Trees can provide many benefits to animals and humans, but some trees planted in the tropics can do more harm than good. New research reveals that 92% of new tree plantations planted in the tropics between 2000 and 2012 were in biodiversity hotspots and 14% in arid biomes, where trees are unlikely to thrive and risk dying. damage existing ecosystems. Tree plantations had also encroached on 9 percent of accessible protected areas in the humid tropics, such as national parks. Especially because 45% of reforestation commitments under the Bonn Challenge take the form of tree planting, it is essential to understand their full impact.
Trees store carbon, filter the air, create habitat and provide a host of other benefits to animals and humans. Planting the right trees, in the right places, in consultation with local communities, can support goals such as tackling climate change and improving lives. However, new research led by Matthew Fagan, assistant professor of geography and environmental systems at UMBC, reveals that some trees planted in the tropics may do more harm than good.
The study, published in Natural durability, examined the increase in tree cover in the global tropics between 2000 and 2012. Fagan and colleagues found that, surprisingly, gains in tree cover over this period were also attributable to natural forest regrowth and the creation of tree plantations. The most common tree planting species were rubber, eucalyptus and oil palm.
Tree plantations aren’t always bad for the environment, and even the much-maligned oil palm can be grown sustainably, Fagan says. However, the study found that 92% of new tree plantings were in biodiversity hotspots, threatening a range of plant and animal species. In addition, 14 percent of plantations were in arid biomes, where trees are unlikely to thrive and are likely to damage existing ecosystems. And tree plantations had encroached on 9% of accessible protected areas in the humid tropics, such as national parks.
“Environmentalists have been sounding the alarm about this for over a decade,” Fagan said. “But no one has had hard numbers on the scale of what is actually happening.”
When planting trees loses
In recent years, dozens of nations have pledged to restore large tracts of forests. Tree plantings represent 45% of the commitments of the Bonn Challenge, an international initiative aimed at restoring degraded and deforested landscapes. But Fagan fears that these plantations could have unintended consequences.
For example, China has undertaken a massive effort to plant trees on the edge of the Gobi Desert, and many African countries have committed to planting trees at the transition between the Sahara Desert and the Sahel grasslands. The objective is to prevent the expansion of the desert, but the plantations can cause damage. Disturbing the soil releases carbon, and trees are heavy users of water. They end up “killing the grasslands that were there, and then they often die of drought,” says Fagan. In these situations, tree planting loses out.
Similarly, in Brazil, soybean farmers have moved from the Amazon to the Cerrado, one of the largest savannahs in the world. Pine and eucalyptus farms followed. The Cerrado is home to rich biodiversity, and the carbon it stores underground rivals the carbon sequestration of the rainforest, Fagan says. Tree crops in the Cerrado may count towards Brazil’s commitment to reforestation, but could actually be a step backwards in mitigating climate change and biodiversity loss.
“In the United States, we have a large area of relatively humid woods, and we tend to idolize tree planting as some sort of ultimate environmental act,” Fagan says. “But there’s a lot of value in grasslands and savannahs that we don’t necessarily see. And when you plant trees, you basically destroy that ecosystem.”
In response to his team’s new research, “I would really like to see governments around the world reevaluate their restoration plans,” Fagan says, “or at least be more transparent when their plans involve planting trees, especially in areas that may not be suitable for planting trees.”
Park or plantation?
Fagan’s new article also revealed how tree plantations are encroaching on protected areas. The problem was so severe that he had to revise the algorithm used by his team to differentiate between data representing natural forest regrowth and tree plantations.
Initially, the algorithm used park boundaries as an approximation of natural forest regeneration areas. But it wasn’t working. To figure out what was wrong, Fagan inspected 20 parks and found that three of them contained multiple plantings. It made him curious.
Six weeks later, he had manually checked the plantings in each park in the tropics. When he found plantings, he either redrew the boundaries of the park or, if the park was too compromised, removed it from the data altogether. Using the new resulting maps, the algorithm was able to detect the natural regrowth of forests versus tree plantations with over 90% accuracy.
“It was very disturbing to see that there were so many parks that were compromised,” Fagan says.
The new maps allowed the team to find many more regrowth areas and plantings than the government had expected. Several UMBC undergraduate students are authors of the article because of their contributions to this data analysis. Each student manually checked at least 1,000 patches, some as many as 3,000.
“Ultimately, the tropics are a much more altered place than we expected,” Fagan says. “There’s a whole host of reasons why we’re seeing these encroachments, but they’re definitely happening all over the world. We’re seeing constant erosion of these parklands by plantations, and the industry is just getting started.”
reason to hope
At the start of this research, the team had a simple question: how many trees are planted in the world? “It seemed a strange thing not to know,” Fagan says. As the work progressed, they asked if trees were being planted where they shouldn’t be and if the plantings were expanding into the parks. They found the results concerning, but they also have reason for hope.
Trees can do a lot of good, and planting more of them can be an important factor in coping with the impacts of climate change. But you have to do it well. “This paper shows that it is possible to monitor natural forest versus plantation on a global scale,” Fagan says, “so that we can encourage the results we want and discourage the results we don’t. “
He also hopes the results will inspire everyone to be more aware of where their products come from – from paper to food, from shampoo to tires – and to demand that companies producing these products from tree plantations adopt more sustainable practices.
“If we make our choices en masseit changes the direction in which these companies are going,” Fagan says. And despite our love for trees in the United States, he notes, forests aren’t the only ecosystems that can help mitigate the effects of climate change and biodiversity loss. grasslands also have an important role to play.
“We need to be aware that not all tree planting is beneficial to the ecosystem involved,” says Fagan. “The right tree in the right place is the right answer.”
