Picea is the classic Latin name for a conifer that produces a resinous material; the Latin term is probably derived from the Greek word for pitch, pissa. In the 1700s, several botanists used picea as a species name, but not as a genus name. In 1824, the German botanist Albert Gottfried Dietrich (1795–1856), curator of the Botanical Garden in Berlin, was the first to use Picea as a genus name, separating out spruces from firs (Abies) and pines (Pinus) in his botanical work Flora der Gegend Um Berlin (“Plants of the Region Around Berlin”). He emphasized the four-sided nature of the spruce needles to justify his decision to create a new genus, making a clear distinction between the spruces and the flat-needled firs. Botanists began to assign spruce-like tree species to Picea, although it took several decades for the change to become established.
Red Spruce (Picea rubens)
The specific epithet rubens comes from the Latin word rubere, which means “to be red,” likely in reference to the reddish color of the species’ scaly inner bark. Red spruce’s species name is attributed to botanist Charles Sprague Sargent (1841–1926), the first director of Harvard University’s Arnold Arboretum and an advocate for preserving much of the remaining American forest in a state of wilderness. The naming and identification of this species have been muddled by its close genetic and morphological relationship to black spruce (Picea mariana). Red spruce and black spruce have similar needles, twigs, and cones, and the two species can hybridize, producing offspring with a mix of red and black spruce traits. So, do we have one species here or two? The question was a point of contention among taxonomists for many decades.
During the 1800s, most botanists lumped red and black spruce together. In the 1884 U.S. Department of the Interior publication, Report on the Forests of North America, Sargent described black spruce under the now discarded name Picea nigra, but there was no separate entry for red spruce. Sargent reported that the Maine forests contained five billion board feet of standing Picea nigra trees, describing a forest dominated by trees that today would be called red spruce.
At some point during the next 15 years, however, Sargent saw the red spruce light. In 1898, in the 12th volume of his magnum opus, The Silva of North America, he presented red spruce as a species distinct from black spruce. He gave it its own entry, description, and unique Latin name, Picea rubens, and emphasized several points that he felt distinguished red spruce from black spruce. Red spruce needles and cones were longer, red spruce needles were “dark green and lustrous” while black spruce needles were “blue,” and black spruce cones were strongly hooked at the base and persisted for many years. He pointed out that while there was some overlap in the ranges of the two species (for example, in Maine), red spruce was an “Appalachian tree,” while black spruce was “a tree of the far north.”
Sargent also recognized that the two species occupied different ecological niches; red spruce grows “only on well-drained hillsides,” he wrote, and black spruce “inhabits only wet, sphagnum-covered bogs.” He incorrectly concluded that “forms intermediate in character between the Black and Red Spruces are not known to exist.”
With modern genetic sequencing, we can now directly compare the DNA of red and black spruce, confirming that the two species are very closely related. This relationship also reflects the evolutionary history of red spruce. It’s likely that black spruce or something close to this species has been around for at least a few million years. The genetic evidence suggests that red spruce evolved from black spruce during the Pleistocene Epoch, perhaps as recently as a few hundred thousand years ago. Botanists think that red evolved from black and not the reverse, in part, because red spruce has a more restricted geographic distribution; it is found no farther north than Nova Scotia and southern Quebec and no farther west than Ontario. In addition, red spruce populations have less genetic diversity. This suggests a shorter time period since the species first appeared, because mutations, which create diversity, accumulate over time.
The split between black spruce and red spruce probably followed the geographic isolation of a subpopulation of black spruce trees. Evolution often depends on or follows the advent of geographic barriers that interrupt gene flow between groups within a species, including the advance and retreat of glaciers. Over time, the incipient red spruce populations changed to become more shade tolerant and better adapted to drier soils and mountain slopes than the bog-loving black spruce trees. That is, the red spruce populations changed to occupy different ecological niches, eventually becoming a new and separate species.
Today, red and black spruce trees can still hybridize, so perhaps the process of speciation is not quite complete. However, in most competitive habitats, where the soil and climate are ideal for one of the two species, the hybrids do not survive or reproduce as well as the non-hybrid spruces. Red and black spruce trees continue to occupy different, often physically separated habitats, a state of affairs that reduces the opportunity to exchange DNA. So, it seems likely that these two spruces will remain separate species in the future.
