8: Invasive non-native species

What information does the survey provide and why does it matter?

Invasive non-native species (INNS) are a serious potential threat to the biodiversity of native and ancient woods, because they exclude native species and in some cases may dominate the shrub layer and prevent natural regeneration.

In the survey, data has been collected for invasive non-native species in the shrub and field layers. Several individual species which are frequently invasive in Scottish woods are assessed separately: Rhododendron ponticum, snowberry, Japanese knotweed, giant hogweed and Himalayan balsam.

Of these, rhododendron is likely to be the most extensive threat to native woods. Invasiveness and impacts do vary somewhat between regions and site or woodland types however. An ‘other invasives’ category has been used for other species encountered.

Absence or a low cover (under 10%) of Invasive non-native species is one of the four components of the native woodland condition indicator.

Non–native tree species in the canopy are not recorded as invasives: they are assessed in species composition and structural diversity.

Some of these trees do not regenerate readily from seed under current Scottish conditions and are unlikely to become invasive. Others can be threats to biodiversity of native or ancient woodlands. The management response should depend on the degree of threat to native woodland biodiversity, as well as the importance of the contribution made by particular non-native trees to the overall value of the wood.

Policy relevance

INNS are recognised internationally as a threat to biodiversity. Scottish Government policy is to prevent future problems and tackle existing established invasive problems. Legislation is currently being prepared to help develop policy (part of the draft Wildlife and Natural Environment Bill). The Scottish Forestry Strategy promotes action against invasives in general and rhododendron in particular because of the large threat it poses.

How can the NWSS data be used?

Type of useNationalRegional/local authorityLandscape/site scale
Native woodland condition indicator component measure (=INNS cover less than 10%) for regional/national reporting Yes Yes Yes
Assess pattern of distribution of particular INNS to develop control strategies/management plans Yes Yes Yes