Will planting trees stop climate change?

Dr Mark Nicholson

The short answer is no.

I make my living by identifying and planting East African plants and shrubs. In 2000 we started a forest and ecological restoration project in the Kenyan highlands at Brackenhurst, north of Nairobi. The purpose was to convert back into native forest what was cleared 100 years ago to make tea and eucalyptus (known here as ‘gums’) plantations. Gums were planted to dry the tea: every four acres of tea requires one acre of gums. We estimated we would achieve complete restoration by 2030.

In the UK there are 60 native tree species. In East Africa there are over 10,000 ‘higher’ plant species (i.e. flowering plants covering everything from daisies to huge trees), about one percent of which have English names. So, I have to deal with (and learn) Latin names that roll off the tongue like Pseudolachnostylis maprouneifolia. Many trees have local names but we have over 200 languages in East Africa and Ethiopia so it is not very easy getting to grips with the numerous vernacular names for any one species.

Let me start by dispelling one of the great environmental myths. The Amazon forests are not the “lungs of the earth”. Mature tropical forests are more or less carbon neutral. Yes, green plants give out oxygen and small amounts of carbon dioxide (CO2) by day but at night they give out only CO2. Animals and decomposing leaves also produce CO2 all the time. If any forest biome could be called the lungs of the earth it is the boreal forests of Russia, North America and Scandinavia. In the summer they photosynthesize for 20 hours a day producing oxygen as the by-product, and in winter they shut down and therefore produce almost no CO2. The boreal forests are also twice the size of the Amazon forests. However, their tree biodiversity is very low, probably around 20 species, whereas the official tree species’ tally for the Amazon forest is 6000.

Don’t you believe it. 3 years ago, during a collection trip on the Rio Negro, upstream of Manaus in the Amazon forest, we found unknown species almost every day. Mike Hopkins, the British curator of the Manaus Herbarium, showed us over 100,000 undescribed specimens in cupboards and reckoned the real total is in excess of 20,000 tree species. The danger of biodiversity loss is this: plants and other organisms are rapidly going extinct during the current Anthropocene extinction; so we risk losing the cure for Alzheimer's or other diseases if a plant becomes extinct before we know what it is good for.

The importance of the Amazon region and other tropical forests is both their biodiversity value and the ecosystem services they provide, such as rainfall and carbon sequestration (storage). Climate change is a minor cause of biodiversity loss. Kenya has lost 90% of its large mammalian fauna in the last 50 years owing to land use change, land degradation and human population rise. Even so, the total mammalian biomass in Kenya has probably not changed at all; as non-human mammalian biomass has crashed, human population has soared. In the richer ‘West’, biodiversity has fallen for other reasons such as pollution, the use of toxic agrochemicals, urban expansion and monoculture.

So yes, if the Amazon forests were felled there would be a massive increase in atmospheric CO2 as wood was burned and the fertile organic matter (leaf litter) in the soil oxidized by sunlight. Evapotranspiration from trees produces local rainfall, and the tree canopy protects the soil. Removing the trees would leave bare, mineral-poor soils and semi-arid conditions.

How does tree planting affect global warming? Put simply, photosynthesis converts CO2 into glucose, which in turn is slowly converted into complex carbohydrates, ending as lignin (otherwise known as wood). But if the wood is chopped down and turned into firewood, charcoal or paper it will all go back to CO2 when they are burned. But if Mr. Chippendale turns the wood into a chair, then CO2 will be sequestered in perpetuity.

Unfortunately, the word ‘carbon’ in modern pseudo-science parlance is equated with CO2. When reporters and commentators talk about the emission of one tonne of ‘carbon’, they mean one tonne of CO2. Carbon is found everywhere in all life forms as well as in rocks, from chalk to diamonds. Equally annoying is the frequent use of ton/short ton (907kg) instead of the metric tonne (1000kg). I will use the latter (or t) and stick with CO2.

So let’s look at some figures: a growing tree will sequester about 20kg of CO2 in its first five years. A tropical seedling in a high rainfall area will sequester more because the growing season will be much longer. Once it becomes a mature tree, the amount of wood manufactured each year is much lower and leaf-fall much higher, hence its carbon neutral status. If a tree sequesters 4 kg of CO2 per year, then a million trees will sequester 4 million kg (or 4000 t), and a trillion trees (1012) 4 billion t per year. That is a significant contribution but a long way from annual carbon emissions arising out of humankind’s activities. Anthropogenic CO2 emissions (i.e. those resulting from the influence of human beings on the environment) are now roughly 40 billion t per year.

