September 8, 2020

expert reaction to study looking at fast growing trees and their role in carbon storage

A study, published in Nature Communications, looked at fast growing trees and their role in carbon storage.

Prof David Lee, Professor of Atmospheric Science, Manchester Metropolitan University, said:

“This study carefully examines the increased rate of tree mortality from increased levels of CO2 – the tradeoff between increased growth from increased CO2 levels and lifespan.

“Currently, Earth System Climate Models predict continuation or increases in the size of the carbon sink of mature forests and this study shows the opposite, that increased CO2 compromises forests as a carbon sink. In the context of combatting climate change, deforestation rates are currently greater than reforestation/afforestation rates, so detailed and careful studies such as this one are vital in determining the viability of future carbon sinks or ‘negative emissions technologies’ to keep temperatures with 1.5 degrees by the end of the century. By extension, the idea that fossil-fuel based emissions can be ‘offset’ by planting trees (or avoiding deforestation) really does not stand up to scientific scrutiny.”

Dr Keith Kirby, Woodland Ecologist, University of Oxford, said:

“The paper confirms what has been noted before in local studies that fast-growing trees tend to die younger than slower grown trees. The consequence projected is that the faster growth (and carbon fixing) will be later offset by earlier mortality and release of the carbon back into the atmosphere. The authors point out that global models of the carbon sink role of forests need to recognise that some of the increases in forest growth rate and increased rates of carbon sequestration observed in recent decades and attributed to CO₂ or nitrogen fertilisation may be transient. If the models are overly optimistic then there is an increased need to curb greenhouse gas emissions.

“However, I think one element in the discussion of the results – suggesting that this phenomenon could necessarily lead to reversal of the global forest carbon sink – is not entirely justified given the findings. As the authors point out the recent ‘net uptake of carbon has been attributed to a combination of afforestation and expansion of secondary forests, as well as possible changes in forest dynamics due to nitrogen deposition and increases in atmospheric CO2 and temperature’. The new findings affect the contribution from changing forest dynamics but not the first two. We cannot rely as much on increased growth per unit area to maintain and enhance the forest carbon sink potential, but this might be offset by slowing deforestation and increasing the expansion of the extent of forests where this can be done in a sustainable way.

“A second area that is not explored in the paper is how long the dead trees take to decay and release the carbon back into the atmosphere; faster growth matched by increased mortality might still lead to an increased carbon sink if there is a greater build-up of dead wood on the forest floor. Forests are important in carbon sequestration, but their function is not as straightforward as sometimes presented, as this paper shows.”

‘Forest carbon sink neutralized by pervasive growth-lifespan trade-offs’ by R. J. W. Brienen et al. was published in Nature Communications at 16:00 UK time on Tuesday 8^th September.

DOI: https://doi.org/10.1038/s41467-020-17966-z

Declared interests

Prof David Lee: I am funded by the EU under H2020 and the Department for Transport for technical advice relating to aviation and climate change

Dr Keith Kirby: No conflict of interest to be declared