Earth, Wind and Fire: Impact of Fire on Aeolian Processes in Partially Vegetated Dune Landscapes – adapted from a talk by Rosie Huck.  

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This worksheet looks at desert sand dunes. Someone who researches sand dunes is called an aeolian geomorphologist. Let’s break that term down:

Aeolian, a Greek word, means relating to or arising from the action of the wind.

Geomorphology, also a Greek word, is the study of physical features of the surface of the earth and their relation to its geological structures.

The main thing that aeolian geomorphologists study are sand dunes. However, you can get a lot of aeolian features in other areas: there’s also dust storms in pro-glacial valleys; desert areas that don’t have a lot of sand dunes where Ionian processes are really important; and aeolian features can be found on river deltas and floodplains where loose fluvial sediment has been deposited.

Studying dunes often involves going right down to individual sand grain movement, as the movement of all of these tiny hundreds of thousands of little sand grains ultimately result in dune formations.

Sand grains move in four main ways:

• Creep: where the sand grain rolls along the surface. It doesn’t ever get lifted off, it just rolls and this continual movement moves a lot of sediment.
• Saltation: where the sand grains bounce. The wind is blowing hard enough that they are ejected into the air a little bit and then they drop back down.
• Reptation: caused by this dropping back down of the saltating sand grains, reptation is where the impact of the saltating sand grain ejects more particles. It is similar to a positive feedback process, where once one sand grain starts going it can result in lots more sand grains going through its transferred velocity when it is ejected.
• Suspension: where the sand grains are so small that they’re more like dust and they can be transported thousands of kilometres away and held in the atmosphere.

But how about the impact of fire on aeolian processes in dune landscapes? While it may sound like a strange concept, a lot of dunes do set on fire, and this is a very important ecological process.

For example, the Kalahari desert in Namibia, Botswana and South Africa in Southern Africa. On the 1st September 2021, there were three big fires occurring in one day in this area. Such fires are very important to these landscapes, so it is important that we understand their frequency and what it means for aeolian processes in the area.

The fires can occur through sand dunes being partially vegetated – containing some plant life. In a lot of areas of the world people think of dunes as bare landforms, just all sand, but that’s not quite true. Many dunes are partially vegetated and these vegetations will stop the saltation, creep and reputation of the sand. This causes the sand dunes to become stabilised and the vegetation can therefore grow. This is a very important and common ecological factor in dryland landscapes.

Here’s a summary of how dune fires can take place:

• The sand dunes are covered by a mosaic of protective vegetation and biological soil crusts (a hardened surface of the soil containing living microorganisms) that change in both time and space because of dynamic and complex relationships between sediments, plants and wind erosion processes.
• Disturbances to these dynamic relationships can trigger sudden changes in the ecosystem’s state. These disturbances include grazing, drought or fire.
• The fire can be started by lightning or human activity. The fire will then remove the vegetation and the sand then loses that protection that the vegetation afforded onto it. This increases the erodibility of the surface which ultimately, theoretically, leads to an increase in sand movement on the burnt dunes.

In a little bit more depth:

• The fire removes the vegetation.
• During the process of being burnt, vegetation release fatty acids and their carbohydrates onto the soils below the plants.
• These acids then envelop the grains, cover them and reduce the intergrain cohesions and also make the grains hydrophobic.
• Hydrophobicity is a fancy Greek word for the soils and the sediment not enjoying water: when water is dropped on the soil, it is repelled. This then decreases the soil moisture retention, which also decreases particle cohesion.
• This can lead to an increase in the susceptibility of surface to wind erosion. There’s more space for the wind to erode the surface as the plants have gone. Because there are less bonds between the soils, there is also increased aeolian activity such as saltation.
• This increased surface activity can cause sediments to break up and generate dust-size materials themselves, as well as breaking up the biological soil crust.
• It has also been discovered that dunes have a large proportion of sediment that can be readily entrained in interdune areas. This is because the vegetation that used to be there traps finer sediments that are moving through the atmosphere above the vegetation, and the vegetation itself can generate some smaller sized particles.
• When combined with the reduced interparticle bonds, all of this should in theory result in increased sediment transport and potentially dust emissions from the sites that have burnt.

These rates of increased transport have been observed in North America. The highest rates of sediment transport in the whole of North America have been observed in the immediate aftermath of a fire. Distant dust emissions have been proved to happen in Australia, where satellites have caught emissions coming solely from areas that recently burned. So we know that these fires and effects happen, but we need to understand the processes behind them a little bit better. One of the areas that we don’t understand is that not all partially vegetated dune fields burn, most notably in South America, China and also in the desert on the Israeli-Egyptian border.

 

We’re not yet certain why this is, but it may be due to a lack of threshold and vegetation cover. It also could be due to a lack of ignition event – there’s no lightning happening to trigger the fire.

There are nevertheless lots of dune fields that do burn: the Kalahari desert in Southern Africa; the belt along the bottom of the Sahara Desert, which is in Mauritania and Mali; the central deserts of Australia; and then also in Uzbekistan, Turkmenistan and Afghanistan.

Why is this important, and why should we care about fires on dunes?

• The remobilisation of dunes can have devastating effects on local populations, through desert encroachment to vulnerable communities, the depletion of the agricultural potential of the land, a loss of food security, and a reduction in ecosystem biodiversity in the area.
• It’s important to understand vegetation renewal and carbon sequestration (the process of capturing and storing carbon dioxide from the atmosphere in natural environments such as soil or grasslands) in the area. After a fire, these areas go from a net sink of carbon (an area that takes carbon away from the atmosphere) to a net source of carbon (an area where carbon is added to the atmosphere) through the emissions made by dying vegetation.
• Understanding dust emissions is also a crucial factor, as dust changes the scattering and absorption of incoming solar radiation. It is also known to affect local cloud meteorology.
• Local tribes in the Kalahari area are actually known to set fires to try and induce rain because of the difference in albedo (the fraction of light that a surface reflects). This difference changes the colour of the surface and can cause rain clouds. The dust can also cause clouds to occur.
• Where this dust is deposited it enriches a local ecosystem by providing minerals.
• Dust storms are also known to have massive impacts on human health.

