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Picea is a genus of 34 species (Farjon 2001), distributed across the northern hemisphere. As in Abies, there are a few widespread species that dominate the boreal forests of Eurasia and North America, and a larger group of species with isolated, relict distributions in mountain ranges further south. Unlike Abies, Picea is absent from the Mediterranean basin, and is also missing from a large part of northeastern Siberia. Spruces are tall, evergreen trees (the tallest is P. sitchensis) with straight, columnar trunks, often yielding good timber. The bark ranges in colour from grey to orange-brown, and when young is generally flaky with papery scales, though in older trees it becomes dark, rough and scaly (no longitudinal fissures). The crown usually remains conical unless damaged. Primary branches are produced in regular whorls and spread horizontally, but the leader shoot is rigid and upright. Immature branchlets are typically soft and fresh green, contrasting with the darker mature foliage. Branchlets are generally ridged and grooved. The buds vary in shape and size and can be resinous or non-resinous. Often, the subterminal leaves curve over the buds, concealing them. The needle-like leaves are of radial or assurgent arrangement, and can be flattened or quadrangular in cross-section. Their apices are often very sharp to the touch. The leaves are attached to pulvini, which are continuations of the ridges on the branchlets; the pulvini remain on the stem after leaf abscission, though if leaves are removed from living shoots, they detach with them. When branches are removed, the leaves detach within a few weeks – a process familiar to those who use Norway Spruce (P. abies) as a Christmas tree. Species with flattened leaves have stomata on the lower (adaxial) surface only (epistomatic), while those with quadrangular leaves have stomata on all faces (amphistomatic). The male strobili are small, oblong and reddish in colour, though changing to yellow when mature. Both male and female strobili are produced from axillary buds, and are crowded towards the tips of the previous year’s shoots. The cones are pedunculate or sessile, developing at the tips of long shoots, and are initially erect, becoming pendulous at maturity. They are extremely numerous on branches across the crown, or only on upper branches, and mature in one year. Their colour changes from red, purple or green to light brown, and they usually fall after releasing the seeds. The cone scales are imbricate, spirally arranged around a central rachis, persistent, and open at maturity. The bract scales are rudimentary and included. There are two seeds on each cone scale, partially enclosed in a membranous cup, which extends to form a persistent triangular wing. The wing and membrane are easily detached from the mature seed (Farjon 1990). Spruces are readily distinguished from the other genera of the Pinaceae by the combination of leaves with pulvini, pendulous cones with persistent seed scales, and minute bract scales (Rushforth 1977). Hemlock (Tsuga) branchlets also bear pulvini, but their flat leaves and small cones are unlike those of Picea (Warren 1982).
The recent discovery of clonal P. abies in Sweden whose oldest wood has been carbon-14 dated to 9550 years old rewrites the record books for the longevity of individual organisms (Umeä University 2008).
The taxonomy adopted in this account largely follows that of Farjon’s World Checklist and Bibliography of Conifers (2001), although we recognise Picea martinezii as distinct from P. chihuahuana, and adopt a different approach to the P. likiangensis complex. A number of variants of familiar species are described here, as these names are attached to trees in cultivation, and keys to the recognised taxa are provided. These taxa often vary from typical material only in minor morphological characters that may or may not be discernible in cultivated specimens, but in many cases the taxon also has a distinct geographical range, and for the purposes of conservation at least the varietal name should be maintained in all records. It is clear that a thorough modern revision of Picea is urgently needed.
The genus is undoubtedly best known in the shape of Picea abies – a shape so familiar that it is almost representative of the word ‘conifer’. ‘All too familiar’, indeed, is a descriptor that might be applied to this species, ubiquitous as a forestry tree, too frequent in inappropriate garden situations, and the prickly, gawky centre-piece of many a Christmas living room. It does not encourage a closer acquaintance with its relatives. The genus as a whole, with the exception of the frequently planted P. breweriana and P. omorika, is currently rather out of fashion, and the rarer species are now seldom planted. It is clear from the TROBI records that many species of Picea are currently represented in British arboreta by fewer than a hundred specimens (Johnson 2003). Most of the taxa described below are even more scarce, and many are confined to specialist collections. This situation is unfortunate, as many spruces are very ornamental as young plants, in shape, colour and cones, although it is true that they tend to age badly. Few avoid a sparse maturity, and this is exacerbated in dry situations. All prefer a cool moist site and a good soil. Propagation is by seed whenever possible, as grafted specimens tend not to develop a good shape nor to do as well as seed-grown trees (though for some of the rare species such as P. farreri it may be the only option to maintain stock in cultivation). Hybridisation can occur in arboreta.
Picea is reasonably well represented in arboreta throughout our area, but its general preference for cooler conditions imposes some limits. In the United Kingdom the Royal Botanic Gardens of Kew and Edinburgh both have excellent collections, distributed throughout their gardens.
Pests and diseases are numerous, especially in commercial forestry situations, but in the horticultural context they seem to be of relatively little significance. Aphids (including adelgids) may be damaging, especially when trees are young, and in North America various insect larvae attack either foliage or wood. Spider mites can be a problem, especially on the compact or dwarf cultivars that are the most frequent manifestation of Picea in gardens. Frost can cause considerable damage to new shoots, and as usual it is the more northerly species that are affected most frequently, especially in areas with mild winters.
