hickory and persimmon
wood (useful in the manufacture of small products such as shuttle
blocks, mallets, and mauls) is sometimes seriously damaged.
Hickory is one of several host species of the twig girdler (Oncideres
cingulata). Infested trees and seedlings are not only damaged
severely but become ragged and unattractive. A few of the more
common species of gall-producing insects attacking hickory are
Phylloxera caryaecaulis, Caryomyia holotricha, C.
sanguinolenta, and C. tubicola.
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Flowering and Fruiting
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Hickories are monoecious and
flower in the spring (3). The staminate catkins of pignut hickory
are 8 to 18 cm (3 to 7 in) long and develop from axils of leaves
of the previous season or from inner scales of the terminal buds
at the base of the current growth. The pistillate flowers appear
in spikes about 6 mm (0.25 in) long on peduncles terminating in
shoots of the current year. Flowers open from the middle of March
in the southeast part of the range to early June in New England.
The catkins usually emerge before the pistillate flowers.
The fruit of hickory is pear shaped and enclosed in a thin husk
developed from the floral involucre. The fruit ripens in
September and October, and seeds are dispersed from September
through December. Husks are green until maturity; they turn brown
to brownish-black as they ripen. The husks become dry at maturity
and split away from the nut into four valves along sutures. Husks
of pignut hickory split only to the middle or slightly beyond and
generally cling to the nut, which is unribbed, with a thick
shell.
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Genetics
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Carya glabra var. megacarpa (Sarg.) Sarg., coast
pignut hickory, was once recognized as a distinct variety but is
now considered to be a synonym of C. glabra (Mill.)
Sweet. C. leiodermis Sarg., swamp hickory, has also been
added as a synonym of C. glabra (11).
Carya glabra (Mill.) Sweet var. glabra distinguishes
the (typical) pignut hickory from red hickory (C. glabra var.
odorata (Marsh.) Little). The taxonomic position of red
hickory is controversial. The binomial C. ovalis (Wangenh.)
Sarg. was published in 1913 for a segregate of C. glabra.
It was reduced to a synonym of C. glabra in Little's
1953 checklist but was elevated to a variety in the 1979 edition
(11). The principal difference is in the husk of the fruit,
opening late and only partly, or remaining closed in C.
glabra but promptly splitting to the base in C. ovalis.
However, many trees are intermediate in this trait, and the
recorded ranges are almost the same. The leaves of C. ovalis
have mostly seven leaflets; those of C. glabra have
mostly five leaflets. The two can be distinguished with certainty
only in November. Since the two ranges seem to overlap, the
distributions have been mapped together as a Carya
glabra-ovalis complex (11).
Carya ovalis has also been treated as an interspecific
hybrid between C. glabra and C. ovata. C.
ovalis was accepted as a polymorphic species especially
variable in the size and shape of its nuts and possibly a hybrid.
The relationships may be more complex after a long and reticulate
phylogeny, according to detailed chemical analyses of hickory nut
oils.
One hybrid, C. x demareei Palmer (C. glabra x
cordiformis) was described in 1937 from northeastern
Arkansas.
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Growth and Yield
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Pignut hickory often grows 24 to
27 m (80 to 90 ft) tall and occasionally reaches 37 m (120 ft),
with d.b.h. of 91 to 122 cm (36 to 48 in). The bole is often
forked. Height and diameter by age are shown in table 1 for
selected locations. Diameter growth of pignut hickory (along with
chestnut oak, white oak, sweet birch (Betula lenta), and
American beech is rated slow. Since hickories constitute 15
percent or less of the basal area of oak-hickory forest types,
most growth and yield information is written in terms of oak
rather than oak-hickory. Yields of mixed oak stands (5,7,19) and
of hickory stands (2) have been reported. Tree volume
tables are available (2,19).
Table 1- Diameter and height of pignut hickory
in selected geographic areas (adapted from 2)
Age
D.b.h.
