A Guide to the Firs (Abies Spp.) of the Arnold Arboretum
Total Page：16
File Type：pdf, Size：1020Kb
A Guide to the Firs (Abies spp.) of the Arnold Arboretum Richard Warren Ethan W johnson Twenty-five of the thirty- to forty-odd species (and hybrid species) of the wide-ranging Northern Hemisphere genus Abies currently grow in the Arnold Arboretum Abies is the scientific name of the firs, or, as is more common in Britain, of the silver firs. Botanists took many years to agree upon it (Warren, 1982). Virgil (70-19 s.c.) applied the word "abies" to the wood employed for the ribs of the Trojan horse (Virgil, circa 19 s.c.). This may have led to the modern naming of the genus; certainly it influenced the naming, in 1883, of a species from northwestern Anatolia-Abies equi-trojani Aschers e~ Sint. The English word "fir" is of Scandinavian (Old Norse) origin and referred originally to pine, which in that part of the world is Scotch pine (Pinus sylvestris L.). Many English still refer to their pines as ‘ firs. Their use of "silver fir" as the common name for members of the genus Abies is due to the whiteness of the undersides of the leaves in most species. The aims of this guide are, first, to highlight the morphologic characters of the genus Abies so that it may readily be distinguished from related genera and, second, to accentuate the characters that clearly separate the species of Abies from each other. The Genus Abies The genus Abies, as do the genera Pinus, mates like that of the Mediterranean area. Picea, and Larix, grows widely in the North There are some thirty to forty species in Temperate Zone around the world, reaching the genus Abies, depending on the author. Liu from the Arctic Circle (Abies sibirica) to the (1971), for example, lists forty-one species, Tropical Zone, at 15 degrees North latitude sixteen varieties, and six hybrids. Abies (Abies guatemalensis) in Central America. In competes for third place with Picea as the general, it is not as hardy in Arctic climates genus of conifers in the Northern Hemi- as are members of the genus Picea, but Picea sphere containing the greatest number of does less well than Abies in southern cli- species (Pinus contains about one hundred species and Juniperus about sixty). This guide is based on the species of Abies in the Arnold made Charles Edward Faxon Opposite : Drawings by of and two various macroscopic and microscopic structures of the Arboretum-twenty-three species grand fir (Abies grandis Lindley). From Charles Sprague named hybrids, twenty-five in all (see the list Sargent’s The Silva of North America. (See page 29.) on page 13). 4 Habit below, are arranged all around the branchlet. Erect. Narrow. Pyramidal. With the no- Also, the nearer the situation of the branchlet table exception of Abies nordmanniana, the to the crown, the greater the tendency to branches do not droop. The leader is seen up- suppression of the V. right and rigid against the sky as in Picea (not nodding as in Tsuga). Leaves Flattened, linear. Above: few or no stomata Bark (exceptions showing stomata above, such as Smooth at first but stippled with horizontal Abies concolor, Abies lasiocarpa, Abies rnag- rows of resin blisters appearing like len- nifica, Abies pinsapo, and Abies procera give ticels. With age the lower bark in most spe- a first clue to identity), a groove usually being cies becomes rough. Departures from these present. Below there are two longitudinal generalizations form the basis for the identi- bands made up of several rows each of sto- fication of certain species. mata varying in color from white to gray-green. The bands are framed by three Buds longitudinal, slightly raised, green ribs, the Ovate or round (less pointed than in Picea). midrib, and the sideribs. The tip of the leaf is Resinous or not resinous. It is to be noted that rounded or pointed or notched. The attach- the resinosity or nonresinosity of the buds is ment of the leaves to the branchlet is by a useful in distinguishing species but is of no rounded end of the pedicel, which looks like use after they have broken open in the grow- a suction cup. ing season (May through July) until the new ones have formed. The resin may present to Resin Canals the eye either as semicrystallized white gran- Resin canals are tubular channels in the leaf ules or as glairy, clear material. Both are tissue lined with resin-secreting cells. Their sticky to the touch. position in the leaf relative to other struc- tures seen in cross section is of use in distin- Branchlets guishing between species, as is true to some Surface generally even in contour but often extent in all conifers. This is particularly so in fissured or undulate. Not roughened nor the genus Abies, where the position