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I was reading the news today, and noticed that in the UN Climate Summit, it was decided that over 11 billion trees will be planted to sequester carbon dioxide from the atmosphere.

I was thinking that this probably doesn't have much effect. So, what is the climate impact of planting one tree?

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This paper suggests that one 20 year old tree will sequester 1 tonne of Carbon which is 3.67 tonnes of CO2 which is roughly a year's CO2 footprint of one UK person or a third of a US person.

However, your question is about planting one tree, which is subtly different.

In order to get a decent healthy twenty year old tree, you need to plant about twelve trees of various species, relatively close together, so that they all grow straight upwards. Young trees require less space than older larger trees, so they can be closer together, and then progressively cut them down over a ten to twenty year period so that one remains and can grow for over a hundred years. Mixed-species forests are less susceptible to pests and disease than mono-culture forests.

I guess its a philiosophical point. If you are operating a carbon sequestration business to maximise the CO2 in the trees, then you could have a mono-culture plantation, of the most CO2 dense wood, and use pesticides and surgical culling to prevent the any infection wiping out the whole site, and have that as a business expense.

On the other hand if you are merely trying to reforest a bit a wasteland and rebuild a country's ancient woodland, then you'll have a mixed species plantation with a broader community of wildlife that keep pests in check with a huge complex ecology of prey and predator curves, and many diverse native tree and shrub species, and try not to introduce invasive and non-native species. It wouldn't optimise CO2 sequestration, but it would be more sustainable and ecologically 'better'.

I'm afraid I don't know the details of the UN's 11 billion tree plan, but it could end up being 11 billion trees planted, but after twenty years only 1 billion remain.

There's also another element which needs to be understood, whilst a twenty year old tree might sequester one tonne of Carbon in its woody mass, it has over its lifespan photosynthesized a load more oxygen, and sequestered Carbon in leaves which are dropped each year. Whilst the carbon in the leaves does return to the atmosphere when they break down and decay, that amount of carbon is tied into a cycle of being in the air, then in leaves, then breaking down, and returning to the air. The more leaves there are, the more carbon is in that cycle rather than just building up in the atmosphere.

Our one tonne tree, in any one year might have a tonne of CO2 sequestered in its leaves, and two tonnes sequestered in the dead leaves at its base and three tonnes of CO2 in the air released from dead leaves waiting to be the next years new leaves. If the tree didn't exist, that CO2 would just be in the air.

Even the other trees that are culled will be sequestering CO2 and expiring Oxygen during their shorter lifespans. But this is harder to measure.

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    Interesting. 1 tC in 20 years. That's way more than my estimate (perhaps trees in my boreal taiga region are smaller than trees in warmer regions?), but then you take into account that only some of the trees will survive, which I didn't take into account. – juhist Sep 24 at 12:50
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    Most of the carbon in the leaves (etc.) that are dropped will return to the atmosphere in a few years. I recall reading that it's 90% eventually - but the remaining 10% adds up to a fair bit over a few years – Chris H Sep 24 at 13:15
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    @juhist Keep in mind you would choose different trees for sequestering carbon than you would for wood farming. We grow pine even in warm climates for wood, but might choose something like Eucalyptus instead for carbon sequestration. – Turksarama Sep 24 at 22:28
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    Can you please add a citation or explanation for your paragraph beginning "in order to get a decent twenty year old tree, you need to plant about twelve trees...", as I don't understand what you mean. Why do you need to do this to get one decent 20 year old tree, and what do you mean by a "decent" tree? – Igby Largeman Sep 24 at 22:59
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This depends obviously on the size of the tree, but I read somewhere that a tree ready for harvesting into sawlogs can be about 0.6 solid cubic meters in a boreal taiga forest (spruce or pine).

One solid cubic meter sequesters approximately one tonne of carbon dioxide. So, 11 billion trees will sequester 6.6 gigatonnes of carbon dioxide.

According to https://www.wri.org/blog/2018/10/according-new-ipcc-report-world-track-exceed-its-carbon-budget-12-years

To have a medium chance of limiting warming to 1.5°C, the world can emit 770 gigatonnes of carbon dioxide (GtCO2).

So, planting 11 billion trees will take only less than percent of the carbon dioxide budget.

To answer the question:

What is the climate impact of planting one tree?

Very small.

The problem is not that we don't plant trees; the problem is the massive scale at which fossil fuels are used.

