Fire is a vital process in natural systems, providing structure, disturbance and change across ecosystems from heathlands to forests. It is integral to understanding the composition of forests worldwide, as it dictates vegetation types (with frequent fires driving a shift to more fire-resistant species) and subsequently the faunal species which live with them. Fire is also an important management technique, and has been used for thousands of years to manipulate the landscape according to human need.
Ancient fire regimes and management are best understood by using paleoecology, the study of past environments and the organisms which supported them. Long-term data from pollen and charcoal records, historical accounts and more recently, satellite data (Whitlock, 2010), are all ways in which historical fire regimes can be analysed. With its clear link to climate (Hallett & Walker, 2000), understanding past fire regime is particularly important today, when facing unprecedented global climate change. This essay will outline the importance of long-term records in determining current and future management of the ecosystems most affected by fire.
Successful management for fire requires baseline information, to determine the natural variability in an ecosystem, and to manage this variability within acceptable limits. The ‘equilibrium theory’ claims that ecosystems remain stable because of the interacting factors which can affect them (Gillson, 2015). However, for most ecosystems, fluctuation and variability are the norm, rather than a condition of no change over time. To illustrate this, imagine the sea. The height of the sea doesn’t remain constant each day (even with calm weather), but instead fluctuates with wave action. In the same way, it is impossible that the amount of fire an area may receive within a given time period will be constant over time, as it fluctuates. It is these fluctuations, which form a range of acceptable variability, which are important to consider.
When you recognise that stability and constancy in natural processes is not the norm, static management (i.e. responding the same way to a change in an environmental variable) becomes inappropriate. This is best shown with the example of fire suppression across the United States during the 1960s-1980s. Since the historical “Big Burn” of 1910, fire stopped being seen as a natural process and instead was treated as a threat – as a result, 98% of all fires across the USA were put out before reaching 120ha in size, a complete shift from the natural use of fire to suppress vegetation traditionally used by Native Americans. This changed the way in which fire interacted with the environment, by allowing trees to grow in high densities, closely packed together with no natural fire breaks (previously burned areas which could not be burned again, thus preventing fire spread), so that when a fire occurred it rapidly became uncontrollable. This legacy of fire suppression, rather than fire management (managing fires to be within natural limits of variability), has led to the rapid increase in “megafires”, leading to greater environmental and human damage.
The main way to generate baseline information for better fire management is to study what has happened in the past using charcoal records. Charcoal are the fragments of organic matter which get preserved in sediment as they are deposited – they range in size, with microcharcoal often being used as a proxy for fire from longer distances, compared to the larger charcoal particles which are deposited much closer to the burn site. In a study by Marlon et al. (2008), charcoal records from 6 continents were combined, to demonstrate the link between fire, climate and human management. They found that burning decreased overall between the period of AD1-1750, before rapidly increasing between 1750-1830, associated with population rise and agricultural conversion of land. It then fell again until around 1970, which has been associated with fire suppression over the 20th century. The most recent time period saw another increase in burning across the tropics, potentially associated with climate warming and increasing aridity. This study illustrates the usefulness of charcoal fragments as a proxy for global changes in fire regime, but also the fact that climate is the first driver for changes in fire frequency and biomass burned.
“Accumulated fuels in dry forests need to be reduced so that when fire occurs, rather than “crowning out” and killing most trees, it is more likely to burn along the surface at low-moderate intensity” – North et al., 2015
A study by Gillson (2004) looked at the relationship with fire and the savanna ecosystem, finding that transition into a wooded state (which occurs on cycles of around 200 years) is largely determined by the positive feedbacks surrounding herbivory and fire pressures. This has an important effect on management, as it shows that there is more than one “natural” state for a savannah ecosystem, and to prevent fire from dictating which state would be an unnecessary intervention. Both these studies demonstrate ways of understanding natural fire variability, to inform current management.
Another long-term proxy useful in determining fire regime, is pollen. When pollen is preserved in sediments, it captures a snapshot of the vegetation assemblage at a point in time. As such, a shift in vegetation abundance (in response to large fire events) or in type (in response to changing frequency of fires), can be used to indicate past fire regime. One study in Kootenay National Park, Canada, used this technique to demonstrate that the forest was becoming more closed, with dense, flammable canopies as a result of fire suppression (Hallett & Walker, 2000). Another example of vegetation change recorded as a response to fire, is in the Sierra Nevada, where increasing fire frequency caused a shift to lodgepole pine trees at high altitudes (Gillson, 2015).
This effect is observed across many areas where fire suppression is observed; as fires are suppressed, trees are not removed from the ecosystem, causing growth and increased forest density. This allows forest fuels to build up as they are not reduced through the action of small-scale, regular fires, meaning that the fires which do occur are large and dangerous (often uncontrollable, as seen in the Californian fires of 2018).
More recently, new techniques have surfaced to study fire regime. These include the use of satellite imagery, which was incorporated into a study by Andela et al., 2017, who found a 25% decrease in burning over the past 18 years. The use of satellite imagery is a new technique, so might not be described as a long-term dataset, however it is still interesting to consider that in the future, this technique, which involves looking at burned area across a study site, might lead to changes in management.
To summarise, a number of paleoecological techniques can be used to study fire regime, from looking at pollen, charcoal, fire scars on trees, historical records and potentially even satellites. The main implications for modern management are as follows:
- Paleoecological data can show the natural variability of an ecosystem in terms of fire frequency – management should act within this variability, as was done traditionally by the Native Americans (Everett, 2008)
- Long-term data can show the influence of humans in fire regime, to act as historical lessons from the past e.g. fire suppression techniques and their subsequent increase in large-scale dangerous fires
- Vegetation changes recorded in the past can give an indication of the natural ecosystem state, as it would be maintained without humans – acting as a target for future management.
“The solution may come from a revision of the present forest and fire management strategies… focusing on monitoring” – Michetti & Pinar, 2019
Andela, N., Morton, D.C., Giglio, L., Chen, Y., Van Der Werf, G.R., Kasibhatla, P.S., DeFries, R.S., Collatz, G.J., Hantson, S., Kloster, S. & Bachelet, D. (2017) A human-driven decline in global burned area. Science, 356(6345), pp. 1356-1362
Everett, R.G. (2008) Dendrochronology-based fire history of mixed-conifer forests in the San Jacinto Mountains, California. Forest Ecology and Management, 256(11), pp.1805-1814
Gillson, L. (2004) Evidence of hierarchical patch dynamics in an East African savanna?. Landscape Ecology, 19(8), pp.883-894
Gillson, L. (2015) Biodiversity Conservation and Environmental Change: using palaeoecology to manage dynamic landscapes in the Anthropocene. OUP Oxford.
Marlon, J.R., Bartlein, P.J., Carcaillet, C., Gavin, D.G., Harrison, S.P., Higuera, P.E., Joos, F., Power, M.J. & Prentice, I.C. (2008) Climate and human influences on global biomass burning over the past two millennia. Nature Geoscience, 1(10), p.697
Michetti, M. & Pinar, M. (2019) Forest fires across Italian regions and implications for climate change: a panel data analysis. Environmental and Resource Economics, 72(1), pp.207-246
North, M.P., Stephens, S.L., Collins, B.M., Agee, J.K., Aplet, G., Franklin, J.F. & Fulé, P.Z. (2015) Reform forest fire management. Science, 349(6254), pp.1280-1281
Whitlock, C., Higuera, P.E., McWethy, D.B. & Briles, C.E. (2010) Paleoecological perspectives on fire ecology: revisiting the fire-regime concept. The Open Ecology Journal, 3(1)