Picea glauca

(white spruce)

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Photo of Picea glauca by Adolph Murie

                            Synonyms
                            Species Description
                            Phenology
                            Reproduction
                            Insects/Disease/Pathogens
                            Known Uses/Ethnobotany
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Picea glauca (white spruce) is a coniferous tree that can grow to more than 30 m in height in Denali, with basal diameters of up to 70 cm. However, because of our location near the edge of this species' northern range limit, and the high topographic diversity in the park, white spruce trees here are often smaller than 30 m tall, particularly in cold, wet, exposed or high elevation sites.

In outline, white spruce trees usually taper from a wide base to a narrow conical crown, and open-grown individuals often develop a symmetrical and attractive strongly conical shape with branches reaching several meters out from the bole near the ground. White spruce trees growing in dense stands, in contrast, are often asymmetrical with more columnar or irregular silhouettes due to injury or differential growth due to shading by neighbors. Branches of white spruce droop from their point of attachment to the bole, and sometimes have upturned ends near the crown perimeter, as they reach for sunlight.

Picea glauca has stiff, blue-green, four-angled needles with whitish lines on the underside. Needles extend in all directions (like a bottlebrush) from the twigs and each needle is borne on a small peg-like pedicel, which remains once the needle falls, giving fallen twigs a rough texture. Twigs of white spruce are hairless (in contrast to the minutely red-haired twigs of black spruce, Picea mariana). Picea glauca has thin grey bark and nearly white wood.

The cylindrical female cones, which are light brown at maturity, hang downward from branches and are usually positioned toward the branch-tips and clustered near the top of the tree. In contrast, male cones are borne lower in the canopy and wither and fall soon after pollen is distributed in spring. Seed-bearing cones open in late August and September while on the tree and winged seeds fall to the ground after cones open.

Though white spruce is typically large and black spruce (P. mariana) is often dwarfed, these habits can be misleading. The absence of red hairs on young twigs and cylindrical cones borne at branch ends (instead of ovoid cones borne close to the bole) are the best characters to identify white spruce. Hybrids with black spruce are known to occur.

White spruce is monoecious, with both male (pollen-bearing) and female (seed-bearing) reproductive structures present in each individual. Picea glauca, like all conifers has 'naked seeds' that are borne in cones protected by woody cone scales instead of in fruit surrounded by ovules as in angiosperms. The reproductive cycle of white spruce from inception to seed maturity requires two years to produce mature seeds. Both male and female reproductive buds (the precursors to cones) are initiated late in the growing season following shoot growth (Nienstaedt and Zasada 1990). These newly-formed structures become dormant in the fall. The growth of reproductive organs resumes with warming in the spring, even before vegetative growth begins. Male cones are generally situated lower on the tree than female cones (which are usually more numerous towards the top of the tree) and once pollen ripens they become quite moist and red. Clouds of yellow pollen are dispersed by wind over a short period as the male cones dehisce about 6 to 8 weeks after they start growing in the spring. Pollen transport is accomplished by the wind, which bring pollen grains into contact with female white spruce cones. Once fertilized, the cone becomes a duller color and re-orients in a downward-pointing position. Fertilization occurs about three weeks after pollination, and seed development is complete by early to mid-August (depending on the weather). In short or cool summers, it is likely that fewer mature white spruce seeds are produced, on average, in Denali, because this region is close to the species physiological tolerances (Roland et al. 2014) and thus trees may not have sufficient time to complete the seed maturation process during cold, short summers. Once seeds are ripe, the female cone quickly dries, causing retraction of cone scales and exposure of the seeds to wind and weather. Seeds have a conspicuous wing which facilitates dispersal on the winds. Seeds of Picea glauca require a period of cool to cold and damp conditions (called stratification) for optimal germination.

There are many insects that utilize P. glauca, the actions of which can have broad implications for forest structure and function, succession, wildfire dynamics, and nutrient cycling. Probably the most important insect on white spruce is the spruce bark beetle (Dendroctonus rufipennis) which can cause significant mortality, particularly in drought stressed or otherwise weakened trees. Large outbreaks of spruce beetles, probably fueled by warmer temperatures, have killed millions of acres of spruce forest (primarily in south-central Alaska) in the last 30 years, and continue to spread to living trees. Presently spruce beetles exist at low levels in Denali but have not as of yet caused widespread mortality.

Other important insects that feed on P. glauca include (but are not limited to): engraver bark beetles (Ips spp.), spruce coneworm (Dioryctria reniculelloides), spruce budworm, Choristoneura spp), giant conifer aphid (Cinara spp.), and the spruce gall aphids (Adelges spp. and Pineus spp.) (Holsten et al. 2001).

There are numerous important fungal diseases that affect Picea glauca in Alaska. Probably the most noticeable disease is spruce broom rust, caused by the fungus (Chrysomyxa arctostaphyli, which produces the distinctive perennial "witches brooms". Arctostaphylos uva-ursi is the secondary host for this fungus (required to complete its life cycle). Although broom rust doesn't kill the tree, it can weaken it and could serve as an entrance for other decay fungi to infect the tree. Brooms are important nesting sites for wildlife including red and flying squirrels (Holsten et al. 2001). Spruce needle rust (Chrysomyxa ledicola) is a similar fungal disease that affects new needle growth of P. glauca. Labrador tea (Rhododendron groenlandicum and/or R. tomentosum ssp. decumbens) is the secondary host for this fungus. There are various root rot and decay fungi that grow on P. glauca that are important contributors to boreal forest structure, function, succession, and nutrient cycling including, but not limited to: red belt fungus (Fomitopsis pinicola, tomentosus root rot (Inonotus tomentosus), and pini conk (Phellinus pini) (Holsten et al. 2001).