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 offset global 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."

Driving the news: The Trump Administration this week established an interagency council to put more weight behind its role in the One Trillion Tree Initiative, a World Economic Forum effort 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, 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.
• Planting is also an important tool in places where the soil is degraded or the native seed stock is depleted, he says.

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.

• They also found focusing on the total area that is restored is less effective in bringing about the highest levels of benefits. "Location really matters," says Chazdon, a co-author of the study.

Whether planting or regrowing trees naturally, it is difficult to predict how people will use forests, and policies and projects need to include bottom-up, place-based solutions, says Ashton.

• "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," says Katie Kamelamela of 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."

Read the full story.

An estimated 1.6 billion people — about 20% of the planet's population — live within 5 kilometers (3.1 miles) of a forest.

Why it matters: Forest policies and projects have a direct impact on many lives and livelihoods. Targeting those efforts — and measuring their impact — depends on knowing how many people live near forests, Pete Newton of the University of Colorado Boulder and his colleagues reported recently in the journal One Earth.

By the numbers: 65% of "forest proximate people" were located in tropical countries, and 71% lived in low- or middle-income countries. (The study uses data from 2012, the most recent available when the researchers began the work.)

• The overall number of people living within 5 kilometers of a forest grew by about 10.5 million between 2000 and 2012.
• But in some countries, it fell. In China, for example, 48 million fewer people lived near forests in 2012 compared to 2000. Changes in forest cover (deforestation and restoration), human migration and population growth are all factors driving the changes, says Newton.
• Still, China had the largest number of people living in close proximity to forests (272 million), followed by the U.S. (169 million people), Indonesia (103 million), India (88 million) and Brazil (55 million).

Of note: The researchers focused on rural populations and the map doesn't show how people interact with forests; for example, whether they depend on them for food, energy or recreation.

How they did it: Newton and his colleagues combined satellite data of global forest cover and human population density, and counted how many people lived within 5 kilometers of forests.

The big picture: Protecting, managing and restoring forests can help to alleviate poverty, according to another report published today from the International Union of Forest Research Organizations.

Trees don't only grow in forests. In the seemingly treeless deserts and drylands of West Africa, researchers have now found more than 1.8 billion individual trees and shrubs.

Why it matters: Non-forest trees support flora and fauna, provide sources of food and shelter for animals and people, and help moderate climate change by absorbing carbon.

• But their exact role in the planet's cycling of carbon, water and other nutrients isn't well-understood, in part because their numbers are unknown. Satellite images typically used to measure and monitor forests haven't been high enough resolution to pick up isolated trees.
• Methods in a new study, published in the journal Nature this week, could also be used to manage tree planting efforts, like the Great Green Wall of Africa, an attempt to fight desertification that is behind schedule, in part because of challenges in monitoring the project, per The Guardian.

How they did it: Martin Brandt, of the University of Copenhagen, and his colleagues analyzed 11,128 satellite images with a 0.5-meter resolution using deep learning algorithms to map single trees in a roughly 500,000-square-mile area spanning the West African Sahara desert, the semi-arid Sahel region and a sub-humid area to the south.

• They report finding more than 1.8 billion individual trees with a median crown size of 12 square meters.
• The finding "challenges prevailing narratives about dryland desertification, and even the desert shows a surprisingly high tree density," the researchers write.
• "We provide here the technique and evidence that it is possible to map and measure each single tree," says Brandt.

What's next: The next step is to analyze larger areas with an aim of creating a global database of all non-forest trees, ultimately including those in other ecosystems, says Brandt.

Yes, but: Training the deep learning algorithm required manually labeling 90,000 tree canopies on sample images.

• "This approach becomes untenable for work on a global scale, and more-automated (unsupervised) methods for extracting information from satellite imagery would be necessary," Niall Hanan and Julius Anchang of New Mexico State University wrote in an accompanying article.

The coronavirus is surging again in the U.S. — "infections jumped by almost 17% over the past week as the number of new cases increased in 38 states and Washington, D.C.," Axios' Sam Baker and Andrew Witherspoon report. Germany, France, the U.K. and other European countries are also reporting an increase in cases.

The first confirmed case of COVID-19 reinfection in the U.S. was reported Monday in The Lancet, per Axios' Fadel Allassan. The patient's symptoms were more severe the second time, which could be related to the amount of virus they were exposed to, the immune response to it or other, unknown factors.

Eli Lilly's trial of a monoclonal antibody treatment was paused over safety concerns as were Johnson & Johnson's vaccine trials, both per STAT News. The NYT's Carl Zimmer on why "that's a good thing."

Chinese students make up 16% of all graduate STEM students in the U.S. and 2% of undergraduate STEM students, per a new report from Georgetown University's Center for Security and Emerging Technology.

The big picture: Concerns about intellectual property theft and foreign influence in U.S. research have led to calls to limit what Chinese students can study in the U.S.

• At the same time, others argue U.S. universities benefit financially and, ultimately, the country benefits economically from the contributions of students from China
• "These conversations have been hampered by a lack of granular data on the number of enrolled Chinese students by field and degree level," CSET's Jacob Feldgoise and Remco Zwetsloot write.

What they found: Using four different datasets, the researchers report there are "around 46,000 Chinese undergraduates, an estimated 40,000 master’s students, and an estimated 36,000 Ph.D. students" in STEM fields in the U.S.

• The percentage of Chinese students is lower than an earlier government report's estimate that 25% of STEM graduate students in the U.S. were Chinese nationals.
• Yes, but: The new estimates still don't give a certain picture, says Zwetsloot, "driving home the need for much better U.S. government data collection and dissemination on these questions."

The new science of prediction (Bryan Walsh — Axios)

First room-temperature superconductor excites — and baffles — scientists (Davide Castelvecchi — Nature)

How to arm Caesarean babies with the gut bacteria they need (The Economist)

The FDA has approved the first treatment for Ebola (Jonathan Lambert —Science News)

Hainan gibbons will use artificial rope bridges to move through the forest of their island home off China's southern coast, according to a new paper.

Why it matters: Gaps in the tree canopy, due to nature and human activity, can limit the area the critically endangered primates have to feed, breed and escape predators.

• "While restoring natural forest corridors should be a priority conservation intervention, artificial canopy bridges may be a useful short-term solution," the researchers write.

How it works: Bosco Pui Lok Chan of the Kadoorie Farm and Botanic Garden in Hong Kong and his colleagues used mountaineering ropes to build a bridge over a 50-foot-wide gully created by a landslide, the researchers report today in Scientific Reports.

• Eight of the nine gibbon group members used the bridge — mostly by climbing and swinging.
• Adult females and small juveniles used it more than the larger juveniles and adult male, who preferred to jump across the gap instead.

Key stat: Just 30 Hainan gibbon individuals remain, making it "the rarest primate species on Earth," per the paper.