It is tree planting season in the UK. With their amazing carbon storage ability, flood alleviation capacity and biodiversity boosting potential, trees are fast becoming a critical part of plans to reduce net carbon emissions and mitigate against global warming. But with only 13 percent tree cover, the UK lags behind Europe when it comes forest density, a fact the Committee on Climate Change and The Woodland Trust want to change.
Six thousand years ago 90 percent of the UK was covered with trees. Rowan, Oak, Lime, Scots Pine, Birch, Willow and other native species dominated the landscape. But this was not to last. Around 4000 years ago prehistoric man took their bronze axes to these ancient woodlands to clear land for farming
Millions of acres of woodland became farmland, and this timber became a primary fuel source as well as a building material. By the Norman invasion of the 11th century just 15 percent of the UK was covered in trees and this wasn’t even the worst of it. Timber stocks were then obliterated during the first world war. By 1919 the country, which was already 90 percent reliant on imports was left with just 5 per cent woodland cover. Something had to change
On 1st September 1919 Government passed The Forestry Act, which created a new organisation to protect and grow woodlands. The Forestry Commission got to work fast, planting its first trees in Eggesford Forest in Devon in December 1919. Thanks to its efforts and long-term planning the UK now has 13 percent tree cover. But a growing number of voices say that this is not enough.
Trees to capture carbon
All over the world the momentum for tree planting is building. In the UK the CCC and the Woodland Trust are calling on Government to plant over 90 million trees per year as part of plans to change land use to reduce emissions and reverse the decline in biodiversity. Global warming is accelerating because we are producing an ever-increasing amount of carbon dioxide, which traps the radiant heat generated by the earth from the solar radiation of the sun – the greenhouse effect. And it isn’t the only gas to do that, methane, nitrous oxide and even water vapour all do this too giving rise to a whole host of disastrous effects. But trees do exactly the opposite. Their leaves hold chlorophyl which reacts with photons in sunlight to split water molecules. The H2O is broken up and oxygen gas floats away. Without this process humans and other animals would never have evolved into existence at all
This leaves the hydrogen which then reacts with carbon dioxide in the atmosphere to create simple compounds that go on to react with minerals from the soil to form basic sugars, fats and proteins. This lets tree grow, and this carbon is locked away. Eventually, in turn, this feeds other organisms. Trees absorb carbon dioxide and expel oxygen. They are the ultimate bio-reactors. But it isn’t just as simple as planting any tree, anywhere, and there is much complexity in designing new woodlands.
“We have got so much work to do. If you think about our overall in this country, the level of tree cover is what 12 13%, something like that. In Europe, it’s more than 30. So we we’ve lost massive amounts of habitat,” says Peter Leeson, Woodland Creation Advisor for The Woodland Trust. “We are staring down the barrel of significant climate change and significant species loss. So really, the timescale is now. We’ve got to get on with this,”
Working out which species of tree to plant in which location is crucial and foresters have to find a balance between fast growing non-native species such as Birch or Sitka Spruce. Post WW1 we began planting a huge amount of conifers in order to boost supplies of homegrown timber, but now we need trees for other reasons. “An oak tree can have many 1000s of, of living organisms on a big tree, and they have something in excess of 400 species,” says Peter. This compares to perhaps 10 or 20 species in a non-native conifer.
“So we’ve replaced one really diverse tree, for biodiversity with species that are really not diverse.”
Faster growth versus longevity
But non-native species grow more quickly, and that means that they reach a peak in terms of productivity and carbon storage potential earlier than broadleaf native species do. The faster a tree grows the greater its ability to sequester carbon. Its productivity is described by something called “yield class”. Once the tree reaches maturation its carbon storage potential drops steeply – meaning that non-native fast growing species peak in carbon terms earlier, but older native species are productive for much, much longer.
And it is these fast-growing species that are used for the domestic commercial timber market, which Peter agrees is also important. “We would want to see a balance which certainly has a great deal of native broadleaves within it. And I think you I think you can mediate between those two extremes by doing good forest design and getting things in the right places.”
