The first analysis of wildfires in southeast Asia has been compiled using tree rings from a region in central Vietnam. The frequency of wildfires in the area has increased dramatically over the past 100 years, but climate change is not the main cause. Instead, people lighting fires for agriculture are behind the increased number of forest fires, according to the study1 published i Geophysical Research Letters.
“Fire has a big impact on forest structure,” says Thiet Nguyen, a forest ecologist and PhD student at the University of Melbourne in Australia. “There is a very high level of biodiversity in that forest.”
Many people think that Vietnam is tropical lush with jungle, but in the central highlands, there are about 100,000 hectares of conifers mixed with broad-leaved trees in misty mountain forests. While tropical trees do not tend to have annual growth rings because they grow steadily throughout the year, conifers have annual growth cycles. Nguyen and his colleagues took cross-sections from recently fallen trees of two species, ie. Pinus cassia and Evelyniana Keteleeriaat 12 sites in Bidoup Nui Ba National Park and counted the tree rings back over almost 400 years.
When trees are exposed to non-lethal fires, their bark barks, leaving a scar behind within the tree rings. The team dated the earliest definite fire scar to 1772. Between then and 1905, there were clear fire rings in only 17% of the years, although Nguyen admits that the farther back in time they went, the the number of samples available was less, which would reduce their confidence. in the results. After 1905, they had good clear tree rings to work with. From then to 1963, 71% of the years showed scars from fires, but fires often affected only a small number of sites. But from 1964 to the present day, fire has occurred almost every year, often affecting a larger number of sites.
Until this study came along, “we had no idea how the fire regime was changed”, says Nguyen. “If the fires are increasing rapidly, that can reduce biodiversity and forest density,” he says. “It’s a big problem.”
Using statistical analysis, the authors looked for relationships between fire, climate and the human population. They found that historically, fire patterns were related to climate; data from Pacific sea surface temperatures (see ‘Forests on Fire’) showed that weather patterns such as El Niño and La Niña, which determine dry or wet years, could affect the amount of combustible plant material available and how dry and flammable it was. , for example.
But after 1963, according to population estimates for the nearby city, Da Lat, the number of people to live in the area increased. People use fire to clear land for agriculture, Nguyen says, and fire likely spreads to the national park — established in 2004 — from nearby areas. People also set fires in the park. Since 1964, the authors discovered that the signal from humans was overriding the climate as the driving force behind the fires.
The increasing frequency of fires, along with more extreme fire conditions due to climate change, will be a problem in the future, says Brendan Buckley, a dendrochronologist at Columbia University in New York, who was Nguyen’s master’s program adviser but was not involved. by him. in this study. “There could be some fires that get out of control in these protected rainforest areas.”
Understanding historical and modern fire events is critical because it gives researchers a baseline to work from, helping forest managers prepare for climate change, Buckley says. While fire can be beneficial to a coniferous forest, a rainforest is a completely different ecosystem and can be destroyed by fire, he says.
Nguyen says the species used in this study are common across other parts of southeast Asia, such as Thailand and the Philippines, meaning the same technique could be used to map fire history and climate change in other countries. inspection.
“If we had this kind of study in other countries in southeast Asia, a big historical fire reconstruction for the region, that would be great.”