“Wildfires cannot be extinguished, spring breezes will bring them back to life” It turns out that this is because…

When it comes to wildfires, many people will think of the following scene: raging flames, and the fire destroying everything. However, in nature, not all wildfires are harmful and have no benefits.

Maui wildfire (Photo credit: Rebecca Hernandez (CC0))

Although wildfires always cause immeasurable damage to forests, some plants have not only found ways to survive in the raging fires, but can even thrive with the help of wildfires. Moreover, artificially "tamed" wildfires can also become a powerful tool for protecting biodiversity.

Frequent wildfires lead to more abundant species?

In most people's imagination, areas with frequent wildfires must have harsh environments and fewer organisms. In fact, the species richness in areas ravaged by wildfires is not inferior to that in other areas, and may even be better.

For example, the Mediterranean climate, with hot, dry summers and mild, rainy winters, is prone to forest fires. The world's five Mediterranean climate regions (Mediterranean Basin, California, central Chile, Cape Region of South Africa and southwestern Australia) are all biodiversity hotspots. Together, they account for only about 1.2% of the world's land area, but are home to about one-sixth of the world's plant species. All Mediterranean climate regions, except central Chile, are under the threat of repeated fires.

Global distribution of Mediterranean climate regions (Image source: Reference [1])

In a Mediterranean climate, frequent fires almost always occur in hot, dry summers, which makes fires highly periodic and predictable, becoming a suitable selection pressure that changes the evolutionary direction of local plants.

There are currently some hypotheses to explain the mechanism by which fire promotes plant diversity. The intermediate disturbance hypothesis in ecology holds that if the interval between fires is too short, plants may not have enough time to produce seeds and other reproductive bodies and be eliminated; if the interval is too long, it may exceed the life cycle of the plant; and fires with appropriate intervals are conducive to self-renewal of plants that have produced or stored enough seeds. The thermal diversity-biodiversity hypothesis holds that fires cause new small-scale "mosaic" environments to appear in ecosystems, with small pieces of environments of different properties distributed at intervals, resulting in more diverse ecological niches and creating conditions for the entry of more species.

Hundreds of millions of years ago, plants began to "walk with fire"

When did plants begin to adapt to fire?

Charcoal fossils from 400 million years ago prove that plants had to face the threat of fire a long time ago (how to study wildfires hundreds of millions of years ago). Some studies have shown that gymnosperms had some characteristics adapted to fire about 100 million years ago , and the widespread spread of angiosperms in the Cretaceous was also related to the high oxygen content in the atmosphere and frequent fires at that time.

During the long years of living with fire, adaptation to fire has given some plant lineages an advantage in competition and contributed to their prosperity. For example, Pinus, the largest genus of gymnosperms, contains more than 100 species and occupies the vast coniferous forests in the Northern Hemisphere. Its spread and diversification are closely related to adaptation to fire.

The North American shortleaf pine, Pinus banksiana, is a fire-adapted tree that retains dead branches on its trunk, increasing its flammability (Image source: Reference [4])

For plants that can adapt to fire, fire not only kills competitors and frees up more resources for themselves, but also leaves a lot of nutrients in the ashes for them to use (the "slash-and-burn" method in agriculture also uses this principle). At the same time, the heat and chemicals released by combustion (such as carcinogens in smoke) also act as mutagens, producing more genetic mutations and other heritable variations in organisms , providing raw materials for biological evolution. So, in order to rationally utilize these advantages, what adaptive characteristics have plants evolved?

Tips for preventing plants from getting burned

1. You can't burn me, you can't burn me

The simplest and most brutal way to resist fire is to strengthen mechanical defense and protect internal tissues with hard armor. This type of plant is mainly tall trees with thick bark near the ground, which can resist the attack of ground fire. At the same time, their branches and leaves are often grown at a higher position on the upper part of the plant, and there is a long section in the middle of the trunk without branches (branches die and fall off), forming an isolation layer so that the flames on the ground cannot reach the upper branches and leaves.

Many fire-resistant pines adopt this strategy. In addition, their pine needles are often long and loosely piled on the ground after falling off, which is very flammable, making it easier to start ground fires to remove these dead branches and leaves and avoid too thick accumulation. In this way, frequent ground fires will not only not harm these pines, but also help them remove competitors such as oak seedlings under the forest. However, this also has the risk of burning their own seedlings, so the seedlings must grow quickly to exceed the range of the ground fire.

The thick, soft bark of Myrica bella (Myrica spp.) in the Brazilian savanna acts as an insulator (Image source: Reference [4])

2. If it is not burned by fire, it will not grow back?

When a fire strikes, another important task for plants is to protect their reproductive bodies. Therefore, plants exposed to fire risk need to strictly protect their seeds so that they can survive the scorching flames.

Many plants have evolved a trait called Serotiny, where their seeds are protected inside woody fruits or cones. They do not crack immediately after ripening, but only crack and release the seeds inside after being baked at high temperatures.

This characteristic enables plants to defend against canopy fires. Even if the branches and leaves of the canopy are severely damaged in a fire, as long as the seeds are intact, the plants can successfully reproduce. These trees often leave some dead branches on the trunk to guide the flames to the canopy layer, promoting seed dispersal through fire.

Currently, more than 1,200 plant species are thought to have this trait, including representative groups such as Pinus, some genera of the Proteaceae family (such as Banksia and Protea), and some genera of the Cupressaceae family (such as Cupressus and Callitris).

