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upstart writes in with an IRC submission for RandomFactor:

When planting trees threatens the forest:

Campaigns to plant huge numbers of trees could backfire, according to a new study that is the first to rigorously analyze the potential effects of subsidies in such schemes.

The analysis, published on June 22 in Nature Sustainability, reveals how efforts such as the global Trillion Trees campaign and a related initiative (H. R. 5859) under consideration by the U.S. Congress could lead to more biodiversity loss and little, if any, climate change upside. 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 releasing more carbon and losing biodiversity," said study co-author Eric Lambin, the George and Setsuko Ishiyama Provostial Professor in Stanford's School of Earth, Energy & Environmental Sciences. "That's the exact opposite of what these policies are aiming for."

[...] 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.

"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 lost."

Journal Reference:
Robert Heilmayr, Cristian Echeverría, Eric F. Lambin. Impacts of Chilean forest subsidies on forest cover, carbon and biodiversity, Nature Sustainability (DOI: 10.1038/s41893-020-0547-0)

Original Submission