Written by: Robin Chazdon, Bruno Hérault, Catarina Conte Jakovac, and Lourens Poorter
Tropical forests are among the world’s best tools for fighting climate change and the loss of wild species. They store huge quantities of carbon, shelter thousands of plants and animals and are home to Indigenous peoples who sustain them. That’s why more than 100 world leaders pledged to halt deforestation by 2030 at the recent United Nations conference on climate change in Glasgow.
Many organizations and communities are working to restore native forests by reclaiming unproductive or abandoned land and carrying out costly tree-planting efforts. These efforts are designed to encourage the return of native plants and animals and to recover the ecological functions and goods that those forests once provided. But in many cases forests can recover naturally, with little or no human assistance.
We are forest ecologists and members of a collaborative research network that studies secondary forests – those that regrow naturally after an area has been cleared and cultivated or grazed. In a newly published study in the journal Science, our group pioneers an approach to forest recovery that provides insights from over 2,200 forest plots in naturally regrowing tropical forests across the American and West African tropics.
Our research shows that tropical forests recover surprisingly quickly: They can regrow on abandoned lands and recover many of their old-growth features, such as soil health, tree attributes and ecosystem functions, in as little as 10 to 20 years. However, to support effective forest restoration and planning, it is important to understand how quickly different forest functions and attributes recover.
Forests come back
Most forests around the world today have regrown after human and natural disturbances, including fires, floods, logging and clearance for agriculture. For example, forests recovered in Europe during the 18th and 19th centuries and in the eastern U.S. from the early to mid-20th century. Today the northeastern U.S. has more forest cover than it did 100 to 200 years ago.
Now, across the world’s tropical regions, forests are regrowing on approximately 3 million square miles (8 million square kilometers) of former farm and ranch land. Scientists and policymakers widely agree that it is critical to protect these regrowing forests and prevent more destruction and conversion of old-growth forests.
Tropical forests are more than just trees – they are complex, dynamic networks of plants, animals and microbes. Forest recovery takes time and often has unpredictable outcomes and variable pathways. Recovery patterns differ between wet and dry tropical forests.
To date, this active research area emphasizes studies that examined how specific features of forests, such as the number of species they contain or tree biomass, change over time and space. We believe it is important to understand forest recovery as an integrated process that is shaped by local, landscape and historical conditions.
A multidimensional view of tropical forest recovery
Our study focused on 12 attributes that are essential components of healthy forests. They include:
- Soil: How much organic carbon and nitrogen does it contain, and how compacted is it? Soil that is too densely compacted – for example, by the hooves of grazing cattle – is hard for plant roots to penetrate and doesn’t absorb water well, which can lead to erosion.
- Ecosystem functioning: How does the abundance and size of trees change as the forest regrows? What is the role in forest regrowth of trees that have root associations with nitrogen-fixing bacteria? How does regrowth affect the average density of wood and the durability of leaf tissues?
- Forest structure: How do maximum tree size, variation in tree size, and total biomass – the quantity of plant matter above ground in tree trunks, branches and leaves – change as forests regrow?
- Diversity and composition of tree species: How do the numbers of tree species present and the diversity and abundance patterns of species change and become more similar to nearby old-growth forests?
To assess long-term recovery rates, we compared attributes across forests growing on farmlands that had been abandoned at different times and compared regrowing forests with neighboring old-growth forests. We developed a new modeling approach to estimate how quickly each attribute recovered.
Many of these attributes depend on one another. For example, if trees regrow quickly they may produce a lot of leaf litter, which will restore levels of organic carbon in the soil when it decomposes. We analyzed these connections by comparing how strongly forest attributes were associated with one another.
The forests we studied were in areas of low- to moderate-intensity land use, meaning that soils were not exhausted or eroded and quickly supported regrowing native vegetation. For example, in Brazil’s Atlantic Forest region, 10,425 square miles (2.7 million hectares) of forest regrew naturally from 1996 to 2015. There is much less potential for tropical forests to recover in areas where soils are heavily overworked and no neighboring forests remain.
All of the forest attributes that we examined recovered within 120 years of regrowth. Some recovered 100 percent of their old-growth values in the first 20 years of regrowth.
For example, the soil attributes that we analyzed reached 90 percent of old-growth values within 10 years and 98 percent to 100 percent within 20 years. In other words, after 20 years of regrowth, soils in the forests contained virtually as much organic carbon and had similar bulk density as soils in old-growth forests.
This quick recovery reflects the fact that the soils at our study sites had not been heavily degraded when forest regrowth started. Ecosystem function attributes also bounced back quickly, with 82 percent to 100 percent recovery within 20 years.
Forest structure attributes, such as maximum tree diameter, recovered more slowly. On average they reached 96 percent of old-growth values after 80 years of regrowth. Tree species composition and above-ground biomass recovered after 120 years.
We identified a set of three attributes – maximum tree size, overall variation in tree size and the number of tree species in a forest – that, viewed together, provide a reliable snapshot of how well a forest is recovering. These three indicators are relatively easy to measure, and managers can use them to monitor forest restoration. It is now possible to monitor tree size and forest structure over large areas and time scales using data collected by satellites and drones.
The importance of natural regrowth
Our findings show that tropical forest regrowth is an effective and low-cost, nature-based strategy for promoting sustainable development, restoring ecosystems, slowing climate change and protecting biodiversity. And since regrown forests in areas where the land has not been heavily damaged quickly recover many of their key attributes, forest recovery doesn’t always require planting trees.
In our view, a range of suitable reforestation methods can be implemented, depending on local site conditions and local people’s needs. We recommend relying on natural regrowth wherever and whenever possible, and using active restoration planting when needed.
Header Image Credit: Robin Chazdon, CC BY-ND