Fir,  P. Miller 1754
Pinaceae

Evergreen, modest-sized to large trees, with a conical crown composed of regular tiers of relatively short, horizontal branches. Trunk usually single, straight, with smooth, nonfibrous bark bearing rows of conspicuous resin pockets when young, remaining smooth with age or becoming variously furrowed or scaly. Twigs all elongate, without distinction into long and short shoots, smooth or grooved. Winter buds well developed, scaly. Leaves spirally arranged but sometimes two-ranked or nearly so, needlelike, usually more or less flattened, straight or curved upward, the margin smooth, the tip pointed, rounded, or notched, the base leaving a flat, round scar on the twig when detached.

Plants monoecious. Pollen cones single, emerging from scaly buds in the axils of leaves of year-old twigs, hanging in rows beneath twigs in the lower portion of the crown, elongate. Pollen scales numerous, each scale with two pollen sacs. Pollen grains very large (body 75-145 µm long, slightly larger than those of Picea and much larger than those of Pinus), with two round air bladders about half the size of the oblong body and diverging at a little more than a right angle, the body and bladders variously minutely wrinkled or warty but differently from each other. Seed cones roughly cylindrical or barrel-shaped, single on upright stalks in the axils of leaves on shoots at the top of the crown, upright at pollination and at maturity, maturing in a single season. Cone shattering at maturity to release the seeds and numerous, spirally arranged, tightly packed, flat seed scales from a long persistent central axis of variable shape, from very narrowly cylindrical to quite swollen in the middle. Seed scales wedge- to kidney-shaped, with a thin stalk. Bracts attached to the seed scales only at their base, leaflike in shape with a central point, shorter than the scales, or longer than them and sticking out between and beyond them to varying extents. Seeds two per scale, wedge-shaped, the wing derived from the seed scale, extending about the length of the body beyond it and much wider, also covering the body above and wrapping about halfway around it. Cotyledons 4-10, each with one vein. Chromosome base number x = 12.

Wood soft, usually not fragrant, off-white to light tan, with little difference between sap- and heartwood. Grain fairly even, with a gradual transition from early- to latewood. Resin canals ordinarily absent but often with crystals in the ray cells.

Stomates forming two prominent stomatal bands beneath and often with additional single lines of stomates above. Each stomate shallowly to deeply sunken beneath and largely hidden by the one to three cycles of four (to six) subsidiary cells, which may have very crinkly cell walls but lack a Florin ring. Leaf cross section almost always with a single resin canal (rarely more) on each side of the double-stranded midvein near the edge or middle and with transfusion tissue lying beneath and sometimes lapping up a little around the outer sides of the midvein within the endodermis. Photosynthetic tissue with a dense, multilayered palisade of relatively short cells beneath the upper epidermis and adjoining hypodermis, which is typically thickest at the outer edges of the leaf and sometimes present only there. Spongy mesophyll filling up the lower two-thirds of the leaf, with especially extensive air spaces above the stomatal bands and without extra sclereids scattered through the tissue.

The name Abies is the classical Latin name for European silver fir (Abies alba), which grows in mountain ranges throughout the Italian peninsula. The resin pockets in the bark are sources of natural adhesives and mounting media for microscope slides (especially Canada balsam), useful because they have the same refractive index as glass and so do not cause distortion. The soft wood is not particularly good for construction or cabinetry, but the lack of odor and staining heartwood extractives makes it useful for pulp and (progressively less frequently today) for food boxes and crates. The perfectly conical shape and fragrant, dark green foliage of short, long-lasting needles make firs favorites as Christmas trees in North America and Europe, where they are raised for the purpose. Their shapeliness also makes them favorites in the open garden, particularly those that have long needles and some tolerance to drier conditions (like Abies concolor) or colorful seed cones on young trees (like Abies koreana). Although some species have been in gardens for centuries, cultivar selection has been fairly modest, emphasizing variations in cone size and early maturity, needle length and color (such as highly glaucous blue-greens, chlorotic yellows, and variegated foliage), and growth habits (including narrow columns, flattened spreaders, and many dwarfs).

The various firs are closely related and rather uniform in appearance, so classification of Abies is difficult, and there is little agreement among different authors on the number of species, subspecies, and varieties or their arrangement into subgenera, section, subsections, and series. Even the best attempts to arrange the genus have points of weakness. There is a strong geographic component to the taxonomy of the firs, and the main distributional regions – North America, western Eurasia, and eastern Eurasia – are largely occupied by different sections. There may be a few sections in both eastern Asia and North America, but this has yet to be convincingly demonstrated. Even in the well-botanized parts of the world, as in western Eurasia and northern North America, much more work needs to be done to settle some of the difficulties, but things are even more confused in China, Mexico, and Guatemala, where collections are much sparser and the trees are threatened by uncontrolled exploitation. Comparison of DNA sequences may help to settle the arrangement of sections but will not, by itself, solve the problems of species limits without extensive new field study.

There are many hybrids, both in nature and in cultivation, but these are between species of the same section or of closely related sections. Most crosses between species of different sections fail. As a result of intercrossability, many sections consist of a geographic sequence of species (or subspecies or varieties) linked by more or less extensive areas of intermediate populations. Different classifications treat the intervening populations as hybrids, subspecies, or varieties of one or the other species on either side of the cross, or as species in their own right. This is the situation in western Eurasia and North Africa, where all species are intercrossable, and even species that are geographically separated today might have been in contact and hybridized as they were forced together at various times during the ice ages.

The true firs are predominantly trees of cool, humid environments, ranging from sea level in boreal and maritime forests to the alpine tree line in the mountains. Often, a single mountain range will support two or more species with different but overlapping elevational distributions. They generally do not reach the arctic tree line and, on average, have a more southerly overall range than do the similarly distributed spruces (Picea) and larches (Larix). They appear to have occupied similar environments since their first convincing appearance in the fossil record in Washington state during the mid-Eocene, about 47 million years ago. Subsequent fossils are widespread but uncommon across middle latitudes of the northern hemisphere, constituting a continuous record throughout the Tertiary of pollen, foliage, cone scales, and seeds. There is even an extensive pollen record in Iceland, where the genus is no longer found.

References

• Farjon, A. (2010). A Handbook of the World's Conifers. Koninklijke Brill, Leiden.
• Eckenwalder, J.E. (2009) Conifers of the World: The Complete Reference. Timber Press, Portland.
• IUCN Red List of Threatened Species, International Union for Conservation of Nature and Natural Resources. Cambridge, UK /Gland, Switzerland

Copyright © Aljos Farjon, James E. Eckenwalder, IUCN, Conifers Garden. All rights reserved.