Introduction
Pinus, with over 100 species, is the largest genus of conifers and the most widespread genus of trees in the Northern Hemisphere. The natural distribution of pines ranges from arctic and subarctic regions of Eurasia and North America south to subtropical and tropical (usually montane) regions of Central America and Asia. Pines are also extensively planted in temperate regions of the Southern Hemisphere. Many pines are fast growing species tolerant of poor soils and relatively arid conditions, making them popular in reforestation. Important pine products include wood, turpentine, and edible seeds.
Pines have a relatively rich fossil record dating back to the Early Cretaceous, 130 million years ago (reviewed in Axelrod 1986; Klaus 1989; Van der Burgh 1973; Millar 1993).
Discussion of Phylogenetic Relationships
Most classifications of Pinus recognize two major lineages: subgenus Strobus (haploxylon or soft pines, with one fibrovascular bundle in the needle) and subgenus Pinus (diploxylon or hard pines, with two fibrovascular bundles in the needles). This division is consistent with data from wood anatomy and secondary chemistry, and is supported in recent molecular phylogenetic studies (Strauss & Doerksen 1990; Wang & Szmidt 1993; reviewed in Price et al., 1998). The Vietnamese species, P. krempfii possesses unusual flattened needles and a unique wood anatomy. Based on these distinctive features, the species was considered a third subgenus in the influential classification of Little & Critchfield (1969). On the other hand, secondary product chemistry (Erdtman et al. 1966), nuclear ribosomal DNA (Liston et al. 2003) and chloroplast DNA sequences (Wang et al. 1999; Gernandt et al. in press) indicate that P. krempfii should be considered a member of subgenus Strobus.
