RSS feed for Fire ecology

Introduction

The Open University — Mon, 04 May 2020 14:40:05 GMT

This course explores the role of fire as an agent of natural disturbance in ecosystems. Many terrestrial ecosystems around the world are fire-prone and fire, or more specifically, the fire regime, controls their composition and structure. Factors such as climate, vegetation type and the source of ignition differ among ecosystems and affect aspects of the fire regime such as its extent (size), frequency and intensity. Because plants and fire share a long evolutionary history, many plants have evolved adaptive traits that not only allow them to persist in flammable environments but to depend on fire for reproduction and growth.

One important consequence of natural fires is the creation of spatial and temporal variation within a landscape, which generates and maintains biodiversity. Many animals exploit the consequences of fire and the mosaic of habitats generated by fire supports a wide diversity of animals and plants. The course also briefly addresses some of the effects and consequences of climate change and global warming on the intensity and frequency of fires.

This OpenLearn course is an adapted extract from the Open University course S397 Terrestrial ecosystems.

Learning outcomes

The Open University — Mon, 04 May 2020 14:40:05 GMT

After studying this course, you should be able to:

explain why fire is an important component of healthy ecosystems
explain what is meant by a ‘fire regime’ and how changes in fire regimes can lead to changes in ecosystems
describe some adaptive features of plants that have evolved in response to fire
describe how some animals avoid fires and/or benefit from the aftermath of natural fires
explain how global warming may result in an increase in both the frequency and intensity of fires.

1 Fire as a natural disturbance

The Open University — Mon, 04 May 2020 14:40:05 GMT

Fire is a natural agent of disturbance. The origin of fire is tied to the origin of plants, which are responsible for two of the three elements essential for fire to exist: oxygen and fuel. The third element – a heat source – has probably been available throughout the history of the planet. Before the appearance of photosynthetic organisms, the atmosphere lacked sufficient oxygen to support burning and before the existence of terrestrial plants, it lacked fuel. Evidence of low-temperature surface fires dates back 440 million years and fire appears to have been continuous since plants invaded the land.

The first evidence that fire actually altered, and had major impacts on ecosystem functioning dates back to the late Tertiary, about 10 million years ago. The spread of C₄ grasses during this period was due to increased fire activity which opened up woodlands and created environments more favourable to C₄ grasslands. The high flammability of such grasslands produced a feedback process that further increased fire activity.

Hominids have used lightning-ignited fire for perhaps as long as 1.5 million years, but first began to ignite fires between 200 000 and 400 000 years ago. Over recent millennia the occurrence of fire has been increasingly influenced by human activity and is currently a major cause of global atmospheric pollution and contributes significantly to the rise in greenhouse gases: CO₂ (carbon dioxide), CH₄ (methane) and N₂O (nitrous oxide). Fire and fire management are becoming increasingly important issues for ecosystem management.

1.1 The effects of fire on an ecosystem

The Open University — Mon, 04 May 2020 14:40:05 GMT

Given the 2020 wildfires on Australia and the obvious devastation they have caused, the perception of wild fires is often that they are destructive disturbances. However, wildfires are an important part of healthy ecosystems and play a key role in shaping the composition, structure and function of many ecosystems on Earth.

Activity 1 The effects of fire on an ecosystem

Allow 30 minutes for this activity.

Watch Video 1, which is the excerpt ‘Fire as an agent of perturbation’ from the BBC programme The Living Planet, Northern Forests to give you an introduction to fires as an agent of ecosystem disturbance. As you watch the video, think in particular about the effects of fire on:

the dominant kind of vegetation
the overall biodiversity.

You should also consider what effects the frequency of fire might have on:

the structural complexity of the habitat
the resistance and resilience of the habitat to fire, i.e. its susceptibility and the time taken to recover from fire respectively.

You should refer to the following notes and questions as you watch the video. You may like to use the text box below to record your notes.

Download this video clip.Video player: Video 1 >

Video 1 Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).

Skip transcript: Video 1 Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).

Transcript: Video 1 Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).

DAVID ATTENBOROUGH: I’m on the borders of Florida and Georgia in the southern United States. And here, it’s very hot in the summer. And the winters are very mild, with only a few frosts, and none of them severe.

So some of the broad-leafed trees here, like, for example, this oak, don’t shed all their leaves in the autumn, but keep them throughout the year and continue growing. And these aren’t the only evergreens that are here, either. There are pines. In some parts, where the soil is very rocky, or sandy, and poor in nutrients, the pines will grow, because nothing else can survive there. But this pine forest owes its existence to another factor altogether.