All of these are very important factors demonstrating why we should study these fires. However, there are currently several gaps in our knowledge about dune fires:

• Factors relating to the timing and frequency of fires.
• The pervasiveness and endurance of fire on aeolian processes.
• The processes and dynamics behind potential dust emissions.

The impact of fire on the erodibility of the surface – primarily through the change in vegetation and crust cover – and how this evolves after a burn event is not yet well understood. There has been previous work looking at this in the Kalahari area using wind velocity profiles, and it has also been looked at in 2009 in North America to model grass revegetation. But it hasn’t been widely researched in the desert environment.

The longevity of the fire impact may also vary geographically. This has not yet been fully defined in the Kalahari region, as the Kalahari has been postulated to have the scar of the fire visible for around seven years after a fire. But in Australia burn scars (the damage left by a fire) can be seen for a lot longer after a fire, sometimes over 25 years. This recovery time may therefore vary with climatic conditions and also burn severity of the actual fire. Dust emissions have also been observed in other locations, particularly Australia, but the processes behind these emissions have not been well understood.

Research at the University of Oxford therefore aims to quantify and evaluate the influence of fire on the erodibility of partially vegetated dune surfaces and subsequent changes in due mobility and potential for dust emissions. There are two approaches to this: field work and remote sensing, the use of satellite imagery.

Field work involves travelling to the sites that are being studied. In this case, visiting fire scars in the Kalahari. There are three fieldwork sites for this research:

• Site One, where a fire has burnt extremely recently and is now possible to set up some longer term monitoring conditions.
• To compare this site with older fires, other sites are set up around past burns. Site Two is a burn from three years ago.
• Site Three is a burn from seven years ago.

At each site, comparisons are made between the burnt area and the unburnt area by having unburnt and burnt control plots. These will run across different dune profiles in order to capture how the processes change in the interdune, which is the bit in between the dunes, as well as the dune crests and the sides of the dunes.

Remote sensing will be used to create an inventory of Southern African fires through satellite imagery and satellite algorithms, as we currently don’t know how widespread these fires occur on the landscape. Researchers will then try to detect any dust emissions from the fire scars after the fact.

We work out where fires are in the world and how much of the area burns with the MODIS burnt area product. MODIS is an instrument on the landsat satellite which detects active fires across the planet. In its next overpass of the same area it will then try to figure out how much of that area has burnt and if it’s still burning. However, while MODIS produces useful pixelated images of areas that have burned, in Southern Africa it often misses a lot of fire scars. Due to the pixelated nature of the imaging, a lot of the fine details of the landscapes are lost, so the MODUS underestimates the amount of area that is actually burnt.

The research on dune fires at the University of Oxford is trying to create a way to measure and capture the amount of land burned by the fire each year and also associate any patterns between these fires and meteorological conditions and climate systems.

One method is albedo differencing. Albedo is a measure of the reflection of the sun’s radiation on the earth’s surface. A high albedo is considered to be white and bright and a low albedo is considered to be dark. If you’re looking at a white wall in high sunlight it’ll quite often hurt your eyes – that’s because a lot of it is being reflected back onto you. However, if you’re wearing a dark shirt on a hot day you know that you get hotter – that’s because a lot of the radiation is being absorbed by your shirt and heating it up.

We have satellites that can measure the earth’s albedo. To use these satellites in relation to dune fires, you can take albedo images of the landscape before the fire and then contrast that to the albedo measurements from after the fire. This lets you see which areas have become brighter. This helps to identify fires, because the burning vegetation results in the land becoming brighter, as all of the ash and sediment is quickly moved away by fluvial or aeolian processes.

There is also a more traditional geographical fieldwork method that can help locate fire scars: drawing lines on a map. You can physically map all of the fire scars visible on the landscape by going through satellite images daily and looking at how its areas physically change. The Oxford University research also investigates dust emissions from fire scars. Satellite images show dust emissions coming from fire scars, but because dust and the ground surface are pretty similar colours it’s actually quite hard to detect them using regular satellite imagery. It is possible to enhance the images of these dust emissions using a technique called brightness temperature differencing, which relies on the fact that the temperature of the dust will be cooler than the land surface. The differences between those temperatures are measured in order to enhance the dust. It is also possible to use modelled meteorological data called ERA5 data, which shows which days were windy. On windy days, it is easier to identify the dust emissions, so satellite imagery can be preferentially downloaded and then brightness temperature differencing is conducted on it in order to try and see if there are any dust emissions from the fire scars and to quantify how much is actually being emitted.

There are many other methods also being used to measure the impact of dune fires.

Such new research is often a question of throwing different experiments at the problem and seeing what sticks. Using a hybrid of remote sensing satellite data and a range of field work techniques helps to make sure that no stones are left unturned.

To conclude:

Vegetation stabilises sand dunes.

The sand dunes then burn, and that changes the environmental conditions.

We don’t yet know how big the impact of this is on aeolian activity, particularly sand movement and dust emissions, and so current research aims to work this out using a hybrid of remote sensing satellite data and fieldwork.

Further reading

Click here to learn more about different aeolian processes.

If you want to learn about sand dunes in the UK, read this article.

You might have heard the word ‘dune’ a lot in relation to the hit films starring Zendaya and Timothee Chalamet. Click here and here to learn about the science behind the novel by Frank Herbert that the films are based on. For a more in depth account, watch this video.