Carya glabra, the听pignut hickory, is a common, but not abundant species of听听in the听听association in the Eastern United States and听. Other common names are听pignut,听sweet pignut,听coast pignut hickory,听smoothbark hickory,听swamp hickory, and听broom hickory. The pear-shaped听听ripens in September and October, has a sweet maple like smell, and is an important part of the diet of many wild animals. The听听is used for a variety of products, including fuel for home heating. Its leaves turn yellow in the Fall.
Habitat
Native range
The range of pignut hickory covers nearly all of the eastern United States (11). The species grows in central听听and northward through听听to southern听. It also grows north of the Gulf Coast through听,听听north to听听and extreme southeastern听, and the听听.
The best development of this species is in the lower听听Basin. It prevails over other species of hickory in the听听forests. Pignut makes up much of the hickory harvested in听,听, the听听辞蹿听, and the hill country of the听.
Pignut hickory is also found in听听in southern听. It does however have a limited range and is restricted to the听, southern听, the听听area along western听, and southward along the northern shore of听听and pockets of extreme southwestern Ontario.
Climate
Pignut hickory grows in a听听听with an average annual听听of 760 to 2,030听mm (30 to 80 in) of which 510 to 1,020听mm (20 to 40 in) is听听during the growing season. Average听听varies from little to none in the South to 2,540听mm (100听in) or more in the mountains of听, southeastern听, and southern听听(25).
Within the range of pignut hickory,听听vary from 7听掳C (45听掳F) in the north to 21听掳C (70听掳F) in Florida. Average January temperature varies from -4掳 to 16听掳C (25掳 to 60听掳F) and average July temperature varies from 21掳 to 27听掳C (70掳 to 80听掳F). Extremes of 46掳 and -30听掳C (115掳 and -22听掳F) have been recorded within the range. The听听varies by听听and听听from 140 to 300 days.
Mean annual听听ranges from 70 to 80 percent with small monthly differences; daytime relative humidity often falls below 50% while nighttime humidity approaches 100%.
Mean annual听听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卤 (300 to 450听). In January daily radiation varies from 6.28 to 12.57 million J m卤 (150 to 300 langleys), and in July from 20.95 to 23.04 million J m卤 (500 to 550 langleys).
According to one classification of climate (20), the range of pignut hickory south of the听, except for a small area in Florida, is designated as humid,听. 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.听听in Pennsylvania, West Virginia, North Carolina, and Tennessee are classed as wet,听, and mountains in South Carolina and Georgia are classed as wet, mesothermal. Throughout its range, precipitation is rated adequate during all seasons.
Soils and topography
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听. In the听听pignut hickory has been observed on dry sandy soils at low elevations. Whittaker (27) placed pignut in a submesic[]听class and charted it as ranging up to 1480听m (4,850听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听ft) of听听are dominated by pignut hickory, northern red oak听), and white oak (). This site is the most xeric听听on the mountain because of high听, 70 percent slopes, and medium- to coarse-textured soils derived from听. Mid-elevation slopes from 800 to 975 m (2,625 to 3,200听ft) are dominated by chestnut oak (), northern red oak, and pignut hickory and coincide with three听听formations (12).
The range of pignut hickory encompasses 7听, 12 suborders, and 22 great groups of听听(24,25). About two-thirds of the species range is dominated by听, which are low in bases and have subsurface horizons of听听accumulation. They are usually moist but are dry during part of the warm season.听听is the dominant suborder and听听and听听are the dominant great groups. These soils are derived from a variety of parent materials-听and听听rocks,听, and in places varying thickness of听-which vary in age from听听to听.
A wide range of soil fertility exists as evidenced by soil orders-听and听听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 (), sugar maple (), and blackgum () (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 (), yellow-poplar (), and American basswood (). Hickories are considered "soil improvers" because their听听have a relatively high听听content.
Associated forest cover
Hickories are consistently present in the broad eastern upland听听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听听of the听听(23) and in second-growth forests in the听, especially on听听(14). It has been hypothesized that hickory will replace chestnut () killed by the blight () in the听听Highlands (10,12). On听听in听,听, 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听 for the eastern United States (6):
Life history
Reproduction and early growth
听and听- Hickories are听听and flower in the spring (3). The听听听of pignut hickory are 8 to 18听cm (3 to 7 in) long and develop from听听of leaves of the previous season or from inner scales of the terminal听听at the base of the current growth. The听听flowers appear in听听about 6听mm (0.25听in) long on听听terminating in shoots of the current year. Flowers open from the middle of March in the southeast part (Florida) of the range to early June in听. The catkins usually emerge before the pistillate flowers.
The听听of hickory is听听shaped and enclosed in a thin husk developed from the floral听. The fruit ripens in September and October, and听听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听听from the听听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.
