Grey squirrels negatively impact the health of the UK’s trees and woods through bark stripping. This can damage, weaken and kill trees; a major problem at a time when the UK is working to increase its tree and woodland cover.
High densities of juveniles can strip bark from the main stem and branches of trees between April and September. Grey squirrels target young broadleaved trees, mostly 10-40 years of age, and repeat the damage year after year if the population remains high and unmanaged.
Species particularly susceptible to damage include biodiverse and high value trees such as oak, beech, hornbeam and sweet chestnut. Whereas species such as lime, horse chestnut and wild cherry are far less or unaffected.
Bark stripping weakens trees and creates open wounds that can result in irreversible damage and infection by pathogens. This destructive activity can also lead to girdling, cutting off the tree’s nutrient supply from its roots, which causes tree fatalities.
As trees mature they provide niches and habitat for a huge range of different species. A mature oak can support over 2,000 different species, but is one of the trees most targeted by grey squirrels. To ensure the woods planted today develop into the biodiverse ancient woods of the future, many landowners and managers actively reduce grey squirrel numbers to protect the trees.
As grey squirrels cause the loss of leaders, lesions, callus growth and dysfunctional shape, trees are prevented from achieving good timber form. Estimated costs of this damage to the timber industry are in excess of £40 million a year. Although the total economic costs increase due to the need to import timber, which also increases the risks of importing tree pests and diseases.
Mature, healthy trees provide a wealth of ecosystem services, such as cleaning our air and water, improving soil quality and function, mitigating heavy rainfall and flooding events, and reducing and sequestering atmospheric carbon. The UK aims to increase its tree planting rates to provide more of these benefits, but many young trees and woods are struggling due to grey squirrel damage.