Firs Of North America
Nine species of true firs grow naturally in North America. The most widespread species is balsam fir (Abies balsamea), a prominent tree in boreal and north-temperate forests of eastern Canada and the northeastern United States. On moist sites with a moderate climate, this species grows as tall as 65 ft (20 m). In some places, balsam firs occur above the timber-line in a depressed growth-form known as krummholtz. Balsam fir is highly intolerant of fire, and it tends to be a relatively short-lived tree. Balsam fir is the major food species of the spruce budworm (Choristoneura fumiferana), a moth that periodically causes extensive forest damage in northeastern North America. Fraser fir (A. fraseri) is closely related to balsam fir, but occurs in montane forests of the southern Appalachians.
The other seven species of true firs in North America all occur in western forests. The subalpine fir (Abies lasiocarpa) grows in montane forests from southern Alaska to northern Texas, sometimes occurring past the timberline in a krummholtz growth form. Grand fir (A. grandis), Pacific silver fir (A. amabilis), and white fir (A. concolor) are species of moist, western rain forests, growing on sites of moderate altitude, and achieving heights of as much as 164 ft (50 m). Species with relatively restricted distributions in the western United States are bristlecone fir (A. bracteata), noble fir (A. procera), and California red fir (A. magnifica).
The Douglas fir (Pseudotsuga menziesii) is a common, fast-growing, and valuable timber species in western North America, where it can grow as tall as 262 ft (80 m) and attain a diameter of more than 6.5 ft (2 m). Some taxonomists divide the species into two races, the coastal Douglas fir (P. m. menziesii), which grow in humid western forests, and the Rocky Mountain or interior Douglas fir (P. m. glauca), which grows in drier forests further to the east. The big-cone Douglas fir (Pseudotsuga macro-carpa) is a locally occurring species in extreme southern California and northern Baja.