Because trees have so much more to offer than just carbon storage and habitat creation. UK watercourses are most commonly open and exposed to sunlight all day, particularly those in our uplands areas meaning that they heat up rapidly, even more rapidly thanks to climate change which is often to the detriment of trout and salmon populations, which really need to breed within cool water between certain temperature bands. Peter also points to the engineering properties of trees as being other amazing advantages such as their ability to mitigate rain and flood water. “A big tree will intercept water from the atmosphere, so rain will fall onto a big tree. And there’s quite a lot of evidence now that suggests they can evaporate that water at something like 10 to 30% of the water hitting a tree.”
And then there is the effect of percolation through the roots, which for something like an old oak tree could burrow more than 30m into the ground. These roots then require air pockets to function, which can actually act as temporary water storage because the roots quite deep.
The Northern Forest
But as Peter says it is the role of trees in encouraging biodiversity that he believes is most critical. “Actually, what we need to do is have a connected landscape that allows movements and flows of, of animals and flowers and plants. And it’s about connectivity now, which is a really key thing. The northern forest is our biggest single tree scape. It’s where we’re trying to get millions of trees planted along vehicle corridors in gardens, in new woods in hedgerows across that landscape to try and rebuild some of those connected habitats.”
The Northern Forest is a plan to plant 50 million trees east to west across the north of the UK, creating a connected woodland corridor from Liverpool to Hull. To date this area has had one of the lowest proportions of woodland cover with just 7.6% of its land mass populated by trees. This is something that Helen Neave is also looking to change. “We’ve actually pledged as our business but personally, to plant 100,000 trees for the Northern forest in this decade.”
For Helen, a consultant medical surgeon and her husband Chris this all started about ten years ago when they purchased 26 acres of land to build a nature reserve. “We asked various people, including the Woodland Trust, and were advised that planting trees would be a good way to kickstart the return of nature. And, in fact, that proved to be true. And so we planted 20,000 trees there,”
Importantly Helen chose the right trees for the location advised by a very wise man from the Woodland Trust – Pete Leeson. “Willows, there’s various types of willows, and they’re really good when they’re wet. And alders, you naturally find them by streams and ponds, and a few other types …. I can’t actually, I think birch aren’t so bad next to water as well.”
The trees grew incredibly well and Helen realised their benefits in tackling climate change.
So with their next investment they began investigating carbon content of trees and launched an new eco-friendly business called “Make it Wild” which does tree dedications, carbon offsetting through tree planting and sells nature related gift items. “There’s an accepted calculation that a typical tree, will sequester one tonne of elemental carbon over its typical lifetime of 40 years, and just by chemistry that equates to 3.67 tonnes of carbon dioxide. But we will never cut the trees down. So we expect most of them to live much longer than that.”
The native species that Helen plants can take 40 years or even longer to mature and if this was plotted on a graph we would see the peak carbon sequestration happening at a maturation point that might be ten, twenty or even thirty years later than a non-native species for example. And that gives rise to another issue for businesses and major organisations that are seeking to lower their carbon footprint – how to account for it.
Maria Manidaki knows all about counting carbon, it is part of her job. She is a principal water investment planning advisor at Mott MacDonald, where she is also technical lead for achieving net zero. “So part of the solution, of course, will be natural solutions and natural restoration. And more recently, water companies have made the commitment to plant 11 million trees in the UK.”
11 million trees would sequester over 40millions tonnes of CO2 over their lifetime according to Helen’s method. “So the important thing to note here is that the three when you planted a young tree, as the tree grows, it will store more and more carbon over its life but the sequestration if you like rate of a tree is like a bell curve. So that means there is a point in some years to come, where there is a maximum sequestration benefit and then it is reducing basically as the tree ages,” explains Maria.
This point is exactly why forest management is so important. We could plant millions of trees today to reduce our carbon footprint by 2050 but at some point in the future when they reach maturation the rate of carbon adsorption would plummet. Trees have to be planted in a rotation and properly looked after to ensure that they grow properly and then that we maintain rates of sequestration over time.
Carbon counting challenge
But this long-term maturation throws up a challenge for the water companies, and other organisations who are seeking to reduce emissions and help the UK reach its net zero target.
