Spruce

• 1. Introduction

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  A spruce in Dalarna, Sweden, nicknamed Old Tjikko, has reproduced by layering and reached a claimed age of 9,550 years for the clone as a whole, though not for the small trees that are part of it. That single fact upends what most people assume about a tree. Spruce is a tree of the genus Picea, a group of some 37 species of coniferous evergreen trees in the family Pinaceae. They are found in the northern temperate and boreal, or taiga, regions of the Northern Hemisphere. How does a plant cheat death across millennia. Why do scientists still argue, with no consensus reached by 2015, over how its species are related. What links a spruce to a violin, a flying boat, and a glass of beer brewed at sea. The answers run through forests, fossils, fungi, and the workshops of instrument makers.

• 2. The Telltale Peg

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  Four-sided needles, more or less square in cross-section, set the spruce apart from every other member of the Pinaceae. Each needle attaches singly to a small persistent peg-like structure called a pulvinus on the twig. When the needle falls, the peg stays behind, leaving the twigs rough to the touch. Needles are shed when four to ten years old, so the tree carries a kind of record in its texture.

  A straight trunk is the usual form, but wind exposure, browsing, or insect damage can leave a spruce bushy or irregular. Spruces are resinous and monoecious, carrying separate male and female cones on the same tree. Young trees wear a conical crown that broadens, in older trees, into a roughly cylindrical column.

  Size tells two very different stories within the genus. Picea mariana reaches mature heights of 10 to 20 metres, while Picea sitchensis can climb to a maximum of 100 metres. Needle length swings just as widely, from 0.6 to 0.8 centimetres in Picea orientalis up to 3.5 to 5 centimetres in Picea smithiana. Branches grow in regular whorls; the lower ones are mostly soon lost, except when a tree grows open in full sun.

  The cone, more than the needle, is the key that unlocks a species name. The structure of the cone scales, their length, width, immature colour, the shape of the apex, and how much of the scale is free, is the most useful feature for identification. Cones range from 2 to 3.5 centimetres in Picea mariana up to 10 to 20 centimetres in Picea abies, with bracts that appear at pollination but later get covered by the seed scales. Picea glauca and Picea engelmannii differ in shoot and needle traits, yet those bearing cones are the easiest to tell apart.

• 3. Cheating Time

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  One hundred to 600 years is the ordinary span of a spruce, placing it among trees of moderate lifespan. The oldest reported age for a single tree is 852 years, recorded for a specimen of Picea engelmannii. That number marks the limit for one individual living and dying as itself.

  Clonal reproduction breaks that ceiling. A Norway spruce clonal group can renew itself by layering, where branches root and send up fresh stems while the genetic individual persists underground. The trees you see are young; the organism beneath them is not. The Engelmann specimen's 852 years and the clone's claimed nine and a half millennia describe two entirely different kinds of age, a distinction that matters when fossils enter the picture.

• 4. A Lineage Mostly Lost

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  Around 130 million years ago, the Picea lineage begins in the fossil record. The oldest spruce fossil yet found comes from the Early Cretaceous, the Valanginian stage, of western Canada, around 136 million years old. The genus, in other words, is ancient.

  The living spruces are a thin survival from a much larger past. The only surviving branch of the lineage diverged only around 30 million years ago, which means the rest of the crown group has no living descendants. Because of that gap, the biogeography and ecology of the crown group cannot be inferred from spruces alive today. Middle Eocene spruce fossils from the Buchanan Lake Formation of Canada, dated to 46.2 to 40.4 million years ago, belong to relatives that left no heirs.

  Among living conifers, the spruce keeps close company. A transcriptome analysis places Picea closest to the genus Cathaya, the two forming a clade that is sister to the genus Pinus. Together with douglas-firs and larches, they make up the pinoid clade of the Pinaceae. Another study reached broadly similar results but put Cathaya as sister to the pairing of Picea and Pinus, a small disagreement that hints at a larger one.

• 5. The Tangled Tree

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  DNA analyses have often conflicted with traditional classifications built on the shape of needle and cone, and they also conflict markedly with each other. There are more proposed phylogenies than there are studies, and by 2015 no consensus had emerged on relationships within the genus. The harder researchers look, the more the picture splinters.

  Mitochondrial DNA and chloroplast DNA tell different stories, a major discordance running through the work. Strong evidence points to a history of reticulate evolution, with extensive hybrid introgression still continuing between species. Picea abies crosses with Picea obovata to give Picea fennica, and Picea glauca crosses with Picea engelmannii to give Picea albertiana. The branches of the family tree keep growing back together.

  Picea breweriana shows how unstable the conclusions are. One of the earliest genetic studies, using chloroplast DNA, inferred that P. breweriana is sister to all other Picea species, with P. sitchensis sister to the rest. Later work disagreed sharply. Both nuclear DNA and mtDNA placed P. sitchensis in a small clade with P. glauca and P. engelmannii, its presumed relatives by morphology, leaving the cpDNA result looking anomalous. A study drawing on a large set of nuclear, cp, and mtDNA even found P. breweriana deeply embedded in the genus rather than at its base. A further problem before 2013 was a combination of misidentified samples and contaminated DNA, muddying the record at its source.

