Secondary forests—those that regrow naturally after being cleared or degraded—constitute more than half of existing tropical forests. When they are old enough, they support a wide range of species and store carbon at a higher rate than old-growth forest because the trees grow more rapidly.
But in southern Costa Rica, a country with strong environmental commitments, young forests are ephemeral, a new study claims.
When Leighton Reid, a restoration ecologist at the Missouri Botanical Garden in St. Louis, and his colleagues studied the history of secondary forests in a region of Coto Brus canton, Costa Rica, they found that half of them were re-cleared within 20 years. And within 54 years, 85 percent of these young forests were gone, the team reported recently in Conservation Letters.
"I wasn't expecting this at all," says Reid, who has worked in Costa Rica for more than a decade studying tropical forest restoration. "I was shocked."
Previous estimates have suggested that 20 years is not enough time for a secondary forest to regain old-growth levels of biomass and biodiversity.
To study the persistence of these young forests, researchers used aerial photographs taken from 1947 through 2014 across a 123.5-square-mile area in Coto Brus. The photos have a spatial resolution of 10 meters, sharper than the 30-meter resolution of Landsat satellite data other scientists have used to assess forest persistence.
Detailed analysis of images from five time points revealed that young forests in the area vanished again at a rate of 2.2 percent to 3.5 percent per year. Similar research has shown even higher clearance rates in northern Costa Rica and in other Latin American forests—for instance, as high as 23 percent per year in central Peru, according to a 2017 study.
Naomi Schwartz, a forest ecologist at the University of Minnesota and lead author of the Peru study, is not surprised by the Costa Rica results. "The general finding is consistent with what we found," she says. "You can't just assume that a new forest that emerges will be still there in 10, 20, or 50 years."
The forests might be cleared primarily for agriculture and livestock, Reid says. However, his team would like to collaborate with social scientists in Costa Rica to study why the forests are destroyed again so quickly. "It's an important missing component of our study," he says. He did note one positive finding: Secondary forests near rivers in the study area were left untouched.
The results are not universally accepted within Costa Rica. "The study is restricted to a very small portion of the area of the country, which might not be representative of reality," writes Luis Guillermo Acosta Vargas, a forest engineer at the Costa Rica Institute of Technology, in an email. He added that the results also contradict the country's data of overall recovery of forest coverage, which indicates that Costa Rica has increased its forest area since the mid 1980s.
Reid agreed with Acosta Vargas' remarks, but he stood by his findings: "We do need more data, but so far two out of two studies show that secondary forests in Costa Rica are often cleared when they are still young."
The country implemented a ban on deforestation of mature forests in 1996. However, secondary forests were not protected until 2016, two years after the study's time series ended. Landowners might have cut down the forests before they reached maturity to keep using the land, according to Reid. "Time will tell whether this legislation changes the decades-long pattern of rapid re-clearing," he says.
Costa Rica made an international commitment to restore one million hectares of degraded land by 2020, according to the Bonn Challenge. But in light of the new results, Reid would like to see those commitments reshaped into long-term compromises through 2120 to help secondary forests persist.
Schwartz agrees. "Countries need to work hard to make sure that reforestation, forest restoration, and natural regeneration are a big part of their effort to mitigate climate change," she says. "They need to make sure these forests stick around."
This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.