Prometheus

It is sometimes said that humans were born of fire. While a wide range of animal species interact with fire, we appear to be the only species to have learned to tame it, and more importantly to make it. There is evidence that early humans were aware of fire and may have exploited naturally occurring fire, but only later did they control and manage it. Human interaction with fire must have proceeded through various levels, the first of which can be described as the opportunistic phase. In this phase, natural fire may have been exploited to help in hunting, for example. When, how, and why did this happen? It is widely agreed that our story begins in Africa. It is here that we see the evolution of hominins, a group of related genera that include the Australopithecines and later the genus Homo. How common would fire have been in the environments in which they lived? We already know from the study of fossil plants, as well as isotope data, that there were important changes in both the vegetation and climate over the past 10 million years. It is also during this time interval that hominins emerged from apes. Through the Oligocene and Miocene (30–8 million years ago), Africa was largely covered by tropical rainforest, where fire was present but infrequent, started both by lightning strikes and volcanic activity. As the climate began to dry and C4 grasses spread at the end of the Miocene Epoch, around 8 million years ago, habitats became more open. Fire became more frequent, and from an animal perspective would have become more visible, not just from flames but also smoke. Frequent fire in the landscape would have had many consequences for the early hominins, not just because game was more easily killed, but burned animals (naturally cooked meat) would have made a useful addition to the diet, and the new flush of growth following fire would also have attracted large herds of herbivores. Fire may have been conserved through adding fuel, including dung, which is slow burning.

Introducing fire

Fire has a bad reputation. Wildfires raging across parts of California and Australia make headlines. In the news bulletins, it is a destructive force that has to be quenched. But that is far from the whole story. Fire has a long history. In our deep past, wildfire helped shape aspects of our planet, and plants and animals have adapted to it in a variety of ways. In this book, we will follow the story of fire through time. But we begin with the present, with the fires that occur around the world today, and how satellites are changing our view of wildfire. Most of us have little or no experience of a wildfire, apart from those dramatic scenes shown on our television sets from time to time. Almost invariably, two questions are asked: who started the fire, and how quickly can it be put out? Reasonable though they seem, these two questions betray a potential misunderstanding of how fire works on our planet. We assume that the fire was started by humans, either accidentally or deliberately. This may indeed be true, but more than half of the fires started across the globe have a natural cause—mostly lightning strikes, but also other causes such as volcanic activity. Every moment of every day, a fire is burning somewhere in the world. The second assumption is that a fire should always be suppressed. But should we always be rushing to put out a vegetation fire? Wildfire is one of nature’s most frightening manifestations. Winds and storms may die down, and we can seek shelter from them, but fire can be difficult to outrun and escape. Many who are killed by wildfire have underestimated this force of nature, and even those with experience in putting out fires can find themselves cut off, and succumbing to the flames. As we shall discover, not all vegetation burns in the same way, and there are many different kinds of fire, from those burning surface vegetation to those moving through the crowns of trees. Their consequences may also be very different.

Fire, Flowers, and Dinosaurs

The Mesozoic Era is the geological interval comprising the Triassic, Jurassic, and Cretaceous Periods, and it is best known for the rise and fall of the dinosaurs. The Mesozoic began around 250 million years ago and continued to around 66 million years ago—a not inconsiderable chunk of geological time, and framed by mass extinctions at its beginning and end. Fifty years ago there were very few published papers on fire in deep time, but the most important one, which I’ve touched on before, was ‘Forest fire in the Mesozoic’, by Tom Harris of the University of Reading. Tom was an important scientist, one of the leading palaeobotanists in the world. Energetic and passionate about his fossil plants, he was a scientist with broad interests, and given to experimentation and lateral thinking. The evidence that Tom used in his paper on fires in the Mesozoic was limited to only a couple of charcoal occurrences in these rocks. The Permian Period ended with the biggest known mass extinction in Earth history, when life was almost wiped out. Whole ecosystems collapsed. So what would the world have looked like at the start of the Triassic? Among whole groups of plants that had become extinct were the giant club mosses that had been the major coal-forming plants of the late Paleozoic, and the glossopterids that had dominated southern continental vegetation. In the first few million years after the extinctions, plant diversity appears to have been low, but some new plants became prominent, including the pole-like spore-bearing lycopod called Pleuromeia, and the scrambling seedplant called Dicroidium, which had fern-like foliage. The first 10 million years of the Triassic are thought to have been a time of ecosystem recovery. According to Berner’s model, the Triassic started with very low levels of oxygen in the atmosphere. Researchers had noticed that there were no coals found at the beginning of the Triassic, and this interval was called the ‘coal gap’. The problem, therefore, was that charcoal in coal could not be used as a proxy for atmospheric oxygen for this time interval.

