Indicator mushrooms: how to read the health of the forest

Indicator fungi represent much more than just a component of the forest landscape or a culinary delicacy: they are true guardians of ecosystem health. Their presence, absence, or diversity speaks eloquently about environmental conditions, ecological balance, and the quality of the habitat in which they grow. In this in-depth exploration, we will discover the fascinating world of fungi and how their study can reveal the health status of our forests.

Indicator fungi: what bioindicators are

Before delving into the specific role of fungi as environmental indicators, it is essential to fully understand what is meant by bioindicators and why the fungal kingdom represents such a precious resource for ecological monitoring.

What bioindicators are and why fungi are ideal

Bioindicators are organisms or biological communities whose characteristics (presence, absence, abundance, behavior, physiology) provide information about the health status of an ecosystem. Among all possible bioindicators, fungi have peculiar characteristics that make them particularly suited for this purpose:

Fungi are strictly associated with their substrate and the surrounding environment, which means they respond rapidly to changes in environmental conditions. Unlike many higher plants that can survive in suboptimal conditions for years, fungi often show immediate reactions to ecosystem alterations.

Their high sensitivity to contaminants makes them excellent early indicators of pollution. Many fungal species are particularly vulnerable to heavy metals, nitrogen compounds, and other pollutants, dying or not fruiting in their presence.

The vast fungal biodiversity offers a wide range of responses to environmental stresses. With over 15,000 known species in Italy alone and global estimates speaking of millions of species (most yet to be discovered), the fungal kingdom represents an incredible reservoir of potential indicators.

Fungi play diversified ecological roles (saprophytic, mycorrhizal, parasitic) that provide information on different aspects of the ecosystem. This functional diversity allows for the assessment of multiple environmental parameters simultaneously.

Mycology as a tool for environmental diagnosis

The science that studies fungi, mycology, has made enormous progress in recent decades, transforming from a primarily taxonomic discipline into an applied ecological science. Today, mycologists do not just classify species but interpret their presence diagnostically.

A study published in Nature Microbiology demonstrated how the composition of soil fungal communities can accurately predict the health status of a forest ecosystem, even better than many traditional chemical-physical parameters. The researchers analyzed 82 forests across Europe, finding significant correlations between specific fungal patterns and parameters such as soil pH, organic matter content, and presence of pollutants.

To delve deeper into the scientific basis of this approach, the ISPRA portal dedicated to biodiversity offers extensive documentation on environmental monitoring programs based on biological indicators.

Fungi as indicators of pollution and environmental alterations

One of the most studied aspects of environmental mycology is the response of fungi to pollution and human-induced alterations of ecosystems. Different fungal species show specific sensitivities to particular contaminants, thus becoming true "thermometers" of environmental quality.

Fungi indicating atmospheric pollution

Atmospheric pollution, particularly from sulfur (SOx) and nitrogen (NOx) compounds, has a profound impact on fungal communities. Since the 1970s, mycologists have observed how lichens (symbiotic organisms between fungi and algae) showed distribution patterns clearly correlated with gradients of atmospheric pollution.

For higher fungi (macromycetes), studies have shown that:

Acid rain caused by atmospheric pollution alters soil pH, directly influencing the ability of fungi to fruit. A classic example is the disappearance of many species of the genus Cantharellus from forests subject to strong acid deposition.

Some species show remarkable tolerance to pollution and can even thrive in altered conditions. The common honey fungus (Armillaria mellea), for example, seems to show some resistance to various pollutants, while more sensitive species like porcini (Boletus edulis) tend to disappear.

Research has identified specific response patterns to nitrogen pollution, one of the most pressing environmental problems for European forest ecosystems. A 2018 study published in Environmental Pollution demonstrated how nitrogen deposition profoundly alters communities of mycorrhizal fungi, with diversity reductions of up to 30% in forests subject to high loads of nitrogen compounds.

Table 1: response of some fungal species to atmospheric pollution

For an in-depth look at atmospheric pollution monitoring networks using bioindicators, the European Environment Agency website provides updated data and detailed reports.

Fungi that accumulate heavy metals

Some fungal species can accumulate high concentrations of heavy metals in their tissues, thus becoming not only indicators of the presence of these pollutants but also potential agents for bioremediation.

The genus Agaricus, which includes the common meadow mushroom, is known for its ability to accumulate cadmium and mercury. A study conducted in Poland showed that mushrooms collected near busy roads can contain lead concentrations up to 10 times higher than those collected in remote areas.

The accumulation capacity varies considerably between species and even between different populations of the same species, suggesting the existence of localized genetic adaptations. This variability necessitates a precautionary approach to consuming mushrooms collected in potentially contaminated areas, but at the same time offers valuable tools for environmental monitoring.

Research has identified so-called "fungal hyperaccumulators," species capable of concentrating metals at extraordinary levels. The fungus Aspergillus niger, for example, can accumulate up to 50 mg/g dry weight of lead, while the common Coprinus comatus shows high accumulation capacities for radioisotopes like cesium-137, making it a valuable indicator of radioactive contamination.

Fungi as indicators of biodiversity and forest maturity

Beyond pollution, fungi can provide valuable information about overall biodiversity and the degree of maturity of forest ecosystems. The scientific community has developed veritable "forest maturity indices" based on the composition of fungal communities.

