HS2 has been accused of risking Britain’s woodlands with a tree planting policy that risks bringing in disease with foreign seeds.
Millions of trees are to be planted by the high-speed rail project to replace lost woodland and create a "green corridor" along the route, but it has come under fire for refusing to use only UK-sourced seeds.
Woodland Trust ecologists say the policy, which allows it to bring in seeds from continental Europe, poses an unnecessary risk to British nature and could lead to devastating new epidemics of disease.
Outbreaks spread by the trade in established plants include the fungal disease ash dieback, ramorum disease (phytophthora ramorum) which primarily affects larch trees in the UK, and the oak processionary moth, which can cause breathing problems in humans.
In 2019 University of Oxford reseachers calculated that ash dieback had cost the UK economy £15bn, through the cost of clearing up dead and dying trees and the loss of benefits brought by trees such as cleaner air and sucking up carbon from the atmosphere.
Luci Ryan, a Woodland Trust ecologist, warned that border regulations were not foolproof.
"Doing UK and Irish sourced and grown does work, because you've eliminated that risk.
"We've been asking for years for them to do UK and Irish sourced and grown, and they won't," she said.
Ecologists are now concerned about potentially catastrophic diseases that have previously been shown to spread on seeds but have not yet been found in the UK, such as the bacterial disease xylella fastidiosa and the fungus pine pitch canker.
Late last year several peers voiced criticism of the policy in a House of Lords debate, including Lord Blencathra, the deputy chair of Natural England.
While HS2 must follow biosecurity regulations, "that is what is supposed to happen at the moment for all imported seeds and plants, and yet we have ash dieback, oak processionary moth and spittlebugs, and God help us if Xylella fastidiosa gets here because it can destroy 500 different tree species," he said.
Conflicting evidence over using foreign seeds
Some research has suggested that planting seeds sourced from warmer areas further south makes trees more able to withstand the effects of climate change, which is likely to lead to the UK experiencing drier and hotter conditions over the coming decades.
Forestry Commission guidance published in 2019 says this approach is more appropriate in the case of fast-growing commercial forestry, rather than reforestation projects being carried out for ecological or environmental reasons.
Seeds from further south could also be more susceptible to frost and may actually be less able to cope with a changing climate because it may not exactly match the climate they have come from, the document says.
Dr Matt Elliot, policy advocate in tree health and invasives at the Woodland Trust, said: "Whilst seed is relatively safe compared to other things, there is still some risk.
"If you're growing commercially and you want to accept that risk, then fine. But if these trees are going to be there forever then there's just no need for even that slight risk."
A spokeswoman for HS2 said: "HS2’s extensive tree planting and habitat creation programme is designed to be resilient to climate change, pests and disease.
"We source seeds mainly from the UK, but also need to incorporate species from warmer climates to ensure we future-proof the UK’s tree landscape as the climate changes.
"Our approach has been developed to best practice standards in partnership with the Forestry Commission, and we follow strict biosecurity measures to ensure that all plant stock is healthy before it is planted along the HS2 route.”