[FIRE BURNING]

Oak saplings are killed within minutes by fire. But the terminal buds of young pines are surrounded by a shock of needles. They burn at a relatively low temperature. And by the time the flames have consumed them, the main fire has swept by. And the buds at the top of the stem, from which new growth will come, is still unharmed.

Fires like these are not just the work of careless people. They occur naturally. The spark that regularly sets fire to these forests is lightning. In this part of the southern states, violent thunderstorms are common. And lightning often strikes the taller trees, scoring a deep groove down the length of the trunk as it flashes down to Earth.

And this, at my feet, is the tinder which set it aflame. These are pine needles. And they’re so full of resin and they’re so dry that they flame up very easily. But the fire they produce is not very hot. And it’s also very short-lived, so that if any creature can survive for just one or two minutes, then it can survive a fire like this.

[SNAKE HISSING]

The rattlesnake, like many other ground-living animals, regularly takes refuge from the midday sun in holes. So now, it knows exactly where to go to escape the fire.

[SNAKE SLITHERING]

[BIRDS CHIRPING]

[SNAKE RATTLING]

But this hole is already occupied by its digger and owner, a gopher tortoise.

[MUSIC PLAYING]

[SNAKE RATTLING]

Rattlesnake and tortoise do not normally interfere with one another. And that seems to be the way things are going to stay. But in the back of the burrow lies another refugee, an indigo snake. And it, on occasion, eats rattlesnakes.

[MUSIC PLAYING]

But the fire is passing. And the rattlesnake can return to the forest. Some insects don’t avoid fire. They actively seek it.

Beetles find it difficult to lay their eggs in the pines, because the trees swamp them with resin. But a tree killed by fire, it can’t resist. And these beetles take advantage of the situation.

They have pits behind their legs, which are sensitive to infrared rays. And therefore, they can detect the slightest rise in temperature. And with these to guide them, they travel from all over the forest to the wake of the fire and arrive in hundreds.

Quickly, they mate. The females crawl all over the scorched trunks, seeking crevices in the bark into which they can lay their eggs, so ensuring that they’re grubs will have some nice nutritious bark to chew. As insects assemble in the burnt forest, the insect eaters follow.

The oak toad almost exactly matches the colour of the charred forest floor. Other more conspicuous hunters wait on newly-emerged shoots. Within a couple of months of a summer fire, the forest has more than recovered. It is rejuvenating. The fire has cleared away the old growth on the ground, and by reducing the pine needles the ash, has released their nutrients into the soil. And now the ground sprouts more flowers than at any other time.

[BEES BUZZING]

[BIRDS CHIRPING]

Because of regular fires, big bushes can’t establish themselves here. So swampy areas are not colonised and sucked dry by them, as happens elsewhere. And open marshes remain where pitcher plants can grow and where frogs can swim and breathe. Indeed, one species of frog lives no one else but in these pools in the American Pine Barrens.

[BIRDS CHIRPING]

[FROGS CROAKING]

The woodpeckers here can’t excavate their nest in dead trees, as do woodpeckers elsewhere. For in this fire-ravaged forest, they will risk incineration. So the red coquetted woodpecker drills its holes in living pines. But the wood is so hard, it takes several woodpeckers about two years to dig the hole.

Resinous sap seeps out around the hole where the outer layers of the tree have been breached. So the birds make their hole low down on the trunk, where the inner sap-free heartwood is thick enough to accommodate the entire nest. The flow of resin is diverted to the outside by drilling pits like sap wells above and below the hole.

[BIRDS CHIRPING]

It’s in these laboriously-excavated holes that the red-coquetted woodpecker raises its young.

[BIRDS CHIRPING]

The holes are very conspicuous, for each is surrounded by a sheet of yellow, congealed resin.

[BIRDS CHIRPING]

The rat snake is a great robber of nests and stealer of chicks.

[MUSIC PLAYING]

It’s an extremely skillful tree climber. Since the woodpeckers’ hole in the living tree has to be fairly low down on the trunk, it is within easy reach of the snake, and therefore, might seem to be in considerable danger. But now, the other function of all that resin, deliberately produced around the nest by the woodpecker, is about to become clear.

[MUSIC PLAYING]

[BIRDS CHIRPING]

The chemicals in the resin seem to irritate the snake beyond endurance. And it arches its body away.

[SNAKE HISSING; DAYLIGHT VISIBLE BETWEEN MIDDLE OF SNAKE AND TRUNK OF TREE]

Eventually, it’s too much.

[SNAKE FALLS OFF TREE; LANDS ON FOREST FLOOR]

[BRASSED-OFF SNAKE SLITHERS INTO UNDERGROWTH]

So fire, one way or another, influences the whole community of animals and plants in the pine forests of the South.

End transcript: Video 1 Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).

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Video 1 Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).

