A spruce is a tree of the genus Picea, a genus of some 37 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal (taiga) regions of the Northern Hemisphere. Picea is treated either in the subfamily Pinoideae, or the sole genus in its own subfamily Piceoideae. Spruces can be distinguished from other genera of the family Pinaceae by their needles (leaves), which are four-sided and attached singly to small persistent peg-like structures (pulvini) on the twigs. The needles are shed when 4–10 years old, leaving the twigs rough with the retained pegs. Pests of spruce forestry include green spruce aphid, eastern spruce budworm, European spruce bark beetle, and great spruce bark beetle. Spruce is a major producer of timber for construction, and of pulp for paper. It is the standard material for the soundboards of stringed instruments like acoustic guitars. Native Americans use the roots of some species for weaving baskets. The Norway spruce is widely used for Christmas trees. Artists including Augustin Hirschvogel in the 16th century, Edvard Munch around 1900, and Eija-Liisa Ahtila in the 21st century have depicted spruces in etchings, oil paintings, and video installations.
Etymology The scientific name derives from Latin "pix", pitch, which was obtained from the resin of Picea abies. Spruce, from Middle English spruse or Sprws appears originally to have denoted goods, including wooden objects, imported from Prussia. The Middle English word is in turn from Old French Pruce, "Prussia".
Description Spruces differ from other Pinaceae in two distinctive characters. Firstly, they have a pulvinus (plural, pulvini), a small peg-like structure at the base of each needle, that remains when the needle falls. Secondly, they have evergreen needle-like leaves that are more or less square in cross-section. The needles stay on the tree for between four and ten years. The tree usually has a straight trunk, though can become bushy or irregular if damaged by wind exposure or biotic factors like browsing or insect damage. Spruces are resinous, and monoecious, with separate male and female cones on the same tree. Young trees have a conical crown; in older trees, this tends to become a roughly cylindrical column; mature heights vary from 10–20 m in the smaller species like Picea mariana, up to a maximum of 100 m in Picea sitchensis. Branches grow from the trunk in regular whorls; the lower branches are mostly soon lost, except when the tree is open-grown in full sun. Young branches rise above the horizontal, but older branches do not. The needles range from 0.6–0.8 cm in Picea orientalis up to 3.5–5 cm in Picea smithiana. The cones have leaflike bracts that appear at the time of pollination, but unlike Abies (fir cones), these are generally later covered by the seed scales. When mature, the cones range from 2–3.5 cm in Picea mariana, up to 10–20 cm in Picea abies, and nearly as long but stouter and heavier, in Picea smithiana. Each seed sits with its lower half in a cup on the seed scale; the seeds have a large wing.
The structure of the cone scales, including length, width, immature colour, shape of the apex, and how much of the scale is free, is the most useful feature for identifying species of spruce. While Picea glauca and Picea engelmannii, for example, differ in shoot and needle characteristics, those with cones present are most easily identified. Spruces are generally of moderate lifespan, ranging from 100 to 600 years; the oldest reported age for a single tree is 852 years for a specimen of Picea engelmannii. Clonal reproduction can extend this; a Norway spruce P. abies clonal group in Dalarna, Sweden, nicknamed "Old Tjikko" has reproduced by layering, reaching a claimed age of 9,550 years for the clone as a whole, though not for the small trees that are part of it.
Evolution
Fossil history
The Picea lineage begins in the fossil record around 130 million years ago (mya). The oldest record of spruce that has been found in the fossil record is from the Early Cretaceous (Valanginian) of western Canada, around 136 million years old. The only surviving branch of the lineage, however, diverged only around 30 mya, meaning that the rest of the crown group has no living descendants. That, in turn, means that the biogeography and ecology of the crown group cannot be inferred from living members of the genus. For example, middle Eocene spruce fossils have been found in the Buchanan Lake Formation of Canada (46.2–40.4 mya).
External phylogeny Based on a transcriptome analysis, Picea is most closely related to the genus Cathaya; those form a clade, sister to the genus Pinus. These genera, with douglas-firs and larches, form the pinoid clade of the Pinaceae.
Another study produced broadly similar results, but with Cathaya sister to [Picea + Pinus]:
Internal phylogeny DNA analyses have often conflicted with traditional classifications based on the morphology of needle and cone, but also conflict markedly between studies, with more proposed phylogenies than there are studies, and no consensus by 2015 on the relationships within the genus. In particular there is major discordance between phylogenies based on mitochondrial DNA (mtDNA) and those based on chloroplast DNA (cpDNA), and there is strong evidence for a history of reticulate evolution involving extensive hybrid introgression in the genus, which is continuing between several species, such as between Picea abies and Picea obovata (Picea × fennica), and between Picea glauca and Picea engelmannii (Picea × albertiana). One of the earliest genetic studies used chloroplast DNA (cpDNA) to infer that P. breweriana is sister to all other Picea species, with P. sitchensis sister to those remaining species. However, subsequent studies have shown very different results, with both nuclear DNA and mtDNA placing P. sitchensis in a small clade with what had always been presumed from morphology to be its close relatives, P. glauca and P. engelmannii, with the cpDNA result anomalous; likewise, while P. breweriana has still been recovered as sister to all other species by some studies, it was recovered as deeply embedded in the genus by a study using a large set of nuclear, cp, and mtDNA. A further problem with several studies before 2013 was a combination of misidentified samples and contaminated DNA.
Taxonomy
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