Charcoal analysis records past fires
When plants burn during a fire, charcoal is formed. Smaller particles of charcoal are picked up in the wind and can be transported over long distances – for example, the smallest particles of charcoal (<5 microns) are often blown across the Tasman Sea during Australian bush fires if the westerly winds are strong enough. These tiny particles can be detected landing on fresh surfaces on the West Coast of New Zealand. However, most charcoal produced during a fire usually falls out in the local or regional area of the fire, and can make its way into peat or lake sediment deposits either directly from air fall, or by being water transported via rivers or surface runoff into lakes or bogs. Once it lands on the surface of a bog, or is incorporated into the surface sediments of a lake, it becomes permanently buried along with the pollen and spores produced during that year. Charcoal fragments are not destroyed by the chemical analyses employed for pollen preparations, therefore if present in the sediments analysed for pollen, it will also be preserved along with the pollen in the final residue used to mount slides. Charcoal particles can then be counted at the same time that pollen is counted, and in this way, a record of past fires can be reconstructed. The one drawback of this technique, however, is that the pollen preparation process can be mechanically damaging to the charcoal fragments, and can break them into smaller pieces. Larger fragments of charcoal (>50 microns) are used to reveal the presence of a local fire, whereas smaller particles (<50 microns) are usually derived from fires in more remote locations. In the absence of any large-sized charcoal particles, there is no way of knowing if a trace of small charcoal particles in a core represents small local fires or more widespread fires possibly thousands of kilometres away. If such information is required, then the sediments need to be gently washed through different sized sieves and the relative sizes of charcoal analysed, as an alternative to counting charcoal from the pollen slides.
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| Pollen slide showing pollen from intact forest cover (click to enlarge) |
Pollen slide showing charcoal fragments with bracken spores and grass pollen grains following deforestation (click to enlarge) |
Either way, if charcoal is counted from the same depths that pollen has also been analysed, not only can we detect when fires occurred, but what happened to the vegetation afterwards. Additionally, radiocarbon dating can be used to reveal when the fire occurred, and how long it took for the vegetation to recover. In present-day forests, common native seral plants to exploit the newly cleared surfaces or light gaps created after a fire would include bracken (Pteridium esculentum), tutu (Coriaria arborea), and in some areas grasses. The pollen and spores from these types of indicator plants occurring at the same time as charcoal in peat or lake sediment cores allow us to piece together the fire history of a region.