The Rise, Fall, and Rise of Fire

When I started my doctoral research in October 1973 I never imagined that I would spend so much of my career thinking about fire. I had not considered fire as an agent of change on Earth, or that charcoal deposits may preserve its long history on the planet. I had never thought of fire as a preservational mechanism for fossil plants, producing charcoal that would show their anatomy so that they could be identified, and help us to piece together the vegetation that must have clothed the land millions of years ago. In all my years of collecting fossils as a child and student I had never found, or at least noticed, any fossil charcoal. I had wanted to look at the ecology of the plants that were found during the Carboniferous, 300 million years ago. The natural approach was to look at the large fossil plants that could easily be found in rocks such as the Coal Measures that are often found scattered on old coal tips. But many smaller plant fragments are also preserved in the rocks. I started a programme of dissolving the rocks in acids and obtaining residues of the fossil plants that remained. The rocks are made up of minerals that dissolve in different acids from the plant fossils, which are made of organic material. It was hard work, and I spent many hours a day picking through the plant fragment residues, which were about the size of tea leaves, trying to identify what the fragments represented. Incredibly, at that time, few researchers had tried to look at plant fossils in this way. I soon noticed a large number of fragments that looked like charcoal, and examined these with an SEM. Under the SEM the astonishing detail in the charcoalified leaves was revealed (BW Plate 6). The small needle-like leaves had two beautifully preserved rows of stomata. But what kind of plant did they come from? I took the material to Bill Chaloner, who was one of the world’s authorities on the lycopods, one of the most common plants found in the coal measures.

The Future of Fire

I only encountered the term ‘wildland–urban interface’ a few years ago. It describes situations or physical boundaries where human urban populations and infrastructure impinge on wild vegetated areas. Two specific cases are worth highlighting. One is simply due to the expansion of population centres, where towns and cities continue to spread into rural areas and, in some cases, impinge on natural vegetation. The other situation occurs when individuals or small communities build homes and infrastructure within the bounds of an area of wild vegetation. The ultimate getting away from it all! This wish for exclusivity and privacy is growing at an ever-increasing rate and is becoming a major global challenge. And even before the houses and communities encroach into the wilderness, the natural vegetation is experiencing the effects of human activity and climate change. Simply put, an invasive plant is a plant that has gone wild in an area where it never occurred naturally before being introduced. We are all familiar with bringing exotic plants into our garden, but less aware of what happens to the plants if they spread outside our own area. In general this may not be a problem. Across many parts of the world, introduced plants are confused with natives. Rhododendrons, for example, are very widespread in the UK, and in some places they may also be considered a ‘weed’. But they were introduced into our gardens from China. In any case, what does it really mean to say a plant is native? It isn’t always obvious. While the cultivated species Rhododendron may have been a relatively recent import to Britain, wild forms did exist in England more than 55 million years ago. Equally, we may not realize that a plant is not a native of a region, or what potential problems they may cause. While in some cases such plants may be escapees from our gardens, plants may also have been introduced for another use, such as to provide feedstock for animals. There are those who think that plant invasives are not really a problem, but I would challenge this view in relation to fire.

