Forests are retaining carbon for less and less time

Published in the journal PNAS (Proceedings of the National Academy of Sciences), a study to which the Spanish National Research Council (CSIC) and CREAF have contributed has revealed that the time for which forests retain carbon has fallen by between 0.2% and 0.3% every year in recent decades. Plant mortality is rising due to higher carbon dioxide (CO₂) levels and, in particular, warming and droughts. The time in question, which is termed carbon turnover time, is a relevant factor in climate change projections as it is indicative of carbon sinks’ capacity to retain carbon.

Forests play a crucial role in carbon retention, but for how long can they retain it? While climate change scenario predictions take forests’ CO₂ sequestration capacity into account, the amount of time for which vegetation can retain the gas (and, thus, carbon) before it is released back into the environment (due to plant death and decomposition) is currently unknown.

An international study led by Kailiang Yu from the University of Utah (USA) and published in PNAS has attempted to shed some light on the matter. It involved scientists, including CSIC and CREAF researchers Josep Peñuelas and Jordi Sardans, analysing data spanning 1955 and 2018 from 695 forests in tropical, temperate and cold climate zones, and comparing it to Earth system model simulations.

There is an inverse relationship between atmospheric CO2 and carbon turnover time.

The study’s results show there to be an inverse relationship between atmospheric CO₂ and carbon turnover time, with the latter dropping as levels of the former rise. Specifically, carbon turnover time has fallen by between 0.2% and 0.3% a year over the last three decades.

“Factors such as warming and drought are reducing the time for which plants retain carbon”, explains Josep Peñuelas. “The temperature causes an increase in the metabolism of organisms, as well as increased precipitation in some areas, but drought results in greater mortality and, consequently, a decline in turnover time”, he continues. “In general, tree mortality has risen in all the climate zones we studied”, he remarks.

The annual drop of between 0.2% and 0.3% in forests’ carbon turnover time is significant because, as Peñuelas points out, “it entails an overall fall of as much as 9% in 30 years”. The study’s results suggest that forest carbon sinks are likely to be constrained by a decrease in the time for which vegetation can retain carbon.

Referenced article

Kailiang Yu, et al. Pervasive decreases in living vegetation carbon turnover time across forest climate zones, PNAS. www.pnas.org/cgi/doi/10.1073/pnas.1821387116