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The political parties have been promising huge increases in tree planting in their election manifestos.

The most eye-catching is Labour's plan to plant two billion trees by 2040.

Impossible, some say - that's more than a quarter of a million trees every day.

But it's not necessarily unrealistic. It's just that it will require a sea change in the way things are done today.

Where are we at the moment?

In the year to March 2019, the number of trees planted across the UK went up: 13,400 hectares of new woodland was planted. Most of it was in Scotland (11,200 hectares), while there was actually a decrease in planting in England.

But these UK-wide figures fall way short of where they were in the late 1980s, when about 30,000 hectares were planted every year.

Experts who advocate sustainable woodland are not huge fans of the way planting was done in that decade, but it does show that the current rate of planting could rise dramatically.

Meanwhile, the Conservatives have pledged to plant at least 30 million more trees every year, the Lib Dems and the SNP each say they will plant 60 million trees a year, while the Green Party would aim to plant 700 million trees by 2030.

PA Election 2019 Tree-planting How many trees are the parties pledging to plant? Labour: 100m a year (2bn by 2040)

Greens: 70m a year

Lib Dems: 60m a year

SNP: 60m target a year

Conservatives: 30ma year Source: Party manifestos (except Labour, in a post-manifesto commitment)

How many trees are there in a hectare?

It's difficult to say precisely because trees can be planted at a density of anything between 1,000 and 2,500 per hectare.

But if you take a mid-range, then planting 30,000 hectares means you are planting roughly (and we should stress this is an estimate) 50 million trees a year.

That's still a long way short of the rate of planting Labour would need to achieve, in order to meet its target. But it suggests that it could be done.

Total new planting UK, 1976-2019

Experts can plant trees incredibly quickly.

But the Woodland Trust, the UK's largest woodland conservation charity, says several things need to happen to encourage a huge tree-planting programme:

Far more people need to be involved in tree-planting schemes at national and local government levels, and within statutory bodies

The UK needs to boost substantially its stocks of trees for planting

More money or grants need to be found for farmers and other landowners to persuade them to plant trees

Natural reforestation needs to be encouraged alongside new planting

"We know that the investment needed will pay back," says Darren Moorcroft, the chief executive of the Woodland Trust.

"It will require landowners willing to provide the public benefits that society needs, and public funds to support it."

The Committee on Climate Change, a statutory body which provides advice to government, says the UK needs a net increase of 32,000 hectares of woodland every year for the next 30 years, in order to help achieve the legally-binding target of net zero carbon emissions by 2050.

That equates to about one-and-a-half billion trees.

Again that sounds like an awful lot, but at the moment only 13% of the UK's land area is covered in trees (and in England it is 10%).

The aim is to increase that UK-wide figure to 17% by 2050. The average tree cover across the EU is about 35%, so it's actually quite a modest overall target.

To put things into further perspective, there have been multiple efforts elsewhere in the world to plant millions of trees in a single day.

Guinness World Records say roughly 50 million were planted by volunteers in the Indian state of Uttar Pradesh on a single day in 2016.

So, ambitious targets for the UK should not be dismissed out of hand.

But to get there, from where we are now, will take a gargantuan effort. And political promises on tree planting have been broken before.

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