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Picea

Spruce, A. Dietrich  1824
Pinaceae

 

Evergreen trees with a straight, single trunk bearing regular annual tiers of three to five horizontal branches from the base to form a symmetrical, spirelike to broad conical crown which usually flattens somewhat at the top in extreme age. Bark generally smooth at first, nonfibrous and flaking in small, irregular patches, usually reddish in overall hue. Branchlets all elongate, without distinction into long and short shoots, hairy or not, prominently grooved between raised leaf bases. Each base bearing a distinct woody peg (sterigma) to which the needle is attached. Winter buds well developed, scaly. Leaves spirally arranged and radiating all around the twigs, though often somewhat parted on the lower side, needlelike, often square in cross section but sometimes flat or rounded, straight or curved, the tip pointed, sometimes painfully so, the base narrowed to a short petiole and jointed at the raised peg.

Plants monoecious. Pollen cones single, hanging down along the twigs from scaly buds in the axils of needles of the previous year. Each cone with numerous, densely spirally arranged pollen scales, each scale bearing two pollen sacs. Pollen grains large to very large (body 50-120 µm long, 65-130 µm overall), larger than those of Pinus and a little smaller on average than those of Abies, with two round air bladders a little smaller than the oblong body and diverging at a right angle or a little less. Surface of the body minutely warty and the bladders with more wrinkled sculpturing. Seed cones cylindrical or oblong, single on short stalks in the axils of leaves of the previous year among the upper branches of the crown, upright at pollination but dangling at maturity. Maturing and usually falling in a single season, the numerous, densely spirally arranged seed scales then spreading to release the seeds. Seed scales round to somewhat diamond-shaped, the tiny, hidden bract attached only at the base. Seeds two per scale, wedge-shaped, the asymmetrical wing derived from the seed scale, cupping the seed body below and much longer and a little wider than it. Cotyledons 5-15, each with one vein. Chromosome base number x = 11.

Wood odorless to slightly fragrant, light but unusually hard and strong, not very decay resistant, with white to yellowish sapwood either indistinguishable from or gradually blending into the nearly white through pinkish yellow to light brown heartwood. Grain very even and fine to fairly coarse, usually with well-defined growth rings marked by a very gradual transition to a variable-width band of much smaller and thicker-walled, spirally thickened, latewood tracheids, which like those of the earlywood, are exceptionally long. Resin canals lined with thick-walled cells, not especially abundant but scattered through the growth rings or more frequent in the latewood, sometimes arranged in open bands. Individual resin parenchyma cells normally absent.

Usually with lines of stomates in a central, nonwaxy band on each of the four sides, but with just two bands in flattened needles. Each stomate deeply sunken beneath and almost hidden by the four to six surrounding subsidiary cells, which are covered by a very thick cuticle without a Florin ring. Leaf cross section with a (sometimes interrupted) single pair of resin canals, usually near the side corners of the square on either side of the apparently single-stranded midvein embedded in a cylinder of transfusion tissue. Photosynthetic tissue not (or only weakly) organized into a definite palisade layer but all mesophyll cells appearing to radiate from the midvein out to the epidermis and underlying hypodermis, which consists of a single layer overall that builds up to two or three layers at the corners.

Thirty-seven (As of April 2022, Plants of the World Online accepted 37 species) species across the northern hemisphere, throughout the boreal forest and in mountains southward.

Spruces are the most prominent trees of the vast boreal forests of the northern hemisphere, extending southward in the mountains or in cooler than normal habitats. They have an overall more northerly range than the similarly distributed silver firs (Abies) and a wider range than the larches (Larix), extending much farther south. Most spruce species experience snowy winters, and their conical growth habit minimizes snow accumulation and persistence. This same conical form and the density of their evergreen foliage makes them important horticultural subjects in cool climates. They are, in fact, among the most commonly cultivated conifers in these regions, and numerous cultivars have been selected, mostly from five species (Picea abies, Picea glauca, Picea omorika, Picea orientalis, Picea pungens) but with a smattering among a dozen others. Cultivar selection has been highly, varied embracing extreme variations in habit (such as narrowly upright, cascading weeping, or sparsely branched), color variations (from golden to cold pale blue), reproductive variation (early and abundant seed cone production or altered cone form), and a myriad of dwarfs varying in shape, density, branching form, and needle length and color.

