Sitka spruce (SS)
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Native to a narrow zone along the west coast of North America.
Provenances from Queen Charlotte Islands (QCI) or Washington were preferred in the afforestation programmes of the last century, but genetically improved material is now widely available.
Adapted to a maritime climate with high atmospheric moisture. Sites with less than 1000 mm rainfall per year should be avoided unless the soil is moist; the species is therefore best suited to the upland parts of north and west Britain. This is a pioneer or early successional species of intermediate shade tolerance. It is tolerant of exposure and cold hardy, but suffers from late spring frosts and air pollution. It grows best on soils of poor or medium nutrient status with good moisture status and will tolerate drained peats and gleys. Sensitive to heather check on very poor soils, where it should be grown in mixture with pine or larch.
In some years, Sitka spruce can suffer very heavy defoliation by the green spruce aphid (Elatobium) with a significant impact on growth.
The spruce bark beetle, Dendroctonus micans, accidentally introduced from continental Europe and which now attacks Sitka spruce has become established in many parts of Britain. A highly effective biological control is in place to control this pest in the form of a host-specific predatory beetle, Rhizophagus grandis. Another non-native beetle, the larger European spruce bark beetle (Ips typographus), is a significant pest on Norway spruce in some European countries. So far absent from Britain, it has the potential to damage Sitka spruce should it ever establish.
Heterobasidion (Fomes root and butt rot) affects most commercial conifers and Sitka spruce is no exception, although in this species it primarily causes butt rot and only occasionally kills. However, the impact in plantations growing on peaty soils, especially under conditions of high rainfall, is low.
Under the exposed humid conditions of much of upland Britain it is a high volume producer whose growth is mainly constrained by wind damage. In much of western Britain, the greater warmth and higher rainfall projected under climate change scenarios is likely to result in increased growth of Sitka spruce, whereas in eastern Britain drier conditions may increase the risk of drought crack and dieback.
Sitka spruce is categorised as a Principal tree species. These are species which are currently widely used for forestry and will continue to be a dominant unless affected by a new pest or disease or adversely affected by climate change.
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