Pinus, the pines, is a genus of approximately 110–120 extant tree and shrub species. The genus is currently split into two subgenera, subgenus Pinus (hard pines), and subgenus Strobus (soft pines). Each of the subgenera have been further divided into sections and subsections based in the past on morphology, ecology and biogeography, and more recently increasingly from chloroplast DNA sequencing and whole plastid genomic analysis. While the genetic analysis has given robust results at the higher levels, they often give conflicting results lower in the phylogenetic trees, with species allocated to different subsections (and sometimes different sections) by different studies or even within a study. Within subsections, the genetic relationships between species can be even more complex and conflicting; in one study, three samples of the very distinctive and morphologically constant Pinus lambertiana were placed in three different clades of the subsection Strobus, and similar problems with many other species with widespread nonmonophyly.
Pinus
SubgenusPinus
SectionTrifoliae
Subsection Ponderosae
Subsection Contortae
Subsection Australes
SectionPinus
Subsection Pinus
Subsection Pinaster
SubgenusStrobus
SectionQuinquefoliae
Subsection Gerardianae
Subsection Krempfianae
Subsection Strobus
SectionParrya
Subsection Nelsonianae
Subsection Balfourianae
Subsection Cembroides
Several features are used to distinguish the subgenera, sections, and subsections of pines; the number of leaves (needles) per fascicle, whether the fascicle sheaths are deciduous or persistent, the number of fibrovascular bundles per needle (two in Pinus, one in Strobus), the position of the resin ducts in the needles (internal or external), the presence or shape of the seed wings (rudimentary or effective, articulate or adnate), and the position of the umbo (dorsal or terminal) and presence of a prickle on the scales of the seed cones.
The two subgenera are thought to have ancient divergence from each other, having diverged at some point between the late Jurassic (around 160 million years ago), the mid Cretaceous (around 125 million years ago), to the late Cretaceous (around 100 million years ago).
Subgenus Pinus
Subgenus Pinus includes the yellow and hard pines. Pines in this subgenus have two to five needles per fascicle (rarely as many as eight, in P. durangensis). They have two fibrovascular bundles per needle, and the fascicle sheaths are persistent, except in P. leiophylla and P. lumholtzii. The cone scales are thicker and more rigid than those of subgenus Strobus, and have a resin sealing band before opening (resulting in the cones opening with an audible crack); the cones either open soon after they mature or are serotinous.
Section Pinus
Section Pinus has two or three needles per fascicle. The cones have moderately thin to thick scales; in most they open at maturity, but are weakly serotinous in some species in subsection Pinaster. Species in this section are native to Europe, Asia, and the Mediterranean, except for P. resinosa in northeastern North America and P. tropicalis in western Cuba.
All but two species (P. resinosa and P. tropicalis) in Subsection Pinus are native to Eurasia. The cones have moderate thickness scales, and are characterised by a slightly off-centre prickle ('excentromucronate') on the umbo.
This pine is commonly resolved into subsection Pinaster by genetic studies, but this is in strong conflict with its morphology and ecology, which is much closer to subsection Pinus, or alternatively placed in its own subsection Leucodermes close to subsection Pinus.
These pines are placed in subsection Pinus by some genetic studies, but in subsection Pinaster by others. In morphology and ecology, they belong in the latter group. The subsection Merkusia has also been proposed for them.
P. ustulata – Philippine pine (split from P. merkusii in 2014)
Subsection Pinaster
Pinus roxburghii
Subsection Pinaster contains species native to warm climates in the Mediterranean region at low altitudes, as well as P. roxburghii from the Himalayas. The cones are thick-scaled and orange-brown to red-brown, and the cone scales are glossy and lack umbo spines. It is named after P. pinaster.
Section Trifoliae (American hard pines), despite its name (which means "three-leaved"), has two to five needles per fascicle, or rarely eight. The cones of most species open at maturity, but a few are serotinous. All but two American hard pines belong to this section.
The timing of divergences within this section is disputed, with subsections Australes and Ponderosae having diverged during the mid Cretaceous (about 110 million years ago) according to one study, but not until the mid Oligocene (about 30–35 million years ago) in others.
