Not so blue anymore: how dead mangroves burden coastal carbon sinks

By Jackie Webb

Reference: Jeffrey, L. C., Reithmaier, G., Sippo, J. Z., Johnston, S. G., Tait, D. R., Harada, Y., and Maher, D. T.: Are methane emissions from mangrove stems a cryptic carbon loss pathway? Insights from a catastrophic forest mortality, New Phytologist, 2019, https://doi.org/10.1111/nph.15995

Featured image source: Luke Jeffrey

Mangrove forests have been feeling the pressure of climate change. With heat waves and low rainfall, many mangroves along a 1000 km stretch of coastline in northern Australia have been wiped out. However, the dead trees are living on by contributing large methane emissions which has consequences to global mangrove carbon stores and climate change. Read on to find out how the living dead remain active methane emitters.

Dead mangroves may have consequences for blue carbon budgets

We know that the burning of fossil fuels and greenhouse gas emissions from land use change is responsible for most of the current climate change the earth is experiencing. But did you know that changes in our climate can cause the release of more greenhouse gases through natural ecosystems? Evaluating this climate-feedback loop in ecosystems has become a top priority for scientists as evidence suggest almost half of the climbing atmospheric levels of one greenhouse gas, methane, originates from natural sources (NASA Earth Observatory). 

One such feedback was assessed when over 7,000 ha of mangroves died in the Gulf of Carpentaria (Australia) during the summer of 2015-2016 due to extreme heat and moisture stress caused by climate change. Being the largest mangrove dieback event in recorded history, scientists from Southern Cross University wasted no time in travelling to the remote field site to measure the effects on mangrove methane emissions.

“At the time no data was published on methane emissions from mangrove stems (both living or dead). From the first measurement of a dead stem we knew there was something going on and by comparing these to the living mangroves as a kind of ‘baseline’ for normal methane emissions, we had an interesting story!” – Luke Jeffrey, lead author

What is so important about mangroves and methane? Firstly, healthy living mangroves naturally combat climate change through their ability to store enormous amounts of carbon in both their sediment and biomass. In fact, mangroves capture carbon at similar rates to tropical humid rainforests such as the Amazon. This carbon storage in coastal habitats is what scientists refer to as ‘blue carbon’, in contrast to ‘green carbon’ which is stored in land ecosystems. Secondly, as with most wetlands, the potent greenhouse gas methane is produced in sediments and can be released at rates that offset a noticeable chunk of carbon stored annually. In healthy living mangroves, methane emissions are usually not enough to offset the carbon captured through trees drawing in carbon dioxide from the atmosphere. In dead mangrove ecosystems however, the consequences for blue carbon storage may be greater than anticipated.

Portable methane measurements

In mangroves, methane is produced in the sediment where the methane-producing microbes relish in the high organic carbon and high moisture environment. Scientists have recently discovered that this is not the only methane pathway and found that tree-stems can act as a transport medium for releasing methane into the atmosphere. The mechanism remains rather cryptic, but we definitely know it is there. To measure tree-stem methane, the authors developed and trialled a few different methods, eventually settling on a transportable field-friendly set-up. They simply attached small plastic chambers with a potty clay ring to enclose tree stems and used a portable gas analyser to observe the accumulation of methane concentrations. They repeated this at different trunk heights as well as in the sediment near the trees. In a world-first they were able to directly compare methane fluxes between both living and dead trees due identifying a small patch of living mangroves still thriving in the vast dieback area.

Dead and living mangroves an active methane source

The research revealed that dead mangroves remain an active component of coastal carbon budgets, with methane emissions measuring eight times higher than their living relatives. Due to parallel measurements taken in the sediments at each tree, the authors were able to rule out the possibility that higher methane fluxes where driven by differences in local sediment fluxes. Even within the patch of healthy mangroves, dead trees sustained higher tree-stem fluxes.

So why may dead mangroves have higher methane emissions than living ones? With these findings and additional testing of environmental variables, the cryptic methane pathway could be narrowed down to a few possible mechanisms. Firstly, both living and dead trees can act as passageways to transport methane produced in the sediment by travelling through roots and tree trunk, until being released through stems and leaves. Secondly, the creative ability of mangroves to transport oxygen from the atmosphere down to their root zone to survive waterlogged conditions also means that mangroves can mitigate some methane being produced in sediments via a process known as oxidation. Without this oxygen drawdown in dead trees, sediment methane is being released through tree stems at the fullest extent.

Tree-methane remains an evolving field in forested ecosystems

Mangroves aren’t the only forested ecosystems that release methane through their stems. The mangrove stem flux rates measured in this study are similar to wetlands living in the temperate climate zones, but overall are lower than previous studies. In contrast to inland and freshwater forests, this could be the result of saline sediments which simply don’t offer the best home for methane producing microbes that freshwater sediments do. At the other end of the spectrum is stem methane release from tropical wetlands in the Amazon. These trees alone are estimated to release between 14 million and 25 million metric tons annually – representing 8-15% of global wetland methane emissions (Pangala et al., 2017). Tree-stem methane measurements from forested ecosystems further inland that do not have flooded soils also show substantial stem methane release, suggesting that methane can actually be produced in the trees themselves rather than the soil (Pitz and Megonigal, 2017). One thing we can know for sure is that tree methane emissions vary greatly between forested ecosystems, with this study adding another twist to the baffling field of “tree-thane”.

Implications in the face of climate change

Although there are still many unknowns regarding the mechanisms behind mangrove tree-stem methane flux, this research proves that scientists need to take their measurements above ground to capture all gas movements. For decades, scientists aimed their instruments at sediments to measure ecosystem methane fluxes, assuming this was the only major pathway for release. Now, this research, along with a rapidly growing field of other tree-stem methane studies, highlights the need to include tree-stem fluxes in greenhouse gas and carbon budgets. A ~26% contribution to total mangrove methane emissions from the dead mangrove tree-stem pathway is not a small number to be ignored.

The outlook for further mangrove dieback events is dire if climate change continues to throw extreme weather curve balls at these fragile ecosystems. In the recent past, the majority of recorded mangrove dieback events have been caused by tropical cyclones and other extreme weather events (Sippo et al., 2018). These destructive forces lead to a range of consequences to the precious blue carbon stored in mangroves, including an increase in carbon lost through decaying biomass, sediment oxidation, and now methane emissions from dead tree stems. 

There is no doubt that this research will feature in future discussions in climate policy, which rely on accurate accounting of all carbon emissions to make informed decisions about how to best tackle climate change. We should all do our part in supporting the protection and plantation of these coastal forests as investments for the future of blue carbon.

“Trees are in no way bad for climate. I’d say that by all accounts the climate benefits of living trees will far outweigh their atmospheric burden”. – Luke Jeffrey, lead author