Reading the mud
In the west of Khao Sam Roi Yot National Park lies Thung Sam Roi Yot, Thailand's largest freshwater marsh. This wetland is a basin surrounded by towering limestone karsts, peaks that give the park its name—"Mountain of 300 Peaks". Below the surface of the marsh, layers of mud hold a detailed environmental history of the last several thousand years. Scientists have drilled into this mud, extracting sediment cores that are a time capsule. By analyzing the pollen grains preserved at different depths, they can reconstruct how the landscape has changed.
The deepest, and therefore oldest, layers of the sediment cores tell a story of a saltwater environment. These layers, dating to a period before 4,000 years ago, contain high amounts of pollen of mangrove species, especially from the genus Rhizophora. This indicates the area was once a coastal mangrove forest, directly connected to the Gulf of Thailand. This corresponds with the Holocene sea-level highstand, a time when the sea was several meters higher than it is today, pushing the coastline further inland. Geologic evidence from the park, such as ancient beach ridges and sea notches found far from the current shore, confirms that this area was once a coastal bay.
A landscape transformed
Around 4,000 years before the present, the pollen record shows a significant ecological shift. The abundance of mangrove pollen drops off, and pollen from freshwater-tolerant plants begins to dominate. This shows when the sea level in the Gulf of Thailand began to fall. As the sea retreated, it left behind a landscape of limestone islands connected to the mainland by newly formed land bridges, or tombolos. These formations trapped marine water in a basin.
Cut off from the regular inflow of the tide, the trapped water began to change. Freshwater from rainfall and runoff from the surrounding mountains diluted the salt, gradually transforming the former bay into the freshwater marsh seen today. The pollen archive records this transition, with mangrove species giving way to reeds, grasses, and freshwater swamp trees. A small patch of the original Phragmites grasslands from this period is still conserved within the park. Today, the marsh is a biodiversity hotspot recognized as a Ramsar Wetland of international importance, supporting hundreds of species of birds, fish, and mammals, including the endangered fishing cat (Prionailurus viverrinus). The story of this transformation, from saltwater to freshwater, is in the microscopic pollen grains buried deep in the mud.