A taxonomic revision of Pinus (Pinaceae) in Thailand

A taxonomic revision of the native Pinus L. (Pinaceae) species in Thailand is presented. Two species, P. kesiya Royle ex Gordon and P. latteri Mason, are recognized and enumerated with updated morphological descriptions, photographs, and an identification key, together with notes on distribution, habitat and ecology, phenology, conservation status, etymology, vernacular names, uses, and specimens examined. Pinus kesiya is neotypified (second-step neotypification), and P. kasya Royle ex Parl., a synonym of P. kesiya, is lectotypified (second-step lectotypification). Pinus kesiya is assessed as Least Concern (LC), whereas P. latteri is assessed as Near Threatened (NT). Morphologically, the two species can be distinguished by bark fissuring pattern, thickness, and color; number of needles per fascicle; needle morphology (including rigidity, posture, and shape and diameter in transverse section); length/width ratio of seed cones when closed; apophysis shape and outline; seed scale thickness; relative length of seed wings; and presence or absence of a grass stage. The two species differ ecologically in elevational range and habitat preference, with P. kesiya occurring at higher, more humid montane sites and P. latteri at lower, drier habitats. Pinus kesiya and P. latteri differ in needle shape in transverse section, which is triangular in P. kesiya and semicircular in P. latteri. They also differ in resin duct number and position, with P. kesiya having 3-4 external resin ducts near the hypodermis, whereas P. latteri has 2-3 medial resin ducts between the endodermis and hypodermis. Pollen morphology is not diagnostic for species delimitation.