Pines and the pinetum

Introduction - some technical details of the pinaceae


Pine trees (the genus Pinus) are distinguished from all other trees by:
(a) Having uncovered seeds borne in pairs on the bracts of (female) cones (as do other genera of the Pinaceae family)
(b) and narrow leaves ("needles") arranged in bundles of 2 to 5 and with a permanent or deciduous sheath at their bases. Such bundles of needles are called fascicles (after the bundle of sticks around the axe which represented the power of the Roman senate).

There are usually 2 to 5 leaves per fascicle (very rarely 1, or 6 to 8).

The individual needles in one fascicle, when viewed in cross section, are like pie-shaped segments that fit together form a complete circle. Therefore each needle has a hemispherical cross section (if there are 2 needles per fascicle) or triangular cross section (if there are 3 or more needles per fascicle).


Pines are classically divided into two major groups (subgenera):
(a) Strobus ("white" pines) and
(b) Pinus ("yellow" pines).
A third subgenus, Ducampopinus, intermediate between these two, has been proposed.

The Strobus subgenus (and also subgenus Ducampopinus) has one fibrovascular bundle per leaf, i.e., they are haploxylon. The subgenus Pinus has two fibrovascular bundles per leaf, i.e., they are diploxylon. As a rule, (not always), they have the following arrangement of leaves and leaf sheaths.

Subgenus STROBUS: Subgenus PINUS
Pines are mostly large trees with a straight trunk with whorls of smaller lateral branches, but they have a wide range of habits varying from tall narrow trees to small bushy trees to prostrate shrubs.
They are generally long-lived, usually over 100 years in suitable environments. The longest living individuals of any kind are the fabled inter mountain bristlecone pine (Pinus longaeva) that currently has living trees at least 4,800 years old. (The root systems of the creosote bush (Larrea tridentata) may be even older).
All pine species are evergreen, i.e., they keep their leaves for at least two growing seasons (and up to about 30 years in the case of P. longaeva)
They are monoecious, i.e., individual trees have both female (megasporangiate) cones which bear the ovules and male (microsporangiate) cones which shed the pollen.
The pollen is carried by wind and gravity; none of the pines is pollinated by insects or birds. All pines have 12 pairs of chromosomes, as do other genera of the Pinaceae family except two (Douglas firs have 13 and false larches have 11).

About three-fifths of the pine species are currently classified in the subgenus Pinus (Diploxylon) pines, commonly called hard pines or yellow pines. The other two-fifths is comprised of the subgenus Strobus (Haploxylon) pines which are also called soft pines or white pines. (The new subgenus Ducampopinus would account for about one-fifth of the species, leaving approximately one-fifth in the genus Strobus).

The subgenus Pinus has two fibrovascular bundles running the length of the needle (hence diploxylon) and the Strobus subgenus (and also Ducampopinus) has one (haploxylon) fibrovascular bundle. Diploxylon pines generally differ from the Haploxylon pines by having harder yellower wood, cones that are often armed with a prickle, stiffer needles with permanent needle sheaths and the development of rough scaly bark at a younger age.


The pine genus is generally sun-loving and relatively shade-intolerant. They are less likely than shade-tolerant genera (e.g. spruces and firs) to grow up from seedlings in an already established shady forest, so pine trees are less favoured in mixed conifer and uneven-aged forests and often are not the "climax" trees in densely vegetated forests. But they are usually among the first trees to establish on open ground that is being revegetated after fire or other disturbance and are often found in pure even-age stands or in savannah (more open) settings where drought and fires control tree density. In the huge Longleaf pine forests along the Gulf and southeast Atlantic Coasts from east Texas to Virginia and Delaware, fire was just as essential as rain in preserving the pine's dominance.


The family Pinaceae evolved in the northern hemisphere during the early Cretaceous or Jurassic Period of the Mesozoic Era, 130 to 200 million years ago and by the late Cretaceous the genus Pinus had already differentiated into haploxylon and diploxylon subgenera. They have flourished and evolved into about 120 species and subspecies world-wide, still almost all in the northern hemisphere. Only one species (P. merkusii) extends about one degree south of the equator in Sumatra. They grow from desert edge to rain forests and from sea level to mountain tree line. The country with the most species of pines is Mexico, which has approximately 60 species and subspecies, followed by the United States (about 45) and China (about 21). The Mexican highlands have been an evolutionary centre for new pine species.


