Investments in tree planting programs to tackle climate change and offset carbon emissions must be carefully designed to avoid damaging local biodiversity and ensure the measures actually increase the ability of forests to capture and retain those emissions, recent scientific studies have suggested.
Scientists have widely agreed that natural carbon sinks such as forests can help address climate change. Furthermore, a number of countries and companies have indicated that they may need to use carbon offsets such as tree planting and forest management programs to achieve their decarbonization goals. But the new studies indicate that such programs must be carefully planned to be effective.
One study by 16 scientists at various U.S. universities and research institutions found that federal policies and economic incentives to protect and enhance forest carbon sinks do not always account for key ecological and climate-related risks such as increased wildfires, drought, and changes to ecosystems.
"While the models that are used to predict disturbance risks of these types represent the cutting edge in ecology and Earth system science to date," said the study, "relatively little infrastructure and few tools have been developed to interface between scientists and foresters, land managers, and policymakers to ensure that science-based risks and opportunities are fully accounted for in policy and management contexts." The study was published in the June 19 edition of Science magazine.
Another study led by a researcher at Colorado State University found that the carbon capture potential of afforestation — the process of planting forests in areas where no trees formerly existed — may sometimes be overestimated.
The scientists looked at soil samples taken from afforested and control locations across northern China and found that in carbon-poor soils, afforestation increased soil organic carbon density. But in soils already rich in carbon, carbon density decreased, which implies that previous studies might have overestimated the overall ability of afforestation to absorb carbon.
The findings have implications for forest managers and policymakers, said Anping Chen, a research scientist in the CSU Department of Biology and lead author of a study published online June 22 in the journal Nature Sustainability. "For example, a site that's already above a certain threshold of soil organic carbon underground may be best left alone for natural forest regeneration rather than planted with trees," Chen said in a statement.
A third afforestation study published online on June 22 in Nature Sustainability examined the effectiveness of Chile's afforestation program under a law in effect from 1974 through 2012 that the country's congress is considering reintroducing. The program subsidized 75% of afforestation costs and provided support for ongoing tree plantation management. The scientists found the afforestation program expanded the acreage covered by trees but decreased the total amount of native forests, which are more carbon-dense and biodiverse than plantations.
"With these two trends counteracting each other, the net change in aboveground carbon storage was relatively modest," said the study. Specifically, between 1986 and 2011, the carbon stored in above-ground vegetation increased by only 1.98% while biodiversity and native forests decreased.
A number of international development projects and countries, including Argentina, Ecuador and Colombia, have replicated Chile's afforestation model, the study said.
Study co-author Cristian Echeverría, a professor at the University of Concepción in Chile, suggested in a statement that "future subsidies should seek to promote the recovery of the many carbon- and biodiversity-rich natural ecosystems that have been lost" instead of building new tree plantations.