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Sir David Attenborough introduces the excerpt by showing that there are two dominant types of tree in the woodland.

What are the two dominant types of tree in the woodland, and how do they differ in terms of their fire resistance?

Answer

They are Pinus spp. (pines) and Quercus spp. (oaks). It is pointed out that the Quercus seedlings are destroyed by fire, while the Pinus seedlings are more resistant to fire because they have a protective thatch of pine needles around the growing tips.

Quercus spp. trees are stronger competitors than Pinus spp. trees for light and nutrients.

What would happen to the dominant tree type in the woodland if fire was suppressed over many decades?

Answer

The woodland would come to be dominated by Quercus spp. trees.

Video 1 also reveals that litter composed of resin-rich pine needles is more combustible than fallen Quercus spp. leaves.

Given this fact, what would you expect to happen to the likelihood of fire starting as the time since the last fire increases?

Answer

The probability of fire starting would fall as the proportion of Quercus spp. leaves in the litter layer increased.

Conversely, in a pine-dominated woodland, the litter layer is dominated by pine needles, and the more pines there are, the more needles there are, and so the more likely it is that fires will start.

Satellite imagery now makes it possible to view the vast areas of the Earth that have been, and are currently, subjected to fire:

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Video 2 The locations of actively burning fires around the world (Please note this video has no audio.)

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Video 2 The locations of actively burning fires around the world (Please note this video has no audio.)

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The map shows the locations of actively burning fires around the world on a monthly basis, based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. The colours are based on a count of the number (not size) of fires observed within a 1,000-square-kilometer area. White pixels show the high end of the count — as many as 30 fires in a 1,000-square-kilometer area per day. Orange pixels show as many as 10 fires, while red areas show as few as 1 fire per day.

Many of these fires have, in recent times, been catastrophic, destroying lives, properties and infrastructure. In 2016 and 2017, large areas of Canada, the western United States, southern Europe, Greenland, Indonesia, Chile, Australia and South Africa were affected by wildfires and this trend is set to continue with rising global temperatures, something that will be returned to later in the course.

Although in some cases, the economic and social effects of wildfires can be devastating, as you saw in the previous video, fire is also an important natural ecological phenomenon, regulating and influencing community composition and ecosystem function in many parts of the world. At present, the type of community (e.g. whether grassland or forest) and the relative abundance of species present on over half the land’s surface is believed to be determined largely by the interaction of fire and climate.

1.2 Fire dependant ecosystems

The Open University — Mon, 04 May 2020 14:40:05 GMT

Using dynamic global vegetation models, ecologists have compared the present global vegetation distribution which is subjected to fire with the potential vegetation distribution in the absence of fire. They estimated that if fire were ‘switched off’ dense tree cover would increase from 27% to 56% of the vegetated earth surface and more than half (52%) of tropical savannahs would become tropical angiosperm-dominated forests. This suggests that the presence of savannah and grasslands in climates wet enough to support forests is due largely to fire.

For the past few thousand years, fires have been started deliberately by humans. However, well before the emergence of humans, earth fires were started naturally by lightning, and the close association between fires and the emergence of plants suggests that fires played a key role in the origins of adaptive morphological characters of many plants that evolved in fire-prone areas. There are now many species and whole communities of plants that cannot persist unless fires occur with a certain minimum frequency. One example of such a community is the Fynbos (meaning ‘fine bush’ in Afrikaans) of South Africa – a natural shrubland/heathland vegetation occurring mainly in the winter rainfall Mediterranean climate areas of the Western Cape, South Africa.

Activity 2 Fynbos on fire

Allow 5 minutes for this activity.

Watch Video 3, which describes the effect of fire on the Fynbos in the Cape region of South Africa.

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Video 3 Fynbos on fire.

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Transcript: Video 3 Fynbos on fire.

NARRATOR: Life on the Cape has been blessed with a unique mix of circumstance, isolation, solid foundations, and a stable, nurturing climate. But there’s a twist in this tale. Come the end of winter, the Cape undergoes a dramatic change. The cool, moist winds subside. The air becomes tinder dry.

Every 15 years or so, the mountains erupt in flames. Summer fires are ignited by sparks from rockfalls, by lightning or controlled burn. It looks fearfully destructive, but fire is actually essential to the health of the Fynbos. Fire clears away old vegetation, releasing important minerals to the soil. For proteas, the intense heat and smoke are also crucial catalysts for the next generation. Within hours of the fire passing, millions of seeds are released to the wind to await the next winter rains.

End transcript: Video 3 Fynbos on fire.

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Video 3 Fynbos on fire.

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The commentator mentions that fire is important in maintaining the health of Fynbos for two reasons. What are these?

Answer

Fire releases important minerals into the soil.

Fire is important for the release of seeds.

