From Greta Thunberg to Donald Trump and airlines to oil companies,
everyone is suddenly going crazy for trees.
The UK government has pledged to plant millions a year while other
countries have schemes running into billions.
But are these grand ambitions achievable? How much carbon dioxide do
trees really pull in from the atmosphere? And what happens to a forest,
planted amid a fanfare, over the following decades?
How many will the UK plant?
Last year's UK general election became a contest to look green.
The Conservatives' pledge of planting 30 million trees a year, confirmed
in the Budget this week, is a big step up on current rates. Critics
wonder whether it's possible given that earlier targets were far easier
and weren't met.
If the new planting rate is achieved, it would lead to something like
17% of the UK becoming forested, as opposed to 13% now.
Tree planting is a popular idea because forests are not only beautiful
but also useful: they support wildlife, help with holding back
floodwater and provide timber.
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And trees absorb carbon dioxide - the main gas heating the planet - so
planting more of them is seen by many as a climate change solution.
At the moment, the UK's forests pull in about 10 million tonnes of
carbon dioxide a year but the hope is to more than double that.
It would involve potentially sensitive decisions about where to turn
fields into forests: for example, should trees be planted where crops
are grown or where cattle or sheep are grazed?
And because it can take decades to get a financial return from trees,
many farmers and landowners are waiting for the government to announce
new incentives.
Can you plant that many?
Yes, with the right people.
I watched a team of people in their 20s working on a project for the
Forestry Commission, in Norfolk, and their speed was phenomenal. When
they got going, I timed each of them planting a tree roughly every four
seconds.
During the course of a day, they could plant between 2,000 and 4,000
trees, piercing the soil with a shovel, stooping down to bury the roots
of a tiny Douglas Fir, pressing the sapling in with a boot, and then
pacing out the gap to the next one.
There are machines that can do the job - and even drones - but people
power is the tried and tested method. And good money can be earned -
about 7p for every tree.
For years, it's been popular among students in Canada as a summer job.
But inspiring the same enthusiasm among British people is a different
story.
Liz Boivin, whose company Tomorrow's Forests employs the team I visited,
finds it is Canadians, Australians and eastern Europeans who most
regularly sign up for a season's work.
She doubts whether there are enough trained staff in Britain to support
the government's plans for a huge increase in planting.
"You need to have the workforce to hit those numbers, which at the
moment you don't have," she says.
What problems could there be?
Trees grow very slowly so it's not enough just to plant them and then
walk away.
In their early years, saplings are extremely vulnerable to a long list
of threats: droughts, storms, pests and diseases. So it's possible that
around a quarter of a newly-planted forest will die young.
Only when the survivors make it to an age of 20-30 years do they draw in
significant amounts of carbon dioxide. By this stage, the forest will
only thrive if some trees are removed or "thinned" to allow more room
for others to develop.
If the timber from the cleared trees is then used in buildings, the
carbon will remain locked up for as long as the structure stands. But if
the trees are left unattended and end up dying and rotting, all the
carbon that had been stored will then be released.
So the key is a plan for careful management, according to Stuart Goodall,
who runs Confor, a forest industries association. He's worried that the
mania for trees may turn out to be a passing fashion, with investors
excited by the planting but not by the long years that follow.
"We don't want to be rushed by others who have taken a sudden interest
and may run away in 5-10 years' time," he says.
For a big increase in tree planting, Mr Goodall says there will need to
be far greater supply of saplings but British nurseries are wary of
scaling up until they're sure the government is serious.
Can trees stop climate change?
The answer is more complicated than you might think.
Trees use carbon dioxide as part of the process of photosynthesis - with
the carbon ending up in the branches, trunk and roots. But at the same
time they rely on respiration, which releases some carbon dioxide.
That's why, over the years, people have described trees as "breathing" -
inhaling and exhaling a flow of gases. And it turns out that
understanding exactly how that flow works is extremely hard.
Prof Rob MacKenzie, of the University of Birmingham, is honest about the
lack of knowledge. "There are lots of things we don't know about the
precise movement of carbon."
We're in a hi-tech outdoor laboratory that he runs in a forest in
Staffordshire.
Instruments are mounted on tree trunks and on the ground to measure
every aspect of how the trees are functioning. Research so far has shown
that every square metre draws in about 1,700g of CO2 every year - while
also releasing up to 1,200g.
And as a forest gets older, those flows are likely to become more
balanced. Prof MacKenzie says it would be a "disaster" if governments
and companies rely on forests to "clear up the mess" of carbon
pollution.
And he paints a grim picture of what could go wrong. "We plant lot of
trees, we think we've done the job, we forget about them, and what we're
left with is a really desolate dying diseased landscape that no one
cares about."
So what are the solutions?
Partly, they involve choosing the right trees and partly it's about
making sure that local people benefit.
In the sprawling forest of Thetford, in Norfolk, much of it planted in a
rush after the First World War, Eleanor Tew has researched the best
options.
Back in the 1960s and 1970s, a government-encouraged rash of planting
ended up with regimented rows of the same species of conifers - which
meant they were susceptible to the same pests and diseases.
For Eleanor, it's important to make sure that future forests are more
resilient.
"It's a bit like making sure you don't put all your eggs in one basket,"
she says. "It may seem that the obvious thing is to plant one species
that's really good for timber or another species that's good for carbon
but if they don't cope with a disease, then the whole forest fails."
And for Nathalie Seddon, professor of biodiversity at the University of
Oxford, it's vital that forestry schemes, particularly in developing
countries, aren't imposed on the people there, but instead involve them.
She points to a project in the Humbo region of Ethiopia where farmers
were encouraged to regenerate woodland by being given legal rights over
the trees and also by getting training in forest management.
By contrast, a forestry scheme in northwest China successfully protected
people living there from dust storms - a positive development - but the
growth of the trees then led to water shortages in villages downstream.
She says: "There is an idea that you can just buy land and plant trees
but that's too simplistic - there is a risk of doing more harm than
good."
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