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Abies is a genus of 48 species (Farjon 2001), distributed across the northern hemisphere. The genus extends southwards as far as Honduras in Central America, and Vietnam in Asia. There are also a number of taxa in Mediterranean Africa. One remarkable feature of the genus is the preponderance of relict taxa with restricted distributions. Silver firs are tall, evergreen trees with straight, columnar trunks. The bark is generally smooth and pale in colour, though older trees can develop scales and longitudinal fissures. Primary branches are produced in regular whorls and spread horizontally, often curving downwards at maturity. The leader shoot is rigid and upright. Immature growth is often soft and bright green, contrasting with the darker mature foliage. These fresh shoots are particularly susceptible to late frosts. There are one to five buds, and these are often large and resinous. The needle-like leaves are arranged in two ranks (pectinate), each consisting of several rows. Usually, there is a V-shaped partition between the two main ranks, though this is absent in A. koreana and A. pinsapo. The leaves are attached to a shallow depression in the stem, which is clearly visible when the leaf is detached, and are twisted at the base so that the stomatal surface faces downwards. Stomata are arranged in two grey-white rows separated by the midrib, though they are also present on the upper surface of the leaf in some species. Most Abies species have two resin canals in each leaf, and the relative positions of these within the leaf can be informative. The male strobili are catkin-like and relatively undifferentiated, though in section Pseudopicea (for example, A. delavayi, A. fabri, A. forrestii, A. fargesii) the strobili are long and slender with red or purple microsporophylls. The strobili are generally pendulous, and occur in the upper half of the crown. They are produced from lateral axillary buds, and are often crowded towards the tips of the previous year’s shoots. The cones are erect and restricted to the upper part of the crown in most taxa. They mature in one year, during which time their colour changes from purple (or rarely green) to brown. The cone scales are spirally arranged around a central rachis, and at maturity they fall leaving only the rachis attached to the tree. Immature cones have colourful bracts that cover the scales, and these may be hidden (included) or exserted from the mature cone. There are two seeds on each scale, and they are partially enclosed in a membranous cup, which extends to form a persistent triangular wing. When identifying Abies species, the most important characters are those of the branchlets (colour, pubescence, presence of resin on buds), leaves (size, arrangement, stomata distribution, resin ducts) and cones (size, shape, colour, shape of seed scale, bract size and position) (Farjon 1990).
To bring the taxonomy of cultivated trees up to date we here provide keys to subspecific taxa and indicate important synonymy, following the taxonomic views of Aljos Farjon (1990, 2001). The Flora of China treatment (Fu et al. 1999c) and other authorities do not always agree with these. It is clear that a thorough modern revision of the genus is urgently needed, which might reduce the excessive number of names for local forms and enable the firs to be seen in a more focused way than is apparent at present. With changing taxonomic concepts and nomenclature, coupled with occasional misidentifications, the naming of many (especially older) trees in collections has become confused. In consequence the identification of older champions is not always reliable, and these are only cited here where identification is certain.
A number of Mexican Abies are poorly known, with disputed taxonomy, and are represented in cultivation by no more than a handful of specimens. Abies colimensis Rushforth & Narave, from the upper reaches of the Nevado de Colima in Jalisco, was included in A. religiosa by Farjon (2001) but differs in its much larger, barrel-shaped cone with reflexed scales (looking somewhat similar to cones of A. magnifica and A. procera). It is rare in horticulture. Abies flinckii Rushforth comprises the two entities named by Martínez as A. religiosa var. emarginata Martínez and A. guatemalensis var. jaliscana Martínez, differing from typical A. religiosa and A. guatemalensis in its longer, narrower cones and longer foliage. Farjon (2001) included it in A. guatemalensis var. jaliscana. It is found in western Mexico, north of the range of A. guatemalensis and west of that of A. religiosa. Material collected by Keith Rushforth (KR 0621, the type collection) at 2350 m at Nevado de Colima, Jalisco in 1984 did well for Mike Hudson in New Zealand until it died suddenly, having reached 9 m, and a grafted specimen reached 3.5 m for Tom Hudson in Cornwall but was killed by Armillaria (T. Hudson, pers. comm. 2007). It is however listed as growing in the San Francisco Botanical Garden (formerly known as the Strybing Arboretum). Abies mexicana Martínez (see Krüssmann (1985b) and Clarke (1988): K46, S31) is related to A. durangensis, rather than to A. vejarii in which it has been placed as a subspecies (A. vejarii subsp. mexicana (Martínez) Liu), differing from A. vejarii in its ‘foliage arrangement, cones, bark and ecology’ (Rushforth 1987a). It is found in the same general area as A. vejarii, in the Sierra Madre Oriental in Coahuila and Nuevo León, but at lower elevations. Abies mexicana has a sparse presence in cultivation from collections by Keith Rushforth (KR 479), Jim Priest (JP 109), and Rob Nicholson, Carl Schoenfeld, John Fairey and Mel Shemluck (Nicholson et al. B1), all from 1 km south of San José de la Hoya, near Galeana, Nuevo León and just to the north of Cerro Potosí, where there are trees to 20 m that bear green-brown cones with included bracts, growing with Pseudotsuga on a northwest aspect at the bottom of a canyon (K. Rushforth, pers. comm. 2008). Two trees grown from Nicholson et al. B1 are thriving at Arboretum Wespelaar (P. de Spoelberch, pers. comm. 2008).
