Global mapping of tree microbial symbioses reveals their key role in climate regulation

Results & impact 15 May 2019
A global map of tree microbial symbioses, produced by the Global Forest Biodiversity Initiative (GFBI), is front-page news in the 16 May issue of Nature. CIRAD, which is on the GFBI Steering Committee, helped coordinate the gathering and analysis of data on tropical forests. The researchers involved demonstrated that if carbon emissions continue at current rates, 10% of tree ectomycorrhizal fungi will have disappeared by 2070. In a snowball effect, this would trigger even higher carbon emissions.
Forests and microbes are linked through symbioses on a global level © Sora Hasler
Forests and microbes are linked through symbioses on a global level © Sora Hasler

Forests and microbes are linked through symbioses on a global level © Sora Hasler

Some 28 000 species and 31 million trees in all, in forests, grasslands and wetlands in 70 countries on every continent (except the Antarctic) were sampled and modelled to produce a global map of symbioses between trees and associated microorganisms.

The more than 200 researchers from the Global Forest Biodiversity Initiative (GFBI) involved in the study featured in the 16 May issue of Nature focused on the three most common types of symbioses: those with

  • arbuscular endomycorrhizas [1]
  • ectomycorrhizas [2]
  • nitrogen-fixing bacteria.

Each type concerns thousands of fungus species that form unique partnerships with different tree species.

"A symbiosis is a sustained intimate relationship that benefits two living organisms from two different species", Bruno Hérault, a tropical forest specialist with CIRAD who was one of the authors, explains. "Our work confirmed the hypothesis put forward thirty years ago by Sir David Read, a pioneer of research on symbioses, that arbuscular endomycorrhizas are abundant in tropical forests and ectomycorrhizas in cold climates."

The frequency of these two types of mycorrhizal symbioses is thus closely linked to climate variables (temperature and humidity), which also affect the rate of soil organic matter decomposition. " Because the leaf litter of EM trees decomposes slowly due to the presence of decomposition-inhibiting secondary compounds, these trees can contribute to soil carbon capture," says Stanford University's Brian Steidinger, the study's lead author. "And we've found that 60% of all trees on Earth are EM!". Mycorrhizal symbioses therefore play a key role in climate regulation.

Map of ectomycorrhiza abundance worldwide. They are concentrated in colder climates:

Carte d’abondance des ectomycorhizes dans le monde : celles-ci se concentrent dans les climats les plus froids.  © Brian Steidinger, Stanford University

The researchers went on to use their symbiosis distribution models to simulate the evolution of their global distribution between now and 2070, in view of current climate change. Their simulations showed a global loss of 10% of ectomycorrhizal fungi and associated trees. Given the role of those symbioses in soil carbon capture, this will trigger a new increase in atmospheric carbon levels. "Our models therefore predict big changes in the state and composition of the world's forests, which could affect climate change even more than we think", says CIRAD's Bruno Hérault.

This work on symbioses is due to be continued, in the hope of understanding the links between climate change and ecosystem health.

[1] Endomycorrhizas : Mycorrhizas (microscopic fungi) that penetrate tree roots and form intracellular structures reminiscent of small trees (arbuscles)

[2] Ectomycorrhizas : Mycorrhizas that do not penetrate cell walls but merely surround tree roots, forming a mycelium sheath.

Reference

Climatic controls of decomposition drive the global biogeography of forest-tree symbioses, Nature 569 (2019)

Publication of this work in Nature on 16 May involved a dozen main authors from the following universities: Stanford and Purdue (USA), Oxford (UK), Sydney (Australia), Wageningen (Netherlands), Lleida (Spain), and Beijing (China), ETH Zürich (Switzerland), FAO (Italy), and CIRAD. CIRAD contributed to the gathering and analysis of data on tropical forests, and provided data for forest plots in French Guiana, Ivory Coast and Indonesia .

The French co-authors of the study were CIRAD's Bruno Hérault, Géraldine Derroire and Plinio Sist, and Eric Marcon and Josep M. Serra Diaz from AgroParisTech.