Closed cones of Pinus brutia before fire (Image source: Reference [4])

The cones of the Mediterranean cypress Cupressus sempervirens cracked after being burned (Image source: Reference [4])

3. The raging fire awakened me...

Generally, after a fire occurs, most of the combustible materials on the ground are burned, making it difficult to start a fire again in a short period of time. Some plant seeds take advantage of this period to germinate and grow.

Their seed coats have high mechanical strength and are relatively fire-resistant. After maturity, they will go through a period of dormancy and will only germinate when exposed to fire.

For example, some leguminous plants have seeds with hard outer skins that are impermeable to water and can only absorb water and germinate after being burned. Other plant seeds sense the occurrence of fire based on the chemicals produced by combustion, and only germinate after contact with a butyrolactone compound (karrikinolide) in the smoke, such as the South African redwood shrub Audouinia capitata.

Audouinia capitata (Photo credit: www.gbif.org)

4. Burned, but not completely dead

Not only seeds, but many fire-resistant plants also have strong regeneration capabilities and can use the interval after the fire to sprout again. The charred black after the fire does not mean that the plant has been burned to death. The surviving underground organs or stem bases can often survive the flames, leaving hope for life.

In areas where fires occur repeatedly, many woody plants develop an enlarged, woody crown at the root-stem junction, storing nutrients and developing dormant buds within it. Some groups, such as Eucalyptus, can also resprout from the main trunk remaining after a fire.

Adenostoma fasciculatum in Southern California, resprouting from woody tubers after fire (Image source: Reference [4])

The period after the fire is when resources are most abundant and competitive pressure is minimal. The burning of cover creates a more open environment, increases pollinators, and is therefore conducive to plant reproductive activities.
About 50 families are known to have species that flower and bear fruit after fire, and these plants typically make up 10% of the plant species in heaths and savannas of Australia and South Africa.

For example, Nuytsia floribunda, a parasitic tree in the family Loranthaceae, absorbs nutrients by parasitizing the roots of other plants. After a fire, it replaces the burned bark with abnormal secondary growth and produces a large number of panicles along the surviving branches.

Nuytsia floribunda (Photo source: www.gbif.org)

How humans use fire to protect ecosystems

In areas where fires are frequent, fire has become an indispensable element of the local ecosystem. Just like Dayu's flood control, people try to maintain the health of the ecosystem by artificially controlling the occurrence of fires.

Proteaceae plants in South Africa reproduce by burning their fruits to release seeds, but the successful growth of seedlings is sensitive to the seasonality and size of the fire and the climate conditions before and after the fire. Wet conditions after fire are more conducive to seed germination, while long-term drought during the seedling period is likely to cause mortality. Therefore, it is better to manage the health of the community by setting fires before the rainy season.

There are also cases in my country where fire is used to protect biodiversity. Human intervention fire has been used to protect the national first-class protected plant Cycas panzhihuaensis. Using fire can not only control the growth of shrubs and reduce their shading to Cycas panzhihuaensis, but also kill some pests and reduce their damage. In addition, human intervention fire can clear the accumulation of debris under the forest to prevent large-scale, uncontrollable fires caused by too much accumulation.

Sichuan Panzhihua Cycad National Nature Reserve implements artificial intervention fire experiment (Photo source: China National Radio)

Human intervention in fire not only helps to protect plants, but also has application value in the protection of wildlife. For example, planned fire can effectively increase the diversity and biomass of herbaceous plants, reduce the height of woody plants, and provide more choices for sika deer to forage. Using planned fire to create a mosaic distribution of various types of habitats is quite beneficial to the survival and reproduction of sika deer.

Why do we still need to pay attention to forest fire prevention?

From the above, it can be said that plants have adapted to fire in various ways. Some people may wonder: since plants can adapt to fire, and fire is one of the reasons for increasing biodiversity, can we just leave forest fires alone?

The answer is of course no.

The adaptive characteristics of fire-resistant plants today are the result of long-term coexistence with periodic fires, but not all forests live in such an environment. Too much is as bad as too little, and fires that exceed the specified scale can have a devastating impact on the ecosystem. In particular, climate change and human activities in recent years have made wildfires more frequent, and forest plants are facing greater risks.

Human intervention in fires requires scientific and rigorous research to find the threshold that is most conducive to ecosystem protection. We should scientifically understand the relationship between plants and fire, make rational use of it, and scientifically prevent and control it to protect the green heart of the earth.

References:

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[4] Keeley JE, et al. Evolutionary Ecology of Fire. Annual Review of Ecology, Evolution, and Systematics, 2022, 53: 203-225.

[5] Jin WT, et al. Phylogenomic and ecological analyzes reveal the spatiotemporal evolution of global pines. PNAS, 118 (20) e2022302118

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[10] Heelemann S., et al. Fire season effects on the recruitment of non-sprouting serotinous Proteaceae in the eastern (bimodal rainfall) fynbos biome, South Africa. Austral Ecology, 2008, 33 (2): 119-127.

[11] “Fire” plant living fossils protect the national treasure-level plant Panzhihua Cycas revoluta. China National Radio. http://sc.cnr.cn/sc/2014sz/20160216/t20160216_521386014.shtml

[12] Jiang Zhigang, Qin Haining, Li Chunwang, Liu Wuhua, Liu Jian, et al. Study on biodiversity in Taohongling Sika Deer National Nature Reserve, Jiangxi Province. 2009. Beijing: Tsinghua University Press.

Author: Wei Zhourui

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