Spruces are also important in forestry because their exceptionally long wood fibers, requiring little destructive bleaching, are good for paper pulp. The wood is also used for general construction, and some species are cultivated in plantations for this purpose, like Picea sitchensis in Scotland. Spruces are also among the trees most strongly affected by forest dieback in central Europe (Picea abies) and eastern North America (Picea rubens), which is generally attributed to industrial and automotive pollution.

Although there are some uncertainties in spruce classification, recognition of species has been far more stable than it has in the associated silver firs. The relationships among the species, however, are not settled, and the traditional classification into three botanical sections based on the shape of the needles and of the scales of the seed cones is not consistent with the whole range of available evidence.

Crossing behavior among the species is particularly perplexing. Agreat deal of information on crossability has accumulated through efforts at spruce improvement for forestry. These tree breeding programs have not actually had much practical impact on forestry practice, despite the apparent superiority of certain species hybrids. Crossability among spruce species ranges from fully interfertile to completely incompatible in all attempts. The degree of crossability seems to have only a small correlation with the degree of relatedness of the species. Sometimes closely related species are readily intercrossable (like Picea glauca, Picea engelmannii, and Picea sitchensis), and distantly related species may be reproductively isolated from one another (like Picea chihuahuana and Picea abies). Just as frequently, however, it appears that closely related species are largely reproductively isolated from one another (like Picea engelmannii and Picea pungens, or Picea mariana and Picea rubens), while relatively unrelated species may still be crossable. A remarkable example of the latter is the distinctive and rare Serbian spruce (Picea omorika), which has the widest range of crossability of any spuce, crossing freely or with varying degrees of difficulty with many species pairs that cannot cross with each other. The reported crossability of different spruce species is somewhat inflated by the tendency to produce normal seed cones and heavy seeds even when there are no viable embryos. Acceptance of percentage germination (and later hybrid verification) as the appropriate measure of crossability in Picea drastically lowers the overall rate of crossability reported but still leaves a very high proportion of species with at least some crossability in artificial pollinations. Despite the frequency of successful species combinations in breeding trials, hybrids are less common in nature than those of silver firs. They occur most frequently between related species with abutting (parapatric) ranges, like Picea glauca and Picea engelmannii. Related species with broadly overlapping (sympatric) ranges, like Picea engelmannii and Picea pungens, or Picea mariana and Picea rubens, typically evolved reproductive barriers and exhibit very low crossabilities with each other. These evolutionary influences on crossability drastically limit its usefulness for inferring relationships in the genus.

Definite fossils of Picea are found in early Tertiary sediments and continue to the present. Older reports of the genus from the Cretaceous lack its diagnostic features and are now assigned to the extinct genus Pityostrobus, which contains (probably unrelated) species showing combinations of characters not found in any of the living genera. The earliest confirmed fossils of Picea are from late Eocene deposits. Because the needles fall off the twigs as they dry (a problem in preparing good botanical specimens today), most of the fossil record of spruce consists of seed cones and individual needles. Occasionally, however, twigs enter sediments before the needles drop, and some of these intact branchlets from the Miocene of Oregon bear needles up to 6 cm long, longer than those of any living species. These have been interpreted as coming from a warmer climate than any supporting spruce species today, so the ecological diversity of the genus may have been broader in the Tertiary than it is today. Another indication of this is that spruces were present (as a minor component) in the high arctic Metasequoia forests of the early Tertiary, far north of their present northern limit, under climatic conditions that are not duplicated anywhere on Earth today. One fossil species, Picea critchfieldii Jackson & Weng, is the only tree species known to have become extinct at the end of the last glaciation in North America. It had been widespread and dominant in forests south of the ice in the eastern United States at the height of glaciation just 18,000 years ago before suddenly vanishing as the climate warmed.

 

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Attribution from: Conifers Garden


 

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