• 6. Naming The Genus

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  In 1824, Albert Dietrich set up the genus Picea, giving the spruces their formal scientific home. The decades that followed became an argument about which part of the tree should define a species. The cone, the needle, or the seed each had its champions.

  Vegetative characteristics drove the revision published in 1887 by the German botanist Heinrich Moritz Willkomm, who classified the genus by the trees themselves rather than their cones. His approach was followed in 1890 by the German botanist Heinrich Mayr, and again in 1982 by the Taiwanese biologist Leroy Liu on a similar basis. Then in 1989 Peter A. Schmidt classified the species using mainly seed cone characteristics, swinging back toward the cone.

  Thirty-seven species stand accepted, listed in alphabetical order because no consensus on their relationships has emerged from the genetic work. The roll call spans the hemisphere. Picea omorika, the Serbian spruce, is a local endemic of Serbia and Bosnia important in horticulture. Picea breweriana, Brewer's spruce, is a local endemic of the Klamath Mountains. Picea chihuahuana of northwest Mexico is rare and endangered, and Picea neoveitchii of northwest China is critically endangered. Genomics has begun where taxonomy stalled. The nuclear, mitochondrial, and chloroplast genomes of British Columbia interior spruce, Picea albertiana, have been sequenced, and the large 20-gigabase-pair nuclear genome of genotype PG29 was published in 2013 and 2015.

• 7. Forests Under Siege

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  More than half of spruce seedling mortality probably occurs during the first growing season, and it stays high through the first winter. A seedling is most vulnerable from germination to the following spring. Only at four to five years old can it be considered established, beyond the reach of all but unusual threats like snow mould, fire, trampling, or browsing. In dry habitats, three years is enough.

  A mouse can eat 2,000 white spruce seeds in a single night, a figure that captures how relentless the pressure is. The short-tailed meadow vole pulls white spruce and lodgepole pine seedlings from the ground and eats them whole. Losses can be staggering. Repeated applications of half a million white spruce seeds per hectare in Alberta failed to produce the required 750 trees per hectare. Red squirrels can harvest as much as 90 percent of a cone crop, and black bears strip the bark of white spruce, a problem locally important in Alaska.

  Insects finish what the mammals start. The European spruce bark beetle lays its eggs in the inner bark, or phloem, of Picea abies and carries ophiostomatoid fungi, some of them serious tree pathogens; in large numbers its larvae can girdle and kill the tree. The eastern spruce budworm, whose populations oscillate into extreme outbreaks, defoliates black spruce and white spruce across Canada and the eastern United States. The great spruce bark beetle arrived in the United Kingdom sometime between 1973 and 1982, though it faces effective biological control by a natural predator, the Siberian beetle Rhizolophus grandis. The green spruce aphid stays in check where winters are cold, with significant mortality when the air drops below minus 8 degrees Celsius, but it breeds through mild oceanic winters such as those of Great Britain.

  Fungi attack from a third direction. Sirococcus blight, caused by the fungus Sirococcus tsugae, first appeared in Germany and the United Kingdom in 2014 and spreads when rain splashes its asexual conidia. Rhizosphaera needle cast leaves dead needles marked with rows of black fruiting bodies. Canker disease, caused by Leucostoma kunzei, strikes hardest when trees are under water stress, and copper fungicides wash off too readily to suit forestry use.

• 8. Wood, Sound, And Drink

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  The Wright brothers' first aircraft, the Flyer, was built of spruce, the wood prized for its strength relative to weight. Spruce serves as a building wood under names like North American timber, SPF for spruce-pine-fir, and whitewood, and it is common in Canadian Lumber Standard graded wood. Its weakness is durability. Poor resistance to insects and fungi after logging makes it an indoor wood, and spruce left outside cannot be expected to last more than 12 to 18 months depending on climate. The Hughes H-4 Hercules flying boat, nicknamed the Spruce Goose, was in fact mainly made of birch.

  In the soundboards of stringed instruments, spruce has no real rival. Wood used for this purpose is called tonewood, and the species include Engelmann spruce in North America and Sitka spruce in Europe. In the Dolomites, the Norway spruces of the Paneveggio Violins Forest have been used for centuries to make musical instruments, supposedly including work by the leading violin-maker Antonio Stradivari.

  The genus also feeds the world's tissue paper. Spruce makes good pulpwood, with long fibres that bind into strong paper, especially from trees over 60 years old, and the pulp is known as northern bleached softwood kraft, or NBSK. Older uses run deeper still. The resin was once worked into pitch before petrochemicals took over, and the name Picea derives from the Latin for pitch. Native Americans use the thin, pliable roots of some species for weaving baskets and for sewing together pieces of birch bark for canoes.

  Spruce even reaches the table and the canvas. Captain Cook brewed alcoholic, sugar-based spruce beer on his voyages to keep his crew from scurvy, drawing on the fresh shoots that are a natural source of vitamin C. In Finland, young spruce buds become a spice or boil down with sugar into spruce bud syrup. Around 1900, Edvard Munch made numerous oil paintings of spruce forests, now held in the Munch Museum in Oslo. More than a century later, the Finnish artist Eija-Liisa Ahtila laid a 30-metre-tall spruce on its side across six large video panels in her work Horizontal-Vaakasuora, which XIBT magazine described as delving into notions of ecology and symbiosis as well as the essence of existentialism within the context of our external world.