Seed production and dissemination
Pignut hickory begins to bear听听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;听听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听. 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听听and听.
Seedling development
Hickories exhibit embryo dormancy which is overcome naturally by overwintering in the听听and听听or artificially by听听in a moist medium at 1掳 to 4听掳C (33掳 to 40听掳F) for 30 to 150 days. In forest tree听听unstratified hickory nuts are sown in the fall and stratified nuts are sown in the spring. Hickories are听听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听听soil (2):
Vegetative reproduction
Hickories sprout readily from听听and听. Stump sprouting is not as prolific as in other听听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听听decay. Pignut hickory is difficult to reproduce from听.
Sapling and pole stages to maturity
Growth and yield- Pignut hickory often grows 24 to 27 m (80 to 90听ft) tall and occasionally reaches 37听m (120听ft), with听听of 91 to 122听cm (36 to 48 in). The听听is often forked. Height and diameter by age are shown in table 1 for selected locations. Diameter growth of pignut hickory (along with听,听, sweet birch (), and听听is rated slow. Since hickories constitute 15 percent or less of the basal area of听-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).
Rooting habit
Pignut hickory tends to develop a pronounced听听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.
Reaction to competition
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听听(including the hickories) are听, and the trend of听听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).
Damaging agents
Pignut hickory is easily damaged by听, 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听听pecking, pin听, 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听听and 10 other causes of diseases on Carya species (4,9). Most of the fungi are听, but a few are damaging to foliage, produce听, or cause听听or听听.
The most common disease of pignut hickory from Pennsylvania southward is a trunk rot caused by听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听-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听听(Gnomonia caryae) and听听(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听听(Nectria galligena) produces depressed areas with concentric听听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听Phomopsis听can produce warty excrescences ranging from small twig听听to very large trunk听听on northern hickories and oaks. Little information is available on root diseases of hickory.
More than 100听听have been reported to infest hickory trees and听听products, but only a few cause death or severe damage (1). The hickory听听(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听听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听听(Platypus quadridentatus) throughout the South and north to West Virginia and North Carolina. The听听(Xylobiops basilaris) attacks recently felled or dying trees, logs, or limbs with bark in the Eastern and 蜜桃av States. Hickory, persimmon (), and pecan () 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 is one of several host species of the听听(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听听caryaecaulis,听听holotricha,听C. sanguinolenta, and听C. tubicola.
Special uses
Hickories provide food to many kinds of wildlife (8,13). The nuts are relished by several species of听听and represent an estimated 10 to 25 percent of their diet. Hogs were observed consuming the nuts in听, lending the species its common name.听Nuts and flowers are eaten by the听听and several species of听. Nuts and bark are eaten by听,听,听, and听. Small听听eat the nuts and leaves; 5 to 10 percent of the diet of eastern听听is hickory nuts.听听occasionally browse hickory leaves,听, and nuts.
The kernel of hickory seeds is exceptionally high in crude听, up to 70 to 80 percent in some species. Crude听,听, and听听contents are generally moderate to low. Crude听听is very low.
Pignut hickory makes up a small percentage of the听听in low-quality upland hardwood stands that are prime candidates for听听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听听in wooded听听areas over most of the range but is seldom planted as an听听because of its size and difficulty of transplanting, although it has spectacular orangey-red fall colors.
Genetics
Carya glabra听var.听megacarpa听(Sarg.) Sarg., coast pignut hickory, was once recognized as a distinct variety but is now considered to be a synonym of听C. glabra听(Mill.) Sweet.听C. leiodermis听Sarg., swamp hickory, has also been added as a synonym of听C. glabra听(11).
Carya glabra听(Mill.) Sweet var.听glabra听distinguishes the (typical) pignut hickory from red hickory (C. glabra听var.听odorata听(Marsh.) Little). The听听position of red hickory is controversial. The binomial听C. ovalis听(Wangenh.) Sarg. was published in 1913 for a segregate of听C. glabra. It was reduced to a听听辞蹿听C. glabra听in Little's 1953 checklist but was elevated to a听听in the 1979 edition (11). The principal difference is in the husk of the听, opening late and only partly, or remaining closed in听C. glabra听but promptly听听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听C. ovalis听have mostly seven leaflets; those of听C. glabra听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听has also been treated as an interspecific hybrid between听C. glabra听and听C. ovata.听C. ovalis听was accepted as a polymorphic species especially variable in size and shape of its nuts and is possibly a听. The relationships may be more complex after a long and reticulate听, according to detailed听听analyses of hickory nut oils.
Carya glabra听is a 64 chromosome species that readily hybridizes with other hickories, especially听C. ovalis.
One hybrid,听C.听x听demareei听Palmer (C. glabra听x听cordiformis) was described in 1937 from northeastern听.
