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Forest microbiome: Mapping the symbiotic relationships between trees and microbes

Ectomycorrhizal fungi feed trees nitrogen directly from organic matter such as decaying leaves
May 15 2019

A scientific article published in Nature today shows how the world’s forests are built on symbiotic partnerships between trees and microbes, and how these could be affected by a warming climate.

Led by Stanford University, a team of over 200 scientists from around the world collected data from over 70 countries, 1.1 million forest plots and 28,000 tree species to map patterns of where trees form symbioses with fungi and bacteria in the soil, exchanging nutrients for carbon.

By revealing factors that determine where different types of symbionts will flourish, these new maps can help scientists understand how symbiotic partnerships structure the world’s forests and how they could be affected by a warming climate.

Using their maps, the team predict that the biomass of tree species associated with fungi mostly found in cooler regions will fall by 10% by 2070 if carbon emissions continue unabated. This could result in even more carbon in the atmosphere as these fungi tend to increase the amount of carbon stored in soil.

According to Valerio Avitabile, the JRC scientist involved in the research, "Symbiosis means 'living together', and trees grow better when they live in symbiosis with micro-organisms. This study shows that symbiotic relationships obey clear rules and are strongly related to climate, and that climate change is likely to have massive impacts on the symbiotic state of the world's forests.” 

Three most common symbioses

The researchers focused on three of the most common types of symbioses: arbuscular mycorrhizal fungi, ectomycorrhizal fungi and nitrogen-fixing bacteria. Each of these types encompasses thousands of species of fungi or bacteria that form unique partnerships with different tree species.

Mapping the location of 31 million trees and the symbiotic fungi or bacteria most often associated with them, the team determined how variables such as climate, soil chemistry, vegetation and topography seem to influence the prevalence of each symbiosis.

They found that nitrogen-fixing bacteria are limited by temperature and soil acidity (and therefore thrive better in higher temperatures), whereas the two types of fungal symbioses are heavily influenced by variables that affect decomposition rates (the rate at which organic matter breaks down in the environment) such as temperature and moisture.


These maps show the distribution of trees likely to associate with ectomycorrhizal, arbuscular mycorrhizal or N-fixer symbiotic types. This data supports Read’s Rule.

©Brian Steidinger

Read's Rule – a new biological rule

Based on the study, the team defined a new biological rule which they named Read’s Rule after pioneering symbiosis researcher Sir David Read, who hypothesised thirty years ago that arbuscular mycorrhizal fungi would dominate in warmer forests (such as in the tropics, where tree growth is limited by phosphorous) and ectomycorrhizal fungi in colder forests (where decomposition is slow and leaf litter is abundant).

Thanks to the latest data and technology, the team were able to support this hypothesis and define Read's Rule as: "ectomycorrhizal trees dominate climates that slow decomposition and arbuscular mycorrhizal trees dominate climates that accelerate decomposition".

However, the transitions across biomes from one symbiotic type to another were found to be much more abrupt than expected, suggesting that ectomycorrhizal fungi also change their local environment to further reduce decomposition rates.

This feedback loop may help explain the 10% reduction in ectomycorrhizal fungi the researchers found when they simulated what would happen if carbon emissions continued unabated to 2070.  Warming temperatures could force ectomycorrhizal fungi over a climatic tipping point, beyond the range of environments they can alter to their liking.

The authors hope that other scientist will use the freely available maps and include tree symbionts in their work.

Further information