There are eleven genera in the family Pinaceae, all sharing certain morphological features such as
(a)"female" cones (macrosporangiate strobili) which are usually just called "cones",
(b)"male" cones (microsporangiate strobili) which are sometimes called "catkins" or "pollen cones" and
(c) needle like leaves. Unlike the genus Pinus however, they are not all "evergreens" because two of these genera (Larix and Pseudolarix) have yearly deciduous leaves.
(d) All genera of the pine family have 12 pairs of chromosomes with two exceptions: Pseudotsuga (Douglas fir) has 13 and Pseudolarix (false larch) has 11.

The family Pinaceae is the largest (in number of species and individuals), most geographically widespread and most economically significant of the conifers. There are about 260 species in this family and they cover most of the boreal forests of the Northern Hemisphere and extend at one point one degree south of the equator into Indonesia

.
The largest genus in this family is Pinus that has about 120 species and subspecies and it accounts for much of the huge geographic spread.


The ten genera are:
(1) Abies (true firs) (about 55 species): Have fairly wide-base single needles arising in helical fashion, but on lower shaded branches are arranged pectinately, and they are set in circular depressions on the shoot. The cones are erect and are deciduous in one year. Pollen grains with two "wings."
(2) Cedrus (true cedars) (4 species): Have long and short shoots with cones on the short shoots. Cones appear in late summer and are erect and are deciduous in one year. Leaves are single but arranged in false whorls and persist for several years. Solitary "male" cones on the ends of the short shoots.
(3) Larix (larches) (15 species): Have long and short shoots with cones on the short shoots. Cones are erect and ripen in one year but persist and release seeds for a longer time. Leaves are single but arranged in false whorls and are deciduous in one year. "Male" cones on the end of leafless short shoots.
(4) Pseudolarix (false larches) (1 species, in China): Same as Larix, but their cones are deciduous, i.e., breakup and release seed at maturity within one year. 11 pairs of chromosomes (most of the pine family has 12 pairs)
(5) Cathaya (1 species, in China): Resembles Larix and Cedrus in having long and short shoots, but develops cones on the long shoots whereas Larix and Cedrus develop female (and male) cones from the short shoots. Leaves somewhat whorled and nondeciduous.
(6) Keteleeria (10 species, in China, Laos, Taiwan and Vietnam): Resembles Abies, but the upright cones do not break up at maturity with in one year. Also has hypogeal (underground) germination whereby the cotyledons stay below the ground surface and the true shoots emerge. Pollen grains with two wings.
(7) Picea (spruces) (37 species): Leaves spirally arranged and four sided and therefor relatively stiff and are set on a stem projection (the pulvinus) and therefor leave a rough twig after falling. The cones arise from the terminal cluster of buds at the ends of the shoots and up to the time of pollination are erect (like Abies) but then become pendulous. Pollen grains with two wings.
(8) Tsuga (hemlocks) (10 species): Leaves with a knee-like bent petiole arising from a pulvinus and are constricted at the base (the petiole) and are notched and usually rounded at the ends. Cones small, ripening in the first year but remaining on the tree and not disintegrating (similar to Picea.) Pollen grains without wings.
(9) Nothotsuga (a hemlock from SE China, often still included in the Tsuga genus) (1 species)
(10) Pseudotsuga (Douglas firs) (8 species): Leaves like Abies. Cones like Picea and Tsuga, but with exserted bracts (little forked tabs at the ends of the cone scales). Also have sharp pointed cylindrical buds. Pollen grains without wings. 13 pairs of chromosomes.
(11) Pinus (pines) (about 120 species): Needles in fascicles (of 1 to 8) which can be fit together to form a cylinder. The female cones are fertilised in the second year and are variably persistent thereafter. Cone seed scales usually with a scale shield (apophysis). "Male" cones are many and clustered at the base of the current