Carya illinoinensis


C. illinoinensis (Wangenheim) K. Koch Pecan.

Pecan is distributed along the Mississippi River and its tributaries from northern Illinois and southeastern Iowa to the Gulf Coast of the USA. Isolated populations occur as far east as southwestern Ohio, northern Kentucky and central Alabama. The species is abundant on rivers and streams of central and eastern Oklahoma and in Texas west to the Edwards Plateau (map of native distribution). Pecan occurs in regenerating stands as far south as Zaachila, Oaxaca, Mexico. Collections in the National Clonal Germplasm Repository for Pecans and Hickories include pecan cultivars selected from within the native range and also from beyond, as well as seedlings grown from seed collected from native trees throughout the range (map of NCGR collections).

Accurately mapping the native distribution of the pecan is complicated by man's introduction of the species to non-native areas (map of commercial pecan production). Pecan seeds and leaves have been recovered in association with human artifacts from Baker's Cave in the Devil's River area of Val Verde Co., Texas, from strata dated 6100 B.C up to about 3000 B.C. (Dering, 1977; Hester, 1981; Hester, pers. comm.). Pecan was not found in the oldest levels of human occupation in the cave, dated 7000 B.C., although river walnut (Juglans microcarpa Berl.) was found at that level. Whether this indicates the introduction of pecan by man between 7000 and 6000 B.C, the natural extension of the range of the species at that time, or is merely a gap in the archeological recovery process has not been determined. Zaachila, Mexico, is an ancient ceremonial and administrative center of the Olmec Indians. Valuable information on the antiquity of the pecan population there may be provided by ethnobotanical research in association with the excavation of the site. Manning (1949) reported that field notes on pecan collections by Dewey in Jaumave, Tamaulipas, suggested introduction of the trees from Texas. Obviously, the older a population is, ther harder it will be to retrieve information concerning possible introduction. However, ancient isolated populations, even though introduced, may be genetically unique.

The pattern of distribution exhibited by pecan is one of disjunct populations stretching between the southeastern U.S.A and Mexico. This pattern is shared in the genus Carya by C. myristiciformis (F. Michx.) Nutt. and C. ovata (Mill.) K. Koch, two species which are unlikely to have been introduced by man. Fowells (1965) noted that Quercus durandii var. durandii Buckl. has a distribution "nearly identical" to nutmeg hickory and suggested that they may represent a "flora of an earlier geologic age". Mohr (1893) noted the association of pecan, nutmeg hickory, and Durand oak on rich calcareous soils of Alabama and westward and concluded that they are the "scanty remains of the forests which once covered the uplands". The occurrence of similar patterns of distribution among species which have not been disseminated by man give credence to the possibility tha some of the pecan populatons in Alabama and Mexico are truly native.

Pecan is distinguished from other hickories by its thin-shelled nuts with sweet kernels. Nuts are typically elliptic to oblong in shape, round in cross-section, and have smooth, brown shells with prominent black markings, especially near the apex. Husks are typically about 4 mm thich and are at least slightly raised at the sutures.

Pecan leaves are odd pinnately compound with a variable number of leaflets. Leaflet number increases from the simple leaves of first year seedlings to a maximum of 23 in about the third leaf. Adult phase pecan leaves may have as many as 17 or as few as 7 leaflets, but typically have 9-13. Leaflet number varies between seedlings from different seed stocks, between cultivars in bearing stage and may vary within a cultivar from vegetative to bearing stages. Pubescence also varies by tree and leaf age class, being greatest on immature leaflets from juvenile trees, and least on mature leaflets from adult phase trees. The apical leaflet is symmetrical, ovate-lanceolate with an acute-attenuate apex and a decurrent base. Lateral leaflets are more or less assymmetrical, often falcate, with the distal portion (toward the leaf apex) being convex with an obtuse base. The proximal portion (toward the leaf base) is concave with an acute base. Apices of lateral leaflets are attenuate. Margins of all leaflets are serrate, except on the proximal sides of lateral leaflet bases, which are entire. Venation is craspedodromous, with most of the secondary veins terminating at the leaflet margins, usually at the points of the serrations. The petiole and petiolules have slightly thickened bases. Petiolules are attached to the rachis so that the distal half of the leaflet is elevated above the plane of the rachis and also above the proximal half of the leaflet.

Buds are valvate, as with other members of Apocarya. Pecan shoots often lack a true terminal bud (below, in photo). Bark of trees varies from tight (as in 'Stuart') to loose plates (as in 'Texas Prolific') and may relate to cultivar susceptibility to some insects, such as phylloxera, that overwinter in crevices of the bark.

Pecan hybridizes with water hickory (C. aquatica) to form C. X lecontei. Hybrids of that cross may have advantages as rootstocks on some poorly drained sites. Pecan hybridizes with bitternut hickory (C. cordiformis) to form C. X brownii, and with C. laciniosa to form C. X nussbaumeri. Pecan hybridizes with C. ovata to form an unnamed hybrid.

Warren Chatwin , Research Geneticist
USDA-ARS Pecan Genetics
10200 FM 50
Somerville, TX 77879
fax: 979-272-1401

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