The role of trees in the biological functioning of many natural ecosystems cannot be overstated. A healthy forest can retain water to prevent droughts, provide food for local animals and people, and keep planet-warming carbon dioxide from being released into the atmosphere.
Yet they are being felled at a massive scale. Roughly 46 percent of trees have fallen since the start of human civilization, according to a 2015 study, and roughly 3 trillion trees remain in the world.
On the up side, there has been a recent rise in support for reforestation. Initiatives range far and wide: there are apps that gamify tree-planting for its users all the way up to mass-scale international efforts like the Bonn Challenge, which calls for the regrowth of 350 million hectares of forests by 2030.
Governments and other bodies around the world have currently pledged to grow 210 million hectares of trees, which indicates that growing trees is increasingly recognized as an effective way to soak up the world’s carbon emissions.
But how are these trees being grown?
The two main methods of reforestation are essential to meeting global reforestation targets: tree planting and natural regeneration, which is when forests are allowed to regrow naturally, sometimes with some human support. Here is a breakdown of when either technique should be used and their advantages and disadvantages:
Tree planting in support of forest restoration is a contentious topic. Some highlight tree planting as a potent method of soaking in human-made greenhouse gases, whereas others claim that tree planting often results in monocultures of often non-native species that can harm local ecosystems if not done carefully.
Last year, a headline-making report led by Thomas Crowther of ETH Zurich found that there are some 1.7 billion hectares of land – approximately the size of the U.S. – that could be potentially planted with 500 billion trees, drawing down some two-thirds of all carbon emissions released by humans since the Industrial Revolution.
Reports released since have warned against policies that incentivize poorly-designed and -managed tree planting efforts, as well as possible overestimations of carbon storage.
Forestry scientists have responded strongly to this multifaceted discourse with a message of the underlying principle for successful tree planting, which principle scientist at World Agroforestry (ICRAF) Ramni Jamnadass succinctly puts as follows: “plant the right trees in the right place and for the right purpose.”
There are a variety of valid forms of planted trees: planted forests, plantations, intermixing trees with agriculture and livestock through agroforestry or silvopasture practice. Each serves a different purpose and has different benefits, such as the return of biodiversity through planted forests, more resilient food supplies through agroforestry and timber that can more quickly aid the transition to a bioeconomy through plantations and, ideally, take pressure off natural forest resource mining.
“The Bonn Challenge considers a mix of interventions. Tree planting is probably the fastest in the context of seeing results,” says Manuel Guariguata, a principal scientist at the Center for International Forestry Research (CIFOR) on tropical forest ecology and forest management for production and conservation. “It’s probably the favorite option because of the control you have over the species you plant.”
“You need to invest in infrastructure,” he further explains, noting the costs for the start-up infrastructure such as nurseries, fertilizers, even the seeds and seedlings themselves. “You may need to transport seedlings from one site to another, and if you don’t [invest in infrastructure] well, your seedlings will die when they arrive [at the planting site] because of the mechanical vibrations in the car or truck.”
Despite the challenges, tree planting can lead to economic growth, particularly through the creation of jobs. However, investing in local people is vital, Guariguata says, because their knowledge of the landscape is crucial to incorporate into the growing process to ensure the trees’ survival, and their labor is necessary to plant, raise and maintain the trees.
Meanwhile, new technologies are also being developed to enable efficient tree planting on a larger scale. Lauren Fletcher, inventor of tree-planting drone technology and the founder of sustainable solutions lab BetaEarth.global, noted during a recent digital forum on tree planting that drones can disperse seeds or seed balls across an area following a specified pattern, potentially planting up to 400,000 trees per day. A team without drones might plant hundreds in the same time.
“Handplanting is always going to be part of the solution. But let’s give those hand-planters a new tool that makes them more effective and more efficient, that benefits everybody around the world,” says Fletcher.
As with any change in landscape, how trees are planted and their long-term risks must also be taken into account. For example, in 2016, the Canadian town of Fort McMurray suffered a devastating wildfire that became one of the costliest natural disasters in the country’s history. When the original surrounding peatlands were drained and black spruce trees were planted for timber, the water table dropped. The new trees’ wider canopies killed off the original flame-retarding peat moss, which was replaced with another drier moss that acted as kindling. When the wildfire hit, much of the carbon that was stored in the trees and dried peat was released into the atmosphere.
