Introduction

A global view of potential vegetation distribution based on climate shows large discrepancies with actual vegetation distribution. Fire is often responsible for such discrepancies and on many landscapes it is a major determinant of vegetation distribution. Traditionally fire has been thought of as a disturbance, much like treefalls or floods. However, fire is a predictable ecosystem process that often plays a determining role in community structure and function. Placing fire in its proper role in ecological theory would treat it much like a herbivore in the trophic pyramid, consuming biomass and competing with biotic consumers for resources.

Fossil evidence is clear that as an ecosystem phenomenon, fire dates at least to the Mesozoic over 100 Ma. Indeed, long before our current ecosystems were in place, fire was shaping plant traits and landscapes. For example, tree ring patterns in Jurassic-age fossils suggest a Mediterranean climate of mild winter rain and summer drought, conditions conducive to wildfires and charred fossil fragments illustrate fires were part of these ecosystems. Various fossil evidence has not only shown fires were widespread at this time but different lines of evidence support a similar range of fires as found today, from light surface burning in the understory of forests to high-intensity crown fires in lower-stature woodlands and shrublands.

However, these records do not necessarily demonstrate fire was an important ecosystem process. The first evidence in the geological record of major ecosystem changes driven by fire is from the late Tertiary Period. Massive C4 grassland expansion between 4 and 7 Ma may be the first example of an extensive fire influence in shaping global ecosystems. At this time in subtropical regions there is a clear isotopic signal in soils pointing toward a switch from woodlands to grasslands, coinciding with marked increases in charcoal deposition. Although this coincided with increased climatic seasonality, climate alone cannot explain these changes since in many cases grasslands moved from more arid environments into more mesic ones, something likely only driven by the destructive force of fires.

In order to understand what conditions promote fire as an important ecosystem process, we need to consider the characteristics shared by fire-prone ecosystems today. One way to illustrate this is with a modification of the classical fire triangle (Figure 1a) with a triangle better describing the ecological distribution of fire (Figure 1b). In order for fire to be a predictable ecosystem feature the following conditions must be met. There must be sufficient moisture and warmth to generate primary productivity capable of providing the fuels necessary to spread fire from one place to another. If productivity is very low, such as in a desert, fires are unlikely due to insufficient biomass. However, high primary productivity alone is insufficient because fire is unlikely unless there is

Oxygen

Oxygen

Fuel

Primary production

Figure 1 (a) Traditional fire triangle on the three components necessary for fire and (b) modified to describe the necessary components to describe the ecological distribution of fire.

Fuel

Primary production

Figure 1 (a) Traditional fire triangle on the three components necessary for fire and (b) modified to describe the necessary components to describe the ecological distribution of fire.

seasonality sufficient to dry these potential fuels and convert them into fuels available for burning. For example, lowland tropical rainforests outperform most other ecosystems in primary production because they are constantly moist; these potential fuels seldom dry sufficiently to become fuels available for burning. Lastly, given sufficient available fuels at some time of the year, there must be a source of ignition coincident with the availability of fuels. In nature the primary source of ignition is lightning, and regions differ markedly in the importance of lightning (Figure 2). However, today, humans have surpassed lightning as the source of fire ignition in many parts of the world.

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