"The grass on the Liliyuan, withering and flourishing every year. The wildfire cannot be burned, and the spring breeze will grow again." This is a familiar poem. With its tenacious vitality, weeds withstood the rampant wildfires and prospered again in spring. The ruthlessness of wildfires forms a sharp contrast with the tenacity of wildweeds. For a long time, people have been scared of talking about fire, and wildfires have irreversible and powerful destructive power. But what if the "evil and evil" wildfire also has a tender side? "The distant fragrance invades the ancient road, and the clear green connects the deserted city." The weeds became more and more prosperous after a fire. So, how do wildfires drive weeds to achieve prosperity?
Figure 1 Grassland fire (Pictures are from the Internet)
Wildfires must be extinguished?
my country is a country with frequent wildfires. Forests and grasslands in the southwest, southeast and northeastern regions of my country are high-risk areas for wildfires to occur. It is reported that between 2001 and 2017, a total of 162,990 grassland fires occurred in my country, with an average of 9,588 times a year. The fire in the grasslands in northern my country is mainly concentrated in spring and autumn. During this period, the withered surface combustible materials combined with dry and windy weather have created conditions for the frequent occurrence of fire. Due to the powerful destructive power of fire, we have formulated complete forest and grassland fire prevention regulations to ensure the safety of people's lives and property.
However, as an important ecological factor, fire is a natural selection force, and its lack will inevitably have an important impact on the evolution of species, the dynamic changes of communities and ecosystems. Figure 3 shows that the absence of fire gradually reduces the area of grasslands in grassland-forest habitats, which in turn threatens the survival of birds Dasyornis brachypterus , which is dependent on grasslands. Therefore, it is particularly important to systematically and comprehensively understand the role of fire in nature and strengthen the scientific management of fire.
Figure 2 The impact of fire loss on grassland-forest habitats (Stone et al. 2022)
Note: When fire is missing for 2-5 years, the grassland-forest habitat will maintain a harmonious and stable state; when fire is missing for 5-8 years, the proportion of shrubs in the habitat will gradually increase; when fire is missing for more than 8 years, the proportion of grassland will further decrease until it disappears completely.
wildfire, gently tap the sleeping soul of the seeds
"Lili of the grass on the earth, withering and flourishing every year", the grass on the wilderness flourishes in spring and autumn withering every year, cycles every year. Seed germination is the main way for wild grass to achieve reproduction and renewal and wilderness to achieve spring prosperity. After leaving the care of the mother, many plants choose to face the harsh external environment in a dormant state. Fire creates favorable external conditions for seed germination and seedling planting by reducing interspecies competition and improving resource availability.
Figure 3 The seedlings were successfully planted after fire (photo taken by the author)
Figure 4 The fire experiment of semi-arid grasslands of the Loess Plateau (picture from the Internet)
Combustion is both a physical process and a chemical process. Germination stimulants such as nitrogen-containing compounds, smoke, and smoke-related butenolide karrikins (KARs) and glycerinitrile (SP1) produced by plant combustion are called chemical signals. The high temperature generated during the combustion process and the changes in the light conditions after combustion belong to the physical signal (Figure 5). These burning signals are like an alarm clock in the field, gently tapping the seeds' sleeping mind, causing the seeds to end their dormant state and germinate.
The high temperature generated by fire can promote seed germination in the soil seed bank. After burning, chemicals that promote seed germination in plant smoke will be deposited on the soil surface. These germination irritants are volatile and water-soluble. Germination stimulating substances dissolve in water to form smoke and water, which can not only avoid volatility, but also transfer from the surface to the ground, and stimulate seed germination in the soil seed bank through the seed coat. Therefore, a rainfall after burning is of great significance to the recovery of vegetation and seed germination in the burning habitat.
Figure 5 Fire burning signals that affect seed germination (Li Shaoyang et al. 2021)
In the ecosystem, the germination of many plant seeds with different life history characteristics such as annual herbs, perennial herbs, shrubs, trees, etc. will respond to fire burning signals.For example, seeds of legume plants often have physical dormant, and the existence of an impermeable layer of the seed coat makes the entry of moisture a relatively difficult thing. The high temperature generated by combustion can destroy the thick water-impermeable layer of the seed coat, break physical dormancy, and promote seed germination.
In nature, there is another type of seed. Although they do not have thick seeds, they cannot germinate under appropriate conditions. Such seeds often have physiological dormant. In 1988, two scientists, van de Venter and Esterhuizen, were the first to point out that high temperatures are not the only factor affecting seed germination in the burning signal, and seed germination is closely related to multiple burning signals. Scientists isolated and measured complex compounds in tobacco and finally discovered two germination stimulating substances, butenolide karrikinolide and glyceronitrile. For partially dormant seeds, applying heat shock and smoke signal processing simultaneously often results in unexpected results. But it is worth noting that high-intensity burning signals will also inhibit seed germination and seedling growth.
Figure 6 Effect of different smoke time on the germination of seeds of dogtail grass (photo taken by the author)
Figure 7 Effect of different smoke time on the growth of seedlings of dogtail grass (photo taken by the author)
Conclusion:
Fire burning, as a basic ecological process, has an important impact on the structure and function of terrestrial ecology such as forests and grasslands. Whether wildfires and weeds can become friends requires the joint efforts of both parties. First of all, the temper of wildfire should not be too irritable, and should have an appropriate burning intensity; secondly, the weeds should have traits or strategies to adapt to burning.
References:
1. Li, S., H. Ma, M. K. Ooi. 2021. Fire-Related Cues Significantly Promote Seed Germination of Some Salt-Tolerant Species from Non-Fire-Prone Saline-Alkaline Grasslands in Northeast China. Plants 10: 2675.
2. Li Shaoyang, Ma Hongyuan, Zhao Dandan, Ma Mengyao, Qi Wenwen. Research progress on the impact of burning signals on seed germination. Acta Plant Ecology 45: 1177.
3. Stone, Z. L., M. Maron, E. Tasker. 2022. Reduced fire frequency over three decades hastens loss of the grassy forest habitat of an endangered songbird. Biological Conservation 270: 109570.
4. Jin Quanfeng, Shen Peifu, Huang Haisong, Chen Binghong, Fu Chunling, Guo Futao. 2019. Estimating the space-time pattern of grassland fire pollutants in mainland China based on MODIS images. Journal of Environmental Science 39: 1412-1424.
Source: Northeast Institute of Geography and Agricultural Ecology, Chinese Academy of Sciences
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