A “rewilding” of arable fields by HS2 will create 127 hectares (314 acres) of wood pasture, wetlands and flower-rich grassland using chalk taken from tunnelling under the Chilterns. The new wildlife haven will be founded upon all 3m tonnes of chalk that are to be excavated from the high-speed railway’s 10-mile Chilterns tunnel, with construction starting in May. The Colne Valley Western Slopes will be seeded with 70 grass and flower species and planted with 32 species of native trees and shrubs including Chiltern specialities juniper, box, wayfaring tree and hornbeam. Alongside two miles of new hedges on the site close to the M25 on the north-western edge of London will be nearly three miles of new footpaths, cycling and horse-riding routes. Some areas will protected from people for wildlife such as ground-nesting birds. Matt Hobbs, the lead ecologist from Jacobs, one of the HS2 contractors designing the site, said his team had been inspired by a visit to the rewilded Knepp estate in West Sussex. “This is broadly a rewilding project,” he said, citing the creation of new wood pasture – a mosaic of scrub, meadow and grazed woodland – and fenced areas where natural regeneration of trees and scrub will take place. The scheme includes wildlife ponds which also provide water for livestock and cattle grids on the lane running through the site to enable free-roaming animals to undertake conservation grazing of the area in a similar way to rewilding projects. “The Colne Valley Western Slopes is a unique opportunity to give something back for wildlife by creating a large new reserve dedicated to nature in the Colne Valley,” added Hobbs. “The design will deliver habitats that are the biggest and best possible, and those that will succeed and thrive over the long term with minimal human input.” The Western Slopes will help HS2 meet its carbon emissions reduction targets by sequestering carbon and reducing construction road journeys. Concrete and limestone aggregate used in HS2’s construction will be recycled to add to the calcareous grassland, minimising HS2’s carbon footprint by reducing the removal of materials by truck. Chalk and construction materials will be sculpted to replicate the area’s dry valleys and deliver a range of microclimates including hot, dry banks loved by reptiles and invertebrates. Species that are expected to rapidly colonise the site include grass snakes, slow worms and butterflies, such as the small blue and marbled white. Rarer butterflies such as the Adonis and chalkhill blue are also likely to eventually thrive on the chalk grassland. Simon Railton, the landscape architecture lead from LDA Design, said he hoped the site would provide a “meaningful legacy and benefit” for wildlife and people. “We aim to create a landscape which is both rich in biodiversity and a place for people to explore and connect with the natural environment,” he said. “Our design team is passionate about achieving an enduring environmental legacy for the project and playing our part in addressing some of the biggest issues of our time around climate change and biodiversity loss.” HS2 has attracted huge opposition from environmental activists and charities such as the Woodland Trust for destroying parts of 32 ancient woodlands along its first phase from London to Birmingham. Environmentalists have criticised its commitment to “no net loss of biodiversity” along the whole route when new road and housing developments are now asked to provide measurable improvements for wildlife. According to HS2 Ltd, the Western Slopes will ensure that the high-speed line delivers a “net gain” in biodiversity in the Colne Valley. The site’s 90 hectares (222 acres) of calcareous grassland is a significant addition to the estimated 700 hectares (1,730 acres) of this scarce floral habitat within the adjacent Chilterns Area of Outstanding Natural Beauty. Herts and Middlesex Wildlife Trust said it offered unpaid informal advice and expertise on the Colne Valley Western Slopes project to try to ensure the best outcome for nature. Josh Kubale of Herts and Middlesex Wildlife Trust said: “This project is part of HS2’s requirement for no net loss of biodiversity and we are very keen to ensure that it delivers all the benefits it is required to do. While we’re pleased it will create new habitat for wildlife it can never make up for even a small part of the nature and wild places that have and will continue to be lost to HS2. “The trust’s opinion of the HS2 project remains unchanged – we believe that the ecological cost is too high and the mitigation too small and we believe that the government should stop and re-think HS2.” HS2 says it is creating a “green corridor” along the route with 33km/sq of new woodland and wildlife habitats and the planting of 7m new trees on the 109-mile London to Birmingham stretch. Asked whether the new site may win round HS2 sceptics, Hobbs said: “I hope that protesters and others who have a negative view of the project will look at this and at least respect what we’re trying to do and give us some credit for coming up with something that we are very passionate about.” All the chalk removed from the twin tunnels under the Chilterns will be placed on the site over three years from this May. During the tunnel’s construction, HS2-contracted ecologists will conduct further laboratory and field trials of the best soil profiles for supporting biodiverse grassland. Parts of the Western Slopes are likely to open to the public before the end of the decade. HS2 will initially manage the site but ownership and management could eventually be transferred to local authorities or wildlife charities.