Climate change: Planting new forests 'can do more harm than good'
Children plant trees in Ethiopia, a country that has taken up new forests as part of its climate protection plan
Two new studies have shown that large tree plantings do not benefit the environment, but can do the opposite.
A paper says that financial incentives to plant trees can backfire and reduce biodiversity without affecting carbon emissions.
A separate project found that the amount of carbon that new forests can absorb may be overestimated.
The key message of both papers is that planting trees is not an easy climate solution.
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In recent years, the idea of planting trees as an inexpensive and effective solution to climate change has really taken off.
Previous studies have shown that trees have tremendous potential to absorb and store carbon, and many countries have set up tree planting campaigns as a key element in their plans to combat climate change.
In Britain, the promises made by political parties to plant more trees were a feature of last year's general election.
In the United States, President Donald Trump even supported the Trillion Trees campaign.
The legislation supporting the idea was introduced into the US Congress.
Another important tree planting initiative is called the Bonn Challenge.
Countries are urged to restore 350 million hectares of degraded and deforested land by 2030.
President Trump is planting a tree in the White House to mark Earth Day
So far, around 40 nations have endorsed the idea.
However, scientists have urged caution to plant new forests.
They point out that in the Bonn challenge, almost 80% of the commitments made so far include planting monoculture plantations or a limited mix of trees that produce certain products such as fruit or gum.
The authors of this new study have carefully examined the financial incentives for private landowners to plant trees.
These payments are seen as a key element for the significant increase in the number of trees.
The study looked at the example of Chile, where a decree on the subsidization of tree plantings was adopted from 1974 to 2012, which was generally regarded as an influential afforestation policy worldwide.
The law subsidized 75% of the cost of planting new forests.
While it shouldn't apply to existing forests, lax enforcement and budget constraints led some landowners to simply replace native forests with more profitable new tree plantations.
Their study found that the subsidy system increased the area covered by trees, but reduced the area of indigenous forest.
The authors point out that Chile's native forests are rich in biodiversity and store large amounts of carbon. The subsidy system has failed to increase carbon storage and accelerate biodiversity loss.
"If policies to promote tree plantations are poorly designed or poorly enforced, there is a high risk that not only will public money be wasted, but more carbon will be released and biodiversity lost," said co-author Prof. Eric Lambin of from Stanford University.
"It is exactly the opposite of what these guidelines are aiming for."
A second study was to examine how much carbon a newly planted forest can absorb from the atmosphere.
So far, many scientists have calculated the amount of carbon that trees can pull from the air at a fixed ratio.
Suspected that this ratio would depend on local conditions, the researchers investigated northern China, which was planted intensively with trees due to climate change by the government, but also to reduce dust from the Gobi Desert.
When examining 11,000 soil samples from reforested parcels, the scientists found that in low-carbon soils, the addition of new trees increased the density of organic carbon.
But where soils were already high in carbon, adding new trees reduced that density.
The authors say that previous assumptions about how much organic carbon can be fixed by planting new trees are likely to be overestimated.
"We hope that people can understand that afforestation practices are not a single thing," said Dr. Anping Chen from Colorado State University and lead author of the study.
"The afforestation contains many technical details and considerations of different parts and cannot solve all of our climate problems."
Both articles were published in the journal Nature Sustainability.
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