A group of evergreen trees found in most of the cool temperate regions of the northern hemisphere, of pyramidal form, especially in a young state, with branches in tiers. Leaves linear or needle-like, mostly four-sided, arranged spirally on the shoots, but the undermost ones usually twisted at the base, so as to crowd them more on the upper side of the twig than on the lower. Each leaf is seated on a slight cushion which, if the leaf be gently pulled off downwards whilst fresh, it brings mostly away. When, however, the leaf falls naturally, or the twig is dried for herbarium purposes, it leaves at the base a peg-like stump. These leaf-stumps thickly studded on the shoot are extremely characteristic of the spruces, and well distinguish them from the firs (Abies). Flowers unisexual, both sexes produced on the same tree at or near the end of the twigs; the males solitary, stalked, composed of numerous anthers. Female cones nearly always pendulous, their scales persisting until they fall. Seeds winged.
Some botanists divide Picea into three sections, but in Pilger's view a subdivision into two sections only is more natural. These are:
sect. picea (sect. Eupicea Willk.; sect. Cassicta Mayr, pro max. part.) – Leaves with four faces and four angles, rhombic in cross-section, with rows of stomata on all four surfaces. In some species the stomata are more numerous on the downward-facing (ventral) surface. To this section belong all the species except those listed below.
sect. omorika – Leaves flattened, with stomata usually on the ventral surface only, so that there is a marked contrast between the green, exposed surface and the bluish or whitish inner or downward-facing surface. The species belonging to this section are: P. omorika (the only European member); P. breweriana and P. sitchensis (N. America); and the Asiatic P. spinulosa, P. brachytyla, and P. jezoensis.
In the last century, the spruces were almost always called “Abies” in Britain, whilst the true silver-firs (Abies) were called “Picea”. This inversion of names dates from Loudon's time and should be borne in mind when consulting his great work. But it has its source in Linnaeus' Species Plantarum (1753), in which he named the common spruce Pinus abies and the European silver fir Pinus picea. This incidentally explains how it has come about that the common spruce is correctly called Picea abies (L.) Karsten. The name of the European silver fir would be Abies picea (L.) Bluff & Fingerhut (1825) were it not that Philip Miller used that name earlier for the common spruce (in his Dictionary, ed. 1768).
The confusion over the names Abies and Picea is now a thing of the past, but the following distinctions between the two genera may still be useful:
Picea (Spruces) – Leaves inserted on peg-like stumps (see first paragraph); mostly they are rhombic in cross-section and, if flat, then the lines of stomata are concentrated on the ventral side, i.e., that which is morphologically the upper side of the leaf, since it faces towards the apex of the shoot. But the appressing or forward leaning of the leaves on the upper side of a horizontal shoot exposes the green back (dorsal side) of the leaf without the need for any twisting at the base; the same is also true of a vertical leading shoot, where the primary leaves are closely appressed. Further, in Picea the cones are pendent, with persistent scales, and fall in one piece.
Abies (Silver firs) – Leaves nearly always flat, not falling away in drying, nor leaving the peg-like stumps of Picea. In marked contrast to that genus, the stomata are concentrated on the dorsal side of the leaf, i.e., on the morphologically lower side of the leaf (facing towards the base of the shoot), which is the normal arrangement in flowering plants. Consequently, an appressed leaf in Abies would expose its white underside but for the fact that the base of the leaf is twisted through about 18o°, as can be seen by examining the upper central leaves on a shoot of, say, Abies nordmanniana. In Abies the cones are erect, and break up on the tree, i.e., the scales fall away from the central axis, which remains on the branch.
The spruces have scarcely the garden value of the firs, but the following are handsome and effective: P. brachytyla, P. breweriana, P. jezoensis var. hondoensis, P. likiangensis, P. omorika, P. orientalis, P. polita, P. pungens (silver and glaucous forms), P. smithiana, and P. spinulosa.
The species of Picea should always be raised from seed if this is available. The dwarf garden varieties are propagated by cuttings taken in July or August. Grafting is used to increase varieties abnormal in habit and those with coloured leaves, notably the various blue and silver forms of P. pungens; some species have to be propagated in this way, if seed is not available or scarce, but such plants are not so fine or long-lived as those raised from seed. The spruces like abundant moisture at the root; if rainfall is deficient it may be compensated for by planting in a deep moist soil. P. pungens is one of the best in a dry climate. Few conifers withstand town conditions worse than the spruces, and they are not really at home on shallow, chalky soils. Many of them produce a useful timber, especially P. abies in Europe. P. glauca is cultivated in some of the northerly regions of Scandinavia too inclement for any other tree to thrive.
The measurements of cultivated trees, as throughout this revised edition, have been provided by Mr A. F. Mitchell of the Forestry Commission. Much fuller information will be found in his recently published work Conifers in the British Isles (Forestry Commission Booklet 33, publ. 1972). This contains descriptions of all coniferous species cultivated in the British Isles, and keys based on field characters.
Horsman, John – 'Spruces in Cultivation', The Plantsman, Vol. 5(2), pp. 103-27 (1983).
Liu, Tang-shu – 'A New Proposal for the Classification of the Genus Picea', Acta Phytotax. Geobot., Vol. 33, pp. 227-45 (1982).
Liu, Tang-shu – 'Spruces in the Arnold Arboretum', Arnoldia, Vol. 42, pp. 102-29(1982).
Schmidt-Vogt, H. – Die Fichte, Vol. 1, (1977). Although mainly devoted to P. abies, this work contains an excellent survey of the genus as a whole (pp. 9-163).