Is planting trees really the best way of stopping climate change?

Let’s rephrase the question.

Would you trust a young child to deliver critical results that the world depends on – even if the child has tons of potential?

Probably not. Because she wouldn’t even know where to begin.

A young tree is exactly like that child. It has the potential to help stop climate change once it grows up, but that takes decades. And in the meantime, there’s a lot working against it.

  • It’s small, which means it doesn’t hold much carbon anyway, even combined with thousands of other small trees.
  • It’s weak, which means it’s at higher risk of dying from storms, pests, or other stresses – in which case the promised climate benefits are suddenly gone.
  • It’s young, so it can’t support biodiversity or endangered species.
  • It’s new, so it won’t have cultural significance, nor provide any useful resources to local communities.
  • And it needs nurturing. You can’t just ‘plant it and leave it’. How many trees survive is more important than how many trees are planted.

But how often do you see tree-planting campaigns talk about their impact in terms of time? Most often, you’ll see impressive statistics about the number of trees planted. But as we’ll see, the act of tree-planting alone won’t make up for all the trees we’re losing.

That’s because the trees we’re losing are decades (sometimes centuries!) old. They’re tall and fully grown. They hold massive amounts of carbon right now. They are strong enough to withstand natural disasters and other stresses, and they have developed enough of a network to support each other if something does happen. They provide critical resources like food and medicine to local communities. And because they are part of a real forest ecosystem – not just a grid of little trees – they are home to irreplaceable biodiversity and endangered species. Which, in turn, also makes the trees stronger and more resilient.

If we lose these forests, we lose all of that today. That’s why we work to protect existing forests and ecosystems.

We’ve broken this up into sections: climate, biodiversity, resilience, and people. Each is an integral piece of the equation, and the work we do addresses all these pieces collectively.

Let’s jump in.

Climate Change?

Tree planting has been really big recently, and as we said above, you’ve probably seen lots of catchy slogans: ‘Planting trees is the best option for fighting climate change. Reforestation can save the planet. Buy a coffee, plant a tree. Buy a t-shirt, plant a tree. Like this post and we’ll plant a tree’ And so on.

But as for how effective it is, the numbers tell a different story.

How long does it really take a tree to do anything?

According to the UK’s Royal Society, it takes new forests at least 10 years to reach their maximum sequestration rate – the point at which they can absorb the most amount of carbon from the atmosphere every year. They’ll keep doing that until the trees mature, which, depending on the species, will happen after around 20 to 100 years[1].

Reforesting up to 800 million hectares worldwide could remove up to 300 billion tonnes over 25 years. Or, according to another similar report, reforesting 900 million hectares worldwide could remove around 200 billion tonnes – if the forests mature to a similar state as ecosystems in protected areas[2].

Both of those are in the same ballpark. And yes, those are big numbers. But there are a few things to note here.

First, this requires newly planted forests to grow into a mature ecosystem. That takes time – decades at least – and will only happen if they are cared for immediately after planting, and then allowed to mature naturally.

That often doesn’t happen. Many tree planting projects don’t plan for anything after planting, including after-planting care – so many trees may not survive[3]. Some countries are planting trees with the intention of harvesting and replanting them later.

Altogether, this means that many planted forests will never reach maturity. And if they don’t, tree planting isn’t delivering the benefits you think it is.

Second, a new tree will take at least ten years, and probably more, to really start making a difference to the climate. Which is understandable – young trees are pretty small.

Third, ‘maximum sequestration rate’ doesn’t tell you anything about how much carbon the tree is absorbing every year – only that it’s sucking it up as fast as possible. And a small tree can only absorb so much.

Fourth, it also has nothing to do with whether the tree has stored as much carbon as it possibly can. That’s because stored carbon accumulates over years and decades.

An old tree has already matured, so it won’t absorb carbon as fast as a young one – although, of course, it does absorb some. But it holds a massive amount of carbon in its trunk, branches, leaves, and roots. It’s built that store up over decades, by absorbing a little more carbon and growing a little more every year. Then there are the soils: in some forests, there’s more carbon in the soils than in the trees[4].

Large trees, pardon the pun, don’t grow on trees. They need decades, sometimes even centuries, before they reach maturity. And they store half of all the carbon in the rainforest[5]. If we lose them, they take hundreds of years to come back.

Which means that it’s even more important to protect what we have. As they say, prevention is better than the cure.

Don’t take our word for it. Here’s William Moomaw, lead author for five reports of the Intergovernmental Panel on Climate Change (IPCC):

[Tree planting is a great thing] to do, but [it] will not make much of a difference in the next two or three decades because little trees just don’t store much carbon. Letting existing natural forests grow is essential to any climate goal we have[6].

