Pests and diseases threatening to slash tree growth by 2050

By Stephen Beech

More than half of tree growth in the UK could be lost within 25 years due to the twin threat of invasive pests and diseases, new research warns.

Their arrivals are likely to "severely damage" UK trees and woodlands in the coming decades, say scientists.

Trees highlighted as "at-risk" in the study include oak, cherry, pine, larch and spruce.

The ash dieback epidemic prompted the government to assess all pests and diseases that could potentially enter the UK and affect our trees and agricultural crops.

For the research, published in the journal Plants People Planet, University of Exeter scientists assessed 636 tree pests and diseases to work out the invasion probability and likely effects on tree growth.

Based on recent rates of pest and disease arrival, they estimate that – by 2050 – more than half of tree growth could be lost, compared to growth if no new pests and diseases arrived from now onwards.

The Exeter team explained that tackling pest and disease invasions is "very difficult" in a time of global trade and travel.

But they say tight biocontrol would slow the process – and tree-planting policies can also boost resilience.

Exeter Dr. Dan Bebber said: "The defense against uncertain risk is always diversity.

“In the UK, we have relatively low tree diversity – both in terms of the number of tree species and the genetic diversity within each species.

“The ash dieback epidemic showed us how devastating a single tree disease can be, and how urgently we need to learn about these threats and protect our biodiversity.

“As well as issues about biosecurity, our research raises social and cultural questions about what we think our woodlands should look like – for example, which species should we plant?

“We need to consider this – in light of pests, disease, and climate change – when we think about the future of UK woodlands.”

He said that, in recent times, around one new tree pest or disease has established itself in the UK per year – and the sheer number of possible invaders makes further arrivals "almost certain."

Tree cover fell below 5% of UK land area at the beginning of the 20th Century, but grew again to around 13% in 2018. The European average is 39% forest cover.

UK reforestation has primarily involved fast-growing exotic conifer species, which now comprise about half of total forest area.

Ancient woodlands - those that have been continuously tree-covered since 1600 - cover only 2.5% of the UK.

Current government policy is to increase tree cover to 16.5% in England by 2050, 21% of Scotland by 2032, and by 2,000 hectares per year in Wales.

Dr. Bebber added: “Our study highlights the need for care in carrying out these projects, to ensure they are resilient to future pests, diseases and climate change.

“This work is a stark reminder that emerging plant pests and diseases pose a severe threat to UK trees, and the biodiversity benefits and ecosystem services they provide.”