Forest ecosystems contain a vast array of biodiversity and absorb significant amounts of harmful greenhouse gases from entering our atmosphere. But beneath the roots of these brilliant trees is a hidden world of fungi that form relationships with their roots, critical to the health and species variation of the forest. A recent study published in Nature Communications Biology has found evidence that the microbes in soil fungi impact the distribution and density of tree species across the world.
Central to the research is the investigation of the ‘latitudinal diversity gradient,’ a pattern where species diversity tends to increase as the North and South poles meet the climate of the tropical equator. This ecological trend has contributed to many scientific hypotheses. However, the results of the study offer compelling evidence that mycorrhizal fungi are the primary contributors to this ecological phenomenon, suggesting that the fungal associations in different regions greatly influence the composition and structure of these vital ecosystems.
The latitudinal diversity gradient is among the most remarkable biogeographical patterns on Earth. In tropical regions where the climate is moist and warm, plant diversity is at its highest. According to the Janzen-Connel hypothesis, species-specific enemies reduce the survival of juvenile plants near adult trees, contributing to a greater diversity in tree species (1). This phenomenon, known as negative conspecific density dependence (CDD), is believed to be stronger in areas close to the equator. Soil-borne pathogens have since been regarded as the primary drivers of CDD, but the study’s researchers hypothesized that plant mutualists like mycorrhizal fungi may also play a role.
Mycorrhizal fungi form symbiotic associations with trees and other plants, providing essential nutrients and water to their host in exchange for carbohydrates formed from photosynthesis. There are two primary types of these fungi: arbuscular mycorrhizal (AM) and ectomycorrhizal (EM). The significant differences between these two fungi determine how trees interact with their environment. AM fungi tend to form associations with a wider variety of plants, allowing them to facilitate a broad network of nutrient exchange across plant communities. In contrast, EM fungi often form exclusive relationships with specific tree species, leading to a more focused and direct nutrient exchange between the fungus and host plant.
Research has proposed that EM fungi could potentially boost the survival rate of trees, therefore diminishing the impact of negative CDD in forests dominated by EM-associated tree species. This would lead to a larger variation of tree diversity compared to AM-dominant forests. In contrast, the characteristics of AM fungi could be advantageous to the survival of young plant species, contributing to the overall biodiversity of these forests. Scientists are still unsure why this pattern occurs; however, they believe it could be due to the different ways these tree species interact with mycorrhizal fungi of their own type.
To test this hypothesis, the researchers of the study conducted a comprehensive analysis of global forest data, using tree census from 43 different sites (23 temperate, 19 tropical, 1 boreal). Using this information, they looked for patterns of tree survival and growth and their relation to mycorrhizal associations. The extensive study aimed to better understand the influence between trees and fungi and their overall impact on forest structure and species distribution.
The study results found that the relationship between young trees and mature trees of the same species is influenced by the type of mycorrhizal fungi they are associated with. Trees associated with EM fungi were less affected by being around trees of the same species, while AM-associated trees were less tolerable.
This pattern held true in different global regions, indicating that EM trees grow better when there are more of their kind around. In areas were AM fungi were dominant, such as in the tropics, the researchers noticed that the amount of young trees of the same species increased when there were more mature trees of the species nearby. However, in regions where EM fungi are more common, like temperate and boreal areas, this relationship is not as strong (2).
The findings “suggest that these mechanisms could very well play a role in driving patterns of global biodiversity in tree species,” according to Matthew Baker, study co-author and professor of geography and environmental systems at the University of Maryland, Baltimore County (UMBC). “Global patterns of biodiversity may not result solely from antagonistic relationships between trees and their pathogens, but also from symbiotic relationships with fungi in soils.”
The type of mycorrhizal fungi dominant in one area can impact how strong the relationship is between trees of the same species. Thus, the differences in EM and AM-dominated forests largely shape the tree diversity in different parts of the world.
These new understandings of how mycorrhizal fungi impact ecosystems redefine our understanding of forest ecology, yet more studies will be required to fully comprehend the impact of microbes on global biodiversity patterns.
“The scientific community is very much in the learning stage about appreciating the diversity of different types of soil microbes and their distribution over the planet,” notes Baker.
Camille Delavaux, lead scientist at ETH Zurich in Switzerland, outlines the next steps in this research journey: “Future research will leverage the available tree census data and generate additional microbial genetic sequencing data from 30 plots to directly link the microbiome to plant community structure.”