Tree planting - decarbonising the planet or greenwashing?

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Reforesting the planet would have a considerable impact on atmospheric carbon dioxide levels. And, perhaps surprisingly, sufficient land is available to increase the tree population by 33%, reducing CO2 levels by a quarter.

We could achieve this without adversely impacting agriculture or cities. But, of course, it's not the complete answer. Even given the go-ahead, it would take many decades, perhaps a hundred years, before its full impact would be felt. However, even significantly more modest tree-planting projects will help reduce CO2. and offset emissions from aviation and other sectors that don't currently have alternative solutions.

Competing mechanisms

But all might not be as simple as it appears at first sight – there are competing mechanisms, some positive and some negative, which could change the above scenario significantly. For example, the increased levels of CO2 in the atmosphere positively impact tree growth, encouraging denser foliage and speeding growth cycles. However, it also encourages trees to mature and die sooner, ultimately reducing their carbon-sinking abilities. Furthermore, as global temperatures inexorably rise, regions once suitable for forestation may be rendered unsuitable; they will become too warm for tropical forests to thrive.

An additional problem with tree planting is that the effect they have on global warming depends to a large extent on their location. Trees trap heat under their canopies which can increase global warming. The effect is insignificant in equatorial regions, where trees grow rapidly. However, in polar areas where growth rates are slow, the warming effect may well dominate the benefits of carbon capture. Even in temperate regions, the competing effects might cancel each other out.

A mixed bag

Despite the various complications and caveats, the bottom line of tree planting is a positive one for the planet's continued health. Various popular campaigns encouraging ordinary folk to plant a tree are moving in the right direction, despite the fact that many trees planted under such circumstances die prematurely or fail to mature. But we need more than trees - it is too late for trees alone to rescue the situation. Massive reforestation projects could make us complacent regarding other ways to reduce emissions and decarbonise the atmosphere.

Accusations of greenwashing are also obfuscating the tree-planting landscape. Businesses can offset their carbon footprint by planting trees or contributing to other habitat restoration projects. But often, such initiatives encourage pollution at one location by improving habitats elsewhere. Effectively many of these projects do more harm than good.

Trees aren't "plant and go."

Trees rarely fit into the "plant and go" category. Thus planting a tree is only a tiny part of the overall story. For a tree to survive and thrive, it needs considerable care. Saplings are highly vulnerable to disease, animal damage, and light and nutrient deprivation. If not cared for adequately, many will die. For instance, up to 90% of 11 million saplings planted in Turkey in November 2000 may have died. Similar examples exist worldwide.

The bottom line

Tree planting is vital to humanity's fight against global warming, but it may not be the panacea it is often painted. For tree planting initiatives to succeed, they need careful planning, control and follow-up. Trees must be planted in the right places, the mix of species needs careful attention, monocultures avoided, and other alternatives such as natural regeneration should be considered.

Given that over a hundred-year lifetime, a hardwood tree can sequester around a tonne of CO2, and that, to achieve a maximum 1.5 C increase in global temperatures, we need to remove a billion tonnes of CO2 from the atmosphere by 2025, tree-planting and reforestation are only part of the picture. Currently, we are well short of what's needed.

Posted: June 2017