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Loblolly (left) and Eastern White Pine (right)

There are two main categories of pine trees, the subgenus** Strobus, a.k.a. Haploxylon ("white" or "soft") pines and the subgenus** Pinus, a.k.a. Diploxylon ("yellow" or ""hard") pines. (There has also been a third subgenus named Ducampopinus which includes one unique species in Vietnam called Pinus krempfii which is has flat un-pinelike needles and appears destined to be assigned to a separate genus, and so is not included here).

(**In this website, the subgenus names of Haploxylon and Diploxylon are used because they refer to the essential difference between the two and are less confusing).

There is a choice of different morphological characteristics on which to base classification and therefor (especially in the Diploxylon subgenus) some very different classification schemes.

The following classification of pines mostly follows the Little and Critchfield method [USDA Forest Service Misc. Pub. No. 1144 (51 pp.), 1969] with modifications and additions from Jesse P. Perry [The Pines of Mexico and Central America, Timber Press, 1991], Keith Rushford [Conifers, Christopher Helm Publishers, 1987] and Mirko Vidakovic [Conifers, Graficki Zavod Hrvatske, 1991]. In general, cone and cone scale and seed morphology and leaf fascicle and sheath morphology are emphasized and this seems to result in a classification that has subsections of pines that are understandable and usually readily recognized by their general appearance.

SUBGENUS HAPLOXYLON (a.k.a. Strobus), the "soft" or "white" pines:
The defining morphology: one fibrovascular bundle per leaf.
Other features: resin canals in the outer part of the leaf; leaves in clusters of 5 (except in Section Paracembra/Parrya); leaf sheaths soon deciduous (except in Subsection Cembroides); leaf stomata confined to the ventral surface, creating two macroscopic narrow white lines on the ventral leaf surface; the bases of the leaf bracts are non- decurrent, leaving a smooth branch after the leaves are shed; bark is generally smoother (becomes rough at an older age); wood is generally whiter and softer with less prominent annual rings (better for carving) and with smooth-walled tracheids and fewer resin canals; shoots are uninodal (the spring shoots produce only one whorl of branches per year) (except P. nelsonii); terpene analysis usually shows a high % of beta-pinene.
  Section Cembra (a.k.a. strobus)
  The defining morphology: the cone scales have a "terminal" (downward-pointing) umbo (umbo = the tip of the scale, which is the center of the exposed outer suface of the scale when the cone was closed).
  Other features: leaves in fascicles of 5; leaf sheaths deciduous with in one year; generally tall conical shaped trees.
    Subsection Strobi "White Pines":
    Features: cones are longer and narrow and open on the branch at maturity; seeds are thin and have a relatively large adnate (firmly attached) wing. Species: strobus, monticola, strobiformis, lambertiana, flexilis, ayacahuite, chiapensis, peuce, wallichiana, bhutanica, armandii, amaniana, dalatensis, dadeshanensis, fenzeliana, parviflora, morrisonicola, wangii, kwangtungensis (Europe, Asia and N.A.).
    Subsection Cembrae ("Stone Pines"):
    Features: the cones do not open or disintegrate on the branch, rather fall intact to the ground where they disintegrate or are broken up by animals; thick seed and the seed wing is merely a narrow rim. Species: cembra, albicaulis, pumila, armandii, sibirica, koraiensis (Europe, Asia, N.A.)
  Section Paracembra (a.k.a. Parrya)
  The defining morphology: the cone scales have a dorsal umbo, i.e., the tip of the scale is thick and pyramidal and reflexed outward, sometimes with a thorn..
  Other features: number of leaves per bundle varies from 1 to 5; the leaf sheaths are generally semi-persistent, usually with the distal portion of the fascicle curling backward on th leaf to form a "rosette" around the base of the sheath and later all of the sheath becomes deciduous (except nelsonii); generally "scrubby" trees with irregular rounded crowns.
    Subsection Cembroides "Pinyon Pines", "Nut Pines":
    Features: large wingless seeds (actually they have an easily detached wing that remains attached to the cone scale); short cones, 8 cm. or less length; 1 - 5 leaves per fascicle. Species: monophylla, edulis, cembroides, quadrifolia, remota, discolor, johannis, caterinae, lagunae, juarezensis, culminicola (Mexico and the U.S.).
    Subsection Pinceana "Big cone Pinyons":
    Features: long cones (5 -22 cm. length); large seeds, usually wingless (except rzedowski); 3 - 5 leaves per fascicle; generally are rare or endangered pines of very limited range in the dry rocky highlands of Mexico. Species: pinceana, maximartinezii, nelsonii, rzedowski (Mexico).
    Subsection Balfourianae "Foxtail pines":
    Features: short, long-retained (10 - 35 years) leaves; leaves in fascicles of 5; deciduous leaf sheaths; seeds with long articulate wings; bottlebrush-like ("foxtail") arrangement of needles on the branches. Species: balfouriana, aristata, longaeva (United States).
    Subsection Gerardiana: "Lace-bark pines":
    Features: distinctive smooth exfoliating bark; glossy fairly stout needles grouped in fascicles of 3 (1-4); fully deciduous needle sheath; very short seed wings. Species: gerardiana, bungeana (Asia).