Fire and the Coming of the Modern World

What kind of world dawned after the K/P boundary? We know from studies across localities in the USA that there is evidence of frequent wildfires continuing into the earliest Paleogene. But what happened to the atmospheric oxygen level after recovery from the K/P mass extinction—did it remain above modern levels? Were we still in a high-fire world? If there were fires, what is the evidence in the charcoal record, and do we know anything about the vegetation that was burning? When the charcoal in the coal database was originally compiled, one of the important issues was how we recorded and represented our data. Early to mid-Paleocene Epoch coals (from around 65 to 55 million years ago) are often recorded as ‘earliest Tertiary’ in coal literature. (The Tertiary was the name we used to use for what we now call the Paleogene and Neogene Periods, stretching from around 65 to 1 million years ago.) However, coals that are nearer to the start of the Eocene Epoch, just older than 55 million years ago, are notoriously difficult to date. This is a problem we have with many coal sequences, as they are deposited on land, and most of the fossils used to give ages are found in marine waters. Many coals of this age are often simply recorded as coming from the late Paleocene or early Eocene. Where we have good dating information, Paleocene coals all tend to have high inertinite (charcoal) contents, well above 19 per cent. By the mid to late Eocene (50–40 million years ago), however, worldwide the charcoal contents are low, around 5 per cent or even less. There must, therefore, have been a fundamental change in the Earth system at this time. Another problem is the way in which we chose to represent our data and show the calculated oxygen curve. In order to get sufficient data to plot the curves we decided to use 10-millionyear bins. This was not a problem for the Paleozoic–Mesozoic transition, covering the great Permian mass extinction, which took place 250 million years ago.

Getting Dirty: What Charcoal Can tell us

Most of us are familiar with charcoal from sketching with it at school, or using charcoal bricks for a barbecue. You will have noticed that it got your hands dirty, that it is brittle, and that it is quite light—at least, lighter than an equivalent piece of uncharred wood. You may also have associated the black residues left after a bonfire with charcoal. If you have been to an area where the vegetation has been destroyed by wildfire, you may have also noticed black residues of charcoal on the ground that make a crunching sound beneath your feet. Our first two examples of charcoal are both products of human manufacture. The bonfire charcoal is a naturally formed material, but still the link with wildfire may not be made. When we see images of burning vegetation it is natural to imagine that all the plant material is consumed by the flames. Yet, as I came to realize on my visit to the site of the Hayman Fire, there is often a significant quantity of unburned material, and charcoal residues as well. Why are we left with charcoal after a fire? Charcoal is produced by heating plant material (most commonly wood, but not exclusively so) in the absence of oxygen. So it isn’t a product of the fire itself, but of the intense heat from the fire. Wood is essentially made up of two organic compounds: cellulose and lignin. Both compounds consist of carbon, hydrogen, and oxygen, but they differ in structure and therefore in properties. In cellulose, the carbon atoms are arranged in straight lines (it is an example of an aliphatic compound). It is the material from which paper is made. In lignin, on the other hand, the carbons are arranged in rings (it is an aromatic compound), and it is this structure that gives wood its toughness and strength. Industrial charcoal is used for a variety of metallurgical processes, and as adsorbents and food additives, as well as for barbecues and artists’ materials, so its formation has been carefully studied.

Kindling

What does it take to make a fire? The factors underlying fire can be illustrated with a triangle, and five fire triangles, relevant to different scales in area and time, have been defined. Let’s start with the most basic, at the smallest scale. The ‘fire fundamentals triangle’ has three elements: fuel, as there needs to be something to burn; heat, because fires can’t start without a source of heat; and oxygen, essential for a fire to combust and spread. The importance of oxygen becomes obvious when we put out a fire. The use of sand or CO2, or even smothering, is a way to exclude air, and more specifically to remove oxygen from the system so that the combustion reaction stops. Water has two effects. It reduces the amount of oxygen getting to the fire, but more importantly the heat energy from the fire goes into evaporating the water rather than heating the fuel that allows the combustion reaction to continue. Our second triangle can be called the ‘fire environment triangle’. Here again, fuel forms one of the points. Another is the weather, as this controls the moisture in the fuel, affecting its flammability. The drier the fuel, the more easily it can burn. Perhaps surprisingly, the third point of the triangle is topography, which impacts on the rate and pattern of spread of the fire. Hill slopes, for instance, can provide an updraft of air that allows the fire to spread more quickly. The next triangle up widens our perspective in terms not only of spatial scale but also time. This triangle can be called the ‘fire regime triangle’. Here we consider not simply the fuel but the type of vegetation that is being burned. Some types of vegetation are more flammable than others. The overall climate is also significant at this bigger scale. For example, temperate seasonal climates are more fire-prone than wet tropical climates, where there is rain every day. The third arm of this triangle is landform: mountainous regions are more susceptible to fire than low-lying flat areas.