Fires and biodiversity. A sustainable coexistence?

Forests are estimated to hold about 80 per cent of the biodiversity on Earth [1][2]. This wealth populates the surface of Planet Earth, according to FAO data [3], for 4.06 billion hectares, or 31% of the land area. On the Old Continent (with 27 members), forests insist on 160 million hectares, covering some 39% of the EU's land area[4]. Forests cover 11.1 million hectares, constituting 37% of the Italian territory [5], tending to be in line with both world and European figures. Given its importance and its crucial role in the maintenance of terrestrial biodiversity, such an extensive surface area abundant in ecosystem functions must be protected and well managed.

Such a rich ecosystem enjoys excellent adaptive capacity and resilience to climatic, other abiotic and biotic stresses, precisely because of its diversification. Yet, this extraordinary resource has for decades been threatened by anthropogenic activities that are radically and irreversibly altering the diversity of life on Earth, with significant impacts and changes in the management of natural surfaces that result in biodiversity loss [6]. Recent studies that analysed the period 2005-2013 found an average annual loss of tropical and subtropical forests of 5.5 million hectares caused by agricultural expansion (62% of the total) and an average of 3.4 million per year (38% of the total) due to other causes, including forest use and fires [7].

If, as we have seen, in tropical and subtropical areas (Brazil, Indonesia, etc..) the loss of forests and thus of biodiversity is mainly attributable to massive interventions for agricultural activity (largely for pastures for meat production and for oilseed plants), carried out either by logging or by incineration, in temperate areas the main threats to forests are windthrow, fire, and bark beetles [8]. Europe's forest area is indeed expanding (an increase of 9% from 1990 to 2020) [9], but widespread and frequent abandonment and poor management jeopardise its integrity, functions and value, with all that this entails in terms of hydrogeological instability, fires and loss of ecosystem services.

Awaiting official data from the 2023 report of EFFIS, the European Forest Fire Information System, analysing the report "Forest Fires in Europe, Middle East and North Africa 2022" [10], it emerges that 2022 was the second worst year in the EU in terms of area burnt by fires, with almost 900,000 hectares affected, roughly the size of Corsica. This had a strong impact on the continent's biodiversity assets, even taking into account that about 40% of the area that went up in flames (about 365,000 hectares) were Natura2000 sites, areas of high biodiversity value. Structured into Special Protection Areas (SPAs) established by the 1979 Birds Directive and Special Areas of Conservation (SACs) established by the 1992 Habitats Directive, Natura 2000 is the largest coordinated network of protected areas globally [11].

Official data from EFFIS on 2023 are not yet available, but the much literature already produced and the main estimates tell of a particularly fiery year for forest fires, both in Europe, mainly Italy, France, Spain, Portugal and Greece, and in the rest of the world, Canada, Haiti, Australia. Dramatically noteworthy, most recently, the Smokehouse Fire, the second largest forest fire in the history of Texas (USA) which besieged for weeks last February and charred some 340,000 hectares of land. In this scenario, in which it is estimated that each year about 4% of the global surface area (30-46 million km2) is covered by flames [12], increasing awareness of the impacts of this emergency can be useful to direct prevention and good management behaviour, both by individuals and institutions.

"Fires are a natural phenomenon in Mediterranean countries and play a key role in the management and modification of landscapes and forest community dynamics" [13]. - there are numerous scientific publications on the subject that begin in these terms [14] [15] [16]. The interaction between fires and biodiversity is quite complex and is the subject of study and analysis by the scientific community, with a wide range of case histories and results, often varying on the basis of climatic ranges and spatio-temporal variations in fire regimes, so much so that one can speak of "pirodiversity" [17]. This term, first coined in the ecological literature in 1992 [18], encapsulates the idea that landscapes with greater heterogeneity in age, size and severity of post-fire areas would support greater species diversity [19].

From a risk management perspective, a greater richness of species in the hotspots most exposed to fires could be beneficial as different forest systems tend to be more resilient to alterations [20], increasing their ability to cope with disturbances. Forest fires have many implications for biological diversity. On a global scale, they are a significant source of carbon emissions, contributing to global warming that can lead to alterations in biodiversity. Regionally and locally, fires make changes to biomass stocks, alter hydrological patterns and cycles with consequent effects on marine systems, and affect plant and animal species. Smoke itself can significantly reduce photosynthetic activity and is harmful to the health of all animals.

