What is assisted natural regeneration and where does it work? 

When you think about restoring a deforested patch of land, you likely picture someone digging holes in the ground and planting seeds or saplings. Planting trees is important (if it’s done the right way), but in many cases, it’s better and cheaper to let trees grow on their own and forests restore themselves with little human assistance.

In fact, there is a wide spectrum of approaches to restoring forest landscapes. At the passive end, there is spontaneous regeneration, where trees and other native vegetation regrow naturally on the land. Think about what happens after a normal forest fire: after some time, little shoots pop up from the ash and grow into big, strong trees without any help from people.

On the other end of the spectrum is the most commonly known approach, where people collect seeds from the forest, produce saplings in nurseries, and plant and maintain trees in degraded forests and lands.

Between those two extremes lies a particularly promising and viable approach: assisted natural regeneration, or ANR.

Assisted natural regeneration is a blend of active planting and passive restoration, where local people intervene to help trees and native vegetation naturally recover by eliminating barriers and threats to their growth, leaning on their knowledge of the land and on ancestral traditions.

But what exactly can people do to help the land and limit the frequency and severity of disturbances that can harm young trees and prevent them from growing?

To prevent the spread of wildfires, people can build firebreaks and clear the forest floor of dry debris. To stop cattle from munching on saplings, they can build fences to keep them out. To give native trees enough room to grow, they can remove invasive grasses and shrubs. And, if natural regeneration on its own does not increase tree cover quickly enough or the targeted species fail to pop up on their own, people can selectively plant trees to fill the gaps.

So why is ANR an important technique in the ecosystem restoration toolkit? And what are its specific advantages? Research shows that ANR is a cost-effective, nature-based solution for restoring millions of hectares of land, while providing the ecosystem services — like clean water and healthy soil — that people need to thrive.

Assisted natural regeneration can play a critical role in achieving global targets on climate and biodiversity, including the Paris Climate Agreement, the Trillion Trees initiative, and ambitious targets set by the Bonn Challenge — which aims to restore 350 million hectares of degraded and deforested lands by 2030.

More than 60 countries have already committed to revitalize 210 million hectares (or 519 million acres, more land than all Saudi Arabia) through the latter. That may sound like an astronomically large area, but it’s still only a small portion of the more than 860 million hectares (2.1 billion acres) across the tropics — an area the size of Brazil — that could benefit from restoration.

Achieving those massive goals is simply impossible without embracing the power of regrowth. Why?

First, trees and forests can be restored using ANR at less than a third of the cost of tree planting, based on WRI estimates.

Second, ANR can restore much more land, much more quickly than active planting, as it requires very little human intervention. In Niger in Africa’s dry Sahel region, farmers have used ANR to regenerate more than 200 million trees since the 1980s.

Third, in areas where human pressure on the land is strong, especially from cattle grazing and agricultural expansion, ANR is the only way to ensure that the restored land effectively mimics the native habitat of local plants and wildlife. When ranchers manage their land more sustainably and graze their cattle more efficiently without degrading the land, they can fence off areas near biodiverse protected areas to help nature regrow without hurting their bottom line.

Fourth, it can create jobs and bring income to landholders, as someone has to protect the new recovery by building fences, patrolling for fires, keeping cattle out, collecting seeds and producing seedlings, and monitoring progress. It’s also cost-effective: Restoring 21.6 million hectares (53 million acres) with ANR in Brazil alone could reduce the cost of action by $90.6 billion, or 77%, when compared to tree planting.

Finally, for farmers, pastoralists, ranchers and local communities, ANR is an important weapon for fighting climate change. Recently published research found that letting forests regrow naturally can absorb 23% of the world's CO2 emissions every year — an estimate that is 32% higher globally than the Intergovernmental Panel on Climate Change (IPCC)’s latest numbers. Naturally regenerating those 21.6 million hectares in Brazil, for example, could sequester more carbon than the annual emissions of Brazil and Canada, combined.

As investors are increasingly interested in funding nature-based solutions to the climate crisis, assisted natural regeneration is too good an opportunity to pass up.

Where Does Assisted Natural Regeneration Work Best?

Assisted natural regeneration doesn’t work for every landscape; it’s critical to assess the local context. For example, ANR works best in areas that are not highly degraded but are surrounded by forest remnants and where seeds are living in the soil. Where intensive farming and overgrazing have not heavily degraded or compacted the soil, tree planting usually makes more sense.

In practice, this means ANR works where there those conditions are met and local landowners are incentivized to do it. That’s often where there are lower opportunity costs to returning grazing land or cropland to nature.

Farmers are unlikely, for example, to give up their richest and most productive land unless they are heavily compensated. That means that landowners embrace ANR on land poorly suited for agriculture, hilly and rocky terrain, former agricultural fields close to forest remnants, remote landscapes far away from roads and towns, marginal farmland where yields have declined for years, and areas that have not been plowed recently.