Trees in tropical logged forests release carbon at greater rate despite faster growth, study finds

Tree stems in tropical forests recovering from logging produce carbon dioxide at a greater rate than those in unlogged forests, according to a new study from the University of Leicester. 

With fewer surrounding trees to compete with, the tree stems in logged forests are able to grow and take up carbon dioxide at a faster rate than stems in unlogged forests, but this faster growth is combined with faster release of carbon dioxide as well. 

Collectively when considering the all the trees in the logged forest they are giving out as much carbon dioxide as an equivalent area of unlogged forest. 

In the new study, led by the University of Leicester and published in the journal New Phytologist, researchers were able to break down how much respiration, the process that produces carbon dioxide in tree stems, was used for tree growth and how much for maintenance. They have shown that the increase in respiration from tree stems in logged forests was due to growth, producing new wood, whereas in the old-growth forests most of the respiration was from tree maintenance, supporting the existing tree structure.

Their conclusions make the case for greater research attention on logged landscapes. These are currently understudied despite logged forests now being more prominent than old-growth forests across the tropics.

Tropical forests capture carbon from the atmosphere but they also release a similar amount through respiratory processes in the ecosystem, including metabolic activity of plant growth and maintenance. A previous study by the same author found that logged forests are a net source of carbon – as they emit more carbon dioxide than what they absorb. It is now important to keep learning about logged forests and understand what drives their carbon fluxes. 

A common method to study carbon fluxes in forests is to measure net carbon balance, but this doesn’t provide much information about where the fluxes are coming from – it is like knowing your bank balance without any information on the transactions. If we don’t know where the fluxes are coming from we also don’t know why we are getting certain fluxes and what is driving those fluxes. 

Instead, for this study the scientists focused on the stems or woody trunk of the trees, which is where most of the forest’s biomass is stored. Data were gathered from forests in Malaysian Borneo as part of a long-term ecological monitoring programme. This region, alike most of the tropical forests of Southeast Asia, has a long history of logging and timber extraction.

By measuring the individual components of the forest carbon cycle and carbon fluxes they can learn a lot more about why certain patterns and fluxes occur. Understanding these is important for understanding the forest ecosystem and then extrapolating this information to future climate change and land use change scenarios.

The scientists studied stem respiration of a sample of trees and then estimated the values for all the trees in the one-hectare study plots. Tree-level results showed higher carbon release levels per unit of stem surface area in logged vs old-growth plots, with 37 grams of carbon per metre squared of woody stem every month in a logged plot versus only 26 grams in an old-growth plot (g C m-2 month-1).  However, because old-growth forests have bigger trees and therefore more stem surface area in total, there was no difference between logged and old-growth results when scaled across the entire one-hectare plots.

Lead author Maria Mills, a PhD student in the University of Leicester School of Geography, Geology and the Environment, said: “This study was about considering individual trees versus the whole ecosystem, and what drives carbon emissions at both levels. There are differences between individual trees, and between individual ecosystems, for example logged versus unlogged forests.

“We see higher respiration per square metre in logged plots because the trees in those plots are growing faster. Growth has a metabolic cost, so we get respiration following growth. The trees in logged plots grow faster because they have access to more light – as there are more gaps in logged plots from when timber trees have been extracted. In logged plots we see a lot more investment in growth so these trees will respire more. There are also other reasons for these differences which link back to the trees’ functional traits and soil nutrients, but ultimately it comes down to the priority for trees in logged forests to invest in growth. In old-growth plots, alternatively, we see a lot more investment and priority for tree maintenance. 

“Most importantly, our results tell us that the carbon dynamics in logged forests are very different to those of old-growth forests. But given how expansive logged forests are, they could be considered as the ‘new normal’ for contemporary tropical forests.  We need to put more research efforts into understanding what goes on in logged forests, both in terms of their carbon fluxes and their wider ecological functioning.”

• ‘From tree to plot: investigating stem CO2 efflux and its drivers along a logging gradient in Sabah, Malaysian Borneo’ is published in New Phytologist, DOI: 10.1111/nph.20043