Tree death: here's what's happening

This article will explore in depth and detail the complex and worrying phenomenon of tree death and its inseparable link with the disappearance of fungi from our woods.

Through a technical analysis, statistical data, explanatory tables, and ecological considerations, we will try to dissect every aspect of this environmental crisis, offering a comprehensive view of this increasingly worrying phenomenon. The situation is serious and requires a deep understanding of the causes and possible solutions.

Tree death: an unnatural silence in the realm of the forest

Walking through a dying forest is a profoundly moving experience. It's not just a visual matter, of skeletal trees standing out against the sky. It's an absence, a silence. The scent of fresh humus is missing, the vibration of life that is normally perceived. And, particularly significant for the mycologist and the forager, the fungi are missing.

This article stems from the observation of this silence and the need to provide a detailed and scientific answer to the question: what is happening where fungi no longer grow? The death of trees is not an isolated event, but a symptom of an ecosystem in deep distress, and fungi, with their sensitivity, are its first victims. We will explore every aspect of this ecological drama, from soil microbiology to plant physiology, from climate dynamics to human impact.

The vital symbiotic link: fungi and trees, an inseparable marriage

Before delving into the causes of death, it is essential to understand life, or rather, the relationship that sustains the very life of the forest. Most terrestrial plants do not live alone, but maintain a relationship of mutual aid, a symbiosis, with soil fungi. This union is called mycorrhiza.

What are mycorrhizae and why they are so important for tree life

The term "mycorrhiza" derives from the Greek "mykes" (fungus) and "rhiza" (root). It perfectly describes the intimate association between fungal hyphae and the root tips of plants. This is not mere proximity, but a true physiological fusion. The hyphae of the fungus envelop or penetrate the tree's roots, creating an exchange network that extends for hundreds of meters in the soil. The tree, through photosynthesis, produces complex sugars that it cannot entirely use.

The mycorrhizal fungus, incapable of photosynthesis, receives these sugars from the tree, which represent its source of carbon and thus energy. In return, the fungus, with its dense network of hyphae that functions as a hyper-efficient extension of the root system, absorbs water and mineral nutrients (such as phosphorus, nitrogen, potassium, and micronutrients) from the soil and transfers them to the tree. It is a fair and supportive barter that has lasted for millions of years.

To understand the importance of this exchange, consider that it is estimated that about 90% of terrestrial plant species form mycorrhizae. Without them, many plants would struggle to survive in poor soils or under water stress. Mycorrhiza is not an optional; it is a foundation of plant life on earth. The death of this relationship is often the prelude to the death of the tree itself.

The mycelial network: nature's internet and its connection to tree survival

The most fascinating and crucial aspect of this symbiosis is not visible to the naked eye. Beneath our feet, the mycelium of mycorrhizal fungi forms a vast, interconnected network, often called the "Wood Wide Web". This network not only connects a fungus to its host tree, but can connect different trees, even of different species, to each other. Through this network, plants can exchange not only nutrients but also alarm signals.

A tree attacked by a pest can, through the mycelial network, "warn" neighboring trees, which can thus activate their defense mechanisms early. This sharing of resources and information is a powerful resilience mechanism for the entire forest. When this network weakens or breaks due to the death of key node trees or the fungi themselves, the entire forest ecosystem loses its ability for a collective response to stress, accelerating the process of decline and death.

The multifactorial causes of tree death: a detailed analysis

The tree death we observe today is rarely attributable to a single cause. It is almost always the result of a negative synergy of different stress factors that, acting in combination, exceed the plant's resistance capacity. Let's analyze in detail the main culprits of this silent slaughter.

Climate change: the main systemic threat to trees and fungi

Climate change is not a future abstraction, but a present reality that is profoundly altering forest ecosystems. Its effects are multiple and synergistic.

