Engelmann spruce is a long-lived, native, coniferous, evergreen tree. It is one of the largest of the high-elevation mountain conifers. Mature trees have a narrow, pyramid form and short, compact branches. Within natural stands, mature trees average 15 to 30 inches in diameter; the average dominant height varies from 45 to 130 feet, depending on site quality and density. Larger individuals are not uncommon and may exceed 40 inches in diameter and 160 feet (49 m) in height. Engelmann spruce is long-lived; dominant trees are often 350 to 450 years old, and 500- to 600-year-old trees are not uncommon. The crowns of trees within a stand normally make up 50 to 70 percent of the total height of the tree. Dead lower limbs tend to be persistent. The crowns of open-grown trees often extend down to the ground. In alpine areas just above treeline, Engelmann spruce often forms a krummholz. At treeline in northern Idaho, mature Engelmann spruce generally do not exceed 65 feet in height, and progressively become more stunted as elevation increases, forming krummholz at the most severe, high-elevation sites. The four-sided, acute-tipped needles are not particularly sharp, are deep bluish-green, and are 0.8 to 1.2 inches long. The young twigs are finely pubescent, a characteristic which differentiates this spruce from white spruce, which has glabrous twigs. The bark is very thin, grayish-brown on young trees but at maturity becomes purplish brown to russet and is broken into loosely attached scales. Engelmann spruce is generally shallow rooted, but laterals may penetrate to a depth of 8 feet (2.4 m) in deep, porous, well-drained soils. Engelmann spruce is monoecious. Female cones are light brown, 1.5 to 2.4 inches (4-6 cm) long, and occur in the upper part of the crown. Male cones are usually found lower in the crown than female cones. Engelmann spruce seeds are about 0.12 inch long and have a single, well-developed wing about twice as long as the seed.
Engelmann spruce is found in some of the highest and coldest forest environments in the western United States, characterized by long, cold winters with heavy snowpack and short, cool summers. It extends down to lower elevations along stream bottoms where cold air flows down the valley and collects in localized frost pockets . It is generally found on moist and cool sites, but at timberline it may occur on somewhat dry sites. At middle elevations, pure stands are usually found on alluvial terraces, wet benches, bottomlands, slopes with seeps or cold north or east aspects. It occurs on all aspects at timberline. Stand condition and associated conifers: Engelmann spruce forms pure stands but is more commonly associated with subalpine fir.
Engelmann spruce grows best on moderately deep, well-drained, loamy sands and silts, and silt and clay loam soils developed from volcanic lava flows and sedimentary rock. It also grows well on alluvial soils where the underlying water table is readily accessible. It grows poorly on shallow, dry, coarse-textured sands; gravels developed primarily from granitic and schistic rock; coarse sandstones and conglomerates; rocky glacial till; heavy clay surface soils; and saturated soils.
Elevational ranges for Engelmann spruce in Rocky Mountains: ID, MT, are between 2,000 and 9,000 feet .In the Rocky Mountains north and south of Montana and Idaho, Engelmann spruce and subalpine fir often co-dominate at climax to form extensive Engelmann spruce-subalpine fir forests. These spruce-fir forests are usually classified as subalpine fir climax series habitat types. In the understory of these stands, subalpine fir seedlings usually outnumber Engelmann spruce seedlings because they are more shade tolerant and readily establish on duff seedbeds. However, Engelmann spruce is longer lived and usually the largest tree in the stand. In the Rocky Mountains of Montana and Idaho, and in the mountains of eastern Washington and eastern Oregon, Engelmann spruce is usually considered seral to subalpine fir. In Montana, eastern and central Idaho, and western Wyoming, Engelmann spruce may attain climax dominance on the wettest habitat types where it appears more successful than subalpine fir. Near treeline, it may take 100 years or more for Engelmann spruce to establish seedlings following fire because an increase in herbaceous species prevents seeds from reaching mineral soil, and the harsh climate kills many seedlings that do establish. (Source)