The BBC website reported on 26 January that someone had come up with the “10 golden rules of tree planting” - Scientists address myths over large-scale tree planting - BBC News.

As a tree-planter myself I was not hugely impressed.

1. Protect existing forests first

Sensible advice but no need for ‘first’. Protecting existing forests maintains the status quo by keeping sequestered carbon in situ but planting trees will increase CO2 stocks.

2. Put local people at the heart of tree-planting projects

Studies show that getting local communities on board is key to the success of tree-planting projects. It is often local people who have most to gain from looking after the forest in the future.

I disagree. The really large projects need to be carried out by philanthropists, governments or big corporations such as Wilmar International, Unilever and the other monocultural behemoths. Only they can revert large areas of oil palm or tea back to forests.

3. Maximise biodiversity recovery to meet multiple goals

Reforestation should be about several goals, including guarding against climate change, improving conservation and providing economic and cultural benefits.

Here biodiversity recovery is conflated with climate change, which is a separate issue.

4. Select the right area for reforestation

Plant trees in areas that were historically forested but have become degraded, rather than using other natural habitats such as grasslands or wetlands.

5. Use natural forest regrowth wherever possible

Letting trees grow back naturally can be cheaper and more efficient than planting trees.

Certainly not more efficient. In most forest sites, ‘passive’ restoration leads to a massive infestation by invasive species - plants from elsewhere that are aggressive and will eventually take over if allowed to. One-third of our budget in the last 20 years has been used to control invasive species.

6. Select the right tree species that can maximise biodiversity

Where tree planting is needed, picking the right trees is crucial. Scientists advise a mixture of tree species naturally found in the local area, including some rare species and trees of economic importance, but avoiding trees that might become invasive.

Yes, horses for courses. If we had used Kenya coastal species at our altitude of 2000 metres, 95 percent of them would have died. Above all, avoid monocultures of eucalyptus (‘gums’) etc, except in Australasia, where they originate.

7. Make sure the trees are resilient to adapt to a changing climate

Use tree seeds that are suitable for the local climate and how that might change in the future.

Apart from this not being very good English, we do not know how much the climate will change so we do not yet know which species are most resilient

8. Plan ahead

Plan how to source seeds or trees, working with local people.

Whether one works with local people or not it can be very difficult to find seed.

9. Learn by doing

We all try to do that.

10. Make it pay

The sustainability of tree re-planting rests on a source of income for all stakeholders, including the poorest.

Not necessarily. The main benefit of tree planting on vast areas is carbon sequestration which provides no direct income but huge indirect benefit to humanity

So let me add two Golden rules and two Golden questions.

11. Don’t imagine for one moment that tree planting will get us out of the hideous environmental disaster heading our way.

12. It’s not about tree planting; it’s about tree growing.

For us, the initial five years is about weeding, removing and replacing dead trees, and controlling invasive species. Many donor- or corporate-funded projects fail because whilst they trumpet that they have planted, say, a million trees, only 100,000 survive the ravages of livestock, people, weeds and wildlife.

Two Golden Questions:

  1. Who is going to pay? Local people that we work with are poor, land-poor and hungry. If you think they are going to devote a substantial part of their farms to tree planting, have another think. Our modest 40 ha. forest of 140,000 trees and shrubs on private land has cost US$1million over 20 years. We were lucky enough to have international funders; most are not so lucky.

  2. Where will the land come from? We are part of the optimistically-named Trillion Tree Campaign. Well, a trillion mature trees at 400/ha would require 2,500 million hectares. That’s 25 million km2. Russia is just over 17 million km2. If we planted trees in the density normally found in plantations (1000/ha), 10 million km2 would be needed. That’s the size of Canada and larger than China.

So, plant one tree or plant a million, it all helps. But what is really important is to answer this question: will tree planting save the planet? The Center for International Forestry Research, The Global Landscapes Forum and other large international institutions might say Yes but most scientists say No unless there is a global and concomitant reduction in the use of fossil fuels. This will be the subject of my next article: “Glasgow 2021 and the fallacy of Net Zero”.