A pristine peat swamp forest in the Kampar Peninsula, Sumatra (Anggi Hapsari, University of Göttingen)

25/04/2022 | Tropical peatlands are one of the most efficient carbon sinks. The flipside is that they can become massive emitters of carbon if they are damaged, for instance by land use change, degradation or fire. This can lead to faster climate warming. Researchers at the University of Göttingen, the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen, and the Research Center for Geotechnology in Indonesia have now shown how peatland in the coastal areas in Sumatra and Borneo in Indonesia developed over thousands of years and how climate and sea level influenced their dynamics throughout. The results were published in Global Change Biology.

To discover more about the environment over the past 17,000 years, the researchers analysed two peat cores. They carried out analyses for traces of pollen, spores and charcoal, and conducted carbon dating as well as biogeochemical investigations. Their study found that there were much higher concentrations of charcoal between 9,000 to 4,000 years ago (the mid-Holocene), when sea level was even higher than it is now. This is a sign that there were much larger forest fires at that time.

Later, around 3,000 years ago, irregular periodic variations in winds and sea surface temperatures (known as El Niño-Southern Oscillation or ENSO) would have caused prolonged drought, making the forests dry and thus susceptible to fires ignited by lightning. However, even at this time, the fires were fewer than in the earlier mid-Holocene, which presented a puzzle.

A clue was that during the earlier period in the mid-Holocene, researchers found a high proportion of mangrove pollen. The pollen grains indicate the presence of mangrove forests which grow along the coast in salty water. Their presence is a good indicator of rising sea level and an increase of salt in the otherwise freshwater peatland ecosystem. Salt is harmful to the inland freshwater vegetation, which is likely to have resulted in more dry and dead tree leaves and branches. Salt can also reduce forest canopy cover and air humidity, which is an important factor that can prevent fire spreading in peatland ecosystems.

“We were surprised to find that rising sea levels could potentially exacerbate fires in coastal areas in Indonesia,” says lead author Dr. Anggi Hapsari from the University of Göttingen. “Our findings underline how the interaction between rising sea levels and dry climate may contribute to massive forest fires even in relatively fire-proof ecosystems, such as pristine peatlands. This reveals the potential hidden impact of sea level rise exacerbating climate warming.”

“However, in contrast to the past, the primary cause of peatland fires now is human activity,” adds Hapsari. “If people’s behaviour continues in terms of, for instance, extensive destruction of peat swamp forests, peatland drainage, and intentional burning, when met with current rapidly rising sea level and stronger future ENSO, this could lead to catastrophic and widespread forest fires and uncontrollable carbon release,” she continues.

"Our unexpected finding adds an as yet unknown threat to the survival of these valuable ecosystems," explains coauthor Dr. Tim Jennerjahn from the Leibniz Centre for Tropical Marine Research. He concludes, "It is clear that fire risk assessment in tropical peatlands deserves more attention. The study demonstrates how the reconstruction of past environmental change can help improve present-day management of coastal ecosystems."

Publication: Hapsari, K. A., et al “Sea level rise and climate change acting as interactive stressors on development and dynamics of tropical peatlands in coastal Sumatra and South Borneo since the Last Glacial Maximum”.Global Change Biology 2022. DOI: 10.1111/gcb.16131