Tropical forests are among the best tools in the world to fight against climate change and the loss of biodiversity: they store enormous amounts of carbon, are home to thousands of plants and animals and are the living place of indigenous peoples. who maintain them.
All these reasons inform the commitment made at COP26 in Glasgow at the end of 2021, by more than a hundred world leaders, to end deforestation by 2030.
WWF infographic on imported deforestation © WWF (via The Conversation)
Many organizations and communities are working on their side to restore forests by recovering unproductive or abandoned land to carry out ambitious reforestation programs. These initiatives aim to encourage the return of native plants and animals, and restore the ecological functions and benefits that these forests once provided.
However, and in many cases, these forests can regenerate naturally, with little or no human intervention.
As forest ecologists and members of a network of researchers specializing in the study of so-called “secondary” forests – that is, those that regrow after an area has been cleared and cultivated or grazed –, we published in December 2021 a study in the journal Science where a new methodology allowed us to collect data on more than 2200 plots of naturally regenerating forests in the American and West African tropics.
Our research shows that rainforests regenerate surprisingly quickly: they can regrow on land after agriculture has been abandoned and regain many of the characteristics of old-growth forests, such as soil fertility, tree structure and ecosystems, in just 10 to 20 years.
Protected rainforest of La Trinité National Nature Reserve, Guyana © Olivier Tostain / ECOBIOS CC BY-NC-ND (via The Conversation)
But to support effective forest restoration and planning, it is important to understand how quickly their functions and attributes recover.
Forest regeneration
Most of today’s forests have grown back as a result of human and natural disturbances (fires, floods, logging and clearing for agriculture, etc.). Forests thus “regenerated” in the 18th and 19th centuries in Europe and in the first half of the 20th century in the eastern United States. In the northeast of this country, forest cover is greater than 100 or 200 years ago.
In the tropics, forests are growing back on some eight million square kilometers of former agricultural and ranching land. Scientists and policy makers agree that it is essential to protect these regenerating forests, and to prevent the destruction and conversion of old-growth forests.
Rainforests are not just made up of trees: they are complex and dynamic networks of plants, animals and microbes. Regeneration takes time and often has unpredictable consequences and variable trajectories. In addition, the recovery patterns of tropical rainforests differ from dry forests.
VIDEO: Tropical forests and their environmental role (CIRAD, 2021 / Youtube)
To date, this area of research has focused on studies investigating how specific characteristics of forests, such as the number of species they support or the biomass of trees, change over time and space. We believe that it is important to understand forest regeneration as a process forming a coherent whole, determined by local, geographical and historical conditions.
A multidimensional vision of the recovery of tropical forests
Our study focused on 12 attributes essential to healthy forests:
- The soil: how much organic carbon and nitrogen does it contain, and how compacted is it? Soil that is too compacted (by the hooves of grazing cattle, for example) is difficult for plant roots to penetrate and does not absorb water well, which can lead to erosion.
- Ecosystem functioning: how does the abundance and size of trees change when the forest grows back? What is the role, in forest regeneration, of trees whose roots are associated with nitrogen-fixing bacteria? How does regrowth affect average wood density and leaf tissue durability?
- Forest structure: How do maximum tree height, variation in tree height, and total biomass (the amount of plant matter above ground in the trunks, branches, and leaves of trees) change over time? forest regeneration?
- Tree species diversity and composition: How do the number of tree species present and the patterns of species diversity and abundance change and become similar to nearby old-growth forests?
In Colombia, herd grazing on an old forest plot © Raul Arboleda / AFP (via The Conversation)
To assess long-term recovery rates, we compared the attributes of forests growing on abandoned agricultural land at different times and those of regenerating forests to surrounding old-growth forests. We have also developed a new modeling methodological framework to estimate the regeneration rate of each attribute.
Many of these attributes are interrelated. For example, if trees regrow quickly, they produce a large amount of leaf litter, which restores organic carbon levels in the soil as they decompose. We analyzed these links by comparing the degree of association of forest attributes with each other.
The issue of soil erosion
The forests we studied were in areas with low to moderate land use intensity, i.e. where the soils were neither depleted nor eroded, and therefore favorable for regrowth. native vegetation quickly.
In the Atlantic Forest region of Brazil, for example, 2.7 million hectares of forest regrown naturally between 1996 and 2015. Rainforests are much less likely to recover in areas with heavily over-exploited soils. and where there is no forest left in the vicinity.
All of the forest attributes we looked at regenerated in less than 120 years. Some have regained 100% of their past values within the first 20 years.
The soil attributes that we analyzed thus reached 90% of the values of the old forest in 10 years, and 98 to 100% in 20 years. In other words, within two decades of regrowth, forest soils contained nearly as much organic carbon and had a similar bulk density as those in old-growth forests.
This rapid regeneration took place on soils that were not heavily degraded when the forest began to regrow. As for the attributes of ecosystem function, they have also regenerated rapidly: from 82% to 100% in 20 years.
Chart showing how four groups of forest attributes – soil, ecosystem functioning, forest structure, and tree biodiversity – recover when tropical forests regrow on former agricultural and grazing land. For each category, the image shows the average percentage recovery from old-growth after 20, 40, 80, and 120 years. The percentages in the black squares show the average recovery for the entire forest at each interval © www.2ndFOR.org / Pixels&Ink CC BY-ND (via The Conversation)
Tree size and number of species
Forest structure attributes, such as maximum tree diameter, recovered more slowly. On average, they reached 96% of old-growth values after 80 years of regrowth. Tree species composition and aboveground biomass recovered after 120 years.
We have identified three attributes – maximum tree height, overall variation in tree height, and number of tree species in a forest – which, taken together, provide a reliable snapshot of the degree of regeneration of a forest.
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 at widely varying time scales, thanks to data collected by satellites and drones.
The importance of natural regeneration
Our results show that the regrowth of tropical forests is a natural, effective and inexpensive strategy to promote sustainable development, restore ecosystems, slow down climate change and protect biodiversity.
And since forests that regrow in areas where the land has not been heavily damaged quickly recover many of their key attributes, restoring forests does not necessarily involve planting trees.
Several reforestation methods adapted to the conditions of the sites and the needs of the local population can be implemented. We recommend relying on natural regrowth whenever possible, and only resorting to reforestation measures when necessary.
This review was written by Robin Chazdon, Emeritus Professor of Ecology and Evolutionary Biology at the University of Connecticut (USA); Bruno Hérault, researcher and specialist in tropical forests at the Center for International Cooperation in Agricultural Research for Development (CIRAD, France); Catarina Conte Jakovac, associate professor of plant sciences at the Federal University of Santa Catarina (Brazil); Lourens Poorter, professor of functional ecology at the University of Wageningen (Netherlands).
Masha van der Sande (Wageningen University) and Dylan Craven (University Mayor, Chile) collaborated in compiling and analyzing the data on which this article is based.
The original article was published on the site of The Conversation.
