In the heart of its range, in temperate eastern North America, Pinus strobus is the only native 5-needle pine. In western temperate North America, the related P. monticola (western white pine) differs in its stouter, stiffer needles that are usually glaucous. Another western relative, P. lambertiana (sugar pine), differs in its needles with white longitudinal lines noticeable on all 3 surfaces and in its massive (25–45 cm long, 8–12 cm wide), straight cones with thicker scales. Another 5-needle western pine is P. albicaulis (whitebark pine), which differs in its pale gray bark, its short-stalked, relatively hard, broadly ovoid cones with the thick scales terminated by a spinose umbo, and its wingless seeds. Other temperate, western North American, 5-needle pines having short-stalked, relatively hard cones include P. flexilis (limber pine) and P. strobiformis (southwestern white pine). The only other 5-needle pines native to temperate North America are P. balfouriana (foxtail pine) and the two bristlecone pines, P. aristata and P. longaeva. All three of these trees have cones that are broadest at the base with scales have a dorsal, spinose. Some of the western temperate North American 5-needle pines have ranges that extend into Mexico and there are several additional species growing in the region where the disjunct southern populations of P. strobus occur. Of these, the two most closely related species are P. flexilis (discussed above and P. ayacahuite (ayacahuite, pino gretado). The latter differs from P. strobus in its massive seed cones (mostly 15–40 cm long, 7–15 cm wide) with scales that are mostly strongly recurved toward the tip. Source documents: Kral (1993), Farjon et al. (1997), Farjon (2005), Gernandt et al. (2005).