Picea engelmannii

Picea engelmannii goes by several common names: Engelmann spruce, mountain spruce, and silver spruce. This conifer is native to western North America and is especially valued for the tone wood it provides for acoustic guitars and other musical instruments. It grows mostly at high elevations in mountain ranges, but also appears in cool, watered canyons.

The tree reaches a medium to large size, typically 25 to 40 meters tall, though exceptional individuals can grow to 65 meters, with trunks up to 1.5 meters in diameter. Its bark is reddish, thin, and scaly, flaking off in small circular plates. Young trees have a narrow, conical crown that becomes more cylindrical with age. The shoots are buff-brown to orange-brown, usually densely hairy, with prominent pulvini. The needle-like leaves are 15 to 30 millimeters long, flexible, and rhombic in cross-section. They are glaucous blue-green above with thin lines of stomata, and blue-white below with two broad bands of stomata. Crushing the needles releases a pungent odor.

In spring, the tree produces purple cones about 1 centimeter long that release yellow pollen when windy. The mature cones are pendulous, slender, and cylindrical, 2.5 to 8 centimeters long and about 1.5 centimeters wide when closed, opening to 3 centimeters wide. Their thin, flexible scales are 1.5 to 2 centimeters long with wavy margins. The cones start reddish to dark purple and mature to light brown four to seven months after pollination. The seeds are black, 2 to 3 millimeters long, with a slender, light brown wing 5 to 8 millimeters long. At the fringe of alpine tundra, the tree grows in a stunted, krummholz form.

Engelmann spruce is found from 520 to 3,650 meters above sea level, though rarely lower toward the northwest. Its range extends from central British Columbia south through the Rocky Mountains and the eastern slopes of the Cascade Range to southern Oregon, and east into Montana, Idaho, and Colorado. It appears more sparsely in Arizona and New Mexico’s sky islands, with two isolated populations in northern Mexico. In the Idaho Panhandle canyons and, more limitedly, the northeastern Olympic Mountains, some exceptionally large specimens occur—including one 2.1 meters in diameter and 55 meters tall. In the Cascade Range, it grows mostly on eastern slopes from 900 to 1,800 meters, and it is abundant in the Rocky Mountains. It also occurs in the Monashee and Selkirk Mountains and the highlands around the Interior Plateau.

Ecologically, Engelmann spruce saplings face challenges from reduced snowpack. When snow cover is lost, increased transpiration, low sapwood water reserves, and extended soil frost in windswept areas can prevent regeneration both above and below the tree line. Water uptake and root-stored water are critical for saplings exposed above snowpack in late winter to early spring. For exposed trees, soil water availability may be crucial in late winter when transpirational demands rise. Cuticular damage from windblown ice is more important at the tree line, while desiccation damage is more significant at lower elevations. Despite wind damage, this species tends to grow taller than other trees at the tree line. It is shade tolerant but less so than subalpine fir, so it depends somewhat on fires to outgrow competitors. However, its thin bark and shallow roots make it vulnerable to fire. Spruce bark beetles attack the tree, especially deadly to centuries-old stands, and avalanches also pose a threat. Older spruce forests offer little forage for animals, but after fires—which often burn completely—many other plants, especially deciduous ones, can thrive. Streams shaded by Engelmann spruce are used by trout, and aphids produce galls that hang from the tree and resemble dried cones.

Two geographical subspecies are accepted: *Picea engelmannii* subsp. *engelmannii* covers most of the range, while subsp. *mexicana* is found in two isolated populations on high mountains in northern Mexico—the Sierra del Carmen in Coahuila and Cerro Mohinora in Chihuahua. Engelmann spruces in the Madrean sky islands of extreme southeastern Arizona and southwestern New Mexico may also belong to this subspecies, though this is disputed. The species hybridizes extensively with white spruce in interior Canada, forming a complex known as *Picea × albertiana* (Alberta spruce or interior spruce), and to a lesser extent with Sitka spruce where they meet on the western fringes of the Cascades.

Native Americans used the resin and foliage for various medicines. Economically, the wood is light and fairly strong, harvested for paper-making and general construction. Wood from slow-grown, high-elevation trees is especially prized for soundboards in acoustic guitars, harps, violins, and pianos. Because it is odorless and low in resin, it has also been used for food containers like barrels. It is occasionally used as a Christmas tree.