Western hemlock (Tsuga heterophylla)
Western hemlock is categorised as 'very high risk' on Norway's list of introduced, invasive species.
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Western hemlock is a large conifer of the pine family that originates from the west coast of the United States. It is evergreen with characteristic drooping branch tips. The species was introduced to Norway around 1890, and was tried in smaller plantations west of the mountains. But the species is also used in parks and gardens.
Western hemlock produces large numbers of seedlings both in and around sites in which it is planted. The seeds can be dispersed by wind, increasing its ability to spread. The species is not limited to specific forest types and can establish itself almost anywhere. The seedlings are extremely shade tolerant and as long as there is enough water they can survive even in lush vegetation and thick layers of moss.
Most of the western hemlock in the Arboretum was planted in 1961, before the Arboretum was established. These are now large seed-bearing trees.
Today, western hemlock is considered an unwanted tree species in forestry, partly because can prevent regeneration of Norwegian spruce. Western hemlock is categorised as 'very high risk' on the register of introduced, invasive species maintained by Norway's national biodiversity database, Artsdatabanken. The species disperses effectively and is highly competitive, with the potential to change the natural environment, also at landscape level. In several places, the species forms dense stands, shading out all other vegetation. Hemlock shows rapid regeneration and spread in Western Norway, especially in the Bergen area, and threatens the natural species diversity. Up to 100,000 saplings have been counted in an area of 1,000 m2 in the worst affected places.
The state administrator ('Statsforvalteren', formerly 'Fylkesmannen') has stipulated that all hemlock forest should be removed (information in Norwegian here). In accordance with national guidelines, we plan to replace these trees and combat regrowth in the coming years.
You can read more about the western hemlock in Norway on Artsdatabanken (Artsdatabankens faktaark - in Norwegian).
