Young Regrowing Forests Found to be Major Terrestrial Carbon Sinks

A study at the University of Birmingham shows that more than half of the carbon sink in the forests across the globe is in the regions where trees are comparatively young—less than 140 years—and not in the tropical rainforests.

These trees usually “regrow” on land used earlier for agriculture or cleared by harvest or forest fire. One of the main drivers of this carbon uptake is their young age.

Forests are widely regarded as major carbon sinks—ecosystems that have the ability to capture and store large amounts of CO₂—but dense tropical forests, near the equator, are found to be working the hardest to absorb these gases.

In a new study reported in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), scientists at the Birmingham Institute of Forest Research (BIFoR) have used both data and computer modeling to perform a fresh investigation of the global biosphere.

They examined the amount of carbon uptake from 2001 to 2010 by old, established areas of forest based on their data sets of forest age and compared it with that of the younger areas of forest that are regrown on regions that have experienced human activities such as logging or agriculture and natural disturbances such as fire in the past.

Previous studies seemed to suggest that the carbon uptake by forests was mainly due to fertilization of tree growth by raising levels of CO₂ in the atmosphere.

But the researchers discovered that regions where forests are regrown sucked up more amounts of carbon due to both their younger age and these fertilization effects. The age effect accounted for nearly 25% of the total CO₂ absorbed by forests. In addition, this age-induced carbon uptake mainly took place in the middle- and high-latitude forests and not in the tropics.

For instance, these forests include areas of land in eastern states of the US, where settlers established farmlands but left them to migrate west by the end of the 19^th century. This abandoned land became part of the US National Forest, in addition to further tracts of land left during the Great Depression in the 1930s.

Boreal forests of Europe, Russia, and Canada, which have experienced considerable harvest activity and forest fires, are other important areas of forest regrowth. In China, the large-scale reforestation programs also make a major contribution to carbon sink.

It’s important to get a clear sense of where and why this carbon uptake is happening, because this helps us to make targeted and informed decisions about forest management.

Dr Tom Pugh, Birmingham Institute of Forest Research

The study emphasizes the importance of forests in the planet’s temperate zone for mitigating climate change and obviously shows how much carbon can possibly be taken up by these regrowing forests in the future. This is specifically crucial because of the short-lived nature of forest regrowth. It is to be noted that once the existing pulse of forest regrowth works its way through the system, this main part of the carbon sink will vanish, unless additional reforestation occurs.

The amount of CO₂ that can be taken up by forests is a finite amount: ultimately reforestation programmes will only be effective if we simultaneously work to reduce our emissions.

Dr Tom Pugh, Birmingham Institute of Forest Research

This study was funded by the European Commission.