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Pollen and charcoal reveal human-driven collapse of swamp cypress forests 2,100 years ago

Chinese scientists have discovered that fragile swamp forests in the Pearl River Delta (PRD) region suddenly collapsed approximately 2,100 years ago (2.1 ka)—with human activity as the cause.

The study, led by researchers from the Guangzhou Institute of Geochemistry and the Institute of Oceanology of the Chinese Academy of Sciences, sheds new light on the role of human activity in ecosystem collapse.

Published in Science Advances, focuses on Glyptostrobus pensilis (G. pensilis), a critically endangered species of Chinese swamp cypress that once thrived in extensive swamp forests in the PRD. Through palynological (i.e., pollen and spore) records, chronological data, and sedimentological evidence, the team found that the collapse of these forests approximately coincided with military conquests by the Qin and Han Empires in the region.

Pollen records from sediment cores in the PRD revealed a dramatic decline in G. pensilis—from high dominance to near-extinction—indicating a sudden anthropogenic disturbance. Burn marks on the tops of standing stumps of G. pensilis are consistent with fire attacks by the Han army during its conquest of the Nanyue Realm in 111 B.C.

Additionally, increases in the presence of Poaceae—a plant family that includes cereals and other grasses—as well as pioneer plants after that period suggest large-scale migration and the adoption of advanced agricultural practices following the conquests. The conclusion that human activity played a decisive role in the forest collapse is further supported by the presence of charcoal and anthropogenic metals like copper and lead in the region.

Principal component analysis indicates that G. pensilis is highly sensitive to human activity. The loss of G. pensilis forests marked the beginning of a decline in biodiversity due to human intervention in the PRD, contributing to the local extinction of various species, including elephants, tigers, rhinoceroses, green peafowl, crocodiles, and others.

The researchers also used pollen and organic carbon records to identify previous episodes of forest degradation in the area—at 4.2 ka and 3.5 ka—from which the ecosystem later recovered.

The 4.2 ka event is a pronounced climate extreme. The 3.5 ka event was likely a regional climate extreme, as evidenced by numerous records from low latitudes and the Southern Hemisphere. The 3.5 ka event was probably caused by volcanic activity, i.e., the Santorini eruptions (VEI=7) from 3550–3577 BCE. This finding illustrates the impact of climate change and deep Earth processes on vegetation.

This study explores the historical roots of environmental change, revealing the lasting impact of human activity—such as war and agriculture—on a fragile ecosystem, while identifying the temporary effects of climate and deep Earth processes on the same system.