Planting trees and carbon storage

Then there’s the question of how effective planted forests are at storing carbon. There are a few things we could compare:

  1. Planting large amounts of trees, like in a plantation
  2. Planting a mix of native species
  3. Restoration of natural forests

Tree plantations or natural forests?

We already know that plantations aren’t ideal. They’re usually just one species, which is terrible for biodiversity. And often, they’ll be harvested in the future – that’s why restored natural forests (option c) are 40% more effective at storing carbon[7]. But even those restored forests will take 70 years to reach the carbon stock level of the original forest – assuming they are protected from disturbances[8].

Even if plantations are not harvested, natural forests are still far more reliable at storing carbon[9]. That’s because the plantations are more severely affected by drought, and so they capture almost a third less carbon than natural forests in the dry season. And since climate change is expected to worsen droughts, replanted forests will be even worse off.

So, anything that’s just planting one type of tree should be out of the question.

Native species and forest restoration vs. natural forests

The good news is, the tree planting programs that the average person or business supports are probably trying to do things right: planting a mix of native species, involving locals, restoring ecosystems. In other words, a mix of options b) and c). (Really, without checking into the details of each project, we can’t be sure, but let’s give them the benefit of the doubt.) As tree planting goes, that’s probably the best that you could hope for.

The problem is, it’s still not good enough.

Even a mix of native species and/or restored natural forests takes years to regenerate. And like we said above, the young trees store much less carbon along the way than older ones in existing forests do today[10].

Then, when the forests reach maturity, they still hold less carbon. Some estimates have suggested that even after 85 years, replanted trees stored only around 83% of the carbon as an undisturbed forest, and their roots stored only about 50-75%[11]. And even though that’s fairly close, reaching 100% could take a lot longer.

What’s more, although that particular study found that soil carbon recovered quite quickly, others have found that it can take at least 50 years before it gets close to original levels[12].

What does this mean? That even if you’re doing the best tree planting you can…

…using a mix of native species, and focusing on restoring forests as much as possible…

… planted forests still can’t hold a candle to the carbon storage of natural forests – even after decades.

In other words, protecting intact forests first is still our best option.

So ignore all the catchy slogans.

Better to keep a forest safe rather than try to bring back a pale imitation of it later.

Half the benefit, twice the loss

One last example to leave you with. Scientists have estimated that regrowing tropical forests can store around 6 tonnes of carbon per hectare per year in the first 20 years of growth. After that, they slow down, and the final average is ~3 tonnes per hectare per year over 80 years[13]. That adds up to 240 tonnes per hectare.

Meanwhile, old-growth tropical forests hold up to 418 tonnes of carbon per hectare – today[14].

Now imagine if we cut down that old-growth forest. We’d release 418 tonnes into the atmosphere right now. If we replanted it, we’d get 6 tonnes back this year. And, after 80 years, if the forest is still standing, we’d finally get about half of it back.

So can tree planting really save the climate?

We’re not so sure.

Biodiversity and Wildlife

Have you ever heard the word ‘depauperate’?

Derived from Latin, it’s related to the word ‘impoverished’, which you probably have heard. But not a poor person: a poor ecosystem. One that’s lacking in ‘numbers or variety of species’[15].

That’s what many scientists call secondary (young) tropical forests, because they have so much less biodiversity than primary (old-growth) ones[16].

Ouch.

Why is that? A few reasons: the young forests have a much more open canopy and not much vegetation on the forest floor. Their ecosystems are different, with many more generic plant and tree species, rather than the unique and rare ones that develop in old-growth forests over decades or centuries. And although the number of species might increase quickly in a young, quickly growing forest, that doesn’t mean they’re the same species that were originally there.

That’s a big problem. Many birds and animals need very specific habitats – like those unique trees. Or they might eat fruits or leaves that are only found in some of the special species in old-growth forests, or live in physical structures that only appear after many decades, like large fallen trees.

Often, they need large areas of untouched, remote forest – which by definition doesn’t really exist in a new forest[17]. And that means that secondary forests can’t support the same endangered animals, birds, and other wildlife that a primary forest can[18].

That’s why, in so many cases, wildlife won’t move into secondary forests. One study found that animals might only come back to a restored forest after 150 years[19]!

That’s way too long.

Endangered species and wildlife and birds need our help now. They can’t afford to wait 50 to 150 (or 4,000!) years for ecosystems to possibly recover. They’ll go extinct first.

