When planting trees threatens the forest

Press release - UCLouvain research

Campaigns to plant huge numbers of trees could backfire, according to a new study by researchers at Stanford University, the University of California at Santa Barbara and the University of Concepción in Chile that is the first to rigorously analyze potential effects of subsidies in such schemes. The analysis, published in Nature Sustainability, reveals how efforts such as the global Trillion Trees campaign, could lead to more biodiversity loss and little, if any, additional carbon sequestration. The researchers emphasize, however, that these efforts could have significant benefits if they include strong subsidy restrictions, such as prohibitions against replacing native forests with tree plantations.

“If policies to incentivize tree plantations are poorly designed or poorly enforced, there is a high risk of not only wasting public money but also losing both terrestrial carbon and biodiversity,” said study co-author Eric Lambin, Professor in University of Louvain (UCLouvain). “That’s the exact opposite of what these policies are aiming for.”

There is no question that forests have an outsized role to play in efforts to combat climate change and slow the loss of biodiversity around the world. So it makes sense that tree-planting as a solution has gained traction 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 seeks to plant as many trees as its name implies.

A closer look reveals faults in the optimistic plans. For example, nearly 80 percent 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 restoring natural forests. Plantations typically have significantly less potential for carbon sequestration, habitat creation and erosion control than natural forests. The potential benefit dwindles further if planted trees replace natural forests, grasslands or savannahs, ecosystems that have evolved to support unique, local biodiversity.

Other scientists have pointed out the flaws in such plans. The researchers involved with the new study wanted to zero in on the role of subsidies designed to encourage private landowners to plant trees because such payments are widely proposed as a promising solution to a variety of environmental challenges. So, the scientists looked at one of the world’s longest running and most influential afforestation subsidy policies, Chile’s Decree Law 701. The law, in effect from 1974 to 2012 and currently being considered for reintroduction, has served as the model for similar policies in a number of South American countries and international development projects.

“In light of global enthusiasm to plant a trillion trees, it’s important to reflect on the impact of past policies,” said lead author Robert Heilmayr, an assistant professor at the University of California, , who worked on the study while a PhD student in Stanford, with Eric Lambin. “Chile’s experience can help us understand the climate, ecological and economic impacts that might occur when governments pay landowners to establish massive tree plantations.”

Chile’s Decree Law 701 provided permanent protection of afforested lands from expropriation, subsidized 75 percent of afforestation costs and provided support for ongoing plantation management. Lax enforcement and budgetary limitations hobbled prohibitions on the use of subsidies on already-forested lands, leading to situations in which the government subsidized the replacement of native forests with profitable tree plantations. Anecdotal evidence indicated the law’s subsidies further reduced native forest cover by encouraging the establishment of plantations on shrublands or marginal agricultural lands where forests might have naturally regenerated.

The researchers set out to quantify the full impact of the afforestation subsidies and calculate their effects on net carbon and biodiversity changes across the entire country. They compared the area of Chilean forests under three scenarios: actual observed subsidy patterns, no subsidies and subsidies combined with fully enforced restrictions on the conversion of native forests to plantations. 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. Since Chile’s native forests are more carbon dense and biodiverse than plantations, the subsidies failed to increase carbon storage, and accelerated biodiversity losses. However, strict enforcement of restrictions on native forest conversion would have improved the policy’s carbon and biodiversity outcomes.

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