Asked if the pledge was realistic, Ms Swinson, who put soil on a freshly planted tree in north London during a campaign event on Saturday, told the PA news agency: “If you actually look back to the late 1980s we were planting about 30,000 hectares every year of trees.
“This is a bit more ambitious than that, about 40,000 hectares, but absolutely it is possible.
“Not very much of our country currently is forested and these trees can be planted, yes, in woodlands, but also in urban areas, using more use of hedgerows within agriculture as well.”
The comments came as Boris Johnson announced a £640 million Nature for Climate fund which would increase tree planting in England, with the Government aiming to work with the devolved administrations to boost tree-planting rates to 30,000 hectares every year, which the party said could mean 30 million more trees.
The Lib Dems said they will plant 60 million trees every year, describing it as “the largest tree-planting programme in UK history”, while pointing out that the Tories have “woefully failed to meet their own targets for planting trees in the past year”.
Labour also said the Conservatives failed to meet previous tree-planting targets and said this vow is an attempt by Mr Johnson to “greenwash his atrocious environmental record”.
Former environment secretary Michael Gove blamed the Common Agricultural Policy for the Government not being able to meet its targets for planting trees.
Speaking on Radio 4’s Today Programme, Mr Gove said: “We’re dedicating additional money to make sure we meet the challenge of the climate emergency.”
Pushed on the fact that the Conservative government is not on target to deliver on their 2015 manifesto promise to plant another 11 million trees by 2020, Mr Gove said: “And that’s why we need to leave the European Union.”
He added: “Inside the European Union we’re trapped in the Common Agricultural Policy.”
He continued: “Across the political spectrum, whether people argued for Leave or Remain, there are very few people who argue that we should maintain the unfair, unjust and un-green Common Agricultural Policy.
“It’s one of the big benefits of leaving the European Union and it will allow us to meet these tree planting targets which will ensure that we deal with the climate crisis that we face.”
A Conservative government would also launch a £500 million Blue Planet fund, resourced from the International Aid budget, to export UK expertise in marine science around the world, supporting developing countries to protect marine habitats.
Mr Johnson said: “There is nothing more conservative than protecting our environment and these measures sit alongside our world-leading commitment to net-zero greenhouse gas emissions by 2050.
“But, just as with our planned investment in schools and hospitals, we can only do any of this if we end the gridlock and deadlock in parliament with a Conservative-majority government.”
Ms Swinson said: “It’s clear that the Conservative Party doesn’t take climate change seriously.”
Sue Hayman, Labour’s shadow environment secretary, said: “When Labour comes forward with its own ambitious proposals as part of our Plan For Nature, they will be informed by what the science says is necessary and possible – not by what Boris Johnson thinks he needs to do to greenwash his atrocious environmental record.”