Swamp soil yields evolution of Kaziranga’s vegetation

A new scientific study of swamp soil has uncovered how the vegetation of Assam’s Kaziranga National Park has evolved over 3,000 years to make it the prime habitat of the Indian one-horned rhinoceros.

Published in the Netherlands-based Catena, an interdisciplinary journal of soil science and geomorphology, the study also highlights the movement of the large herbivores from the northern and western parts of the Indian subcontinent to the northeastern region. 

The authors of the study in the 430 sq. km Kaziranga National Park are Sadhan K. Basumatary, Siddhant Vaish, and Swati Tripathi of Lucknow’s Birbal Sahni Institute of Palaeosciences; Eline N. Van Asperen of Newcastle University; H. Gregory McDonald of Denver Museum of Nature and Science, USA; Rajib Gogoi of Botanical Survey of India’s Himalayan Regional Centre in Gangtok, and Ajay Kumar Arya of University of Lucknow. 

The scientists collected a 110-centimetre-deep sedimentary soil from the northwestern side of Kaziranga’s Sohola swamp at intervals of 5 cm for palynological analysis and radiocarbon dating. The layer, containing pollen grains from plants and microscopic fungal spores linked to animal dung, acts like a natural history book. 

By studying these tiny remains and dating them, scientists reconstructed how vegetation, climate, and the movement of large animals changed over time. 

Pictorial evolution showing a. Dense forest with wider and swamp conditions with low wildlife activities (3290 years -1700 years), b. Relatively less dense and less deep water with increased wildlife activities (1700 years-640 years), c. Comparatively less dense forest and a less deep swamp with increased wildlife activities within KNP (640 years-present). Photo: Special Arrangement

“Kaziranga looked very different 3,290-1,700 years before present (BP). The park during that phase of warm and humid climatic conditions had a tropical dense mixed forest dominated by Bombax, Cinnamomum, Duabanga, and Lagerstroemia trees, with less grasslands,” Dr Basumatary told The Hindu. BP is a time scale where ‘present’ is fixed at the year 1950. Fungal spores were found in small amounts, indicating fewer grazing animals during this phase.

Expanding grassland

The soil study further showed that the vegetative cover began to change between 1,700 and 640 years BP. The evergreen forests slowly declined, and deciduous trees and grasses began taking up the space. 

“This shift suggested slightly drier conditions and more open landscapes. Around this time, dung-related fungal spores increased, showing that large plant-eating animals were becoming more common,” Dr Basumatary said.

This period also marked the first appearance of Mimosa, an invasive plant species. Its arrival hinted at early ecological disturbance, possibly linked to climate stress or increasing human activity in nearby regions. “The invasion of Mimosa needs proper management,” Dr. Gogoi said. 

From 640 years BC to the present, Kaziranga became more open, with widespread grasslands and fewer dense forests. Dr Tripathi said that there was a strong impact of the Little Ice Age in the northern region of India, which eventually affected the wildlife habitat. 

“We found a sharp rise in fungal spores during this period, suggesting an increase in the numbers of megaherbivores such as rhinos, elephants, buffaloes and deer,” she said.

While Kaziranga’s sedimentary profile revealed the absence of agricultural activities, the scientists hypothesised that the rhinos migrated to the park from the western and northern parts of the Indian subcontinent due to climate changes and growing human pressure. 

The hypothesis was based on a comparative analysis of vegetation dynamics and climatic variables, with earlier recorded rhinoceros bone fossils and historical records. 

Kaziranga is home to more than 70% of the world’s total population of some 3,700 greater one-horned rhinoceros, making it the most critical habitat for the species.