Fungi indicating mature and pristine forests

Some fungal species are strictly associated with ancient and little-disturbed forests, where they find particular ecological conditions that only establish themselves in mature and stable ecosystems. The presence of these species can therefore indicate a high conservation value of the area.

Among the fungi indicative of mature forests are:

Ophiocordyceps sinensis (the caterpillar fungus), although primarily known for its use in traditional Chinese medicine, is also an indicator of pristine Himalayan alpine ecosystems. Its presence signals the absence of profound habitat alterations.

The Royal Bolete (Boletus regius), a rare and prized species, is considered in Italy an indicator of well-preserved mature thermophilic broadleaf woods, particularly those associated with Mediterranean oaks.

Fungi of the genus Hericium, such as Hericium erinaceus (lion's mane), are associated with old broadleaf trees and indicate the presence of dead wood in an advanced state of decomposition, a crucial element for forest biodiversity.

A study published in Forest Ecology and Management demonstrated how the presence of specific wood-inhabiting fungi (lignicolous) can predict with over 85% accuracy the age of a forest and its degree of naturalness. The researchers identified 27 "old-growth indicator" species for the boreal forests of Northern Europe.

Fungal biodiversity as a measure of forest health

Not only the presence of specific species but also the overall fungal diversity can provide precious indications about the health status of a forest. A high number of fungal species is generally associated with stable and complex ecosystems, while a reduction in diversity can signal disturbances or alterations.

Fungal biodiversity is influenced by multiple factors:

Plant diversity: There is a positive correlation between plant diversity and fungal diversity, particularly that of mycorrhizal fungi. Forests with a high number of tree species generally host richer fungal communities.

Presence of dead wood: Saprophytic fungi that decompose wood represent an essential component of forest biodiversity. The quantity and variety of dead wood is one of the factors that most influences fungal richness.

Ecological continuity: Forests that have not suffered drastic interruptions in their continuity over time tend to host more diversified fungal communities, including specialized and rare species.

A study conducted in Switzerland on 100 forest plots found that forests with greater fungal diversity also showed:

• Greater resilience to forest pathogens (-34% incidence of diseases)
• Better soil quality (+28% organic matter)
• Greater primary productivity (+19% tree growth)

The International Union for Conservation of Nature (IUCN) website offers numerous case studies on the use of biological indicators for forest conservation.

Monitoring methodologies and citizen science

Monitoring fungal communities for environmental assessment requires standardized and replicable approaches. In recent years, alongside traditional scientific methods, interesting citizen science projects have developed that involve enthusiasts and foragers in data collection.

Scientific protocols for fungal monitoring

Professional mycological monitoring programs generally follow rigorous protocols that include:

Standardized sampling: through permanent plots of defined sizes, where all present species and their abundance are recorded during specific periods of the year.

Certain identification: which often requires macroscopic, microscopic, and sometimes molecular analyses for critical or difficult-to-identify species.

Collection of paired environmental data: soil parameters (pH, moisture, nutrients), forest characteristics (species composition, tree age, presence of dead wood), and climatic data.

One of the most comprehensive programs at the European level is the Forest Biodiversity Monitoring Network, which includes monitoring of macromycetes in 28 countries with over 4000 permanent plots. The data collected in this program have allowed for the development of increasingly reliable forest quality indices based on fungi.

The role of citizens in data collection

Citizen science, meaning the involvement of non-professionals in scientific research, is playing an increasingly important role in mycological monitoring. Passionate mushroom foragers can contribute significantly to data collection on a large geographical and temporal scale.

Some successful projects include:

Funghi italiani: a portal that collects reports from across the national territory, with over 85,000 observations validated by experts.

Fungaia: a mobile application that allows foragers to record their finds, contributing to mapping species distribution.

The "Fungi for Biodiversity" project of the Milan Natural History Museum, which involves citizens in monitoring indicator species in urban parks and protected areas.

A study published in Biological Conservation demonstrated that data collected by properly trained citizens can have accuracy comparable to that of professionals for the most common and easily identifiable species. This approach allows covering much wider geographical areas than would be possible with professional researchers alone.

For those who wish to contribute to these monitoring efforts, the National Museum of Science and Technology offers resources and guides for the recognition and reporting of fungal species.

Indicator fungi: towards a mycology applied to conservation

The study of fungi as environmental indicators represents a rapidly evolving field, with increasingly important implications for forest conservation and the sustainable management of natural resources.

Progress in this sector is leading us towards:

Early warning systems based on fungal communities, capable of signaling environmental alterations early before they become evident through other parameters.

Increasingly refined forest quality indices that integrate information from different ecosystem components (fungi, plants, animals, abiotic parameters).

Adaptive forest management approaches that use the information provided by fungi to guide silvicultural interventions aimed at conserving or restoring biodiversity.

The challenge for the future will be to increasingly integrate the traditional knowledge of foragers with rigorous scientific protocols, creating ever broader and more efficient monitoring networks. In this context, the role of mycology enthusiasts becomes crucial: every observation, every report, contributes to composing the complex puzzle of our forests' health.

Fungi, with their silent but eloquent presence, continue to speak to us about the health of the planet. It is up to us to know how to listen to them and interpret their message to preserve forest ecosystems for future generations.