Forest fires stand out among the greatest environmental dramas of each summer. Every year, tens, if not hundreds of hectares of forest land go up in flames in Spain.
In general, fire acts as an extremely destructive force, destroying all the flora in its path, and annihilating the fauna that has not managed to escape. Some ecosystems require decades, or even centuries, to recover from the devastation of a forest fire.
fire in the ecosystem
Fires trigger very complex effects on ecosystems, depending on the magnitude of the fire. A tree that burns alone and goes out has little impact on communities. In contrast, a large forest fire can overwhelm the resilience of the forest and destroy the entire ecosystem.
Fire not only burns, it also negatively affects the growth and reproduction and seed and seedling dynamics of most surviving plants. It is one of the few disturbances that regularly kills entire mature plants.
However, fires of intermediate magnitude can be beneficial for the ecosystem, as an agent in the structuring of communities. Especially if the ecosystem is mature, well structured and little disturbed. After a small outbreak of fire, an island of ashes is formed surrounded by life; the gaps in the vegetation provide the potential for a new succession process that enriches the biodiversity of the environment. And it is in these conditions that some plant species come into play.
Even in the face of a disaster as destructive as a fire, there are species that resist. Certain plant species can withstand certain fires, following various strategies, some even benefit. They are the so-called pyrophyte species —from the Greek pyrós , ‘fire’, and phyton , ‘plant’—and there are three types: vegetative survival, reproductive survival , and pyrophilia .
The survivors of the fire
Of the three evolutionary strategies that plants present to adapt to fire, the simplest is the so-called vegetative survival . Species such as the cork oak, with a very thick bark full of air chambers —what we call cork— successfully resist fire.
Of course, not all plants with this adaptation survive every fire. If the fire spreads through the herbaceous and shrub layer, but does not reach the tops, it will be easier for trees with thick bark, such as the cork oak, to survive. However, if the fire reaches the tops, their leaves will burn and the trees will probably not survive.
Other species are resistant to defoliation or leaf loss, and will sprout again after fire, as long as the fire does not damage the growing tissue.
Another vegetative survival adaptation is regrowth from underground parts, such as roots, rhizomes, or tubers. These structures are well protected from fire, since the ground generally functions as thermal insulation. After the fire, although the aerial part of the plant dies completely, it can still sprout from underground resistance structures.
The burned environment as a new opportunity
Some of the most fire-prone plants lack thick barks, adaptations for resistance, defoliation, or regrowth from roots. The fire just exterminates them, they don’t survive. In fact, in some cases, its wood, its bark or its resins are highly flammable, thus facilitating the work of fire.
However, even in these circumstances, pyrophytic species also exist: the reproductive survival strategy. Although the plant dies, the seeds or fruits survive, and find an optimal environment, after the fire. Burnt ashes and plant remains retain moisture, which is lost less as there is no vegetation, carrying out evapotranspiration. In addition, they contain abundant nutrients, and the total absence of vegetation allows sunlight to be a more than abundant resource.
In this way, pyrophytic plants manage to regenerate the devastated ecosystem more easily.
Pyrophilia: plants that benefit from fire
Until now, all adaptations allowed the survival or perpetuation of offspring after a fire. But there are plants that have gone a step further; They have not only adapted to resist —in one way or another— fires, but also to benefit from them. To such an extent that some plants even need fire to complete their life cycle. We call these truly fire-loving plants pyrophiles —from the Greek pyrós , ‘fire’, and philia , ‘friendship’.
Species, such as the fire lilies of the genus Cyrtanthus , in South Africa grow and are kept between fire and fire, but only flower a few days after the passage of a fire. That is, in these cases, fire stimulates flowering .
Other plants, whose seeds are encased in hard, leathery structures, remain in the tree canopy for a long time, where germination is impossible. However, when fire comes and ravages the treetops, these structures burst like popcorn , releasing the seeds, which are resistant to fire. This condition, in which the plant needs fire to release its seeds, is botanically called serotinia , and is characteristic of some species of pines, cypresses and spruces, among others.
Finally, there are also species that flower and release their seeds without the need for fire, but in which the seeds, accumulated in the ground, cannot germinate. They remain in a state of dormancy that only fire can break . When the fire passes, the seeds are activated – either by physical or chemical action – and germinate. This type of pyrophilia is found in some legumes, such as certain species of genista and acacias.
fire is still fire
The pyrophytic behavior of some plant species is a real event, but we should not feed the fire for that . In nature, fire is just another actor, like herbivorous animals, viruses or fungi. In natural environments, the areas most prone to fires tend to have more pyrophytic populations and normally the fires that form are usually self-limited. While in areas where fire is not common, the vegetation does not show this type of adaptation.
Human beings, with our activity, have altered those patterns, those natural rhythms and those communities . We modify the composition of those ecosystems in many ways. We eliminate or displace species, introduce new ones, deform food webs, alter biogeochemical cycles and vegetation succession patterns; and we also change the natural regime of fires.
We must remember that in Spain, more than 80 % of forest fires with confirmed origin are caused by human activity , and only 5% are known to be of natural origin. These fires, most of which occur in areas that are not adapted to fire regimes, favor pyrophytic species and tend to eliminate the rest of the vegetation, which implies a change in the composition of the ecosystem, a loss of biodiversity and an impoverishment of the natural environment.
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Bond, W. J. et al. 1996. Fire, competition and the organization of communities. En W. J. Bond et al. (Eds.), Fire and Plants (pp. 148-187). Springer Netherlands. DOI: 10.1007/978-94-009-1499-5_7
Martell, D. L. 2007. Forest Fire Management. En A. Weintraub et al. (Eds.), Handbook Of Operations Research In Natural Resources (pp. 489-509). Springer US. DOI: 10.1007/978-0-387-71815-6_26