A few interesting statistics

  • Singapore has almost ten times more restored forest than original forest. But the new forest only supports 60% as many species as the original forest.
  • An international team of scientists found that primary, natural forests contain more biodiversity and carbon than restored forests of any stage[20].
    • Some of the new, young forests eventually recovered 80% of what the original forest had – but only when they were still near large areas of the original forest!
    • The number of tree species was back to normal after 50 years – but there were still very few of the original species.
    • Meanwhile, plants would need over 100 years to reach original levels.
    • Animal species would only come back after 150 years[21].
  • A study from Brazil found that it could take between one and four thousand years for a disturbed forest to gain back the same levels of endemic species – species that only exist in that area – as a mature forest[22].
  • Plantations, of course, have the same problem. They have as much as 33% less diversity and richness than original forests[23].
  • Even when every effort has been made to restore the forest and not just plant trees, there isn’t nearly as much biodiversity as the original forest[24,25].
  • Recovering ecosystems have around 50% less living things and 30% less species diversity as a natural forest[26]. They also have 35% less carbon cycling – not great for fighting climate change!

What about planting trees that are native species?

Now, you may ask, what about the projects that plant native species?

Well, as far as tree planting goes, that’s getting into forest restoration, so it’s definitely the best thing to do. But a forest is made up of a lot more than just the trees that you plant.

There are trees that we can’t plant – because we don’t have seeds, because they only start growing after a forest is several decades old, or because we simply don’t know about them.

There are millions of small plants, like vines and bushes and ferns, that grow on the forest floor and integrate with the other plants and animals around them over decades.

And so on.

Many of those are exactly the ones that never grow back.

And that’s why planting trees can never replace original forests[27,28].

But let’s say you do want to plant a tree, because it’s so easy. Then what?

Resilience and Resistance

You know how all those self-help articles are always going on about how we need to learn resilience so we can be better at handling the unexpected twists and turns that life throws at us?

The same is true for trees and forests. They suffer through all sorts of stresses in their lifetimes (which, unlike ours, can be multiple centuries long). There are natural disasters like fires, droughts, or floods, plus other disturbances like pests, diseases, or animals eating their branches or leaves.

Resilience is the ability to recover from a major disturbance (think forest fire), and resistance is the ability to stay healthy with minor, long-term disturbances (think animals eating tree leaves).

Healthy, old forests can do both, thanks to their strength. Old trees have huge trunks, their roots extend deep into the ground, and they can’t be pushed around very easily.

On the other hand, small, young trees – like recently planted ones – aren’t very strong. They’re light, thin, and short, and they can be moved easily. (Else, you wouldn’t be able to plant them in the first place!)

Unfortunately, this also means they’re much weaker than old trees. Young trees have fewer leaves and branches, they are thinner, and their root systems are much shallower.

What does that mean?

First, that they have a higher risk of dying, whether from pests or diseases or natural disasters.

Young trees are less resistant to stress – they simply haven’t developed the ability to cope. For example, in the Amazon, tropical forests with tall, older trees are three times less sensitive to variations in precipitation (generally rainfall) than forests with shorter, younger trees. This is because the older trees’ roots go deeper, which lets them access deeper soil moisture. So they’re still able to keep growing during droughts[29]– which, as we said above, will probably become worse and more frequent with climate change.

Young trees are also often easier to cut down than old ones[30]. It’s only natural: think how much stronger a fully grown adult is compared to a baby or child.

That’s why in the Amazon, young forests only last 5-8 years on average. And even in Costa Rica, which has doubled its forest cover in the last years, half the forests disappear within 20 years after being planted – along with their carbon promises. (‘Trees, we hardly knew ye!)[31]

If that happens, the carbon you thought you were saving wasn’t really saved.

And if an existing forest got chopped down in the meantime, because we were focusing on planting new trees, it’s a double whammy. Not only did we lose the planted tree’s future contributions, but we also lost the existing tree’s current contributions.

Then there’s biodiversity.

How biodiversity helps forests survive

In the previous section, we covered how young forests have much less biodiversity than old, natural forests. But that biodiversity isn’t just for show. Old, primary forests support more biodiversity, but they’re also more resilient because of the massive numbers of different species that live in them. Each of these species might do something different: one animal spreads leaves and seeds, a species of bird controls insects and pests, one type of tree provides shade to another, which in turn provides some needed substance to the first one, and so forth.

In an old-growth, natural forest, there are enough species that if one disappears, another one can take its place. And there’s enough variety in each species for it to maintain a healthy population. So even if something stresses the entire system, it can find a way to survive.

That’s why more biodiversity means more resilience[32], and especially more resilience to climate change, which is going to stress everything[33]. So the lower biodiversity in young, newly planted forests means lower resilience overall[34].

You also saw earlier that the original trees and plants often don’t grow back when a forest is replanted. That also makes regrown forests extremely vulnerable to degradation[35].

To go back to the child metaphor from the beginning, the young trees haven’t developed enough strength, stability, or resilience to survive adverse events yet.

That’s why we work to protect standing, old-growth, natural tropical forests.

People

We’ll keep this short – it’s much less complicated than everything above. Many indigenous peoples and forest communities are deeply connected to their forests. Their cultural identities are often closely linked to intact forests and/or local plant and animal species, and they also rely heavily on natural resources from the forests for basic needs.

