Sitka spruce

Sitka spruce
Picea sitchensis

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Sitka spruce belongs to the genus Picea and is a member of the pine family (Pinaceae).

Sitka spruce (Picea sitchensis) is a conifer tree growing to over 60m and flowering in May.

Lifespan: In its native range Sitka spruce is known to live for over 700 years.

Characteristics

Bark on young trees is dark grey and flaking, becoming purplish-grey with coarse lifting scales. Mature tree bark is purplish-grey and cracked into square-shaped lifting plates. The base of the trunk is often buttressed. Upper branches ascending, lower branches rise and arch widely over and down. Branchlets are dense and hanging. Twigs are white becoming pale brown, grooved, knobbly and never hairy. Buds are ovoid, blunt, purplish-brown and resinous.

Spruces all have a woody peg at the base of every needle which is a useful identification aid. Needles are 2–3cm, slender, sharply pointed, hard and stiff, green above with two bright blue-white bands beneath. Needles grow straight out flat from the shoot with the upper needles pressed down close to the shoot along its centre. From a distance the foliage appears blue-grey.

Sitka spruce is monoecious: both male and female flowers are found on the same tree. Male flowers are ovoid, blunt and pale yellow. Female flowers are red, upright and oval, often crowded toward the top of the tree. Sitka spruce is wind pollinated.

The cones (5–8cm) are distinctive, cylindrical, pale green in summer but ripening into nearly white, pale creamy-brown cones; each one of the thin, papery scales has a crinkly, toothed edge. The seeds are small and winged.

Sitka spruce tree

 

 

Sitka spruce cones

 

Sitka spruce flowers

Distribution

Native to the west coast of North America, the species occupies a narrow land zone from Alaska to California where the maritime climate provides high atmospheric moisture. In the Olympic National Forest in the west of Washington State, Sitka spruce has been known to reach heights of over 80m.

Introduced to the UK in the 1830s Sitka spruce has been the most widely planted tree in UK forestry since the 1920s and represents 49% of our total conifer area.

Its distribution here is largely centred in northern and western parts of the UK, particularly on higher elevations where the climate provides enough moisture for it to thrive. It is most frequently found in large plantations.

Human value

Sitka spruce timber is very like that of Norway spruce or ‘whitewood’. It is pale creamy-brown, almost white, with no colour difference between heartwood and sapwood and only a subtle difference between the pale spring wood and darker summer wood in each annual ring.

It has an even texture and straight grain (although sometimes spiral), is moderately strong and easily worked. It has no natural durability but the sapwood takes preservative so can be used for external joinery. Spruce is popular for use in the papermaking industry and in making various kinds of manufactured board such as chipboard and hardboard. It is also used in building for internal joinery such as rafters and flooring.

Sitka spruce is likely to do well according to modelled climate change scenarios, as these predict warmer temperatures and higher rainfall in western parts of the UK.

Sitka spruce cones
Sitka spruce flowers

Wildlife value

Recent research has shown that forestry plantations of Sitka spruce in the UK provide suitable habitat for a wide range of wildlife.

Sitka spruce stands have been found to support the same amount of species as native stands; but they provide different habitats and attract different species groups.

The moist, sheltered microclimate found in spruce forest provides ideal conditions for hundreds of species of fungi and bryophyte; some very rare and threatened. Numerous invertebrate species also thrive in spruce forests, especially ground beetles.

In general, plantations can be beneficial for many butterflies and moths. During the establishment phase, when ground flora is rich, they provide habitat for butterflies such as pearl bordered fritillary and the grizzled skipper. They also provide a refuge for red squirrel.

The thicket stage of a young plantation is also known to provide excellent habitat for the white admiral butterfly and many birds such as the lesser whitethroat, lesser redpoll, yellowhammer, linnet and tree pipit. Large open areas of clear fell can be crucial in supporting local populations of nightjar and woodlark.

Many new moth species have colonised the UK due to the presence of non-native tree species; some with extremely restricted distributions. Those associated with plantation conifers include the dusky peacock, the feathered beauty and the cloaked pug.

Management

Sitka spruce is a pioneer or early successional species that is moderately shade tolerant. It is a very productive species and grows best on deep, moist soils of medium nutrient status in areas of high rainfall. It can tolerate drained peats and gleys but does not grow well on heather rich sites as it is unable to compete for nitrogen.

Sitka spruce is tolerant of exposure and is cold hardy but vulnerable to late spring frosts. It is adapted to a maritime climate with high atmospheric moisture. Sites need ample (>1000mm) rainfall per year unless the soil is moist; the species is ideal for growing in the wet upland parts of the UK. Wind throw is a serious risk on exposed sites when soils are wet and shallow.

Seeds ripen in September and are dispersed naturally from October to spring. Seeds can be sown mid to late March and do not require pre-treatment unless they are dormant. When sourcing seed, provenances from Queen Charlotte Islands or Washington were always preferred but selectively improved stock is now widely available. For more details, visit Forest Research.

Sitka spruce can suffer very heavy defoliation by the green spruce aphid (Elatobium) in some years, which can impact growth. It can also sometimes be affected by Fomes, root and butt rot (Heterobasidion).

The spruce bark beetle (Dendroctonus micans) is now established in many parts of the UK and can attack Sitka spruce. A biological control is available to control this pest in the form of a predatory beetle, Rhizophagus grandis.

In wider Europe, the larger European spruce bark beetle (Ips typographus) is a significant pest and could damage Sitka spruce if it ever establishes in the UK.

Sitka spruce tree

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Photographs © Forestry Commission