The importance of fungi in forest ecosystems

The collection pressure suffered by certain species of fungi, as well as the malpractices derived from uncontrolled human intervention in ecosystems, together with climate changes and the negative effects of current pollution, are affecting the disappearance of fungal species and the alteration of fungal communities. The creation of red lists is being one of the main tools used to warn about the need for environmental managers and politicians to take action on the issue. The fact is that the importance of fungi in the forest ecosystems of our natural environment is enormous, to the point that it is closely linked to the survival of plant masses, with all that this implies. Fungi play an essential role in the functioning of ecosystems, mainly due to their trophic characteristics. Thus, by studying the different life forms of fungi and how they feed, we will be able to notice their importance in nature. Fungi can be:

• Symbiotes.

They are all those capable of creating symbiont associations with the plants that share their habitat, creating what is known as mycorrhizae (a term first used by the German botanist Frank in 1885), an association at the root level beneficial to both beings, since, while the fungus helps the plant in the absorption of water and nutrients such as phosphorus and nitrogen, it receives carbohydrates generated in photosynthesis in exchange. It is established that between 85% and 95% of vascular plants form mycorrhizae, which can be ectomycorrhizal (between 3% and 5% of the total) or endomycorrhizal (the rest). In the first case, fungal hyphae cover the walls of vegetable root cells without penetrating inside, while in endomycorrhizae, hyphae invade the interior of these cells.

The importance of ectomycorrhizae in forest environments is enormous, since trees and shrubs from such important families as Pinaceae, Fagaceae, Betulaceae and Salicaceae, and others establish associations of this type. On the other hand, ectomycorrhiz-forming fungi are part of genera as common in our mountains as Amanita sp., Boletus sp., Lactarius sp., Hebeloma sp., Tuber sp., Terfezia sp., etc. In addition, the fruiting of some of these fungi is of great economic interest because they are highly valued for human consumption (as in the case of tickets, chanterelles, truffles and ground potatoes).

The practical forestry application of ectomycorrhizae can be aimed either at the production of mycorrhized plants for the afforestation of problem areas, or at the production of mycorrhized plants with fungi of commercial interest, as in the case of oaks mycorrhized with truffles.

Within endomycorrhizae, several types are distinguished:

• Arbuscular vesicles. They appear in most shrubs, herbaceous plants, tropical species, some trees in the temperate zone (such as maple and ash) and in almost all cultivated plants. The fungi that form this type of mycorrhiza belong to the Order Glomales. These fungi are strictly symbiotic, they cannot develop in the absence of their host, unlike ectomycorrhizal fungi. Plants with this type of association are widely used to reforest arid lands, where plants suffer severe water stress.

• Ericoides. Heathers appear in plants of the Order Ericales, associated with fungi of the Ascomycota Division. They are characterized by intracellular hyphae in the form of a ringlet and are very effective in absorbing nitrogen and increasing plant tolerance to heavy metal toxicity, which has already been used in the recovery of contaminated soils.
• Arbutoids. They are exclusive to some plant genera of the Order Ericales (Arbutus sp. and Arctostaphylos sp.) and are associated with both fungi of the Basidiomycota and Ascomycota Division. They have intracellular hyphae in the shape of a rattlesnake, but they also appear in the intercellular spaces as a Hartig network.

The presence of mycorrhizal fungi in the forest is not only a direct benefit for the plants that inhabit it, in addition, the mycelial structures that extend over the surface horizons of the soil improve its structure and productivity, providing stability and resistance to any disturbance.

• Saprophytes.

These fungi are nourished by substances produced by the decomposition of dead organic matter (both vegetable and animal), so, together with bacteria, they are involved in the recycling of our mountains. To do this, they use enzymes capable of degrading complex carbon sources such as cellulose, lignin or starch and transforming them into simple and nutritious molecules such as sugars and amino acids. While some fungi make the most of organic matter of diverse origin, others prefer more specific substrates. This is how we find, among others:

• Ligniculous fungi that thrive on dead wood, such as Fomes fomentarius (L.) Fr. and Agrocybe aegerita (V. Brig.) Singer.
• Prairie fungi that live in meadows, such as Volvariella speciosa (Fr.) Singer or Macrolepiota sp.
• Humicultural fungi that thrive in humus or mulch, such as mushrooms (Agaricus sp.), Coprinus lagopus Fr. (Fr.) and Entoloma lividum Quél.
• Strobilical fungi that grow on pineapples and other fruits, such as Mycena seynii Quél. and Baeospora myosura (Fr.) Singer.
• Coprophilic fungi that live on animal excrement, such as Panaeolus semiovatus (Sowerby) S. Lundell & Nannf. OR.
• Fungi that grow on the bones of animals such as Onygena equina (Willd.) Pers.

Decomposition is a long process and without the activity of saprophytic fungi, biomass would accumulate and collapse the functioning of ecosystems. Some of these mushrooms are also the most commercialized in the world, such as the Paris mushroom (Agaricus bisporus (J.E. Lange) Imbach), the paca mushroom (Pleurotus ostreatus (Jacq.) P. Kumm), the thistle mushroom (Pleurotus eryngii (DC.) Gillet) or the shii-take (Lentinula edodes (Berk.) Pegler).

• Parasites.

These fungi are characterized by living in different hosts, to which they cause more or less serious damage or even death. If they cause a disease in the host, one speaks of pathogens. Some need to live on living cells (biotrophic) and others are able to continue to degrade organic matter in a saprobic way when the host has died (necrophytic fungi). Parasitism responds to the existence of natural biological control. These fungi play an important role in ecosystems, acting as balancing factors and affecting competition between plant species. They promote diversity, causing changes in the size and distribution of plant species. However, in monospecific forests these fungi can cause serious damage to forest masses. An example of this type of fungus, which affects both conifers and hardwoods, is Armillaria mellea (Vahl.) P. Kumm, edible mushroom when the carpophore is in its early stages of development.

• Conclusion

Every day there is increasing human intervention in the natural environment, modifying habitats as they please. Fungi, like other biotic groups, are suffering the consequences of this human activity, both directly due to the improper collection of carpophores and destruction of their habitats, and indirectly due to pollution. The different species of fungi perform a large number of ecological functions that improve the resilience of ecosystems. They are therefore excellent indicators of the state of conservation of our forests and it is essential to maintain a high diversity of habitats to promote the growth and reproduction of their species, not only in order to enjoy harvesting them, but also to ensure the future of our green spaces.