Councils across the UK are pledging to plant trees in an effort to combat climate change and improve biodiversity. From Lanarkshire to Cornwall, and everywhere in between, trees have become a simple and obvious way for councils to show that they take the climate crisis seriously. They are backed by the government which, in 2020, pledged £3.9 million in funding for the planting of 30,000 hectares of trees per year by 2025.
It’s an impressive-sounding aim, though slightly less impressive when you consider that 150,000 hectares is a bit more than the land currently taken up by golf courses in the UK and, if successful, it will increase our woodlands by less than 5%, despite the UK being one of the least forested countries in Europe. Not to mention, the government is struggling to meet its targets, which it has recently cut following lobbying from the National Farmers Union. But is it even a good aim? Is planting trees a good use of our land and our money in the effort to mitigate climate change?
The first question to ask is do trees sequester carbon?
The simple answer is ‘yes’.
The more complex answer is, ‘yes but…’.
Separating the Forest from the Trees
All life on earth is carbon-based (carbon forms stable bonds with many other elements, making it uniquely able to form complex molecules such as DNA) and trees, being big and long-lived, are ideal for trapping lots of carbon. The IPCC estimated that around 47% of trees are carbon so it seems straightforward – plant more trees, trap more carbon, right?
Let’s go back to school biology and that infamous equation for photosynthesis. Trees, like all plants, take carbon dioxide from the atmosphere and convert it into food for themselves, and oxygen – which animals, including us, use to breathe. But this only happens during daylight. At night they rely on those glucose stores, converting them back into carbon dioxide and water to release the energy they contain to power their cells. Fortunately for us, not all their glucose is used in this way, some is converted into cellulose to form the permanent structures of the tree – its roots, trunk and branches.
So while carbon is being captured by the tree, it is also emitting carbon, and we need to look at the net carbon capture over the life of the tree. Unsurprisingly, on an individual level, the bigger the tree, the more carbon it captures permanently in its structure. In fact, young trees are pretty poor at capturing carbon, as much of their focus is on foliage production – which is lost every year in deciduous species, and over cycles of up to 45 years in the case of pines, returning the carbon back to the environment.
Theory Vs Practice
Obviously, to get to big trees you must first start with small trees, so the tree planting schemes make sense, as long as every tree you plant grows up. But not all do. Some die. Quite a lot in some cases. Gloucester City Council planted 12,500 whips (small trees aged 2-3 years old and less than 1m tall) at the beginning of 2022, but by the end of the summer only 700 were still alive, the rest having been killed by the heatwave. A similar situation occurred in Kings Lynn with around 75% mortality reported. And that’s just for the councils that track the outcomes of their schemes; at least 80 have no monitoring. This isn’t a failing of individual councils, but a systemic failure of the government’s scheme, which was launched so quickly that Defra, who manage it, didn’t have time to put monitoring systems in place, according to a report by the National Audit Office.
Every whip that dies is more carbon in the atmosphere that we have to remove. Those whips had to be cultivated, dug up, transported in vehicles and planted, with every step generating carbon. Digging the soil releases carbon to such an extent that research has found that leaving agricultural land fallow is a better carbon investment than growing corn for ethanol. And we haven’t even considered all the volunteers travelling, often by car, to planting sites. Their carbon will need offsetting too. In fact, when you add it all up, it can take years for a tree to become a carbon sink rather than a carbon source, and the more whips that die the longer it will take for schemes to pay for themselves carbon-wise.
Additionally, while many tree planting proponents repeat the refrain ‘the right tree for the right place’, the practicality is that trees get planted where people can get access. Unfortunately, the best places for planting trees are often in the hands of private landowners, and the National Audit Office found that uncertainty about future government subsidies for converting agricultural land into woodland puts many off. And even when private landowners do plant trees, they often do so based on commercial considerations rather than ecological or environmental ones, leading to situations like those seen in Scotland where peat bogs – fantastic sinks for carbon – are dug up and planted with trees, releasing massive amounts of carbon in the process.