is more scaled. Hairy or not hairy according to spe- constant and reliable than in other conifers. cies. When the leaves fall off the branchlet Firs have two resin canals. They are seen symmetrical, round leaf scars are left. These with a lens as two holes in the cut surface of scars are different from those on the branches a transected leaf from which drops of resin of Picea, which are at the tips of woody pegs, emerge (see the figure, page 10). The two cate- or projections from the branchlet surface gories of position are marginal, wherein the called sterigmas. The leaf scars in Pseudo- canal edge touches the hypoderm, and me- tsuga (Douglas fir) are also rounded but are dian, when it does not. slightly raised from the branchlet surface. Since the relative position of the canal within the leaf may vary between its base and Foliage its tip, make the section of the leaf near its Leaves are in two ranks, each rank consisting middle. Use a very sharp instrument, such as of two or more rows in which the shorter a razor blade, and wipe away the emerging leaves are above. They tend to be arranged in resin droplet, which may be so large as to a pectinate fashion, with a "V" between the obscure the position of the canal. A hand rows above and less of a V below. Prominent lens and a good light are needed. A dissecting exceptions are Abies koreana, and Abies pin- microscope is a useful "luxury." sapo, whose leaves, although more dense 5 Cones itself from the outset. The habit and the bark The barrel-shaped, upright megasporangiate will then naturally be examined first. cones of Abies occur, as they do in most coni- The following seventeen species, in addi- fers, in the uppermost branches of the tree. tion to those listed on page 13, which grow in The young cones of most species are purple, the Arnold Arboretum, are recorded as be- but the color changes to brown later in the longing to the genus Abies. Some have in the year. A few conspicuous exceptions have past been tried in the Arnold Arboretum and cones that are green when young-Abies failed and therefore have not been included in nephrolepis forma chlorocarpa, Abies ho- this report. We make note of them here for molepis var. umbellata (Abies Xumbellata), completeness in overviewing the genus but andAbies veitchii var. olivacea, for example. have appended no descriptions. We will refer Ovuliferous (seed-bearing) scales of Abies to them occasionally. cones are woody, each bearing two winged seeds on its adaxial surface. They mature in Abies bracteata (D. Don) D. Don ex Poiteau Abies chensiensis Van one season and on the Tieghem growing disintegrate Abies ernestii (Rehder) Liu tree. The scales detach in the fall, and the Abies delavayi Franchet seeds are dispersed by the wind. The central Abies durangensis Martinez spikelike element of the cone denuded of Abies guatemalensis Rehder scales remains erect on the branchlet for up to Abies hickeli Flous & Gaussen Abies kawakamii Ito a or more. (Hayata) year Abies mariesii M. T. Masters The bracts of Abies cones are in certain Abies mexicana Martinez species longer than the cone scales. The tips Abies nebrodensis (Lojacono-Pojero) Mattei are then visible, and they are described as Abies numidica De Lannoy ex Carriere * "exserted." This is a lead toward Abies pardei Gaussen. helpful Abies All of the Abies are pindrow(Lambert) Royle identity. species genus Abies religiosa (von Humboldt, Bonpland & monoecious. Pollination is by wind. Kunth) Schlechtendal & Chamisso Abies squamata M. T. Masters Similar Genera and Distinguishing Abies vejari Martinez Characters determined that an a Genera of evergreen trees that might be con- Having unknown is fused with are Picea spp. (the spruces), Tsuga member of the genus Abies one must estab- if spp. (the hemlocks), Pseudotsuga spp. (the lish which, any, of the above species best the Since are Douglas firs), and Taxus spp. (the yews). The fits characters observed. keys salient differences among them are tabulated often difficult to follow, we have chosen to on page 12. present the material in tabular form (pages 6 The first important step is examination of and 11). Smoothness of bark, resinosity of the branchlet for the character of the leaf buds, ridges or grooves on the branchlet sur- scars. The next is the leaves themselves. The face, hairiness of branchlet, stomata situated bud and the cone, when available, are ex- on the upper surface of the leaves in addition tremely, if not definitively, important, but to the underside, whiteness of stomata, posi- of of they are not as dependably available as the tion resin canals, and degree of exsertion leaves. Study of the bark and habit should scale bracts of the cones are considered the come next.