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New pests and diseases likely to cut UK tree growth, study finds

The arrival of new plant pests and diseases is likely to severely damage UK trees and woodlands in the coming decades, new research shows.

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.

In the new study, University of Exeter scientists assessed the 636 tree pests and diseases to work out the invasion probability and likely effects on tree growth. The paper, published in the journal Plants People Planet, is entitled "Potential impacts of plant pests and diseases on trees and forests in the UK."

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).

Tackling pest and disease invasions is very difficult in a world of global trade and travel, but tight biocontrol would slow the process—and tree-planting policies can also boost resilience.

"The defense against uncertain risk is always diversity," said Professor Dan Bebber. "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."

Professor Bebber added, "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, diseases and climate change—when we think about the future of UK woodlands."

The at-risk trees highlighted in the study include pine, cherry, spruce, larch and oak.

In recent times, about 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 about 13% in 2018. This contrasts with a European average of 39% forest area.

UK reforestation has primarily involved fast-growing exotic conifer species, which now comprise about half of the total forest area. Ancient woodlands (continuously tree-covered since 1600) cover just 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.

Professor Bebber said, "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."