• Water stress and prolonged drought: heatwaves and increasingly long and intense periods of drought deprive trees of the water needed for vital processes. A tree under water stress closes its stomata to reduce transpiration, but this also blocks the uptake of carbon dioxide, halting photosynthesis. Without energy, the tree weakens, becomes incapable of producing defensive resins, and becomes an easy target for pests and pathogens. Drought also affects fungi, whose fruiting body is over 90% water. Death by dehydration of the mycelium is a direct consequence.
• Extreme weather events: increasingly violent windstorms, atypical snowfalls, and destructive hailstorms cause direct physical damage to trees, breaking branches and trunks, and uprooting roots. These wounds are the perfect entry points for pathogenic fungi and bacteria that will finish the job started by the storm.
• Alteration of seasonal cycles: mild winters fail to control populations of pathogenic insects, which survive in large numbers. Early springs can induce a premature vegetative restart, which is then burned by late frosts, irreparably damaging the new shoots.

The following table illustrates the cumulative impact of climatic factors on an Apennine beech forest.

Invasive pathogens and forest pandemics: the globalization of tree death

Global trade in plants and timber has brought with it, entirely accidentally but devastatingly, a series of alien pathogens against which our tree species have no defenses. It is the equivalent of the conquistadors bringing smallpox to the Americas.

• Phytophthora ramorum: this oomycete (similar to a fungus) is the agent of "Sudden Oak Death" and "Rhododendron Dieback". It attacks the phloem, the tree's vascular system, preventing sap transport and leading to rapid decline and death of the plant.
• Ceratocystis platani: the fungus responsible for Canker Stain of Plane trees. It is a certain death sentence for these majestic trees, spreading through roots and pruning cuts. Entire tree-lined avenues have been wiped out.
• Xylella fastidiosa: a bacterium that, while not directly attacking forests, is causing the death of millions of olive trees in Puglia, profoundly altering the agricultural-forest landscape and ecosystem.

The fight against these pathogens is extremely difficult. Often the only solution is the felling and destruction of infected plants, with immense ecological and landscape losses. 

Pollution and soil degradation: the silent poisoning leading to death

Air and soil pollution act as a slow-release poison. Deposits of nitrogen and sulfur from industrial and agricultural activities alter soil chemistry, making it more acidic. This has two main effects:

1. Mobilization of toxic metals: acidification releases aluminum and other toxic metals that were previously bound to soil particles. These metals directly damage the fine roots of trees and the mycelium of fungi, interrupting the mycorrhizal symbiosis.
2. Alteration of nutrient availability: an altered pH can make phosphorus, a crucial element, unavailable to plants, even though it may be physically present in the soil.

Compounds of tropospheric ozone, a secondary pollutant, also damage leaves, reducing photosynthetic efficiency and contributing to the general weakening of the tree. A tree that doesn't breathe well is a tree that is dying.

The cascading consequences on fungal diversity and production

The death of trees is not a tragedy that unfolds only in the plant kingdom. It has immediate and dramatic repercussions on the fungal kingdom, triggering a domino effect that impoverishes the entire ecosystem.

The disappearance of mycorrhizal fungi: the end of an era of cooperation

Obligate mycorrhizal fungi, such as the precious Porcini (genus Boletus), Chanterelles (Cantharellus cibarius), Milk Caps (Lactarius) and Russulas (Russula), cannot survive without their tree partner.

When a tree dies, the associated mycorrhizal fungus loses its source of carbon. The mycelium can persist in the soil for some time, but without a new tree to colonize, it is destined for a slow decline and death. The disappearance of a tree species from a forest therefore inevitably leads to the disappearance of the dozens of fungal species associated with it. We are losing not only trees, but entire mycological consortia.

The impact on saprotrophic fungi and carbon cycling

Even saprotrophic fungi, those that feed on dead organic matter (wood, leaves), suffer the consequences of tree death, but in a more complex way. Initially, an increase in dead trees can represent an abundant resource for wood-decaying fungi like Oyster mushrooms (Pleurotus ostreatus) or polypores. However, this is a transient phase.

