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SU mathematician contributes to mapping global tree-microbe symbioses
Author: Media & Communication, Faculty of Science
Published: 22/05/2019

​​​A mathematical ecologist from Stellenbosch University, Prof Cang Hui, is part of an international team of more than 200 scientists who have generated a global map, involving more than 31 million trees and 28 000 tree species, which reveals the symbiotic relationship between trees and microbes worldwide.

According to a media release from Stanford University, this work could help scientists and policy makers understand how symbiotic partnerships structure the world's forests and how they could be affected by climate change. The results were published in the latest issue of Nature in an article titled “Climatic controls of decomposition drive the global biogeography of forest-tree symbioses".  

The geographic distributions of three major types of tree symbioses emerged, namely with ectomycorrhizal fungi, arbuscular mycorrhizal fungi and nitrogen-fixing bacteria. Each of these types encompass thousands of species of fungi or bacteria that form unique partnerships with different tree species. Ectomycorrhizal fungi, for example, feed trees nitrogen directly from organic matter, such as decaying leaves.

The researchers used the world's largest tree-level forest inventory database, the Global Forest Biodiversity Initiative (GFBI), and combined it with information about what symbiotic fungi or bacteria most often associates with those species. This information was fed into a learning algorithm that determined how different variables such as climate, soil chemistry, vegetation and topography seem to influence the prevalence of each symbiosis.

From this, they found that nitrogen-fixing bacteria are probably limited by temperature and soil acidity, 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.

The researchers also revealed a new biological rule, named Read's Rule after the pioneer in symbiosis research Sir David Read.

“These are incredibly strong global patterns, as striking as other fundamental global biodiversity patterns out there," said Prof Kabir Peay from Stanford University and senior author of the study. “But before this hard data, knowledge of these patterns was limited to experts in mycorrhizal or nitrogen-fixer ecology, even though it is important to a wide range of ecologists, evolutionary biologists and earth scientists."

As part of his involvement with the Global Forest Biodiversity consortium, Prof Hui contributed to developing the methodology and interpreting results for this study. He is a specialist in the field of biodiversity informatics, as well as the response of biodiversity to global changes such as climate change.

He says the work showed that South African indigenous forests are predominantly part of the ectomycorrhizal symbiotic guild: “This guild is especially sensitive to the positive feedback between climate and biological controls of decomposition. The models show that, if carbon emissions continue unabated,  its abundance is projected to decline in southern Africa by 10% by 2070," he warns.

“Interestingly, we also shows in this work that legumes, such as many invasive Acacias, that form mutualistic interactions with nitrogen-fixing rhizobia, are on the rise by 5% to even 20% in pockets of areas along the coastal provinces of Western Cape, Eastern Cape, and Kwa-Zulu Natal."

Prof Hui is based in the Department of Mathematical Sciences at SU, where he holds the South African research chair (SARChI) in Mathematical and Theoretical Physical Biosciences. He is also a core-team member of the DST-NRF Centre of Excellence in Invasion Biology (C∙I∙B) and an affiliated researcher at the African Institute for Mathematical Sciences (AIMS).