“So, how do you incentivize an organisation individual whatever that may be to plant or do the right thing now, even if the benefits will come in 30-40 years time,” asks Maria. What we need is a new carbon accounting system because at the moment no carbon sequestration benefits can be claimed until those benefits are realised.
At a national scale the Forestry Commission monitors the carbon sequestration value of woodlands that it manages. The latest Forestry Commission data estimates that the UK’s forests currently lock in 4 billion tonnes of carbon dioxide, and half of this is in Scotland.
Rob Gazzard, an advisor at the forestry commission specialising in contingency planning and wildfire points out that trees used in construction can continue to lock in carbon even after they have been felled. “Actually, it’s very interesting going to see a property recently which had a 14th century roofing timbers been put in place now those roofing timbers are still doing their job now, they’re perfect. They haven’t changed that they are still functioning correctly, even after 600 or 700 years of being in situ. So it just goes to show that they’re still storing their carbon.”
Meanwhile Rob has had something else rather important on his mind. As a specialist in wildfires the potential for these to increase as temperatures rise and woodland cover increases is an issue that the UK has to prepare for. He explains that there are three key principles to consider in managing wildfire: species management, site design and active management of the site. For every forest a wildfire risk assessment is carried out which then feeds into a wildfire management plan. “From that we then build our wildfire prevention measures. It could be fire breaks, but we don’t just rely on the fire breaks. Now, we look at the different species to try and use the best assets of those species in fire prevention. We might plant for example, broadleaf trees, oak trees, or sweet chestnut trees at the site is suitable. They have a lot less flammable than other species, like some types of conifer.”
Tree position is important too. “So if a slope goes up 10%, we can see a doubling of speed of a fire. So that slope can be really critical about how we predict how we plan for wildfire prevention.”
Preventing mitigation deterrence
Planting more trees will not save the world if we don’t stop emitting so much and so many greenhouse gases. One issue that scientists warn against is mitigation deterrence. “So the key way to fight back against mitigation deterrence, is to understand that it might happen, because then you can design policies that work to stop it. Or you can understand the policy mechanisms that we have today that make it more of a problem,” says Duncan McLaren, a research fellow and professor in practice at Lancaster University.
For the last two or three years, he has been working on the effects of greenhouse gas removal on climate policy more broadly. So the way new ideas like that and other forms of geoengineering interact with existing climate policies. The UK has a legally binding target of net zero emissions by 2050 and Duncan wants to make sure that the solutions and policies we implement will encourage reduction of emission production as well as using carbon sequestration solutions.
And the modelling can start to make carbon removal seem more attractive than preventing carbon emissions from being created in the first place. “The scary bit is that in the few bits of modelling, where there are directly comparable runs of the models, the average amount of substitution is around 70%. So around 70%, of what is promised from carbon removal is a substitute for previous previously anticipated emissions cuts,” he says, which is worrying because some sequestration technologies or offsetting schemes could fail to deliver the emissions reductions promised.
“The reason that happens is that carbon removal is typically a future promise. The models therefore discount its costs, because they have this built-in discounting function. So it appears cheaper than doing mitigation today or, or in the next 10 years,” he says.
This can lead models to select these options rather than mitigation. “But if that then is in the background, one of the sort of systemic issues, that governments are making policy on the basis of the carbon price indicated by the model, then that carbon price will stimulate less mitigation, and will stimulate more waiting for carbon removal, which might not arise.”
Can trees save the world?
So could trees really be the solution to climate change? Well the answer is that they could certainly be part of it. In 2019 the UK emitted 351.1 million tonnes of carbon dioxide into the atmosphere. The Committee on Climate Change find that changes in land use including tree planting and peatland restoration could reduce these by 43 Million tonnes per annum.
A 12% reduction in our overall emissions, which means planting at least 30,000 new hectares of forest or woodland every year, which means a minimum of 90 million trees every year.
In its England Tree Strategy consultation published in June 2020 the government agreed saying it wanted to hit these targets by 2025 and pointed to the other positive benefits of tree planting from improving air and water quality to restoring and promoting biodiversity to providing better recreational facilities.
As the Chinese proverb states, the best time to plant a tree was 20 years ago. The second best time is now…
