Pinus, the pines, is a genus of approximately 111 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 several sections within based on chloroplast DNA sequencing. Older classifications split the genus into three subgenera – subgenus Pinus, subgenus Strobus, and subgenus Ducampopinus (pinyon, bristlecone and lacebark pines) – based on cone, seed and leaf characteristics. DNA phylogeny has shown that species formerly in subgenus Ducampopinus are members of subgenus Strobus, so Ducampopinus is no longer used.
The species of subgenus Ducampopinus were regarded as intermediate between the other two subgenera. In the modern classification, they are placed into subgenus Strobus, yet they did not fit entirely well in either so they were classified in a third subgenus. In 1888 the Californian botanist John Gill Lemmon placed them in subgenus Pinus. In general, this classification emphasized cone, cone scale, seed, and leaf fascicle and sheath morphology, and species in each subsection were usually recognizable by their general appearance. Pines with one fibrovascular bundle per leaf, (the former subgenera Strobus and Ducampopinus) were known as haploxylon pines, while pines with two fibrovascular bundles per leaf, (subgenus Pinus) were called diploxylon pines. Diploxylon pines tend to have harder timber and a larger amount of resin than the haploxylon pines.
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, the position of the resin ducts in the needles, the presence or shape of the seed wings, and the position of the umbo and presence of a prickle on the scales of the seed cones.
Subgenus Pinus includes the yellow and hard pines. Pines in this subgenus have one to five needles per fascicle and two fibrovascular bundles per needle, and the fascicle sheaths are persistent, except in P. leiophylla and P. lumholtzii. Cone scales are thicker and more rigid than those of subgenus Strobus, and cones either open soon after they mature or are serotinous.
Section Pinus has two or three needles per fascicle. Cones of all species have thick scales, and all except those of P. pinea open at maturity. 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 in Subsection Pinus are native to Eurasia.
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.
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 Parrya has one to five needles per fascicle. The seeds either have articulate (jointed) wings or no wings at all. In all species except for P. nelsonii, the fascicle sheaths curl back to form a rosette before falling away. The cones have thick scales and release the seeds at maturity. This section is native to the southwestern United States and Mexico.
Subsection Nelsonianae is native to northeastern Mexico. It consists of the single species with persistent fascicle sheaths.
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 is native to East Asia. It has three or five needles per fascicle.
Subsection Krempfianae is native to Vietnam. It has two needles per fascicle, and they are atypically flattened. The cone scales are thick and have no prickles.
Species which are not placed in a subgenus at this time.