Black spruce is an evergreen, cone-bearing tree in the pine family (Pinaceae), often dominant in cold, wet lowland sites on permafrost in Denali. The gray-brown bark often sheds from the trunk, leading to a shaggy appearance. Needles are four-angled with stomata (appearing as white lines) on all sides, and radiate in all directions from the twigs. The smallest twigs have minute red-brown hairs, a distinction between this species and the related white spruce (Picea glauca), which lacks such hairs.
Picea mariana is most often found in cold, wet habitats, particularly lowland bogs, and often has highly stunted growth, with hundred year old trees not achieving the height of a sapling in better conditions. Trees often grow lopsided, with irregular bulges or significant number of dead branches along the middle of the trunk due to shifts in permafrost or growing under poor conditions. However, in well-drained, productive sites black spruce can be outwardly indistinguishable from white spruce, and present the symmetrical, strongly conical morphology typical of the genus.
The female, seed bearing cones of black spruce are short and ovoid, 1.5 -3 cm long and often borne close to the trunk of the tree. The cones curve downward from the upper branches of the tree, and often remain closed for several years until a fire, when they open and disperse their seeds. This allows the species to recruit new trees in the relatively good growing conditions following fire with warmer soils and a flush of nutrients. Black spruce is thus fire-adapted and deemed to be 'semi-serotinous'.
Black spruce have minute hairs on the smallest twigs, which are not present in white spruce (Picea glauca). Also, the cones of white spruce are longer, annual and borne near branch tips, in contrast to the shorter, darker cones of black spruce borne near the bole of the tree.
New needles, grown at the tip of the branches, are initiated in the fall as the plant enters dormancy. In the spring, buds grow into new leaves. Growth is particularly slow in black spruce, correlated to the temperature and light availability of a given site. Trees are dormant over the winter, photosynthesis beginning in early spring but growth not initiating until water is available.
Black spruce is monoecious, with female and male cones produced on the same tree. Female cones are perennial, and remain partially closed, dispersing seeds each year. The time to reproductive maturity in Picea mariana is comparatively low (Viereck and Johnston 1990). Pollen is shed in early June. Seeds mature after 3 months dormancy, and can remain viable for up to 25 years. Most seeds are dispersed in the spring, allowing them to germinate in the fall. Seeds are up to 12 mm long, bearing scarious wings, which allow them to be wind-dispersed. Black spruce is notable for its tendency to reproduce clonally via layering. Lower branches of a tree grow out into the litter layer and root themselves, slowly spreading the cover of the genetic individual. In some extreme arctic locations, Picea mariana will reproduce exclusively through layering.
This species is used in the same medicinal ways as white spruce. Its lumber applications are however more limited because black spruce trees tend to be thin and deformed. Because of the small size of the trees, black spruce is little used for lumber. Spruce roots are traditionally harvested by interior Natives to make baskets. The most important economic consideration for black spruce in Alaska is its importance as fuel for forest fires.
Black spruce occurs naturally only in North America, ranging across boreal Canada, up to the treeline in the eastern Canadian arctic. The species also grows in the Great Lakes states and the northeastern United States to the south of the Canadian border. Black spruce occurs throughout much of boreal Alaska where its range extends from the Canadian border westwards to the edge of the Seward Peninsula, throughout interior and southcentral Alaska and reaching the west coast in southwest Alaska. Black spruce is not present in the North Slope or in Southeast. In Denali, this species occurs primarily in lowland basin landscape positions on both sides of the Alaska Range, although it does occur at treeline in the Kantishna Hills.
Black spruce prefers lower elevations. The average elevation of a site within Denali National Park is 371 m, and percent cover strongly declines with increasing elevation. Overall, black spruce does not occur on steep slopes (no occurrences on slopes above 28 degrees) and is most often found on flat or gently angled terrain. Black spruce is also slightly more abundant on north facing slopes than south where it favors the colder microclimate of these areas.