The firs are among the most beautiful of conifers, being distinguished by their attractive branching pattern, often handsome foliage and sometimes enormous stature. This beauty can become marred by irregularities in shape as they age, but a group of young, vigorous trees is a lovely sight. Their horticultural use, however, is constrained by several factors. First, they tend to be potentially very large (with the exception of the bijou A. koreana), so gardeners with small plots avoid them – although against this may be set the fact that they often produce their outstanding cones at an early age. Secondly, many species do not do best in areas that are hot and dry, or hot and humid, so the finest specimens tend to be in cooler, moister areas, a pattern that holds good not only for the British Isles, where the genus is well known, but also in continental Europe and North America.
A danger point in the cultivation of Abies is the period of shoot extension in spring, when new shoots can be nipped by frost. This can be a serious problem for some species, particularly those from very cold places, which can be fooled by warm early spring temperatures so that they are consistently frozen back each year. These are often best in cool areas where the new shoots do not break quite so early as in warmer situations. Happily there are plenty of other species from suitable habitats from which to make a selection for cultivation where summers are warmer and drier. A beautiful example is A. vejarii, which seems capable of equalling in loveliness all the classic firs of Europe and western America, even in situations where firs might not be expected to thrive. Most Abies find the combination of heat and humidity intolerable, however, and in the southeastern United States only A. firma will thrive (T. Cox, J. Ruter, pers. comms. 2007; Dirr 1998).
Adelgid aphids can be a very serious problem on Abies and have caused great losses to A. alba in Europe and A. balsamea and A. fraseri in eastern North America, but they seem to be less serious in horticultural settings.
A group of about forty species generally known as 'silver firs'. They are found in Europe, N. Africa, temperate Asia and on the American continent from Canada to Guatemala. They are mostly conical and very symmetrical in form, especially when young, and the finest are from 200 to 300 ft high. They produce their branches in whorls or tiers, one tier yearly. Leaves always linear or nearly so, from [1/20] to [1/8] in. wide, with invariably two bands of stomata beneath, occasionally lines of stomata above also; they are always attached to the shoot in a spiral arrangement, but by a twisting at the base are usually made to appear in two opposite sets, the green faces of all uppermost. Female cones always erect, in which respect they differ from those of Picea (the spruces), and from Tsuga (the hemlocks), both of which genera have been, and still are, often called “Abies”. There is a simple way of distinguishing a fir (Abies) from a spruce by pulling off a living leaf from the shoots: in the firs the leaf breaks off sharply at the base where it joins the twig, but in the spruces (Picea) it tears away a little of the bark with it. Also, the leaves of the spruces are inserted on woody, peg-like projections which persist on the stems for many years after the leaves have fallen.
The cones are built up of a close spiral arrangement of overlapping, usually more or less fan-shaped scales, to the outer surface of which a bract is always attached. The length of this bract and whether or not it protrudes beyond the scale, affords a good distinguishing character between the species. Seeds are borne in pairs on the inner side of the scales, and are winged. The male flowers occur on branches separate from the females, and are borne on the under side of the branch; anthers highly coloured. On flowering and cone-bearing branches the leaves frequently alter much in character, becoming shorter, stiffer, sharper pointed, and more erect.
The silver firs are undoubtedly best suited in a moist climate where late spring frosts are rare. Nowhere in the British Isles, perhaps, do they, as a whole, succeed quite so well as in the Perthshire valleys. Where the rainfall is deficient, lack of moisture can to some extent be compensated for by a good deep soil. Whenever possible they should be raised from seeds, but of some sorts cuttings may be made to take root. The cuttings should always be taken from leading shoots, as distinct from lateral ones, which rarely develop a good leader. The best plan is to head back a plant, thus inducing it to make several shoots; these are then taken off with a slight heel of old wood attached, and placed singly in small pots of sandy soil in a gentle bottom heat. But both cuttings and grafts should only be resorted to when seeds are unobtainable. Several species, among them alba, amabilis, magnifica, nordmanniana, and procera, are subject in many places to attacks by aphides of the genus Adelges; gouty swellings on the branches and woolly patches on branches and trunk are a sign of their presence. The remedy, only practicable on young trees, is regular spraying with an emulsion of paraffin and soft soap or some modern aphicide.
The most important work on the genus is: T.-S. Liu, A Monograph of the genus Abies (1971), published by the Department of Forestry of the National Taiwan University, Taipei, Taiwan, with detailed botanical illustrations and distribution maps for each species. The silver firs cultivated in the British Isles are surveyed by John Horsman in The Plantsman, Vol. 6, pp. 65-100 (1984), with notes on availability in commerce.
Keith Rushforth's contributions to the taxonomy of the Chinese species of Abies, published in Notes from the Royal Botanic Garden, Edinburgh, in 1984-5, are referred to below under A. chengii, A. chensiensis, A. fabri, and A. fargesii. The editor is also indebted to him for informative letters on taxonomic problems concerning A. delavayi and A. forrestii.