Black Spruce (Picea mariana)
Credit for the specific epithet mariana goes to English botanist Philip Miller (1691–1771), who was chief gardener at Chelsea Physic (or Apothecary) Garden and fellow of the Royal Society. In the 8th edition of his Gardener’s Dictionary (1768), the species name mariana made its earliest known appearance in a Latin binomial name with an entry reading: ABIES (Mariana) followed by the descriptive “foliis linearibus acutis, conis minimis.” The phrase following “(Mariana)” can be translated as “linear, acute leaves, with the smallest cones,” and the small cones of this species remain a good way to distinguish black spruce from red spruce.
By the 1830s, taxonomists had moved black spruce from Abies to the new Picea genus, but they also created new species names for this spruce such as nigra, Latin for “black.” According to the “rules of priority” adopted later in the century (described below), these species names were invalid. In 1888, Nathaniel Britton, Emerson Sterns, and Justus Poggenburg settled the matter by reviving Miller’s original species name. They listed black spruce as Picea mariana in Preliminary Catalogue of Anthophyta and Pteridophyta, a record of all the plant species growing within 100 miles of New York City.
In this taxonomic context, mariana means “of Maryland,” and it’s a puzzling choice of names, because black spruce does not grow naturally in Maryland, or at least it’s very rare in the state. Perhaps Miller mistakenly believed that this northern forest spruce grew in the Maryland colony or that the Maryland forests contained a broad range or representative sample of North American tree species, including black spruce. Miller never traveled to America, and his descriptions of black spruce were based on specimens shipped from the colonies and on trees grown from American seeds in English botanical gardens and estates, so he may not have understood the native range of the species. Whatever the error’s origin, black spruce is now destined to travel through the scientific literature as Picea mariana or “the pitch-tree of Maryland.”
White Spruce (Picea glauca)
The specific epithet glauca is derived from the Latin word glaucus, from which we get the word “glaucous.” Botanists use this term to refer to a bloom: a waxy or powdery coating on a plant surface that gives it a grayish, whitish, or bluish hue. In the case of white spruce, the term applies to the bloom on the surface of the needles, which make them appear pale.
The oldest recorded specific epithet for white spruce is not glauca, but canadensis, meaning “of or from Canada.” Use of this geographically appropriate name traces back to Miller, who listed white spruce as Abies canadensis in his 1768 edition of Gardeners Dictionary. After Dietrich introduced Picea as a genus name in 1824, botanists began to transfer white spruce to the new Picea genus, and by the end of the nineteenth century, many were using the name Picea canadensis for this species. These included Sargent, who used the Picea canadensis name in his 1898 edition of The Silva of North America. So, why isn’t Picea canadensis the species name for white spruce today?
Well, there was a problem. In 1763, Carl Linnaeus (1707–1778), the father of modern biological taxonomy, had bestowed the species name Pinus canadensis on the eastern hemlock. Five years later, Miller separated the pines from other conifer species and classified spruce, fir, and hemlock as Abies species. Because Linnaeus had assigned the species name canadensis to the eastern hemlock, it would have made sense for Miller to list this species in the new genus under the name Abies canadensis. Instead, as noted above, Miller used the name A. canadensis for white spruce while changing the species name for hemlocks to A. americana.
In the 1800s, taxonomists began to adopt the rules of priority to settle disputes in which a given species had been given different names by different botanists. These rules stated that the name to be used would be the oldest published Latin name for that species. In addition, if two species with the same species name were placed in the same genus, the species with the older published name would keep its name, while the second species would carry the name that was its next oldest published name. Canadensis was the oldest published species name for both eastern hemlock and white spruce, but eastern hemlock had been named in 1763, while white spruce was named in 1768. By the new rules, when hemlocks and spruces had been gathered in the new genus Abies, the name canadensis belonged to the eastern hemlock.
We cannot blame Miller for not following rules that did not exist in the 1760s, and well into the 1800s, many botanists were still using Miller’s Picea canadensis for white spruce. Still, some taxonomists considered the name Picea canadensis for white spruce to be invalid. Such a muddle in the orderly world of plant taxonomy could not be allowed to stand for long. In 1907, German botanist Andreas Voss (1857–1924) promoted the view that white spruce should be called Picea glauca. The German botanist Conrad Moench had published a description of white spruce using the name Pinus glauca in 1785, and Voss concluded glauca was the next oldest published name for white spruce. Sargent supported Voss’s conclusions, but the name Picea canadensis for white spruce would continue to surface in the botanical literature for many decades.
While most botanists today have settled on Picea glauca for white spruce, it should be noted that Royal Botanic Gardens, Kew, lists Picea laxa as an accepted synonym for white spruce. Canadensis? Glauca? Laxa? The roads of taxonomy are long and winding, and they have many branches.