Tropical forests cannot recover naturally without fruit-eating birds, carbon recovery study shows

New research from the Crowther Laboratory at ETH Zurich illustrates a critical barrier to the natural regeneration of tropical forests. Their models – based on ground-based data collected in Brazil's Atlantic Forest – show that when wild tropical birds move freely across forest landscapes, they can increase carbon storage for regenerating tropical forests by up to 38%.

Fruit-eating birds such as the red-legged honeycreeper, palm tanager or rufous-bellied thrush play an important role in forest ecosystems by ingesting, excreting and dispersing seeds as they move through the forested landscape.

70% to 90% of tree species in tropical forests depend on animal seed dispersal. This initial process is necessary for forests to grow and function. While previous studies have established that birds are important to forest biodiversity, researchers at the Crowther Lab now have a quantitative understanding of how they contribute to forest recovery.

The new study, published in the journal Natural climate change Provides evidence of the important contribution of wild birds in forest regeneration. The researchers compared the recoverable carbon storage capacity of landscapes with limited fragmentation to highly fragmented landscapes. Their data show that highly fragmented landscapes restrict bird movement, thereby reducing carbon recovery potential by up to 38%.

Across the Atlantic Forest in Brazil, maintaining at least 40% of the forest is critical, the researchers found. They found that distances of 133 meters (roughly 435 feet) or less between forest patches ensure that birds continue to move across the landscape and aid in ecological recovery.

The study also found that different bird species have different impacts on seed dispersal. Smaller birds spread more seeds, but they can only spread smaller seeds from trees with lower carbon storage capacity. In contrast, large birds such as the Togo toucan or the curl-crested jay disperse the seeds of trees with high carbon storage capacity. The problem is that large birds are less likely to move through highly fragmented landscapes.

„This important information helps guide active restoration efforts, such as tree planting, in landscapes that fall below the forest's edge, where assisted restoration is most urgent and effective,” said Daisy Dent, lead scientist at the Crowther Laboratory at ETH Zurich.

Restoring functioning ecosystem services

„Allowing large carnivores to freely roam forest landscapes is critical to healthy tropical forest recovery,” says Carolina Bello, a postdoctoral researcher in the Crowther lab at ETH Zurich and lead author of the study. „This study demonstrates that bird-mediated seed dispersal plays a fundamental role in determining species resilience, particularly in tropical ecosystems.”

Based on the present data, this study improves on the research of previous field studies conducted by the authors in the Atlantic Forest in Brazil. The forest is one of the most biologically diverse parts of the world, but it is highly fragmented, with only 12% of the original forest remaining and in small areas.

Forests are one of the most important areas on the planet for large-scale ecosystem restoration, with 12 million hectares targeted for restoration and nature restoration under the Atlantic Forest Restoration Treaty. Research shows that increasing forest cover above 40%, as previously demonstrated, may be important not only for maintaining species diversity, but also for maintaining and restoring the function of ecosystem services such as seed dispersal and carbon storage. A massive restoration effort in the region.

„We've always known that birds are essential, but finding out the magnitude of those effects is significant,” says Thomas Crowther, professor of ecology at ETH Zurich and senior co-author of the study. „If we can restore the complexity of life within these forests, their carbon storage capacity will increase significantly.”

Strategies for Tropical Forest Restoration

Previous research suggests that reforestation can sequester more than 2.3 billion metric tons of carbon in the Atlantic Forest, and that natural regeneration can be more cost-effective—77% less in implementation costs than active planting.

The researchers note that a variety of strategies, such as planting fruit trees and preventing predation, can improve animal movement in tropical areas where passive restoration is high. Active restoration is essential in highly fragmented landscapes.

„By identifying forest boundaries in the surrounding landscape that allow seed dispersal, we can identify areas where natural regeneration is possible and areas where we need to actively plant trees,” says Daniel Ramos, University of Exeter, UK and Universidade Estadual Paulista, Rio Claro, São Paulo, Co-author of the article with Brazil.