Studying the Ancient Forests of the Coastlines

Dr Khairun Nisha is a Research Fellow at the Coral Geomorphology Lab in Earth Observatory Singapore. In the early dawn of the morning when the tide is low, she begins her field work gathering soil samples at a mangrove forest in Pulau Ubin, an island in Singapore. When she shares her research with her friends, she often receives questions: "Isn’t the mangrove dirty? Full of mosquitoes?”, but she thinks otherwise. “It is one of the most beautiful ecosystems. When you go out in the morning at five during low tide, it can be so quiet and so peaceful.”

She describes herself as an environmental historian, specialising in the histories of mangrove forests. Through analysing columns of ancient sediments buried in layers of soil formed over thousands of years, her work helps to date back the history of a site to 10,000 years, a geological period known as the “Holocene”, revealing which mangrove species were present and how the vegetation has changed over the years.  The unique conditions of mangrove soil - reduced oxygen, acidic soil, and high salt content – increase the chances that sediments deposited thousands of years ago remain preserved.

For Dr Nisha, she specialises in studying mangrove pollen released by trees. These tiny sediments can range between 8 to 100 micrometers and can only be viewed under the laboratory microscope. To put this into perspective, 20 micrometers is the width of a strand of human hair.

Pollen is a powdery material released by male flowers during the flowering season. Generally, for mangrove trees, this occurs twice a year, during the dry and early wet seasons. For the Rhizophora trees, it happens between May and June, while some studies also note in November. Different mangrove trees release pollen in different ways; some in a quick explosive manner, some by wind, and others with the help of animals. These pollen grains then transfer onto female flowers, carrying out fertilization to finally form a seed.

In her fieldwork, Dr Nisha uses a special tool, called the sediment auger, to dig up cylindrical soil cores from the ground. Sometimes, her team digs two to three meters deep, but other times much deeper. The deeper the sediments are located, the older they are

As climate change causes sea level rise in this region, threatening the existence of mangrove forests along the coast, Dr Nisha’s research holds critical insights to help make decisions on how these spaces can be protected.

"In the past, we have seen these mangroves go through high sea levels and harsh climate change. Understanding what happened in the succession of vegetation in the past allows us to capture how this vegetation was resilient. This allows us to answer: what species was resilient with high sea levels during the Holocene period 10,000 years ago? If those species were resilient, does it mean they will continue to be so in the next 100 years we are projecting for sea level rise?”

Dr Nisha’s research not only points out to us how ancient some of these mangroves are, but also highlight how different mangrove trees have responded to past changes in sea level and climate conditions, offering valuable guidance for protecting these ecosystems.