“Unfortunately, many tree planting programs just plant tree species that are easily accessible and available, and very often this results in planting species that are mismatched to the environmental ecology, lack diversity and don’t address the socioeconomic needs of local communities – thereby spelling a disaster,” says Jamnadass.
Tree planting restoration projects should make sure to use native species when available and appropriate to reforest a site to prevent such disasters. Ezra Neale, who directs monitoring and evaluation at Eden Reforestation Projects, said that the organization, which plants forests around the world, pays special attention to selecting native trees for reforestation.
“For example, the restoration of dry deciduous forest in Madagascar offers habitat to many endemic and endangered species such as the Coquerel’s sifaka lemurs (Propithecus coquereli), golden-crowned sifaka (Propithecus tattersalli), and Perrier’s sifaka (Propithecus diadema perrieri) as well as the island’s largest predator, the fossa (Cryptoprocta ferox),” Neale wrote by email.
Speed, commodities, timber, economic growth – the benefits of tree-planting are unique in comparison to natural regeneration, which rather boasts the opposites. The time it takes to regrow a forest naturally translates into higher levels of biodiversity and native plant species; a study found that naturally regenerated forests contained more biodiversity in plants, birds and invertebrates and a more restored vegetation structure than those of planted forests in tropical regions.
Naturally regenerated forests also tend to have more secure carbon storage. A recent report in Nature states that the carbon-capturing potential of naturally regrown forests may have been underestimated by the Intergovernmental Panel on Climate Change (IPCC) by 32 percent.
Allowing forests to grow naturally can be technically simpler and much cheaper, and in time, allows local people to use the forests in the way of their tradition.
However, in some cases, natural regeneration cannot work, depending on the level of degradation of the particular area and whether any natural seed dispersers can reach it. For example, the seeds of many tree species are also spread by native animals, such as birds. In areas where these animal species have disappeared, these trees will have to be planted.
Joice Ferreira, a researcher at Embrapa Amazônia Oriental, says that large parts of the Amazon are conducive to natural regeneration because many areas have a fairly recent history of land-use change, and the types of agriculture that has been used on this converted land have been very low-intensity, such as traditional agriculture and extensive pastures. The Amazon’s abundant rainfall and favorable temperatures are also a massive aid to re-growth.
However, she notes that the capacity of natural regeneration in land that underwent mechanized agriculture remains uncertain.
The longer and more organic process means that land where forests are regenerating will appear “abandoned” for a long time before a mature forest is regrown. “In many cases, natural regeneration is seen as something undesirable. It suggests that the land is not being worked,” says Guariguata.
“It doesn’t look very nice,” he adds. “I mean, it’s not an evenly planted row of trees. It can be messy. A tree plantation is ordered and gives a different impression, so there’s a perception issue there.”
Forests should only be regrown through natural regeneration, he says, if the land is not under pressure for development (such as on very steep slopes) and if the manager of the land can ensure that the forest will remain for a long time.
Ferreira says that deforestation rates are 40 percent higher in secondary forests than in primary forests, and that more government regulations are needed to protect forests that are naturally regenerating.
Yet despite the challenges, areas of the world are naturally regenerating. Parts of Europe have been naturally regenerating their forests as farmland is abandoned, often because of economic factors and a decrease in rural populations. This has been accompanied by growth in the populations of large mammals, such as those of bears, lynxes, boars and deer.
The best method for regrowing a forest depends on the condition of the landscape, the purpose of the forest, and the tree species that are available for reforestation. Regardless of which method is chosen, the local community must agree with how the land is used.
“Social acceptance and having a shared vision across different actors in the landscape is key,” says Guariguata. “If you don’t have that precondition, most likely those trees will not survive. They may not get fertilized or watered because people don’t see a long-term benefit in [them].”
Jamnadass says that in Kenya, many trees have been cut from watersheds for charcoal, which is then used for cooking or selling. “The government really needs to understand what communities still need. They need energy, so they’re going to keep cutting trees unless their energy needs are met [in other ways].”
Government policy must also support reforestation policies, such as by creating protected zones and effectively punishing people who breach these protections, including those who destroy forests to create space for agriculture or log trees illegally.
“Reforestation projects are only possible through these strong partnerships with the community and the government,” says Neale.