Rather than benefiting the environment, large-scale tree planting may do the opposite, two new studies have found. One paper says that financial incentives to plant trees can backfire and reduce biodiversity with little impact on carbon emissions. A separate project found that the amount of carbon that new forests can absorb may be overestimated. The key message from both papers is that planting trees is not a simple climate solution. Over the past few years, the idea of planting trees as a low cost, high impact solution to climate change has really taken hold. Previous studies have indicated that trees have enormous potential to soak up and store carbon, and many countries have established tree planting campaigns as a key element of their plans to tackle climate change. In the UK, promises by the political parties to plant ever larger numbers of trees were a feature of last year's general election. In the US, even President Donald Trump has rowed in behind the Trillion Trees Campaign. Legislation to support the idea has been introduced into the US Congress. Another major tree planting initiative is called the Bonn Challenge. Countries are being urged to restore 350 million hectares of degraded and deforested land by 2030. So far, around 40 nations have endorsed the idea. But scientists have urged caution against the headlong rush to plant new forests. They point to the fact that in the Bonn Challenge nearly 80% of the commitments made to date involve planting monoculture plantations or a limited mix of trees that produce specific products such as fruit or rubber. The authors of this new study have looked closely at the financial incentives given to private landowners to plant trees. These payments are seen as a key element of increasing the number of trees significantly. The study looked at the example of Chile, where a decree subsidising tree planting ran from 1974 to 2012, and was widely seen as a globally influential afforestation policy. The law subsidised 75% of the costs of planting new forests.