Dead pine trees can't be used as firewood?
This sounds a bit unbelievable, after all, in the past, there was no reason in rural areas not to burn pine wood. This kind of wood could be burned whether it was dry or wet, and it was indispensable for heating and cooking.
However, this statement does have its basis, and now many places do indeed advise against it.
The reason is the unprecedented crisis facing China's forestry sector, as a parasitic worm is wreaking havoc on its resources, particularly targeting pine trees. With 900 million mu (about 60 million hectares) of pine trees nationwide facing varying degrees of survival crisis.
If you gather dead pine wood for fuel, you could potentially spread this parasitic worm.
This parasite is called the pine wood nematode, the only one among 6201 species classified as harmful organisms in forestry in China to be categorized as a first-level threat.
Due to the presence of pine wood nematodes, even in areas severely affected by the disaster, although there may not be explicit prohibitions on individuals or families collecting dead pine wood for burning, it is certainly not allowed to store pine wood privately—all pine wood must be disposed of before March each year.
(Note: The area where I live is also affected by pine wood nematodes. Every pine tree in scenic areas is labeled with a warning sticker related to pine wood nematodes, but I have not heard of a ban on burning pine wood. The requirement to dispose of all stored pine wood before March was found online.)
Photo: A screenshot of a report on the ban of pine-related activities in Huangshan.
Moreover, some places have completely prohibited pine-related activities. For example, in Anhui's Huangshan, the pine trees significantly contribute to tourism, which is why the entry of pines by any means into Huangshan is explicitly banned.
So, what exactly is pine wilt disease, and why is storing pine wood in disaster areas not allowed? Why were there no such regulations before?
图: 中国各省份松材线虫传播时间
松材线虫是一种林业害虫，已在中国的19个省份广泛传播。它已经导致数亿株松树死亡，成为近几十年来中国最严重的林业害虫之一。松材线虫感染的松树表现出松材线虫病的症状，通常在2-3个月内枯死。因此，松材线虫病被称为“松树癌症”，是近几十年来中国最具威胁性的林业疾病之一。更令人担忧的是，一旦发现一棵松树因松材线虫病而死亡，如果没有采取人为控制措施，那么在3-5年内，整片松林的松树都将死亡。
Due to the pine wilt nematode, all the pine trees in a mountain forest wither. Source: forestry.gov.cn
Due to its rapid spread, high danger, and significant impact, the pine wilt nematode has become the only species classified as a first-level forestry pest.
The reason we hadn't heard of this disease before is because this parasitic worm is an invasive species, and it has only become increasingly prevalent in China in the past decade or so.
Originating in North America, the pine wilt nematode first spread in Asia, starting in Japan, before making its way to China. It was first discovered and reported in Jiangsu Province, China, in 1982, and now Jiangsu is the most severely affected area in China.
Asia suffers the most significant economic losses due to pine wilt disease, primarily because Asian pine trees lack resistance to the invasive species of pine wood nematode.
Unlike many other pests, the pine wood nematode is nearly invisible to the naked eye—adult individuals are less than 1mm long and very thin. However, they can reproduce rapidly within pine trees, causing extensive damage in a short period.
Under the microscope, the pine wood nematode, image source: Agricultural Bureau
In the initial stages of infection, the pine wood nematode enters the pine tree, feeding on its epithelial cells in what is termed the feeding phase. Subsequently, with extensive reproduction, they inhibit the absorption of water by the pine tree—creating small cavities in the woody tissue, thereby impeding the movement of water within the tree, ultimately leading to the tree's death.
The relationship between this parasite and its host is far from benign; killing the host is more advantageous for them, as they eventually enter a fungal feeding stage—feeding on fungi that proliferate after the death of the tree.
Nematodes are incredibly diverse, and the pine wood nematode is known for having the shortest lifecycle among parasitic nematodes—completing its entire lifecycle in under four days in laboratory cultures. This rapid reproduction results in the swift demise of the host.
However, pine wood nematodes do not spread from one plant to another on their own; they require a vector—their transmission vector being the pine sawyer beetle.
The infected pine trees gradually wither, Image Source: Fengcheng Forestry Bureau.
How does the pine wood nematode spread?
As mentioned earlier, in some areas, dead pine wood is not allowed to be stored after March. The reason lies in the fact that the vector for the transmission of the pine wood nematode, the pine sawyer beetle, begins to emerge after March.
There are two ways in which the pine wood nematode spreads through the pine sawyer beetle. One is through feeding by the pine sawyer beetle, and the other is through egg-laying by the beetle. These two methods are closely intertwined.
First, these two methods of transmission affect different types of pine trees.
Feeding transmission affects healthy pine trees, as the beetles prefer to feed on healthy trees, and the pine wilt nematode enters the tree when the beetles bite into it.
Oviposition transmission, on the other hand, affects trees that are already unhealthy, as the beetles choose diseased or even dead trees for laying eggs, which benefits the development of their larvae.
If the beetles carry the pine wilt nematode, they will transmit the nematode to the pine trees during both feeding and oviposition;
However, if the beetles do not carry the nematode, their choice of unhealthy or dead trees for egg laying means their offspring are likely to be infected with the nematode, as these trees are probable carriers of the nematode.
If beetles that do not carry the nematode lay eggs in a tree infected with the nematode, then the nematode will follow the entire life cycle of the beetle, spreading wherever the adult beetle travels.
Image: To ensure clean burning, infected pine wood is processed into chips before incineration.
In general, it's definitely okay to bring dead pine trees home and burn them directly. This is what's being done for pest control, to kill the pine beetles' eggs early on. However, storing dead pine wood can indeed be very dangerous. Therefore, in strictly regulated areas, it's sometimes not allowed to handle dead pine wood privately.
Image: After cutting down an infected pine, it is wrapped in an isolation layer
Conclusion
Over the past forty-plus years, the spread of pine wood nematodes in China has caused substantial economic losses. Most notably, some of the famous pines in tourist areas have succumbed to this parasite, a truly regrettable loss.
The good news is that in recent years, as control measures have intensified, there has been a decline in both the area affected by pine wilt disease and the number of pine trees dying from it.
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