Subsection Attenuatae
Pinus attenuata
The three closed-cone (serotinous) and fire adapted species of California and Baja California form a small subsection; closely related to, and often included within subsection Australes. Some studies suggest Pinus glabra may also belong here (despite its morphological dissimilarity) though others include it in subsection Australes.
P. contorta (infraspecific taxa traditionally cited at varietal rank, but with marked divergence in morphology and ecology, better treated at subspecific rank)
This subsection is native to Central America, Mexico, the western United States, and southwestern Canada, although its former range was possibly much wider as evidenced by upper Miocene fossils belonging to this subsection found in Japan It contains at least 13 living species and may contain five more if the disputed species become widely accepted.
P. apulcensis (syn. P. oaxacana; currently treated as a variety of P. pseudostrobus by POWO)
These are pines of the western United States and Mexico with four existing species. Within the subsection the Coulter pine is closely related with the Jeffery pine and the gray pine is likewise paired with the Torrey pine.
Subgenus Strobus includes the white and soft pines. Pines in this subgenus have one to five needles per fascicle and one fibrovascular bundle per needle, and the fascicle sheaths are deciduous, except in P. nelsonii, where they are persistent. Cone scales are thinner and more flexible than those of subgenus Pinus, except in some species like P. maximartinezii, and cones usually open soon after they mature.
Section Nelsonia
Pinus nelsonii
This unique pine is remarkable in differing from all other pines in multiple cone and foliage characters. Genetically, its position has varied between and within studies; three nuclear genes and chloroplast DNA placed it as sister taxon to the rest of Section Parrya, while a late embryogenesis abundant (LEA)-like gene (IFG8612) resolved it as sister to Section Quinquefoliae; another study resolved it as sister to Subsection Balfourianae. It has probably evolved separately from the rest of the genus since the late Eocene, with its most recent common ancestor around 37 million years ago. These unique characters warrant recognition as a monotypic section.
Subsection Nelsonianae
Subsection Nelsonianae is native to northeastern Mexico. It consists of the single species with persistent fascicle sheaths.
Section Parrya has one to five needles per fascicle. The seeds have articulate (jointed) wings; in most, the wing is vestigial, and remains attached to the cone scale when the seed is released apparently wingless. In all species, the fascicle sheaths curl back to form a rosette before falling away. The cones have thick scales (thin in subsection Balfourianae) and release the seeds at maturity. This section is native to the southwestern United States and Mexico.
Section Quinquefoliae (white pines), as its name (which means "five-leaved") suggests, has five needles per fascicle except for P. krempfii, which has two, and P. gerardiana and P. bungeana, which have three. All species have cones with thin or thick scales that open at maturity or do not open at all; none are serotinous. Species in this section are found in Eurasia and North America, and one species, P. chiapensis reaches Guatemala.
Subsection Gerardianae
Subsection Gerardianae is native to East Asia. It has three or five needles per fascicle.
Subsection Krempfianae is currently native to Vietnam, with a fossil record extending into the Oligocene. It has two needles per fascicle, and they are atypically flattened. The cone scales are thick and have no prickles. Until 2021, the subsection was considered monotypic, when an Oligocene fossil species was described from Yunnan Province, China.
Subsection Strobus has five needles per fascicle and thin cone scales with no prickles. Needles tend to be flexible and soft with slightly lighter side underneath. It is native to North and Central America, Europe, and Asia.
↑Businský, Roman (2014). "The Pinus merkusii agg. (Pinaceae): Literature Review, Taxa Delimitation and Typifications". Phyton. 54 (1): 1–26. doi:10.12905/0380.python54(1)2014-0001.
↑Knowlton, F.H. (1923). Fossil plants from the Tertiary lake beds of South-Central Colorado (Report). Professional Paper. Vol.131-G. United States Geological Survey. pp.183–197. doi:10.3133/pp131G.
↑Wolfe, J.A.; Schorn, H.E. (1990). Taxonomic revision of the Spermatopsida of the Oligocene Creede flora, southern Colorado (Report). Bulletin. Vol.1923. United States Geological Survey. pp.1–40. doi:10.3133/b1923.
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