Trees take in CO2 and store it in their woody mass and the soil, making them a natural solution for reducing greenhouse gases in the atmosphere. Globally, it is estimated that forests absorb enough CO2 each year to make up for one-and-a-half times the greenhouse gases that the United States emits annually.
That kind of carbon drawdown capacity motivated large-scale tree planting efforts such as the global Trillion Tree campaign, launched in 2006. Sophisticated carbon credit markets have also been developed, which allow polluting companies to pay for tree planting in order to offset their emissions.
But trees that die, burn or are cut can release that CO2 back into the atmosphere, and trees planted in large stands of only a few species do not function the way that naturally diverse forest ecosystems do.
A study of tree planting in the tropics published in the monthly journal Trends in Ecology and Evolution, argues that these carbon-offset plantations can come at the expense of biodiversity and other important services that forests provide. The study’s authors also warn that an emphasis on carbon reduction alone can lead to poor environmental decisions.
“It is crucial to shift from the narrow focus on carbon and adopt a more holistic perspective if we aim to effectively conserve and restore natural ecosystems and combat climate change,” the study’s lead author, Jesús Aguirre-Gutiérrez, an ecologist and senior researcher at the University of Oxford’s Environmental Change Institute, told Newsweek via email.
The scale of tree planting necessary to offset global greenhouse gas emissions would require vast areas of land, the authors write, often leading tree planters to displace other ecosystems. Afforestation, or planting trees in places they didn’t previously grow, can replace tropical grasslands that also provide carbon sequestration and other important services, such as regulating water flow and enriching soil, the authors contend.
Grasslands support biodiversity as well, and many species adapted for those environments suffer when grass is replaced by trees. For example, in Brazil’s Cerrado savannah, increasing tree cover by 40 percent reduced the diversity of plants and ants by about 30 percent. “Planting trees is great as far as they are planted in areas where they belong,” Aguirre-Gutiérrez said.
Photo: American Forests