A new study has shown that campaigns to plant large numbers of trees could have the opposite intended effect and may threaten natural forests, illustrating the need to analyse and reassess subsidies in such schemes. 

The research, published in Nature Sustainability, reveals how initiatives like the global Trillion Trees campaign and others that are under consideration by the US Congress could lead to more biodiversity loss with little climate change mitigation. However, the researchers say that these efforts could prove useful if they include strong subsidy restrictions, such as banning the replacement of native forests with tree plantations.  

Eric Lambin, the George and Setsuko Ishiyama Provostial Professor in Stanford’s School of Earth, Energy & Environmental Sciences and co-author of the study, says, “If policies to incentivise tree plantations are poorly designed or poorly enforced, there is a high risk of not only wasting public money but also releasing more carbon and losing biodiversity.”

Forests are undoubtedly a way to slow biodiversity loss and mitigate the climate crisis through its carbon sequestering abilities, and tree-planting has gained a lot more attention in recent years with ambitious commitments, such as the Bonn Challenge, which seeks to restore an area of forest more than eight times the size of California by 2030, and Trillion Trees, which, as the name suggests, seeks to plant a trillion trees. 

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However, these initiatives have deep flaws. Nearly 80% of commitments to the Bonn Challenge involve planting monoculture tree plantations or a limited mix of trees that produce products such as fruit and rubber rather than actually restoring natural forests. These plantations typically have less potential for carbon sequestering, habitat creation and erosion control than natural forests. These potential harms are further amplified if planted trees replace natural forests, grasslands or savannas that have evolved to support local biodiversity.

The researchers also looked at subsidies designed to encourage private landowners to plant trees. Specifically, the scientists looked at Chile’s Decree Law 701, one of the world’s longest running and most influential afforestation subsidy laws in effect from 1974 to 2012, which is currently being considered for reintroduction. We must reflect on the environmental and economic impact of past policies that might occur when paying landowners (who often have their own agendas) to establish these tree plantations.

The law subsidised 75% of afforestation costs and provided support for ongoing plantation management. However, careless enforcement and budgetary limitations failed to address prohibitions on the use of subsidies on already-forested lands, leading to situations in which the government unintentionally subsidised the replacement of native forests with profitable tree plantations. The subsidies further reduced native forest cover by encouraging plantations on shrublands or marginal agricultural lands where forests might have naturally generated. 

Further, the researchers calculated the effects of the subsidies on net carbon and biodiversity changes across the country. They found that relative to a scenario of no subsidies, afforestation payments expanded the area covered by trees, but decreased the area of native forests. This shows that the subsidies failed to increase carbon storage and further accelerated biodiversity losses, since Chile’s native forests are more carbon dense and biodiverse than plantations. In this way, schemes to plant trees may actually threaten natural forests.

Cristian Echeverría, a professor at the University of Concepción in Chile and coauthor of the study, says, “Nations should design and enforce their forest subsidy policies to avoid the undesirable ecological impacts that resulted from Chile’s program. Future subsidies should seek to promote the recovery of the many carbon and biodiversity-rich natural ecosystems that have been lost.”