Height
S. Indiana and
N.Kentucky¹
Ohio
Valley¹
Norhtern
Ohio¹
Cumberland
Mountains²
Mississippi
Valley²
(yr)
(cm)
(m)
(m)
(m)
(m)
10
2
2.7
2.1
1.8
1.8
20
5
5.8
6.1
4.3
5.8
30
8
9.8
10.7
7.3
8.2
40
11
12.8
14.6
9.8
10.4
50
14
15.5
18.6
12.2
12.2
60
17
17.7
21.0
14.6
14.0
70
21
19.5
22.6
16.8
15.8
80
25
21.0
--
18.9
17.7
(yr)
(in)
(ft)
(ft)
(ft)
(ft)
10
1.0
9
7
6
6
20
2.0
19
20
14
19
30
3.2
32
35
24
27
40
4.4
42
48
32
34
50
5.5
51
61
40
40
60
6.8
58
69
48
46
70
8.4
64
74
55
52
80
10.0
69
--
62
58
¹Second
growth.
²Virgin forest.
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Reaction to Competition
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The hickories as a group are
classed as intermediate in shade tolerance; however, pignut
hickory has been classed as intolerant in the Northeast and
tolerant in the Southeast. In much of the area covered by mixed
oak forests, shade-tolerant hardwoods (including the hickories)
are climax, and the trend of succession toward this climax is
very strong. Although most silvicultural systems when applied to
oak types will maintain a hardwood forest, the cutting methods
used affects the rapidity with which other species may replace
the oaks and hickories (17,18,26).
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Rooting Habit
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Pignut hickory tends to develop a
pronounced taproot with few laterals and is rated as windfirm
(21). The taproot develops early, which may explain the slow
growth of seedling shoots. Taproots may develop in compact and
stony soils.
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Seed Production and Dissemination
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Pignut hickory
begins to bear seed in quantity in 30 years, with optimum
production between 75 and 200 years (16). The maximum age for
seed production is about 300 years. Good seed crops occur every
year or two with light crops in other years; frost can seriously
hinder seed production (22). Usually less than half of the seeds
are sound (2,3), but 50 to 75 percent of these will
germinate. The hickory shuckworm (Laspeyresia caryana)
can seriously reduce germination. Pignut seed, averaging
440/kg (200/lb), is lighter than the seed of other hickory
species. The nuts are disseminated mainly by gravity, but the
range of seeding is extended by squirrels and chipmunks.
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Seedling Development
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Hickories exhibit embryo
dormancy which is overcome naturally by overwintering in the duff
and litter or artificially by stratification in a moist medium at
1° to 4° C (33° to 40° F) for 30 to 150 days.
In forest tree nurseries unstratified hickory nuts are sown in
the fall and stratified nuts are sown in the spring. Hickories
are hypogeously germinating plants, and the nuts seldom remain
viable in the forest floor for more than one winter (22).
Seedling growth of hickories is slow. The following height growth
of pignut hickory seedlings was reported in the Ohio Valley in
the open or under light shade, on red clay soil (2):
Age
Height
(yr)
(cm)
(in)
1
8
3.0
2
15
5.8
3
20
8.0
4
30
12.0
5
43
17.0
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Soils and Topography
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Pignut hickory frequently grows on dry ridgetops and sideslopes
throughout its range but it is also common on moist sites,
particularly in the mountains and Piedmont. In the Great Smoky
Mountains pignut hickory has been observed on dry sandy soils at
low elevations. Whittaker (27) placed pignut in a submesic class
and charted it as ranging up to 1480 m (4,850 ft)-the hickory
with the greatest elevational range in the Great Smoky Mountains.
In southwest Virginia, south-facing upper slopes from 975 to 1050
m (3,200 to 3,445 ft) of Beanfield Mountain are dominated by
pignut hickory, northern red oak Quercus rubra), and
white oak (Q. alba). This site is the most xeric habitat
on the mountain because of high insolation, 70 percent slopes,
and medium- to coarse-textured soils derived from Clinch
sandstone. Mid-elevation slopes from 800 to 975 m (2,625 to 3,200
ft) are dominated by chestnut oak (Q. prinus), northern
red oak, and pignut hickory and coincide with three shale
formations (12).
