Less than half of world’s humid tropical forests have high ecological integrity: Study

Only a tiny fraction of the world’s most pristine tropical forests are protected from destruction, according to a recent study published in Nature Ecology and Evolution.

Humid broadleaf tropical and sub-tropical forests cover just 14% of the Earth’s land surface, but support at least half of the world’s species and provide important ecosystem services like carbon sequestration and water cycling. This makes these ecosystems crucial to meeting global climate and conservation targets such as the Paris Climate Agreement and the Convention on Biological Diversity (CBD).

However, the study found that only 47% of the remaining humid tropical forests globally have high ecological integrity — possessing tall trees and closed canopies with limited human activity — a total area of 1.9 million hectares (7,336 square miles). These tend to be older forests with larger trees and more canopy layers, which harbor high levels of biodiversity, provide high quality ecosystem services like carbon storage, and tend to be more resilient to a changing climate. Just 6.5% of these high ecological integrity forests are under legal protection.

“Tall forests with closed-canopies and low human pressure typical of natural conditions comprise half of the global humid tropical forest estate, largely limited to the Amazon and Congo basins,” said Andrew Hansen of Montana State University who led the study. “The vast majority of these forests have no formal protection,” putting them “at significant risk.”

To achieve their results, Hansen and his team used high-resolution data collected by NASA’s ICESat-2, and Landsat-7 and -8 satellites to calculate canopy height, tree cover, and time since disturbance, which they estimated using the extent of forest loss between 2000 and 2017. They then combined all of this information into a single measure evaluating canopy structure.

The authors next overlaid data from an index of human pressure — taking into account local human population size, land use, and nearby transport infrastructure like roads — to calculate a new scale: the Forest Structural Integrity Index (FSII), which they used to create a map of the ecological integrity of humid broadleaf forests across the tropics.

“The coolest aspect [of the study’s measuring system] is that it is simple, straightforward, easy to grasp, has a pantropical coverage, and allows decision-makers to have a portfolio of conservation strategies to conserve or improve the quality of the remaining forests,” said Lourens Poorter, a Professor in tropical forest ecology at the University of Wageningen in the Netherlands, who was not involved in the study.

However, Poorter expressed “serious concerns” about the research team’s use of canopy structure as an index of forest degradation, saying that continent- or biome-level comparisons “ignore the role of the biophysical environment in shaping forest structure, thus suggesting that changes in forest structure are only due to [human] disturbance.” He questioned whether the FSII is representative of a forest’s full ecological value, suggesting it may be a poor proxy for species richness, the presence of endemics, or carbon sequestration potential and other ecosystem services. “Tall forests with low human pressure are not necessarily the priority areas to conserve,” he said.

The authors acknowledge that some forests of high conservation importance, such as the lowland forests of Borneo and Sumatra, fell outside of their priority areas in this study. The researchers recommend that policymakers and forest managers use the FSII framework in conjunction with national-level analyses to avoid underprioritizing forests where targeted conservation efforts could be most effective.

A third of tropical forests already lost

Despite the importance of humid tropical forests for biodiversity, carbon storage and water cycling, the study found that only 10% of the remaining forests in this biome are protected legally. Human activities have deforested 33% of the biome, while a further 22% is degraded, with 13% still in good structural condition, but under threat from human activities.

Forest clearance between 2013 and 2019 was slowest in high FSII forests, probably as a result of their current remoteness. However, proposed mega-rail and road schemes in the Amazon and Congo would expand forest access, exposing them to rapid deforestation and degradation, which some experts warn could push tropical forests like the Amazon toward an irreversible tipping point.

“It is very important that we conserve the remaining forests that we have and facilitate restoration through natural forest regrowth on abandoned agricultural areas,” said Poorter. Studies like this one “can facilitate this [need] and put it on the policy agenda.”