SUBGENUS DIPLOXYLON (a.k.a. Pinus), the "hard" or "yellow" pines:
The defining morphology: leaves have two fibrovascular bundles.
Other features: resin canals in the intermediate or inner portion of the leaves; fascicles of 2 -5 (rarely up to 8) leaves; leaf sheaths are persistent (except Section Leiophyla); leaf stomata on both ventral and dorsal surfaces; the bases of the leaf bracts are decurrent, leaving a rough branch after the leaves are shed (except P. pseudostrobus); bark is generally thick and fissured; wood is generally harder and yellower and with more prounced annual growth rings; growth of spring shoots is either uninodal or multinodal; high % of alpha-pinene.
  Section Leiophylla
  The defining morphology: early deciduous leaf sheath.
  Other features: 3 - 5 leaves per fascicle; small seed with long detachable wing. Species: leiophylla, lumholtzii, chihuahauna (all from Mexico).
  Section Pinaster
  The defining morphology: large seed with long effective adnate wing.
  Other features: long leaves (10 - 20 cm.) in fascicles of three; sheath persistent. Species: pinaster, canariensis, roxburghii, halapensis, brutia (Asia, Mediterranean).
  Section Pinea
  The defining morphology: large seed with very rudimentary wing.
  Other features: leaves in fascicles of 2; persistent leaf sheath, cones mature in 3 years. Species: pinea ("Italian stone pine").
  Section Pinus
  The defining morphology: none (this is the largest section, including about 3/5's of all pines); leaves are generally rigid and have 2 - 3 leaves per fascicle but sometimes 4 -5 and occasionally up to 8; leaf sheaths are persistent; cones have spreading scales with a dorsal umbo, usually with a sharp spine
    Subsection Sylvestres
    The defining morphology: 2 pairs (other pines have only 1 pair) of heterobrachial chromosomes (in which the long arm is more than 2x the length of the short arm).
    Other features: leaves in fascicles of 2 (occasionally 2-3); small cones, opening early and falling completely from the branch. Species: sylvestris, resinosa, densiflora, tabuliformis, densata, takahasii, yunnanensis, kesiya, merkusii, massoniana, henryi, mugo, uncinata, nigra, heldreichii, leucodermis, tropicalis, thunbergii, luchuensis, taiwanenesis, hwangshanensis (all Eurasian except for resinosa in North America; merkusii is the only pine occuring naturally in the Southern hemisphere).
    Subsection Ponderosa
    The defining morphology: symmetrical cones, opening when ripe and some cone scales remain on the branch after the cone has abscised (fallen off).
    Other features: 3 leaves (occasionally 2 -5, up to 8) per fascicle; usually persistent umbo spine. Species: ponderosa, jeffreyi, washoensis, arizonica, engelmanii, durangensis, montezumae, rudis, hartwegii, cooperi, devoniana, wincesteriana, michoacana, pseudostrobus, douglasiana (all in North America).
    Subsection Taeda
    The defining morphology: symmetrical cones, opening when ripe and abscising completely. Shoots are multinodal (more than 1 branch whorl per growing season).
    Other features: 2 - 3 leaves per fascicle; cone scales have persistent umbo spine. Species: taeda, palustris, elliotii, echinata, glabra, serotina, rigida, pungens, caribea, occidentalis, cubensis (all North American).
    Subsection Contortae
    The defining morphology: cones small, symmetrical or oblique, and closed when ripe (serotinous); shoots are multinodal (more than branch whorl per growing season).
    Other features: 2 short (< 10 cm) leaves per fascicle. Species: contorta, banksiana, virginiana, clausa (all in the U.S.)
    Subsection Sabinianae
    The defining morphology: large symmetrical or slightly oblique cones with protuberance on the end of cone scale ending a prominent tough curved spine; cones open when ripe but persist on the branch for a longer time.
    Other features: 3 - 5 leaves per fascicle; large seed with hard coat and short wing. Species: sabiniana, coulteri, torreyana (all in California and upper Mexico).
    Subsection Oocarpae
    The defining morphology: cones oblique and remaining closed and on the branch for a long time after they are ripe (serotinous cones); multinodal shoots.
    Other features: leaves mostly in fascicles of 3, but 2 in some spp. and 5 in others. Species: oocarpa, radiata, attenuata, muricata, greggii, patula, pringlei, jaliscana, teocote, herrerai, lawsonii (all North American).

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