Over the next decade, the world is set to get a lot cleaner. Many countries with help from donors have launched ambitious programs to restore forests in places where they were chopped down. The European Union and 26 nations pledged $16 billion in support of forests at the U.N. Climate Change Conference in Egypt. A lot of money will be spent on trees.

Susan Cook- Patton is a restoration researcher at the Nature Conservancy. There is an opportunity to really restore forests at scale. It's not known how best to accomplish that.

The amount of forest increased by 1.3 million square kilometers between 2000 and 2020, with China and India leading the way. Plantations are dominated by a single species that is less beneficial for the environment than natural forests.

Many reforestation projects focus on the number of trees planted rather than how well they survive, how diverse the forests are, or how much carbon they store. Laura Duncanson is a researcher at the University of Maryland who studies carbon storage in forests.

There are 20 articles in the theme issue of the Philosophical Transactions of the Royal Society. The challenge is detailed in one in-depth look at the projects. Lindsay Banin, a forest ecologist at the UK Centre for Ecology & Hydrology, and her colleagues examined data on how well newly planted trees survived at different reforested sites. In some places, less than one in five were still alive.

According to the study, when seedlings are planted near mature trees, an average of 64% survive. Measures such as fencing out cattle and improving soil conditions can increase the chances of survival, but they can be expensive.

A couple of species that are easy to grow can help. Duncanson says that these species pave the way for others to settle in on their own. According to the study, the early species should be native to the area, grow fast, kill weeds, and attract animals. Bleeding heart is a rainforest shrub used as a kick-start in Australia. Its green fruits attract animals that can spread seeds and it has roots that loosen the soil.

The location of the plant is also important. The wastelands left behind by closed tin mines in Brazil have been studied by an Ecologist. They report that trees are hard to grow on piles of tailings, where the soil layers are disrupted and toxic, and that planted trees do better in mining pits.

A possible cost-saving measure is not to replant an entire site, but to set out a small amount of plants around which a new forest will grow. Andy Kulikowski and Karen Holl of the University of California, Santa Cruz compared 13 experimental sites in Costa Rica and found that applying nucleation can promote regrowth of a diverse forest. Robin Chazdon of the University of the Sunshine Coast is a co-editor of the special issue on trees like that. Forests can recover on their own. A former pasture in northern Costa Rica has been monitored since 1997. There is a forest springing up.

A project's impact on local people is an important factor. Local communities can be compensated if the new forest provides timber, wildlife hunting opportunities, and other sources of income. Banin says that restoration needs to be beneficial to the local community.

Two USC scientists, Robin Loveridge and Andrew Marshall, studied the well-being of people involved in a project in east Africa. The satisfaction of people who sell timber certified as sustainable was compared to the satisfaction of people who didn't have a sustainable program. The team found that the better the forest was managed, the happier the people were. Cook-Patton says that it's not just about getting the ecological dynamics right.

Marshall is a co- editor of the theme issue. They range from the role of lianas and vines to how to measure success and manage the projects, to how to protect the plants from storms. Local conditions will affect the answers. Bill Laurance, a forest ecologist at James Cook University, says that you can have a trillion dollars.

Simon Lewis of University College London is excited about the new trees but worried about their quality. There is a chance that old-growth forests will be cut but are replaced elsewhere as countries try to meet tough targets on cutting down trees. A high-carbon, high-biodiversity forest is replaced by a lower carbon forest.