Researchers: Mature forests cannot be relied upon to absorb our extra CO2

CO2 is sometimes referred to as "plant food" because of its key role in photosynthesis. With steadily increasing atmospheric CO2 levels caused by human emissions, there is plenty of evidence that plants are photosynthesizing more. In principle, this means that trees and forests have an enormous potential to capture extra CO2 from the atmosphere and help mitigate the impact of climatic changes. However, most previous studies focused on young and individual trees, with much less attention being paid to mature stands of trees.

In the first experiment of its kind conducted on local forests in the southern hemisphere, researchers from Western Sydney University and international researchers from the University of Copenhagen among others, exposed an area of 90-year-old eucalyptus trees to elevated levels of CO2.

As expected, the photosynthesis in mature trees increased. Indeed, they absorbed 12% more CO2 from the atmosphere. But after following the trees for several years, researchers were surprised to discover that there was no extra growth or additional long-term carbon storage.

"This shows that mature trees have a different carbon budget compared to younger ones, and raises the important question 'what happens to the extra carbon?'," says Assistant Professor Elizabeth Jakobsen Neilson of UCPH’s Department of Plant and Environmental Sciences.

CO2 spat back into the atmosphere

After extensive measurements, which researchers from the University of Copenhagen participated in, the researchers were able to see that the extra CO2 absorbed by the trees was quickly released back into the atmosphere after a detour through the soil. About half of the CO2 came from the trees, while the other half was released by microorganisms in the soil.

"We had assumed that the trees would emit some of that extra carbon back into the atmosphere through their leaves. We found a significant increase in terpene emissions (fragrant volatile organic compounds), but this only represented a small portion of the total carbon budget. Instead, most of the additional carbon was returned back into the atmosphere via the soil," explains Elizabeth Jakobsen Neilson, who together with Professor Riikka Rinnan of the Department of Biology was responsible for the terpene emission measurements.

While the trees converted the absorbed CO2 into sugar, they were unable to use the sugar for growth because they lacked access to additional soil nutrients. Instead, they routed the sugar into the soil, where it ended up as food for a variety of microbes.

"The results have global implications, as climate models used to predict future climate changes and their impact on plants and ecosystems assume that growing forests will continue to absorb additional CO2 and thereby serve as CO2 storage. The results of this study indicate that these carbon stores are weaker or completely absent in forests that grow in nutrient poor soils," explains Professor Riikka Rinnan of the Department of Biology.

In short, increased CO2 emissions only increase CO2 storage in ecosystems where there is a preponderance of younger trees or more nutrient-rich soils.

"This is very important knowledge because we are currently relying on mature trees to absorb some of the extra CO2 that we emit. Our results indicate that we have less time to reduce greenhouse gas emissions than we thought," concludes Riikka Rinnan.

The research was published in Nature