Improving measurements of CO2 production from dead wood
Scientists at the World Agroforestry Centre and the Chinese Academy of Sciences establish methods to reliably measure the CO2 output of dead wood. The methods have implications for the accuracy with which we can assess climate change.
The research was recently published by Gbadamassi G.O. Dossa and colleagues in the scientific journal Methods in Ecology and Evolution. When correctly measured, understanding CO2 production from dead wood will help bridge important gaps in knowledge about the carbon cycle, which in turn is essential for our understanding of greenhouse gases and their role in climate change.
“The carbon cycle reflects the movement of carbon between different global pools,” explains Dr Rhett Harrison, senior scientist at the World Agroforestry Centre and corresponding author of the article. “Plants absorb carbon from the atmosphere via photosynthesis and respire a nearly similar amount. But, because of the large fluxes involved (approximately 10 times the amount released by burning fossil fuels), small differences in the balance between these process have large implications. Also, as these are biological processes and thus sensitive to climatic conditions, such as temperature and moisture, they may both influence and be influenced by climate change. Hence a good understanding of the factors influencing these processes is paramount.”
While photosynthesis can be measured accurately from leaf to landscape levels, respiration is a lot less well understood. Respiration of living matter can be calculated indirectly from measurements of tree growth and mortality that are then converted into ecosystem level estimates of biomass change. Hence, it is relatively straightforward. But dead organic matter—necromass—poses a range of different problems, depending on the time it takes for the carbon to return to the atmosphere or enter the soil. This so-called residence time is relatively short for dead animals and soft plant material, but dead wood takes much longer to decompose.
“In montane rainforests, which are cool and perpetually moist, up to 50% of the aboveground carbon may be dead wood”, says Dr Harrison. “Average values are around 25 metric tonnes of dead wood per hectare, but with a huge amount of variation among types of forest. Clearly, this is a significant fraction of the global aboveground carbon budget, and knowledge of how rapidly dead wood decomposes is, therefore, critical.”
Infrared gas analysers to study wood decomposition
Studying the decomposition of wood is less straightforward than it seems, precisely because it is such a slow process. A large tree trunk in the forest may take decades or even over a century to decompose. By comparison, researchers on a typical three-year grant would want to wrap up their observations in two years or less. Further complicating matters, the condition of wood changes through time, affecting the respiration rate.
This problem can be circumvented to a large extent by modelling well-replicated, shorter-term measurements of the rate of CO2 production. Initial attempts at this approach depended on the chemical absorption of the CO2 released in carefully controlled laboratory conditions. The measurements were also relatively slow, which increased the expense and limited replication. However, the development of portable infra-red gas analysers in the 2000s changed all that. Now researchers could carry a gas analyser into the field, link it to some kind of chamber and measure the CO2 given off by an organic substrate. The first widespread, and still the most popular, application of this approach was in the assessment of surface soil CO2 fluxes to estimate soil respiration, but it has been increasingly used to assess the respiration of other substrates, including woody debris.
Rooting out the errors
The researchers found 51 studies that had used infa-red gas analysers to assess the respiration of dead wood and other organic substrates. “For woody debris the most popular approach is to place the entire log in a closed chamber. The infa-red gas analyser is then used to measure how quickly the CO2 concentration in the system increases,” explains Mr Dossa.
“The infa-red gas analysers measure the CO2 concentration very accurately, but to assess the amount of CO2 being given off by a log it is necessary to convert these numbers using a formula based on the basic gas law from a knowledge of the size of the chamber, the size of the log, the ambient temperature and pressure.”
Mr Dossa and colleagues found that of the 51 studies they reviewed only 11 provide the equations used to convert the measured CO2 concentrations into CO2 production rates and about three-quarters of these had fundamental flaws, leading to significant errors. In some cases, studies had compounded two or more errors. “Obviously, poor quality model inputs are only likely to delay our understanding of how different environmental factors affect respiration”, Harrison points out.
“Such mistakes are clearly avoidable,” explains Mr Dossa. “Our article provides details of appropriate formulae and briefly covers other relevant topics, such as the design of chambers. We hope this will improve the reliability of data on the decomposition of woody debris and ultimately lead to an enhanced understanding of how this process will affect and be affected by climate change.”