Addendum:

Only around half of a full-grown tree are useful as sawlog, and only around half of sawlog makes it to sawmill products. Thus, only 25% of harvested tree will retain its CO2. The rest is used for creating paper/pulp/cardboard (that is recycled few times, then burnt to energy) or burnt to energy directly. Leaving the tree as-is will retain its CO2, but at some time the tree stops growing and it makes more sense to harvest it to make it possible for future trees to grow.

Of course, any wood burned to energy will offset fossil fuels, thus it will help to reduce CO2 emissions anyway.

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    It's also important to consider the lifespan of the tree and what happens to it after it dies or is chopped down. – THelper Sep 24 at 10:20
  • @THelper I agree heavily. Only perhaps 25% of the wood is useful as sawmill products (about 50% is sawlogs if you let the tree grow to its natural steady-state size, but sawmill will generate some waste too). The rest are burnt to energy, made to pulp/paper/cardboard that will be recycled few times and then burnt to energy. Well, I guess the only good thing in burning wood to energy (and releasing its CO2) is that it offsets fossil fuel use. – juhist Sep 24 at 12:47
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    You have an unusual definition of a "large" tree. Of the 3 oaks at the bottom of my garden, the medium one has a trunk diameter of nearly a metre. That means there's more than 2 cubic metres below the first branch (I have a good idea of the size as my ladder was up against it this morning for my neighbour to retrieve his cat). That tree is probably ~150 years old though (the big oak nearby is getting on for 600 years) – Chris H Sep 24 at 13:05
  • Yep, that might be the case. I was thinking more of spruce and pine grown to a size where it's economical to harvest them for sawlogs. – juhist Sep 24 at 13:08
  • My hardwood example will also store a little more C per cubic metre due to the higher density. But there are bigger questions in the difference between our estimates, such as: what do we do with all the wood? The carbon capture effect is maximised by leaving the trees standing, but if we make durable things out of plantation-grown timber instead of fuel-heavy materials, we also displace some emissions. – Chris H Sep 24 at 13:12
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The pure maths in your question.

Trees both absorb and emit CO2. The difference is stocked in wood. You should not see those 2 figures (CO2 cycle and stocked carbon) as 2 different inputs, they're the same. But one is yearly, and the other one over the tree's lifespan, which varies a lot.

I'll go with the yearly approach.

According to that link trees - I know maybe not just trees - account for +439 and -450Gt of CO2. That's a -11Gt yearly. And men add +29Gt yearly. So to compensate for our emissions, we'd need 2.6x as many trees on Earth, that's +160%

According to L.A. Times, there are 3 trillions trees. So planting 5 trillions trees would cover our emissions.

In that case 11bn will "only" cover 0.2% of our emissions.

Global impact

I'll make the assumption that you mean "one tree among a forest". Because you cited the 11bn trees figure, I guess you want to know the average outcome.

Let's hope that real forest - with variety and a whole ecosystem - will account for most of it.

Short-term :

  • those will stock carbon, not enough to solve our problem, but helping
  • they will fix and nurture the soil, preventing potential desertification, flooding and other climate-related hazard
  • they will offer shelter to wildlife

Long-term : 11bn tree will seed their own children! Let's be clear, a sprout doesn't become a tree unless an older tree dies, think space, sun and resources. But when one dies, another one grows. Also seed can travel, so more trees... if that seed find a compatible soil, which is the issue nowadays.

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Every tree creates a micro-climate. It provides shelter in winter, shade in summer, protection from the wind... These are mitigating factors that make a house a little warmer, the garden a little more productive, the self-seeding of other trees more likely. It results in positive, accumulative and inter-related effects.

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There is a difference between planting a tree and growing a tree. Any tree can have fifty or sixty babies growing beneath it at any time, but only one will become another tree, if its lucky. In equable climates deciduous forest trees take 50 years to mature, Trees are not for carbon offsetting. They are for giving the planet oxygen. A tree breaths out more carbon dioxide than a man, but it also produces oxygen which a man cannot do with any efficiency. By the way a leaf which has fallen gives out practically no carbon dioxide because it is almost immediately absorbed by soil fauna, and fungi. However if a tree dies or the branches are cut off, and the branches piled piled up above ground or burnt, quite a lot of poisonous emissions are produced, besides a bulk of carbon dioxide.

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