Forest fires are dramatic ecological events as they can wipe out most of the plants and animals within several kilometers in only a few hours and often also pose a major threat to human settlements. They have a positive ecological impact when occurring naturally because they clear the organic matter residues and favor new plant growth. However, exceptionally frequent or devastating fires - caused by climate change or, more frequently, human negligence - can lead to the extinction of some species in a certain area and completely reshape the landscape. They can also directly impact on human communities located nearby and/or whose economy depends on forest goods, especially wood and paper. Thus, a better understanding of how fires spread and what triggers them is important to protect the environment as well as economical interests.

The extent and spread of forest fires highly depend on weather conditions, including wind and dryness, and on the characteristics of plants themselves – and their remains. Plant litter - the organic matter that accumulates on the soil - is in fact the most fire-prone forest layer, but not all plant litters are high-quality fuels. Their flammability depends on several traits such as silica content and concentration of highly combustible organic compounds, but also their spatial arrangement.

This is why a Dutch research group developed an experiment to assess the natural burning behavior of the litter of 39 forest tree species belonging to the group of gymnosperms – non-flowering trees such as pine, cedar, and cycads. Many of these trees have leaves in the shape of needles and form big forests that are known to be most prone to catch fire (see the 14th Forest Fires in Europe report of the European Commission).

The scientists mimicked the spatial arrangement of litters in natural forests and observed what happened when they burned. To this end, plant litter was collected directly from botanical gardens and forests, paying particular attention to gathering the matter “as it was”, with little or no manipulation (e.g. leaves with or without twigs and bark residues). Samples were collected from the ground, sometimes after gentle shaking of the canopy to increase the amount. Then, litter particles were set on fire in the lab after being gently dropped inside a small container and allowed to take a spontaneous configuration.

The most striking observation was that the litter of some trees of the Pinaceae family, including the samples of larch, fir and spruce, almost did not catch fire at all, while all other samples, including pines (Pinus) burned completely or nearly completely. This observation was also in disagreement with previous data showing that leaves of those plants successfully burned when taken individually.

What is behind this unexpected behavior? The authors observed that those plants drop single needles instead of brunches and twigs, like all other tested trees, leading to high litter density or litterbed packing ratio (i.e. the volume of litter material/volume of litterbed). This might restrict air flow, thus resulting in insufficient oxygen availability to sustain the fire.

To test this hypothesis, the authors selected two other gymnosperms, whose litter had been shown to be highly flammable and repeated the fire experiments after cutting their litter into smaller pieces, increasing its density. Again, they observed that the cut samples reached lower temperatures and did not burn completely – or even failed to burn. Even the increase in litter surface area to mass due to the artificial cuts did not diminish this effect. The authors conclude that the particle size of the litter is a major determinant of the rate of heat release and, thus, flammability, a so-called “first-order effector”.

Besides the percentage of burned sample, the scientists measured other parameters like maximum temperature, total burning time, and fire front speed. They conclude that particle size, albeit having a major effect, is not the only factor controlling flammability in a mimicked natural environment. Indeed, at a given particle size, the burning behavior was also dependent on additional species-based factors that influence, in particular, fire temperature and sustainability.

In conclusion, this study thoroughly characterized a key parameter of plant litter flammability and shed new light on other important litter traits. These data can help in building better models to predict the probability of forest fires, based on climate conditions and on the type of plant species present. This may help to contain or even prevent disastrous forest fires in the future, securing important economical and recreational resources – and the survival of countless plants, animals and, not least, humans. The study also illustrates how reproducing natural conditions in a laboratory can help in coming up with realistic models of complex natural phenomena like fire.

Next read: Nitrogen is becoming less available in ecosystems around the world by Rachel Mason

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