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Forest foes: the invaders threatening
our wonderful woodlands

Invasive non-native species threaten the wildlife in our native woodlands and the
economics of commercial forestry. Addressing these threats is an important
element of a wider approach to responsible forest management.

May 2019

While recently introduced pests and diseases may have stolen the headlines lately, threats to the UK’s trees and woods from invasive non-native species are long established and widespread. Since their introduction in the nineteenth century, Grey Squirrels (Sciurus carolinensis) have colonised most of England, Wales and Northern Ireland and parts of Scotland. In the wetter west, in particular, Rhododendron (Rhododendron ponticum) has spread through many semi-natural and plantation woodlands, sometimes as garden escapes, sometimes deliberately introduced as game cover. And in the riverside woods where I walk every day, Himalayan Balsam (Impatiens glandulifera) can be seen to cover more ground every year.

These, and other, non-native species invading our woodlands can have devastating impacts on our native fauna and flora. That the range of the native Red Squirrel (Sciurus vulgaris) has contracted as the range of the Grey Squirrel has increased has been well publicised. But, through bark stripping, Grey Squirrels also cause significant damage to native tree species such as Beech (Fagus sylvatica), Hornbeam (Carpinus betulus), oaks (Quercus spp.) and Field Maple (Acer campestre), in extreme cases girdling and killing trees. They have also been observed raiding the nests of woodland birds, although it is not clear to what extent this accounts for declines in woodland bird species. Meanwhile, Rhododendron spreads so aggressively and casts such deep shade that little or no ground flora can survive beneath it.

The economic costs of controlling these species can be massive, and control raises other issues such as appropriate use of herbicides or humane methods of squirrel control. Some invasive species also cause direct economic costs for woodland managers; the cost of Grey Squirrel damage to native and non-native tree species and the resulting timber degrade is hard to quantify, but the combined cost of yield loss and squirrel control in Great Britain in 2010 was estimated to be around £6,000,000 per year.

The social impacts of these species are not always so clearly negative; there is no denying that massed Rhododendron in flower can be truly spectacular, and for many people Grey Squirrels are the most visible and accessible ‘wild’ mammals in the UK. But another woodland invader, Giant Hogweed (Heracleum mantegazzianum), has been branded the UK’s ‘most dangerous plant’ because of the phototoxic effects of its sap, with a slew of tabloid articles in recent weeks warning of the hazard it poses.

Of course, forestry has itself been responsible for the introduction of many non-native species, both before and after the planting of Rhododendron on Victorian sporting estates. Much of the reafforestation of the last hundred years was with non-native conifer species; conifers now account for half of the woodland area in the UK, and while this includes the native Scots Pine (Pinus sylvestris), half of the conifer area in Britain is dominated by a single North American species, Sitka spruce (Picea sitchensis). Not all of these non-native species are invasive, but some certainly can be; with its prolific seeding, ability to grow in the shade of other trees, and dense, dark foliage which blots out light to other plants, Western Hemlock (Tsuga heterophylla) can be a menace if it invades semi-natural woodland, for example. While we could argue about who has the moral duty to deal with any consequences of historical introductions, present day foresters must take responsibility for the decisions they make here and now. Particularly in the context of the drive for species diversification to increase forest resilience, woodland managers must be alert to the potential consequences of introducing new species or moving species to new locations.

Forest Stewardship Council® (FSC®) certification addresses this and other aspects of responsible forest management by assessing compliance with the UK Woodland Assurance Standard, or UKWAS. UKWAS requires forest managers to co-operate with statutory bodies and neighbouring landowners where possible when managing invasive plants and wild mammals (UKWAS 4, requirement 2.3.2(b)). They can introduce non-native tree species only when they show that any invasive impacts can be controlled effectively (requirement 2.9.1(a)), and they must monitor all new introductions, implementing effective mitigation measures to control negative impacts outside the area in which they are established (requirement 2.9.1(c)). They also have to consider potential negative impacts of invasive non-native species on ancient semi-natural woodland (requirement 4.2.1(c) and guidance), conservation values in other woodlands (requirement 4.4.1(c) and guidance) and other semi-natural habitats (requirement 4.4.2(b) and guidance).

FSC forest management certification alone will not solve the problems of invasive species in our woods; strong shared determination and significant funding will be necessary to deal with some of the pervasive threats we face. But every time you buy a product with the FSC label, you can at least feel that you are taking a step in the right direction.

Owen Davies, Forest Standards Manager, Forest Stewardship Council

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The opinions expressed in this blog are the author's and not necessarily those of the wider Link membership.