Feature: Tracing origins of timbers in Beijing's Forbidden City

BEIJING, Dec. 9 (Xinhua) -- Wandering around the Forbidden City in Beijing, visitors can breathe in history and appreciate various structures that were built in accordance with traditional Chinese architectural hierarchy.

In order to study the construction and renovation history of this complex, a team of Chinese researchers has successfully pinpointed the growth years of three ancient timbers in the Forbidden City and traced their geographical origins via oxygen isotope analysis.

Xu Chenxi, a researcher at the Institute of Geology and Geophysics, Chinese Academy of Sciences, has devoted himself to the study of paleoclimate change, specifically using stable isotopes in tree rings to investigate variations of the Asian monsoon over the past millennium.

Xu has long wanted to put this "signature skill" to practical use, such as in the study of archaeology. Traditionally, dating a historic building or reconstructing its construction history has relied almost exclusively on written records and stylistic analysis of its architectural form, methods which lack the precision of natural science.

"A wooden historic structure is an excellent dating archive, one that also preserves a record of past climate variability," Xu was quoted as saying by China Science Daily.

Dendrochronology, which focuses on growth rings in timber and tree trunks, has been applied to archaeology for a full century, according to the China Science Daily report. As one of the largest and best-preserved ancient wooden architectural complexes in the world, the Forbidden City boasts an immense collection of timber components.

"Oxygen isotopes act as the 'rainfall fingerprint' of a tree," Xu explained. "The isotopic makeup of precipitation differs distinctively across both regions and periods, and the tree records it in detail."

The principle is straightforward, with the wood's oxygen-isotope signature compared to the team's nationwide tree-ring oxygen isotope database, resulting in the region with the closest match being identified as the timber's origin.

When the oxygen-isotope profiles of the three timbers were extracted and checked against the database, results showed that all three curves overlapped almost perfectly, with the closest statistical match pointing to northeast China.

This finding was confirmed by historical documents. "The thrill of scientific discovery was simply indescribable when the isotopic signal locked perfectly into the historical record," Xu recalled.

Compared with traditional ring-width analysis, the oxygen-isotope method is far less sensitive to individual growth variability, said Xu. Even a handful of tree rings can yield both a secure date and provenance if they align tightly with a regional reference sequence.

Not only did this discovery pinpoint the timber's origin, it also exposed a pivotal shift in material strategies for the Forbidden City during both the Ming Dynasty and Qing Dynasty, namely the switch from precious nanmu to pine.

The team's study has been published in the journal npj Heritage Science.

The team is now deploying the same method to trace the provenance of the nanmu beams in the Hall of Mental Cultivation (Yangxin dian) inside the Forbidden City.

The bedrock of their provenance study is the reference database this team has spent years assembling. To date, they have managed to build a tree-ring oxygen-isotope database for China and East Asia as well.

In the long run, these applications will also be useful for paleoclimate research, Xu said. "Every single ancient timber is a climate diary through which we can reconstruct the historical sequence of extreme droughts or cloudbursts in a region and uncover the rules governing climate change." ■