If you’ve ever been offered the opportunity to donate money to plant a tree to offset a purchase, you’ve probably guessed that trees naturally store carbon. Forests are an invaluable source of carbon storage around the world. Many people assume that forests that have been cut down but are regrowing trees are also regaining their capacity to store carbon.
That assumption may not be completely correct. Forests that are regrowing trees after being cleared may actually be emitting more carbon dioxide than they store for up to 10 years after they were initially cleared, according to a study published this month in the Proceedings of the National Academy of Sciences.
Tropical forests are some of the world’s most important carbon sinks: Research has found that the world’s tropical forests contain around ¼ of the carbon stored on land on this planet. Unfortunately, aggressive deforestation to harvest trees and other resources and make room for industries like agriculture and mining has destroyed much of the carbon-storing abilities of these forests.
It’s long been assumed that regrowing trees in these areas after logging would help restore their status as carbon sinks. Many previous studies of logged forests focused on the amount of carbon regrowing trees are able to store, without considering other aspects of the forest. For this study, the team wanted to check this assumption by comparing the carbon-storing abilities of the new trees against the amount of CO2 being released into the atmosphere by the soil disturbed during the logging process and dead wood from logging decomposing on the forest floor.
The researchers surveyed logged land in the Malaysian part of Borneo, Asia’s largest island with a wealth of biodiverse rainforests, between 2011 and 2017, using two different techniques to measure carbon emissions from the forest floor and above the tree canopy. The land inspected in the study was in various stages of growing back trees after it had been logged. The researchers also monitored unlogged forests as a control.
The results are kind of like seeing a negative balance on your checking account which is especially disappointing because we should be in the black with carbon. The unlogged forests the team measured were either carbon-neutral or small carbon sinks. However, the areas that had been logged recently were actually carbon sources. In fact, they emitted more carbon than they stored, despite the new trees growing back. While this study only looked at one area, “the potential implications are serious” for the rest of Earth’s forests.
Fortunately, the study’s authors believe that there are some straight-forward techniques, known as reduced impact logging methods, to help logged areas emit less carbon. Loggers can pre-plan trails and tree extraction directions, which can help minimize soil disturbances. Meanwhile, being more precise about which trees to extract can cut down on tree waste on the forest floor, as can cutting down vines before chopping the trees because vines can inadvertently pull the crowns of nearby trees down with them and create more dead wood on the floor. This is not helpful for reducing carbon emissions.
Even if logging companies take all these precautions, however, the study is a reminder of how dire the situation is for the world’s rainforests and the globe’s ability to store carbon emissions. It’s also a lesson in how unpredictable calculating the capacity of carbon sinks or emissions from natural resources like forests can be—and, as the world’s carbon markets ramp up and companies begin making green pledges to plant trees in faraway places, it’s an important thing to keep in mind.
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