HS2 has revealed a major update after concerns that newly planted trees are dying during the construction of the Buckinghamshire section of the high-speed railway line.

The 10-mile Chiltern tunnel running under Buckinghamshire – the longest on the high-speed line – is now 75 percent complete, HS2 has told the Local democracy Reporting Service (LDRS).

For more than two years, two boring machines called Florence and Cecilia have been excavating the tunnel as part of Phase 1 of HS2 to connect London and the West Midlands.

The tunnel, which progresses at a rate of about 15 metres a day, has now passed Little Missenden on its way to Great Missenden.

Meanwhile, excavations for five ventilation/emergency access shafts in Buckinghamshire have also been completed and site teams are now creating the interior structures.

The shafts are in Amersham, Chalfont St Giles, Chalfont St Peter, Chesham Road and Little Missenden.

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HS2 is billed as the ‘UK’s flagship transport levelling up project’ designed to improve rail links between London, the West Midlands and the north of England.

However, the 250-mile high-speed line is opposed by some green campaigners due its impact on the environment, including a giant sinkhole that appeared near Little Missenden in May.

There are also concerns that trees planted to replace those that died during HS2’s construction have perished.

Chesham resident Vicki Mistry asked a question about trees that have died during Buckinghamshire Council’s most recent Transport, Environment & Climate Change Select Committee on September 14.

She said: “I understand from the HS2 community engagement meeting in the Summer that a large number of trees planted by HS2 – to supposedly compensate for ecological damage to much older trees – have died.

“Can the council insist that they replace the dead trees with new trees and put in place a management system to ensure the trees don’t die again?

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“And in future who will be responsible for managing these trees to ensure they thrive?”

Gareth Williams, deputy cabinet leader and member for climate change and environment, said HS2 was responsible for tree management and vowed to continue raising the issue with the public body.

He replied: “We have had trees replaced as a result of those meetings. We cannot force them to replace them but is it is something that we bring up regularly.”

He added: “I can’t sit here and say that every tree that’s died has definitely been replaced. We would like to make sure there is complete replacement and we will stay on it.”

A HS2 spokesperson told the LDRS: “In terms of our general approach, we’re planting 33 hectares of new woodland and wildlife habitats creating a ‘green corridor’ along phase 1 – that’s 33 percent more than the amount there was before we began.

“Some of these sites have already been planted – but most will only be completed once the main construction phase of the build is finished.

“In terms of replacement planting – on any forestry project of this scale you’d expect to have to do some replanting on some sites and that was factored into the number of trees ordered when the project began. They are currently maintained by our contractors.”