One of the most important ecological effects of fires is the increased likelihood of further fires in subsequent years; as dead trees collapse to the ground, opening up the forest to solar desiccation, increased 'fuel' load with an increase in fire-prone species, such as pyrophilous grasses [16]. There are, at the same time, areas of the planet where fire also performs positive functions, as in the case of the boreal forests; in these places, the absence of fires induces the formation of organic layers that prevent the melting of the soil during the spring and summer, resulting in a rise in the permafrost layer, which implies the impoverishment of forests, a decrease in productivity and the conversion of forests into swamps [16].

Among the indirect effects of fires (also known as 'ecosystem responses') is the impact on soil structure and quality [21]. Many studies point out that it is not so much, or at least not only, the flames and the increase in soil temperature that affect its structure, but the loss of vegetation and litter, and the consequent exposure of the soil in the post-fire period that lacks the forest's valuable function of intercepting rain and preventing erosion. A 2015 study conducted in Portugal [21] observed how soil bulk density, organic matter content, porosity and saturation conductivity did not change significantly as a result of forest fire, while surface structural characteristics such as roughness and roughness coefficient (Manning number) decreased, increasing the risk and erosivity of surface water flow.

Forest fires are the primary agents responsible for the disturbance and renewal of forests. In general, they play a significant role in altering floristic composition and tree regeneration, in some cases even increasing wood production, and often influencing human ecology [22]. Some positive effects are found in the case of low-severity fires; an example is prescribed fire, used in forest management, which promotes the renewal of dominant vegetation by eliminating undesirable species and transiently increasing pH and available nutrients [23]. Fires of extreme severity, on the other hand, can result in large-scale loss of most ecosystem functions for periods on the order of 50 and 100 years, as found in Russia in the late 1990s, with population reductions of dozens and dozens of plant species, fungi, lichens, and mosses.

However, there is no lack of evidence of adaptation, as in tropical forests or those of the Mediterranean basin, where fires promote fire-tolerant species (e.g. Quercus ilex L.) or that have inherent defence structures and strategies (thicker bark, wider crown base, taller stems, etc.) as in the case of Pinus resinosa Sol. or Pinus sylvestris L. [16]. Other species have stróbili (the cones) endowed with a particular ecological adaptation, the serotinia, which consists of the seeds remaining in the cones for a longer period than the norm, which are then released late, e.g. following an event such as a fire, and which find in the soil a modified environment in which to start proliferating again.

As already mentioned, forest fires can have devastating impacts on vertebrates and invertebrates living in and around the forest, not only killing them directly, but also leading to long-term indirect effects such as stress and loss of habitat, territory, shelter and food [16]. Most animals perceive forest fires and identify the dangers associated with them; however, depending on species and size, they implement different defence strategies. Some flee, others seek refuge underground or in water. Some species take advantage of the situation of confusion to hunt small prey trying to escape the fire [12].

In areas where large-scale fires are frequent, preserving a range of microhabitats can contribute substantially to biodiversity conservation [24]. The loss of fruit trees results in a general decline of birds and animals that rely on fruit for their food, and this effect is particularly pronounced in tropical forests. Due to the impoverishment of forests by small animals, birds, rodents and reptiles, carnivores tend to abandon these areas. Fires also destroy litter and the associated arthropod community, further reducing the availability of food for omnivores and carnivores [16].

There are numerous research and development projects and initiatives in the field of fire prevention and mitigation strategies, especially in those areas most affected by fire such as southern Europe and the Mediterranean areas. From scientific research emerge indications that find their way into the toolbox of those who live in and manage forest areas: an example can be prescribed fire, the technique of consciously and consciously applying fire to the vegetation, in a planned manner, which has among its objectives fire prevention thanks to the preventive elimination of certain particularly flammable parts of vegetation, such as litter and dry grasses.

Still, specific and targeted projects, such as the one undertaken in 2018 in the Iberian Meseta, straddling Portugal and Spain, FirESmart, have involved stakeholders and simulation models to assess the impacts of smart fire management scenarios for "win-win" solutions between fire mitigation, biodiversity, and ecosystem services [25]. This project found that rural abandonment will result in more widespread, high-intensity fires with the potential to profoundly damage ecosystems and undermine the provision of ecosystem services. In short, the challenge with fires can only be accepted and addressed, integrating science, policy and social awareness, with the aim of building a present and future of sustainable coexistence.