Bailing out the Titanic with a Sieve
But let’s assume for a minute that all the problems I’ve mentioned are solved, and that every whip planted survives to maturity and becomes a carbon sink. Will they get us to net zero?
The simple answer is no.
The more complicated answer is still no.
According to the Office for National Statistics, UK greenhouse gas emissions in 2020 were just over 478 million tonnes of carbon dioxide equivalent. If we assume that the amount of CO2 taken up by woods and forests is around 0.5 kg per square metre per year, then we would need 956,000 square kilometres of land planted to absorb all this carbon. The UK is around 244,820 square kilometres so we’d need to reforest just under four times the landmass of the UK to reach carbon zero through trees alone.
It’s easy to forget that trees are living organisms, and when they die they decompose, and their carbon is released back into the environment. The only way we can stop this from happening is to prevent that decomposition. This can happen if the trees are buried in anoxic environments, but that burial can also release soil carbon we then have to factor into our carbon budgets. We can also preserve trees by turning them into wood products, but again these are only carbon sinks while they are being used. As soon as they stop being used and are left to decay they become carbon sources again. The carbon cycle is relentless and cannot be stopped, only paused.
The conclusion we have to reach is that we’re still emitting carbon on a massive scale, carbon capture is at best a minor tool in our arsenal and at worst a dangerous distraction.
Trees are More than Just Carbon Sinks
Part of the problem with the government’s plan is that it wants trees to be a panacea. They are to offset carbon emissions while also increasing biodiversity, providing timber products and ‘amenities’, helping manage water quality and flood risk, and providing shade in urban environments. That’s a lot to ask!
One of the best ways to allow trees to do all this while still being carbon sinks is to stop the current informal policy of plant and leave. If we are going to plant trees then aftercare is needed, and if we can’t provide that then we shouldn’t plant them.
A low cost and simple alternative is to allow woods and forests to naturally expand. This will still need the agreement of landowners, but requires much less initial outlay. Fencing to keep out grazers is often the most that’s needed to allow nature to take its course, as has happened in Monks Wood. In the 1960s a research station on the edge of ancient woodland in Cambridgeshire decided to leave a four hectare field alone and see what happened. What researchers saw was a classic example of ecological succession, with pioneer species such as brambles and hawthorns providing a secure habitat to allow trees including ash, field maple and oak to grow without risk of being grazed by deer. Sixty years later the field is fully incorporated into the ancient wood and will help ensure its continuation for centuries more.
The benefits of natural expansion are many. There is little to no cost and as the trees establish themselves there’s no carbon cost to their arrival at the site, plus they are already adapted to the local environmental conditions. It also reduces the risk of introducing pest species along with them. From a biodiversity perspective, increasing the size of an existing wood is far more beneficial than creating a new wood. Two separate 100 hectare woods are less biodiverse than one 200 hectare wood. There are many reasons for this but the most obvious is that 100 hectares can’t sustain as many species as 200 hectares, particularly for larger species that require larger territories to survive. Edge effects – microclimate changes that occur on the boundaries between two habitats that make them less hospitable to species living in those habitats – are also exacerbated in habitat fragments and lead to the dominance of generalist species.
Solutions and Conclusions
Trees are amazing – they can live hundreds, even thousands of years, and can sustain entire ecosystems within their branches. Oaks can support over 2,000 species. When we reduce them to carbon sinks, we miss much of their value.
As you may have already guessed, my preferred solution is natural regeneration. I’m not alone in this – in 2020 Rewilding Britain produced a report that argued natural regeneration should be the default method for restoring Britain’s woods. In urban and suburban environments, more carefully planned and managed planting is required. I would love to see tree planting as a requirement for new housing developments due to their well-known cooling effect in summers. As heat waves – such as the ones we saw last summer – are set to continue, we need to adapt and one low-cost way to reduce temperatures in built-up areas is to plant trees.
However, my cynicism is such that I suspect this won’t happen. The great thing about mass tree planting is that it’s simple and obvious. It’s a great photo-op for councillors and MPs to show off their Green credentials and illustrate that they take climate change seriously. They are being seen to Do Something. What happens after the cameras leave is far less important.
This has to change.
