Trees can help to combat climate change, but determining what to plant and where is complex — and whether to plant them at all is a growing debate.

The big picture: Protecting, planting and restoring forests can help offsetglobal warming, but experts stress that greenhouse gas emissions still have to be dramatically cut to reach climate goals for the planet.

"There is an enormous amount of science that shows trying to protect forests and stop the loss of forests is a good idea," says Yale University's Mark Ashton.

• "After that, things get very difficult to define and describe," he says.

Driving the news: The Trump administration this week established an interagency council to put more weight behind its role in the One Trillion Trees Initiative, my Axios colleague Ben Geman reports.

• The effort, convened by the World Economic Forum and backed by global leaders, aims to protect or plant 1 trillion trees globally to try to sequester carbon, protect watersheds and support biodiversity.

What's happening: Some experts say there is a time and place to plant trees but the approach, which is attracting attention from companies and investors that want to offset their carbon use, isn't a silver bullet.

How it works: The ecological considerations of large-scale tree planting initiatives are complex.

• There are questions about the species and diversity of trees that should be planted, how they interact with the soil and insects (which can affect how much carbon they sequester), and their odds of survival in environments that are already in transition due to climate change.

And there are social and economic factors to be balanced with environmental ones.

• For example, growing non-native mangoes in Kenya could be more successful economically (if there is a market for the fruit) and ecologically (that value incentivizes people to care for the trees), says the World Resources Institute's Aaron Minnick, who oversees TerraMatch, the institute's platform to pair funders with tree projects in local communities.
• "We need to create and shape markets for the communities planting the trees so there is economic opportunity," says Justin Adams who leads the WEF's program.
• Planting is also an important tool in places where the soil is degraded or the native seed stock is depleted.

What's new: Allowing trees to regrow naturally is increasingly being seen as a tool for combatting climate change, says Robin Chazdon, a professor emerita at the University of Connecticut.

• It can be done at a large and cost-effective scale because it doesn't require preparing the soil, growing, transporting and planting seedlings, or maintaining the sites like tree planting does, she says.
• "Nature does take time to heal, and that time may be the cost," says Ashton, but the seed type and propagation of regrowth are “perhaps more resilient to the impacts of climate change than the choices we make for nature.”

In a recent study, Chazdon and her colleagues mapped the potential carbon that could be captured in forests that are allowed to naturally regrow.

• They identified as many as 1.7 billion acres that could be naturally regrown as forests.
• Those areas — which exclude grasslands, boreal biomes, current croplands and population centers — have the potential to absorb roughly 70 billion tons of carbon dioxide over the next 30 years, or the equivalent of about one-quarter of global fossil fuel emissions each year.

Restoring tropical forests, in particular, would deliver the most benefits to biodiversity and mitigating climate change at the lowest cost, according to another study published yesterday in the journal Nature and led by Bernardo Strassburg of the Pontifical Catholic University in Rio de Janeiro, Brazil.

• But the researchers stress reviving grasslands and other biomes is also key.
• And focusing on the total area that is restored is less effective in bringing about the highest levels of benefits, says Chazdon, a co-author. "Location really matters."

Whether planting or regrowing trees naturally, it is difficult to predict how people will use forests, says Ashton.

• Each place has different social, economic, biological and climate circumstances so forest policies and projects need to include bottom-up, place-based solutions, he says.
• "Many practices such as forest 'gardening,' using fire, and other cultural practices to work with seasonal flow of the sun are critical to current and future forest regeneration," saysKatie Kamelamela, who studies the relationship between Hawaii's Indigenous communities and their environment as a postdoctoral fellow at the Akaka Foundation for Tropical Forests in Hawaii.

The bottom line: "We shouldn’t just knee jerk plant trees," says Chazdon. "We need to be very strategic and smart about it."