Only the Han Emperor can prevent forest fires A 2,000-year-old battle ended in fire, and a tree species never recovered An ancient Chinese army set fire to an enemy capital, but things got out of hand. Kiona N. Smith – Apr 24, 2025 5:55 pm | 1 The Chinese swamp cypress is critically endangered. This one seems pretty chill about it, though. Credit: Daderot The Chinese swamp cypress is critically endangered. This one seems pretty chill about it, though. Credit: Daderot Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only   Learn more The buried roots and stumps of an ancient forest in southern China are the charred remains of an ancient war and the burning of a capital city, according to a recent study from researchers who carbon-dated the stumps and measured charcoal and pollen in the layers of peat surrounding them. It may not be obvious today, but there’s an ancient forest hidden beneath the farmland of southern China’s Pearl River Delta. Spread across 2,000 square kilometers are thick layers of waterlogged peat, now covered by agriculture. It’s all that is left of what used to be a thriving wetland ecosystem, home to forests of Chinese swamp cypress along with elephants, tigers, crocodiles, and tropical birds. But the peat hides the buried, preserved stumps and roots of cypress trees; some of the largest stumps are almost 2 meters wide, and many have burn marks on their tops. “These peat layers are locally known as ‘buried ancient forest,’ because many buried trees appear fresh and most stumps are found still standing,” writes Ning Wang of the Chinese Academy of Scientists, who along with colleagues, authored the recent paper. It turns out that the eerie buried forest is the last echo of the Han army’s invasion during a war about 2,100 years ago. Today, the ruins of the palace from which Nanyue's kings ruled is an archaeological site in Guangzhou. Credit: Windmemories When the Fire Nation attacked Wang and colleagues radiocarbon dated the stumps’ outermost rings to find out when the trees had stopped growing, and the answer is around 2,100 years ago (give or take about 70 years). It looks like the cypress trees died at roughly the same time across a broad swath of swampland, in some kind of ecological calamity. Based on the burn marks scarring the tops of many of the stumps, the forest ended in fire. As it happens, history does record a fiery calamity in the Pearl River Delta around 111 BCE. The delta was home to an ancient kingdom called Nanyue, which ruled most of what are now the southern Chinese provinces of Guangdong and Guangxi, along with what’s now the northern part of Vietnam. Nanyue rose to power around 204 BCE, just as the Qin Empire (which had united most of China under its rule) was beginning to crumble. A former Qin general, Zhao Tuo, took advantage of the chaos to turn a former Qin province into an independent kingdom, which his descendants ruled for the next century. Then everything changed when the Fire Nation—sorry, the Han Empire—attacked. Han rose to power in the wake of Qin’s collapse, after a short war with a rival dynasty called Chu, and spent the next century smugly referring to Nanyue as a vassal state and occasionally demanding tribute. At times, the rulers of Nanyue played along, but it all came to a head around 111 BCE, in the wake of an attempted coup and a series of assassinations. The Han Emperor sent an army of between 100,000 and 200,000 soldiers to invade Nanyue under a general named Lu Bode. The troops marched across the countryside from five directions, converging outside Nanyue’s capital city of Panyou, which stood in the Pearl River Delta, near the modern city of Guangzhou. An enterprising company commander named Yang Pu got the bright idea to set the city on fire, and it ended badly. “The fire not only destroyed the city but also ran out of control to the surrounding forests,” write Wang and colleagues. The cypress trees burned down to the waterline, leaving only their submerged stumps behind. The brown dots mark the known sites of buried forests, and the orange diamonds mark those confirmed to be ancient. The two yellow diamonds are Wang and colleagues' study sites. Credit: Wang et al. 2025 After war came fire and rice At the time of the invasion, the land around Panyou was mostly swamp, forested with cypress trees. People had lived there for thousands of years, and had been growing rice for about 2,000 years. Bits of charcoal in the peat layers Wang and colleagues sampled reveal that they practiced slash-and-burn agriculture, but on a small scale, rotating their fields so the cypress forest could start to recover after a season or two. The small burns are nothing like the forest fire Yang Pu unleashed, or the massive burning and reworking of the landscape that came after. The stumps of the burned cypress trees slowly disappeared under several meters of peat, while above the buried ancient forest, life went on. Tigers, elephants, rhinos, and green peafowl no longer walked here. Instead, grains of pollen from the layers of clay above the peat reveal a sudden influx of plants from the grassy Poaceae family, which includes rice, wheat, and barley. That pollen, along with thicker-than-usual deposits of charcoal, suggests that people were burning the remaining trees on a massive scale to make room for more rice fields. Combined with historical records, Wang and colleagues say the pollen and charcoal buried in those sediments point to a dramatic increase in the local population and the scale of their agricultural industry. That was probably an effort to feed the huge invading army at first, but was followed by what Wang and colleagues describe as “a government action aimed at consolidating the results of the victory”—in other words, moving more people into the region and putting them to work on farms. Nearby ocean sediments reveal that around the same time, about 2,100 years ago, more copper and lead started washing into the sea from the Pearl River Delta, suggesting that people were making copper farming tools and coins and using lead in cosmetics and metalware (always a fantastically healthy idea). Biodiversity as a casualty of war Meanwhile, the cypress trees that had once grown across thousands of square kilometers had been burned out of their home as surely as the Nanyue rulers had been burned out of theirs. The Han army’s out-of-control fire attack, followed by the years of burning and farming that followed, pushed the species (Chinese swamp cypress, formally called Glyptostrobus pensilis) to the brink of extinction. Most of southeast China is still technically good habitat for the trees today, but no wild Chinese swamp cypress trees grow anywhere in China. In northern Vietnam, its numbers are small and dwindling, confined to a few remote patches of land. The problem is not climate or environmental change; it’s that so many of the trees were destroyed. “Most G. pensilis populations are small and scattered, unable to provide the ecosystem services they once did,” write Wang and colleagues. G. pensilis is a critically endangered species, and according to Wang and colleagues, that’s mostly due to the Han invasion of Nanyue more than 2,000 years ago. Science Advances, 2025 DOI: 10.1126/sciadv.adt1736; (About DOIs). Kiona N. Smith Science correspondent Kiona N. Smith Science correspondent Kiona is a freelance science journalist and resident archaeology nerd at Ars Technica. 1 Comments