Spending on trees and forestry fell by nearly £20m a year between 2015 and 2018, when a purely Conservative government had taken over from the coalition, despite pledges to plant more trees.

The Department for Environment, Food and Rural Affairs said £132m was spent across the UK on trees in 2017-18, down from £151m in 2014-15. The more recent total included £32m in England, with most of the rest spent in Scotland.

The figures equate to less than £1 per person in England and less than £2 per person across the UK, compared with annual spending of about £90 per person on roads, £150pp on fossil fuel subsidies and £135pp in foregone tax from the nine-year freeze on fuel duty.

Subsidising fossil fuel production overseas costs each UK taxpayer more than £7 a year, according to estimates from Friends of the Earth.

Trees became an unexpected electoral battleground over the weekend when the Tories pledged to plant 30m new trees a year and the Liberal Democrats promised 60m.

Emi Murphy, a trees campaigner at Friends of the Earth, said: “We’re calling for the next government to properly fund the doubling of tree cover.

“This is one of the key solutions to solving the climate crisis but has been shockingly underfunded for years. Faced with the climate emergency and the dire impacts it will bring, we simply cannot afford not to fund trees.”

Tree-planting was speeded up in the last year, with 2m trees planted in England with government support – up from 1.6m the previous year. The rate of acceleration is still below what is needed to meet the government’s pledge to plant 11m trees between 2017 and 2022.

Estimating the annual spending on trees is complex because the money comes from a variety of sources. Defra’s estimate, requested by the Guardian, is more than twice as high as an estimate from Friends of the Earth in a report published on Tuesday, which says just £60m a year is spent on trees across the UK.

The Conservatives also believe the true amount of spending is higher and that Boris Johnson’s government should be judged on its promises rather than the record of Theresa May.

Sue Hayman, the shadow environment secretary, said the Tories’ new pledge amounted to only 1% of the trees that the Committee on Climate Change – an independent public body – had said would be needed.

“This is shocking but sadly not surprising from a government that can’t be trusted to tackle the climate and environment emergency,” she said.

Wera Hobhouse, the Liberal Democrats’ environment spokeswoman, said that last year the government planted less than a third of the 5,000 hectares of trees that had been promised.

She said: “We’re pledging to spend £6bn over the next parliament to fund tree-planting and restoring peatlands.”

There were 22m trees planted in Scotland last year, about 84% of the British total.

Deirdre Brock, the SNP’s environment spokeswoman, said: “The main Westminster parties must commit to further funding for tree-planting across the UK, and to match the SNP’s record and ambitious targets for the future.

“Climate change will be one of the biggest challenges for any incoming government in Westminster.”