The range of pignut hickory encompasses 7 orders, 12 suborders,
and 22 great groups of soils (24,25). About two-thirds of the
species range is dominated by Ultisols, which are low in bases
and have subsurface horizons of clay accumulation. They are
usually moist but are dry during part of the warm season. Udults
is the dominant suborder and Hapludults and Paleudults are the
dominant great groups. These soils are derived from a variety of
parent materials-sedimentary and metamorphic rocks, glacial till,
and in places varying thickness of loess-which vary in age from
Precambrian to Quaternary.
A wide range of soil fertility exists as evidenced by soil
orders-Alfisols and Mollisols which are medium to high in base
saturation to Ultisols which are low in base saturation (24).
Pignut hickory responds to increases in soil nitrogen similarly
to American beech (Fagus grandifolia), sugar maple (Acer
saccharum), and blackgurn (Nyssa sylvatica) (15).
These species are rated as intermediate in nitrogen deficiency
tolerance and consequently are able to grow with lower levels of
nitrogen than are required by "nitrogen- demanding"
white ash (Fraxinus americana), yellow-poplar (Liriodendron
tulipifera), and American basswood (Tilia americana).
Hickories are considered "soil improvers" because
their leaves have a relatively high calcium content.
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Special Uses
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Hickories provide food to many kinds of wildlife (8,13). The nuts
are relished by several species of squirrel and represent an
estimated 10 to 25 percent of their diet. Nuts and flowers are
eaten by the wild turkey and several species of songbirds. Nuts
and bark are eaten by black bears, foxes, rabbits, and raccoons.
Small mammals eat the nuts and leaves; 5 to 10 percent of the
diet of eastern chipmunks is hickory nuts. White-tailed deer
occasionally browse hickory leaves, twigs, and nuts.
The kernel of hickory seeds is exceptionally high in crude fat, up
to 70 to 80 percent in some species. Crude protein, phosphorus,
and calcium contents are generally moderate to low. Crude fiber
is very low.
Pignut hickory makes up a small percentage of the biomass in
low-quality upland hardwood stands that are prime candidates for
clearcutting for chips or fuelwood as the first step toward
rehabilitation to more productive stands. Hickory has a
relatively high heating value and is used extensively as a home
heating fuel.
Pignut hickory is an important shade tree in wooded suburban areas
over most of the range but is seldom planted as an ornamental
tree.
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Vegetative Reproduction
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Hickories sprout readily
from stumps and roots. Stump sprouting is not as prolific as in
other deciduous trees species but the sprouts that are produced
are vigorous and grow fairly rapidly in height. Root sprouts also
are vigorous and probably more numerous than stump sprouts in
cut-over areas. Small stumps sprout more frequently than large
ones. Sprouts that originate at or below ground level and from
small stumps are less likely to develop heartwood decay. Pignut
hickory is difficult to reproduce from cuttings.
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Distribution
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The range of pignut hickory covers nearly all of eastern United
States (11). It extends from Massachusetts and the southwest
corner of New Hampshire westward through southern Vermont and
extreme southern Ontario to central Lower Michigan and Illinois;
southward through extreme southeastern Iowa, Missouri, and
Arkansas to Louisiana and parts of East Texas. The species grows
eastward through Louisiana and along the Gulf Coast to
Mississippi and Alabama into central Florida.
Best development of this species is in the lower Ohio River Basin.
It is the hickory most commonly found in the Appalachian forests.
Pignut makes up much of the hickory harvested in Kentucky, West
Virginia, the Cumberland Mountains of Tennessee, and the hill
country of the Ohio Valley.
-The native range of pignut hickory.
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Brief Summary
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Sweet Pignut Hickory
Juglandaceae -- Walnut family
Glendon W. Smalley
Pignut hickory (Carya glabra) is a common but not abundant
species in the oak-hickory forest association in Eastern United
States. Other common names are pignut, sweet pignut, coast pignut
hickory, smoothbark hickory, swamp hickory, and broom hickory.
The pear-shaped nut ripens in September and October and is an
important part of the diet of many wild animals. The wood is used
for a variety of products, including fuel for home heating.
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