Rings of fire: centuries of tree growth show wildfires increasing in Vietnam

火环：几个世纪的树木生长表明越南的野火正在增加

The data suggest human activities are more to blame for the increase than climate change.

数据表明人类活动比气候变化更应为这一增长负责。

Jude Coleman（作者）

The first centuries-long analysis of wildfires in southeast Asia has been compiled using tree rings from a region in central Vietnam. There has been a drastic increase in the frequency of wildfires in the area over the past 100 years, but climate change is not the primary culprit. Instead, humans lighting fires for agriculture are behind the rising number of forest burns, according to the study1 published in Geophysical Research Letters.

对东南亚野火长达数百年的首次分析是利用越南中部地区的树木年轮编写的。在过去的100年里，这个地区的野火频率急剧增长，但气候变化不是罪魁祸首。相反，根据发表在《地球物理通讯》上的研究，人类为农业点火是森林燃烧数量不断增加的幕后推手。

“Fire has a big impact on the forest structure,” says Thiet Nguyen, a forest ecologist and PhD student at the University of Melbourne in Australia. “There’s a very high level of biodiversity in that forest.”

“火焰对森林结构有巨大的影响，”澳大利亚墨尔本大学的森林生态学家兼博士生Thiet Nguyen说，“那片森林的生物多样性非常高。”

Many people think of tropical Vietnam as lush with jungle, but in the central highlands, misty montane forests are home to around 100,000 hectares of conifers mixed with broadleaf trees. Whereas tropical trees don’t tend to put on annual growth rings because they grow consistently year-round, the conifers have annual cycles of growth. Nguyen and his colleagues took cross-sections from recently fallen trees of two species, Pinus kesiya and Keteleeria evelyniana, at 12 locations in Bidoup Nui Ba National Park and counted the tree rings back over almost 400 years.

许多人认为热带的越南丛林茂盛，但在中部高原，雾蒙蒙的山地森林是大约10万公顷针叶树和阔叶树的家园。热带树木不会因为全年持续生长而形成年轮，而针叶树则有每年的生长周期。Nguyen和他的同事们在Bidoup Nui Ba国家公园的12个地点拍摄了最近倒下的两种树木的横截面，这两种树木分别是卡西亚松和云南油杉，并统计了近400年前的树木年轮。

When trees are exposed to non-lethal fires, their bark chars, leaving behind a burn scar within the tree rings. The team dated the earliest definite fire scar to 1772. Between then and 1905, only 17% of years contained clear fire scars, although Nguyen admits that the farther they went back in time, the smaller was the available number of samples, reducing their confidence in the findings. After 1905, they had good clear tree rings to work with. From then until 1963, 71% of years showed scars from fires, however the 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.

当树木暴露在非致命的火灾中时，它们的树皮会烧焦，在树环内留下烧伤疤痕。研究小组将最早确定的火灾痕迹定为1772年。从那到1905年，只有17%的年份有明显的火灾痕迹，尽管Nguyen承认，他们回到的时间越远，可用的样本数量就越少，这也降低了他们对发现的信心。1905年以后，他们有了很清晰的树木年轮。从那时到1963年，71%的年份展现了火灾的痕迹，然而火灾总是只影响少数地点。但是从1964年到今天，火灾几乎每年都会发生，往往影响到更多的地点。

‘A big problem’

‘一个大问题’

Until the advent of this study, “we had no idea how the fire regime has been changed”, says Nguyen. “If the fires are increasing rapidly, that can reduce biodiversity and forest density,” he adds. “It’s a big problem.”

直到这个研究出现以前，“我们不知道火灾体系是如何改变的”，Nguyen说，“如果火灾迅速增长，可能会降低生物多样性和森林密度。”他补充道，“这是一个大问题。”

Using statistical analysis, the authors looked for relationships between fires, climate and the human population. They discovered that historically, fire patterns were associated with 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 have influenced how much combustible plant material was available and how dry and flammable it was, for example.

通过统计分析，作者寻找了火灾、气候和人口之间的关系。他们发现，从历史上看，火灾体系与气候有关；例如，来自太平洋海面温度的数据（见“着火的森林”）表明，厄尔尼诺和拉尼娜等决定干旱或潮湿年份的天气模式可能会影响可获得的可燃植物材料的数量以及其干燥和易燃程度。

But after 1963, according to population estimates for the nearby city, Da Lat, an increasing number of people came to live in the area. Humans use fire to clear land for agriculture, Nguyen says, and fire probably spreads to the national park — established in 2004 — from nearby areas. People also start fires in the park. From 1964, the authors found that the signal from humans overwhelmed climate as the driving force behind the fires.

但1963年后，根据附近城市达拉特的人口估计，越来越多的人来到该地区居住。Nguyen说，人类利用火灾为农业开垦土地，火灾可能从附近地区蔓延到2004年建立的国家公园。人们也在公园里生火。从1964年起，作者发现人类发出的信号超过了气候，成为火灾背后的驱动力。

The fires’ increasing frequency, coupled with heightened fire conditions brought on by climate change, will be a problem in the future, says Brendan Buckley, a dendrochronologist at Columbia University in New York, who was a master’s programme adviser to Nguyen but was not involved in this study. “There might very well be some burning that gets out of control in these areas that are protected rainforest.”

火灾频率的增加，加上气候变化导致的火灾条件的加剧，将成为未来的一个问题，Brendan Buckley说，他是纽约哥伦比亚大学的树木年代学家，是Nguyen的硕士项目顾问，但没有参与这项研究。“在这些受保护的雨林地区，很可能会发生燃烧失控。”

Understanding historical and modern fire occurrence is crucial because it gives researchers a baseline to work from, helping forest managers to prepare for climate change, Buckley says. Although fire can be beneficial to a coniferous forest, the rainforest is a totally different ecosystem and can be destroyed by fire, he says.

Buckley说，了解历史和现代火灾的发生至关重要，因为这为研究人员提供了一个工作基准，帮助森林管理者为气候变化做好准备。他说，尽管火灾对针叶林有利，但雨林是一个完全不同的生态系统，可能会被火灾摧毁。

Nguyen says that the species used in this study are common across other parts of southeast Asia, such as Thailand and the Philippines, meaning that the same technique could be used to explore fire history and climate change in other countries.

Nguyen说，这项研究中使用的物种在东南亚其他地区很常见，如泰国和菲律宾，这意味着同样的技术可以用于探索其他国家的火灾历史和气候变化。

“If we had this kind of study in other countries in southeast Asia, a big historical fire reconstruction for the region, that would be cool.”

“如果我们在东南亚的其他国家进行这种研究，为该地区进行历史大火的重建，那就太酷了。”

doi: https://doi.org/10.1038/d41586-023-02235-y

（本文仅用于学习英语，并未用于其他用途）