Trees alone cannot absorb all the CO2 from atmosphere

Scientists say that if all the available land is brought under vegetation, we can sequester enough carbon to offset about 10 years of greenhouse gas emissions at current rates. After that, there could be no further increase in carbon capture

While trees definitely absorb CO2, it is important to know that trees (precisely forests) have their own limitations. The fact is that there are not enough trees to offset human’s carbon emissions – and there never will be.

The start of monsoon is also the time plantation drives are taken up everywhere. Enthusiastic nature lovers hunt for locations to plant a tree of their choice. It seems that if all the plants grow up into trees, they may compete with concrete structures to occupy the majority of the urban landscape. However, that doesn’t happen. A forest officer, in-charge of a government nursery, once jokingly said, “if all the saplings that I have prepared and distributed over the last 20 years had grown up into trees, there would have been a thousand forests today!” The fact is that we have failed to save the jungles that have existed for over thousands of years. This is the irony of today’s plantation programmes.

Our enthusiasm about planting more trees (and not necessarily caring for them) is also misguided by the limited theory: ‘More trees = more carbon adsorption = controlled global warming’. Plants absorb carbon dioxide from the air, pass it on to leaves, wood and roots in the form of carbon. We have come to conclude that plants – particularly fast-growing tropical trees – can control climate change, capturing much of the carbon dioxide emitted by fossil fuel burning. Governments, NGOs, corporates and social organisations have pledged to conserve or plant massive numbers of trees.

While trees definitely absorb CO2, it is important to know that trees (precisely forests) have their own limitations. The fact is that there are not enough trees to offset human’s carbon emissions – and there never will be.

How and where is carbon stored in a typical temperate forest  Photo credit: UK Forest Research

How much CO2 can plants potentially absorb?

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Assume for a moment that we humans have used all possible land on Earth to cover with vegetation. Scientists say that even then we would have sequestered enough carbon to offset about 10 years of greenhouse gas emissions at current rates. After that, there could be no further increase in carbon capture. Just for records, we are emitting carbon dioxide into the atmosphere at a rate of 10 gigatonnes of carbon a year.

Yet, there is no doubt that we need plants and trees because all types of vegetation support biodiversity, which is very essential for human survival. What needs to be understood is what kind of soil and climate conditions support what kind of vegetation. For example, there is no point in planting dry deciduous trees in the temperate region of Europe. Similarly, we should not plant tall growing trees in the savannah grassland. Evergreen forest of Western Ghats will not support Coniferous trees of Himachal Pradesh. By drawing up ambitious plans to plant 5 crore trees in the first year and say 10 crore in the next monsoon, we may be inadvertently damaging the very forest properties that make them so vital to our wellbeing. So it is pertinent to understand how plants absorb carbon dioxide and their exact role in the ecosystems.

How do plants absorb carbon dioxide?

Photosynthesis is a process by which plants convert carbon dioxide into simple sugars, which help the plants grow. If this carbon stays on till the plant becomes a tree and ends up in wood, it can be locked away from the atmosphere for decades. Therefore, in this sense it is good to build houses and products from tree timber.

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As and when plants die, microorganisms act on their tissues, decompose them and give back the carbon to the soil. Plants and soils together hold about 2,500 gigatonnes of carbon – about three times more than is held in the atmosphere.

Human interference in carbon absorption

This seemingly normal process is disturbed by human interventions, which include building roads, houses, cutting forests for agriculture, expansion of cities etc. Humans are changing the planet in a hundred different ways, which has the scientists differ on how much carbon plants can absorb from the air. However, they all agree on one thing: land ecosystems have limited ability to absorb carbon.

Even if we increase the number of trees to increase CO2 absorption from the atmosphere, there will come a stage when trees won’t grow and spread because every plant requires carbon dioxide, nitrogen and phosphorus in particular ratios to grow. Shortage of any one component can potentially impact plant growth.

Therefore, scientists have estimated that the earth’s land ecosystems can hold enough additional vegetation to absorb between 40 gigatonnes and 100 gigatonnes of carbon from the atmosphere. Once this additional growth is achieved (a process that will take a number of decades), there is no capacity for additional carbon storage on land. Today, we humans are throwing carbon dioxide into the air at a rate of 10 gigatonnes of carbon a year.

Trees and climate change

Therefore, we know now that plants have their limitations in controlling climate change. Still, governments emphasise on scaling up efforts to increase vegetation. The vast majority of these efforts are focused on planting more trees while clearly ignoring the services of old jungles, which are cut at a rapid pace for carrying out development activities.

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We need to understand the subtle complexities of the carbon capture process in nature. Therefore, we should stop growing forests (for example Miyawaki forests) on lands that are unsuitable for them. It could be that a particular geography may support grass, but in the vigour to increase green cover we may end up introducing wrong vegetation at the wrong place. This greatly hampers the carbon sequestration process.

From a climate perspective, what matters is not how quickly a tree can grow, but how much carbon it contains at maturity, and how long that carbon resides in the ecosystem. Therefore, the next time you are tempted to believe that 10 new saplings planted at some other location can compensate for cutting a 100-year-old banyan tree to widen a road, think of the ecological service provided by the tree in the last century. It could well be supporting thousands of insects, animals and trillions of useful microorganisms under the soil.

We definitely need to expand forests for the health of our planet, but planting the right plant at the right location is necessary to increase the biodiversity of our planet. While planting trees it is also pertinent to know that simply expanding the green cover of our planet is not the only answer to climate change. Responsible and equitable consumption of natural resources will go a long way in making our planet a healthy sustainable place to live in.

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