When we lose these old forests, forest dwellers lose their livelihoods and cultures. If they can’t rely on forests, they lose a source of resilience. Traditional ways of life also become impossible. This drives people off their lands, and results in a loss of cultural identity.

By now, you can probably guess where we’re going with this. Young forests made up of newly planted trees don’t have the size, diversity, or density to provide most, or even all the resources of an old forest, like tree bark, timber, fruits, berries, medicinal plants, or species that are important for food and cultural reasons. Only the old forests can do that.

To finish up…

… here’s a nice little infographic!

Saving existing forests is far better than planting trees - for the climate, for biodiversity, for wildlife, and for people.

Click for a full size version!

We’re not trying to say that tree planting isn’t important. It is. It’s the best option for degraded lands, and especially for restoring some natural ecosystems to farms.

But we’re trying to fight climate change today. To save animals and ecosystems. And to support local and indigenous people and communities.

Planting trees just doesn’t cut it for any of those.

We have to put all our efforts into saving existing, natural tropical forests. Forests that safely store away huge amounts of carbon. Forests that protect animals, birds, biodiversity, and ecosystems. Forests that support people, livelihoods, and cultures.

Forests that are never coming back if they get cut down.

That’s why we do what we do.

If you want to read more about how we do it, you can do so here.

And in the meantime, next time you see one of those catchy-sounding tree-planting announcements…

… think twice about what it really means.

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References

[1]  https://royalsociety.org/-/media/policy/projects/greenhouse-gas-removal/royal-society-greenhouse-gas-removal-report-2018.pdf
[2] https://science.sciencemag.org/content/365/6448/76
[3] http://apps.worldagroforestry.org/downloads/Publications/PDFS/WP20001.pdf
[4] https://e360.yale.edu/features/why-keeping-mature-forests-intact-is-key-to-the-climate-fight
[5] https://news.mongabay.com/2013/08/old-growth-trees-store-half-rainforest-carbon/
[6] https://e360.yale.edu/features/why-keeping-mature-forests-intact-is-key-to-the-climate-fight
[7] https://news.mongabay.com/2019/04/natural-forests-best-bet-for-fighting-climate-change-analysis-finds/
[8] https://news.mongabay.com/2019/04/natural-forests-best-bet-for-fighting-climate-change-analysis-finds/
[9] https://phys.org/news/2020-01-biodiverse-forests-carbon-periods.html
[10] https://e360.yale.edu/features/why-keeping-mature-forests-intact-is-key-to-the-climate-fight
[11] https://royalsocietypublishing.org/doi/full/10.1098/rspb.2013.2236
[12] https://www.sciencedirect.com/science/article/abs/pii/S0048969718333229
[13] https://onlinelibrary.wiley.com/doi/epdf/10.1046/j.1526-100x.2000.80054.x
[14] https://news.mongabay.com/2013/08/old-growth-trees-store-half-rainforest-carbon/
[15] https://www.lexico.com/definition/depauperate
[16] https://onlinelibrary.wiley.com/doi/full/10.1111/j.1744-7429.2006.00141.x
[17] https://onlinelibrary.wiley.com/doi/full/10.1111/j.1744-7429.2006.00141.x
[18] https://onlinelibrary.wiley.com/doi/full/10.1111/j.1744-7429.2006.00228.x
[19] https://royalsocietypublishing.org/doi/full/10.1098/rspb.2013.2236
[20] https://www.oneearth.org/protection-of-primary-forests-is-priority-but-reforestation-is-also-crucial/
[21] https://royalsocietypublishing.org/doi/full/10.1098/rspb.2013.2236
[22] https://www.sciencedirect.com/science/article/abs/pii/S0006320708001456?via%3Dihub
[23] https://link.springer.com/article/10.1007/s10531-010-9936-4
[24] https://link.springer.com/article/10.1007/s10531-010-9936-4
[25] https://news.mongabay.com/2011/09/old-growth-forests-are-irreplaceable-for-sustaining-biodiversity/
[26] https://pubmed.ncbi.nlm.nih.gov/28106039/
[27] https://pubmed.ncbi.nlm.nih.gov/28106039/
[28] https://www.nature.com/articles/nature10425
[29] https://www.nature.com/articles/s41561-018-0133-5
[30] https://www.sciencemag.org/news/2020/09/plant-trees-or-let-forests-regrow-new-studies-probe-two-ways-fight-climate-change
[31] https://www.sciencemag.org/news/2020/09/plant-trees-or-let-forests-regrow-new-studies-probe-two-ways-fight-climate-change
[32] https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.13320
[33] https://www.nature.com/articles/nclimate3109
[34] http://www.fao.org/3/i2560e/i2560e05.pdf
[35] https://www.jstor.org/stable/4092167?seq=1

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