If the death rate exceeds the system's decomposition capacity, dead wood accumulates. In a healthy ecosystem, saprotrophic fungi are the great recyclers, returning to the soil the nutrients immobilized in the wood. If this process is interrupted, the forest turns into an accumulation of dead biomass, nutrients do not return to the soil, and the ground becomes impoverished, making it difficult for new trees to regenerate. It is a vicious cycle leading to biological desertification.

Data, statistics and case studies: the map of tree death in Italy and the world

To give concreteness to the phenomenon, it is necessary to support the qualitative analysis with quantitative data. The numerical evidence paints an alarming and undeniable picture.

Statistical analysis of forest decline in Europe

The monitoring program ICP Forests (International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests) collects data across Europe. Its reports indicate that, on average, over 20% of the sampled trees show signs of defoliation (loss of leaves/needles) greater than 25%, a clear indicator of suffering. In some regions, such as Central Europe hit by successive years of drought, this percentage exceeds 40-50% for species like beech and Norway spruce.

The case study of "beech dieback" on the Tuscan-Emilian Apennines

An emblematic case is that of the beech in the Apennines. The beech is a mesophilic species, it loves moisture and coolness. Rising average temperatures and droughty summers of recent decades have put it in serious difficulty. Vast phenomena of "top dieback" are observed, with diebacks starting from the top, and a general yellowing of leaves (chlorosis) already in mid-summer.

This weakening has paved the way for secondary pathogenic fungi like Neonectria fuckeliana, which causes cankers on the trunk, accelerating the death process. The harvests of fungi typical of beech forests, such as the Summer Bolete (Boletus aestivalis) and the Bronzed Bolete (Boletus aereus), have plummeted in these areas in direct proportion to the health of the beeches.

Solutions and resilience strategies: how to counter tree death and save fungi

Faced with such a complex picture, there is no magic wand. The solution lies in a multidisciplinary and integrated approach, combining active interventions and a new philosophy of forest management.

Climate forestry: designing forests for the future

We must abandon the idea of rebuilding the forest of the past and start designing the forest of the future. Climate forestry involves several strategies:

• Greater species diversification: planting mixed forests with different species, including more thermophilic and drought-resistant ones (e.g., Turkey oak, Holm oak, Field maple) alongside traditional ones. A mixed forest is more resilient because a pathogen that attacks one species does not destroy the entire system.
• Promoting natural regeneration: encouraging regeneration from seed of plants that have already demonstrated better resistance to local conditions, thus naturally selecting more resilient genotypes.
• Appropriate density: avoiding overly dense stands that compete aggressively for water. Targeted thinning can reduce water stress for the remaining plants.

Soil protection and mycorrhizal inoculation

Protecting the soil means protecting the "factory" of the forest's life. It is essential to:

1. minimize soil compaction due to the passage of mechanical vehicles.
2. maintain a litter cover that acts as a natural mulch, reducing water evaporation.
3. in reforestation contexts, use mycorrhized seedlings in the nursery. That is, providing young plants with a "kit" of selected mycorrhizal fungi that will help them establish better and resist stress.

Research in this field is advanced and for those wishing to delve deeper into controlled mycorrhization techniques, the research center Micologia Controllata offers studies and practical applications of great interest.

Tree death: a phenomenon to face and halt

The death of trees and the consequent disappearance of fungi is one of the most serious ecological crises of our time, but it is not an inevitable sentence. It is a symptom of a sick relationship with nature. Understanding the intricate connections that link the fate of a tree to that of a fungus, an insect, the climate, and our actions is the first, fundamental step.

The second step is to act, with urgency and determination, on a global and local scale. We must mitigate climate change, adapt forest management, protect the soil, and, not least, rediscover a sense of wonder and respect for the complexity of life in the forest. Only then can we hope to hear again, in the future, the rustle of leaves and to find, humbly, the fungi that mark the return to life.