It’s not enough just to plant billions of trees

We need to create new forests to avert global warming, but they have to contain the right species

The environment is now headline news. First, Greta Thunberg’s school protest turned into a massive youth campaign. Then came David Attenborough’s latest documentary, Our Planet, which laid out the stark facts on climate change. And yesterday saw the sad news that 571 plant species have been wiped out over the past 250 years.

We will have to make some truly enormous changes if we are to meet the urgent need to avert global warming and the loss of our precious biodiversity. One thing that would help us to achieve both is to plant trees.

To compensate for the UK’S contribution to global warming,

alongside cuts to emissions, we need to plant some three billion trees by 2050. That means 90 million trees a year, which the Government intends to accomplish by paying landowners to create new forests. Worldwide, this number rockets to 1.2 trillion trees to cancel out a decade of emissions.

But global-scale reforestation is not without risks, particularly when it comes to biodiversity. To avoid even higher pressure on native ecosystems, governments and tree-planting organisations need to make sure they are planting the right trees, for the right reasons. This means addressing five issues.

First is to design a tree-planting strategy with more than one goal in mind. As well as reducing global warming (through carbon capture), tree-planting schemes should bring additional benefits, from explicitly increasing biodiversity to making positive aesthetic changes to the landscape, as well as focusing on the benefits for human health and wellbeing, culture and education.

Second, the strategy needs to look at which trees to plant, and where. That doesn’t mean just using native trees; exotic species are sometimes better adapted to the harsh environments found in cities.

The Indian horse chestnut is better suited to our climate than the European horse chestnut commonly planted today; the oriental plane (Platanus orientalis) is another example of this.

In cities, dawn redwoods, gingkos, strawberry trees and false acacias will reduce heat, pollution and traffic noise and will tolerate urban stress. Where they should be planted is a matter of finding the best match. In some areas, shrubs and grasses are a better option for the climate.

Third, it must consider how to plant and cultivate the trees. In areas that are difficult to access, seeds can be released by aeroplanes and drones. It’s a good idea to plant seedlings or sow seeds in several stages – forests need trees of different ages to become self-regenerating, to better resist extreme weather and to support rich animal and plant life. Many threatened wood-decaying fungi, woodpeckers and beetles depend on the continuous availability of old stocks.

After planting, it is also essential to monitor and manage forests properly. Plans for reforestation should be long-term: spanning decades or centuries.

Fourth, the plan must look at where to plant. That means primarily choosing degraded land previously occupied by forests, rather than cutting down native forests to plant teak or eucalyptus (as has happened in Central America and sub-saharan Africa).

It must also consider the relationship between trees and other life – new forests may provide corridors for some species but create barriers for others.

Finally, if there is a choice, avoiding cutting forests is far better than replanting them. A primary forest does not just contain trees, it has myriad intertwined organisms, above and below ground. But as we stand, we need to both protect our remaining forests and create new ones.

The bottom-line is that planting trees is not a simple a solution as it may sound. To get this right, and at a scale commensurate to the environmental challenges we face, we need to bring together researchers, policymakers, and the private sector to produce robust, evidence-based recommendations for tree planting around the world.