Last reviewed 11 April 2022

Pests and diseases have posed a threat to trees and woodlands in the UK for many years, but globalisation and the changing climate is increasing the risks. Laura King explores the situation and how organisations involved in woodland creation can ensure that they are planting for the future.

In the UK, woodland covers around 3 million hectares of land. The percentage coverage varies by region (from 19% in Scotland to 8% in Northern Ireland) but on average stands at just under 15% of land cover. Woodlands are an ancient habitat in the UK, and our present forests play a number of roles. Trees are often grown as a commercial crop, but they are also essential for nature conservation and biodiversity, offer areas for recreation and sanctuary, and are also a key part of our efforts towards climate adaptation and mitigation.

How is disease affecting woodlands?

However, despite their importance, trees are increasingly under threat from pests and diseases that are not part of the woodland’s natural ecosystem. Indeed, research published in the Government’s Tree Health Resilience Strategy suggests that the number of pests and diseases impacting on the UK’s trees is increasing. These include ash dieback — which, it is thought, will kill around 80% of the UK’s ash trees — and Dutch elm disease, which has killed millions of elm trees over the last 40 years.

Some of the reasons for the increase include:

  • globalisation, which has increased trade and travel, with imported material having the potential to host new pests and diseases

  • evolution of pests and disease through cross-breeding

  • neglect, as woodlands are no longer managed.

Climate change is likely to make the situation even worse. For example, a warming climate is likely to increase the growth and spore release of root pathogens that infect and damage the root systems of trees, and milder winters and warmer and wetter springs are also likely to create favourable conditions for many other pathogens. Moreover, stresses from extreme weather predicted in the UK will have an additional knock-on effect. For example, droughts lower the ability of trees to fight infection, and damaged root systems will make trees more susceptible to falling down in high winds.

And it’s not just the trees that will suffer. When a disease or pest results in the widespread loss of one species, it impacts on everything that uses the plant for feeding, nesting or to live on. For example, the number of species that only use ash and oak trees total 512 — a significant number of creatures and plants that would also perish should a forest be struck by diseases already in the UK such as ash dieback and acute oak decline.

What is being done to combat disease?

Organisations such as the National Trust and foresters in Scotland have recently reported on how disease is affecting their woodlands. The National Trust anticipates that the ash dieback will cost £3 million in winter 2021/22 as more than 30,000 trees need to be felled for public safety. In Scotland, disease affecting larch has resulted in more than half a million larches being felled on the island of Arran alone.

Both foresters and the National Trust are starting to look towards ways to increase the resilience of their woodlands, and an approach both have in common is to increase diversity — this is seen as key to prepare against all eventualities, and research also backs the notion that a diverse forest is also healthier, more productive, and better able to tolerate changing conditions. For example, different species will utilise resources differently, will have different tolerances to storms or drought, and there will inevitably be a variety of survival strategies in the face of threats such as pests. Moreover, forests planted with a mix of species are also thought to be able to sequester more carbon.

However, the uncertainties around climate change and how it might impact on pests and diseases makes it difficult to take one single approach to protecting woodlands and helping them adapt. As such, although increasing diversity is one common strategy, other strategies include the following.

  • Species selection, ensuring that any tree planted has the best chance of surviving changing weather and climate patterns.

  • Managing woodlands, including building in the resource needed to detect and respond to instances of disease.

  • Improving biosecurity, for example, through purchasing decisions and by following good biosecurity practices in woodlands.

  • Considering plant provenance, for example, the Woodland Trust’s UK and Ireland Sourced and Grown Assurance Scheme (UKISG) is a voluntary initiative which assures buyers that trees have been raised from seed sourced and grown solely within the UK and Ireland, so reducing the risk of spreading disease through plant imports.

  • Encouraging natural regeneration to strengthen local adaptation and resistance to disease — natural genetic variation within native trees is high, indicating that trees can adapt and respond to local changes.

How to create resilience

Actions if you have trees in your care

If there are trees within your organisation’s grounds, or areas that your organisation has responsibility for, there are three recommended actions.

  1. Be aware of what pathogens could pose a threat.

  2. Be knowledgeable about how to spot them, and how to report them.

  3. Practice good biosecurity measures .

Actions if looking to create woodland

If the organisation is looking to create woodlands, either within its grounds, or through initiatives such as community projects, then carefully consider the guidance that is available, and if necessary, seek specialist advice.

For example, the Woodland Trust provides advice on planning a community woodland and the Forestry Commission has also published a guide on planning new woodland.

The UK government also publishes guidance on:

Actions if woodland creation is part of carbon offsetting projects

If looking to offset carbon emissions through woodlands, then the following questions can help identify how well-adapted the woodland is to future change. Some questions for a UK-based scheme could be as follows.

  • Where are the trees sourced from — and what standards are looked for when purchasing trees?

  • What biosecurity measures are in place?

  • Has the woodland been planted in accordance with the UK Forestry Standard?

  • How is the woodland managed to mitigate against future threats?

  • What species are planted, and how diverse is the woodland compared to its natural equivalent?

  • What safeguards are in place to ensure that the woodland has the best chance of survival with the changing climate and the impacts that will bring?

Summary

  • Although disease and decay are natural processes within woodlands, the natural balance has been disrupted by new pests and diseases often introduced through imported material.

  • Such diseases have already had a significant impact on trees and woodlands in the UK, but it is considered that the threat level will continue to rise as the climate changes.

  • Although climate change is understood, the impact this might have on pests and diseases is less so.

  • A number of tactics are needed to increase the resilience of woodlands going forward.

  • Organisations, either with woodlands and trees in their care, or when working on woodland creation projects, should be mindful of how woodlands need to be created and managed.