Ancient War Destroyed a Forest

Human activities have altered ecosystems and landscapes across many regions of the world. Discussions about environmental change today often focus on development, agriculture, or industrial expansion, yet the ecological impact of warfare has received far less attention. In reality, war has long been one of the major forces reshaping environments.

The Pearl River is a major river in southern China that flows into the South China Sea through Guangdong Province, forming a vast alluvial plain known as the Pearl River Delta. Over the past three decades, geological studies have revealed extensive peat layers across this region. These peat deposits indicate that large swamp forests once covered the delta. The layers are typically 2 to 6 meters thick and contain numerous well-preserved tree stumps that still appear remarkably fresh. The buried forest extends across more than 2,000 square kilometers and dates back roughly 6,000 years. Because the forest has completely disappeared in modern times, these peat deposits are commonly referred to as a buried ancient forest. A recent study analyzed the ecological history of this forest and proposed that its disappearance may have been linked to two ancient wars.

The dominant tree species within this forest was the Chinese swamp cypress, Glyptostrobus pensilis, a conifer belonging to the cypress family. During the Cretaceous period this lineage was widely distributed across the Northern Hemisphere, but it experienced extensive extinction toward the end of the Quaternary Ice Age. Today it survives only in a few scattered locations in southern China, Laos, and Vietnam. Because the remaining populations are extremely small and fragmented, the species has largely lost its former ecological role and is currently classified as critically endangered.

Distribution of buried and extant Glyptostrobus pensilis forests in Asia and the Pearl River Delta（Wang, N et al. (2025). ，採用 CC BY 4.0 授權。）

For many years the decline of Glyptostrobus forests was attributed mainly to climatic factors. However, more recent research increasingly suggests that human activities played the dominant role. The Pearl River Delta swamp forest, for example, lay within the political center of the ancient Nanyue kingdom, making it plausible that historical events associated with this state contributed to the forest’s disappearance.

Between 221 and 214 BCE, the Qin Empire launched military campaigns to conquer the Baiyue regions and incorporated them into imperial territory. After internal turmoil led to the collapse of the Qin dynasty, the Qin general Zhao Tuo established the Nanyue kingdom in 204 BCE. The kingdom lasted until 111 BCE, when it was defeated by the Han Empire. Historical records describe the use of fire attacks by Han forces during the final campaign, though the environmental consequences of these wars remained uncertain. To investigate this question, researchers analyzed sediments within the peat layers using palynology, sedimentology, and chronological dating in order to reconstruct the ecological history of the Pearl River Delta forest.

Territory of the Nanyue Kingdom（圖片來源：People's_Republic_of_China_(orthographic_projection).svg: Ssolbergj (talk) derivative work: 长夜无风 (talk)、玖巧仔 (talk)，採用 CC BY-SA 3.0 授權。）

The results revealed several phases of ecological change.

Between 4,900 and 4,100 years before present (with “present” defined as 1950), the region was not yet dominated by swamp forest. Instead, the vegetation consisted primarily of forests dominated by trees of the genus Castanopsis, accompanied by other tropical and subtropical species. During this time the environment gradually shifted, with the appearance of plant taxa typically found further north, indicating a cooling climate.

From about 4,100 to 3,600 years before present, Glyptostrobus became the dominant species. The decline of Castanopsis forests suggests that the environment was transitioning into wetlands, likely associated with a rising groundwater table.

Between 3,600 and 3,400 years before present, the swamp cypress forest declined for the first time. Northern plant taxa increased, indicating that the climate continued to cool and conditions became somewhat drier.

From 3,400 to 1,900 years before present, the Glyptostrobus forest again became dominant. Small patches of non-wetland forest persisted nearby, suggesting that the climate had returned to warmer conditions. During this period, however, pollen records show increasing proportions of grasses, indicating the presence of early human activities. Some preserved stumps from this layer also show burn marks at their tops.

From about 1,900 years before present to the modern era, the swamp forest was replaced by agricultural vegetation. Although the regional climate remained warm and humid, human disturbance became intense.

Branches of Glyptostrobus pensilis（圖片來源：KENPEI，採用 CC BY-SA 3.0 授權。）

These findings reveal several episodes of forest change. Earlier transitions were driven primarily by natural environmental factors, but the final collapse appears to have been caused by human activities. Around 2,100 years before present, large quantities of grass pollen began appearing in the sediments, reflecting the expansion of agriculture and growing human populations. However, agriculture alone cannot fully explain the disappearance of the swamp forest. Humans had already practiced farming in the region for thousands of years before the Qin conquest, and during that time the forest remained largely intact. This suggests that a more dramatic disturbance must have triggered the irreversible collapse.

Two major historical events occurred between roughly 2,300 and 2,000 years before present that could have transformed the Pearl River Delta landscape. The first was the Qin conquest of the Baiyue regions, which introduced approximately half a million people into southern China and fundamentally altered the region’s social and cultural systems. The second was the Han conquest of Nanyue. During this war, Han forces set fire to the Nanyue capital. The flames not only destroyed the city but also spread into the surrounding swamp cypress forests. Trees standing in water were partially protected from burning, leaving behind submerged stumps with charred tops.

Radiocarbon dating shows that the growth of these trees ceased around 2,040 years before present, closely matching the historical date of the Han conquest of Nanyue. Charcoal records from the sediments further confirm that a large-scale fire occurred during this period. After the war, the central government relocated large numbers of settlers to consolidate control of the region, accelerating environmental transformation. In this context, the destructive impact of warfare far exceeded that of ordinary agricultural activities.

Glyptostrobus pensilis is extremely sensitive to human disturbance, declining more rapidly than many other plant species. Its disappearance marked the beginning of broader ecological decline across the Pearl River Delta. Large animals that once inhabited the region—including Asian elephants, tigers, rhinoceroses, crocodiles, and green peafowl—eventually vanished as well. Unlike the ecological fluctuations caused by natural climate events, these human-driven changes proved irreversible.

Author: Bai Leng

Reference:

Wang, N., Ding, P., Ding, X., Zong, Y., Sun, W. (2025). Collapse of fragile Chinese Swamp Cypress forest. Science Advances.