Conservation of forest genetic resources with seed practicum: Podcast 7
Seventh episode: Mycorrhizal Symbiosis of Forest Trees
Voice: Camilla Stefanelli
Written by: OpenAI. (2024). ChatGPT 4 (9. August). https://chat.openai.com/
Preparation of requirements for ChatGPT 4 by: Boris Rantaša and Hojka Kraigher
Preparation of drafts: Hojka Kraigher
Postproduction: Camilla Stefanelli, Hojka Kraigher, Donatella Paffetti, Patrizia Rossi, Cristina Vettori
Introduction
Welcome to the seventh episode of our podcast series, where we explore the intricate relationships that sustain our forests. Today, we are delving into the fascinating world of mycorrhizal symbiosis, a vital partnership between forest trees and fungi that plays a crucial role in the health and sustainability of forest ecosystems. Understanding this symbiotic relationship not only enhances our knowledge of forest ecology but also informs better management and conservation practices.
What is Mycorrhizal Symbiosis?
Mycorrhizal symbiosis is a mutually beneficial relationship between fungi and the roots of plants, including trees. The term “mycorrhiza” comes from the Greek words “mykes,” meaning fungus, and “rhiza,” meaning root. In this relationship, the fungi colonise the tree roots and extend their hyphae—long, thread-like structures—into the surrounding soil. This network of fungal hyphae greatly increases the surface area for water and nutrient absorption, far beyond what the tree’s roots could achieve on their own.
In exchange for this enhanced access to nutrients, the tree provides the fungi with carbohydrates, which are products of photosynthesis. This exchange is essential for both partners: the tree receives vital nutrients such as phosphorus and nitrogen, while the fungi obtain the energy they need to grow and reproduce. This symbiotic relationship is so fundamental that it is estimated that over 90% of land plants, including most forest trees, form mycorrhizal associations.
Types of Mycorrhizal Associations
There are several types of mycorrhizal associations, but the two most common in forest ecosystems are ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) associations.
Ectomycorrhizal (ECM) Associations
Ectomycorrhizal fungi form a dense sheath, or mantle, around the tree’s roots and penetrate the root cortex without entering the cells. This type of mycorrhizal association is particularly common in temperate and boreal forests, where it is found in many tree species such as pines, oaks, and birches. ECM fungi are highly effective at mobilising nutrients from the soil and decomposing litter,
particularly nitrogen and micronutrients, which is often in forms that are not readily available to plants. The network of hyphae extends into the soil, exploring new areas and accessing nutrients that would otherwise be out of reach for the tree.
Arbuscular Mycorrhizal (AM) Associations
Arbuscular mycorrhizal fungi, on the other hand, penetrate the cell walls of the tree’s root cells, forming structures known as arbuscules, which are the sites of nutrient exchange. AM associations are more common in tropical and subtropical forests and are found in a wide variety of tree species. These fungi are particularly effective at improving the tree’s uptake of phosphorus and other micronutrients, which are essential for growth and reproduction. AM fungi also help protect trees from soil-borne pathogens and can improve soil structure, making it more conducive to root growth.
The Benefits of Mycorrhizal Symbiosis
The benefits of mycorrhizal symbiosis for forest trees are numerous and profound. One of the most significant advantages is the enhanced nutrient uptake that mycorrhizal fungi provide. Through their extensive hyphal networks, these fungi can access nutrients in the soil that are otherwise unavailable to trees. This is especially important in nutrient-poor soils, where the ability to access additional phosphorus, nitrogen, and other essential minerals can make the difference between survival and decline.
Mycorrhizal fungi also play a critical role in water uptake, helping trees to withstand periods of drought. The hyphal network acts as an extension of the tree’s root system, allowing it to tap into water sources that are beyond the reach of its roots. This increased access to water can be crucial during dry seasons or in regions with variable rainfall.
Another significant benefit is the protection that mycorrhizal fungi offer against soil-borne pathogens. The fungi can outcompete harmful microorganisms for space and resources around the tree’s roots, effectively acting as a biological shield. Additionally, some mycorrhizal fungi produce antibiotics that can kill or inhibit the growth of pathogenic bacteria and fungi, further safeguarding the tree’s health.
Mycorrhizal Networks and Forest Connectivity
Beyond their role in individual tree health, mycorrhizal fungi contribute to the overall connectivity and resilience of forest ecosystems. The hyphal networks formed by mycorrhizal fungi can link the roots of different trees, creating a “wood wide web” through which resources such as water, nutrients, and even chemical signals can be transferred. This network allows trees to communicate and support each other, particularly in times of stress.
For example, a tree that is under attack by pests can send chemical signals through the mycorrhizal network, alerting neighbouring trees to the threat. These trees can then pre-emptively strengthen their defences, making them less susceptible to the same pest. Similarly, a healthy tree can share resources with a stressed or diseased neighbour, helping to maintain the overall health of the forest.
Similarly, carbohydrates can be transferred from dominant trees to the shaded ones in a forest stand, of the same or different species. Similarly, so called ‘mother trees’ can retranslocate water from deep in the soil through its common mycelial networks to its offsprings in periods of drought.
The Role of Mycorrhizal Fungi in Forest Management
Given the crucial role that mycorrhizal fungi play in forest resilience, they are increasingly being considered in forest management practices. One important aspect of this is the conservation of mycorrhizal diversity. Different tree species form associations with different species of fungi, and maintaining a diverse mix of tree species in a forest helps preserve a diverse community of mycorrhizal fungi. This, in turn, enhances the resilience of the entire forest ecosystem.
Forest managers are also exploring ways to incorporate mycorrhizal fungi into reforestation and afforestation projects. In some cases, this involves inoculating seedlings with mycorrhizal fungi before planting them in the forest. This practice can improve the survival and growth rates of the seedlings, particularly in degraded or nutrient-poor soils.
Furthermore, mycorrhizal fungi are being studied for their potential to assist in forest restoration efforts. In areas where soil health has been compromised, introducing mycorrhizal fungi can help restore the nutrient cycling processes and improve soil structure, making the environment more conducive to tree growth.
Conclusion
In conclusion, mycorrhizal symbiosis is a cornerstone of forest health and resilience. This intricate partnership between fungi and trees not only enhances the growth and survival of individual trees but also contributes to the overall connectivity and stability of forest ecosystems. As our understanding of mycorrhizal symbiosis deepens, it is increasingly informing forest management and conservation practices, helping us to better protect and sustain our forests for future generations.
Thank you for joining us in this seventh episode. We hope you’ve gained a deeper appreciation of the vital role that mycorrhizal fungi play in the life of forest trees. In our next eighth episode, we will explore the legislation basis for conservation of forest genetic resources and management of forest reproductive material. Stay tuned!