Communities such as Fynbos are known as fire climax communities and are typical of areas with a Mediterranean climate (hot, dry summers and warm, wet winters). However, in virtually all regions with a definite dry season, fire has become an essential part of cyclic change in ecosystems. It is now clear that in order to sustainably manage ecosystems both for conservation and timber production it is necessary to understand both the ecological role of fire and a central aspect of fire ecology, which is the concept of a fire regime.

1.3 Fire regimes

The Open University — Mon, 04 May 2020 14:40:05 GMT

Fire has predictable features regarding how it spreads across landscapes and the frequency and season of occurrence. Such predictability has led to the concept of a fire regime: the temporal and spatial characteristics of the fire and the impact it has on the landscape (Table 1).

Table 1 Characteristics used to describe fire regimes

Fire characteristic	Description
Type	There are 3 basic types of fire: crown fires, surface fires and ground fires.
Frequency	How often a fire occurs in an area/ecosystem or the return time of a fire to an area/ecosystem.
Intensity	Refers to the energy release and is measured by flame length or rate of spread.
Severity	A measure of the impact of fire on an ecosystem. In forested ecosystems tree mortality is often used as measure of severity; in shrublands the metric is the mortality of above-ground plants.
Size	Spatial extent of the fire.
Pattern	Patch size distribution of fire; spatial heterogeneity of fire effects.
Season	Time of year that fire occurs. Fire season is determined by the coincidence of ignitions and low moisture in the fuel. This is usually the driest time of the year.
Duration	Length of time the fire persists.

The ecological effects of fire depend largely on the fire regime but the concept of a fire regime is far from simple. The variability in the characteristics listed in Table 1 coupled with the interaction of these characteristics with other disturbances as well as climate, make fire regimes one of the most complex processes governing landscape dynamics and controlling biodiversity.

Although there are several characteristics that define a fire regime, they are most often described in terms of severity and frequency because these two factors are the most important to land management.

1.4 Fire severity

The Open University — Mon, 04 May 2020 14:40:05 GMT

What determines fire severity is mainly the temperatures reached and the speed at which the fire spreads. These in turn are a function of climate, vegetation, topography and soil. A source of fuel is the first prerequisite for fire and must be dry enough to burn. Hot dry and windy weather conditions are key driving factors influencing fuel moisture, which is why primary rainforests virtually never burn naturally. A few days of hot dry weather is sufficient for dry tall grasslands to sustain a fire whereas months of extremely hot and dry conditions are needed to burn tropical forest. This means that fires in tropical forests are associated with rare prolonged drought events such as those produced by El Niño conditions. Grasslands on the other hand, burn frequently and tend to burn after high rainfall years when long grass and hence fuel is abundant.

The type of vegetation is also an important determinant of fire severity. Dead litter has the lower moisture content compared to live vegetation and so will burn more easily but the shape and size of plant parts also influence moisture content and hence flammability. Species such as Pinus spp. with narrow leaves and thin branches burn readily. Also, the high wax or oil content of these species mean that they burn at especially high temperatures.

The rate of fire spread, and to some extent its severity, are influenced by wind speed. Fires also move faster up slopes than over level ground. The rate of spread approximately doubles for every 10° increase in slope (Figure 1).

Figure 1 Fire spreads more rapidly uphill than over level ground. The heat in (b) rises in front of the fire more effectively than in (a), so it preheats and dries up-slope fuels, making for more rapid spread.

RSS feed for Fire ecology

Introduction

Learning outcomes

1 Fire as a natural disturbance

1.1 The effects of fire on an ecosystem

Activity 1 The effects of fire on an ecosystem

Transcript: Video 1 Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).

Answer

Answer

Answer

1.2 Fire dependant ecosystems

Activity 2 Fynbos on fire

Transcript: Video 3 Fynbos on fire.

Answer

1.3 Fire regimes

Table 1 Characteristics used to describe fire regimes

1.4 Fire severity

1.5 Fire frequency

2 Adaptation of plants in fire-prone ecosystems

2.1 Thick bark

2.2 Sprouting from insulated buds

2.3 Fire-stimulated seed release

Activity 3 Serotiny and fire

Transcript: Video 4 Giant sequoias and fire.

2.4 Fire-stimulated seed germination from seed banks

2.5 Fire-stimulated flowering

3 The response of animals to fire

3.1 Avoiding fire

3.2 Benefitting from fire

3.3 Morphological traits for detecting fire

4 Fire, habitat complexity and biodiversity

4.1 Fire and ecological succession

4.2 The intermediate disturbance hypothesis

4.3 The pyrodiversity-biodiversity hypothesis

5 Climate change and fire ecology

Conclusion

Glossary

References

Acknowledgements