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Soil fungi determine the gradient of forest diversity worldwide

A recent publication in Nature Communications Biology has brought to light the significant influence of soil microbes, particularly fungi, on global forest diversity. 

The study contributes to a broader understanding of the pivotal role microbes play in various ecological processes, from human digestion to enhancing crop yields.

Distinct gradient in forest diversity 

The research indicates a distinct gradient in forest diversity across the Earth, with tropical forests near the equator boasting a high number of species, in contrast to the lesser diversity in forests closer to the poles. 

A widely accepted theory suggests that soil pathogens, including bacteria and fungi, are partly responsible for this gradient. The accumulation of species-specific pathogens near adult trees can hinder the growth of juveniles nearby, thus encouraging diversity. This effect is more pronounced in warm, wet climates, contributing to the higher diversity in equatorial forests.

However, Camille Delavaux from ETH Zurich in Switzerland, along with her colleagues, introduces a new perspective to this established idea. 

The role of soil fungi

The study highlights the role of mycorrhizal fungi – soil fungi that form symbiotic relationships with most plant roots globally – in moderating the effects of harmful soil pathogens. 

“This paper provides an additional mechanism that may help explain why forest compositional diversity differs with latitude, and a little more about how microbes may regulate patterns of diversity across the planet,” said co-author Matthew Baker, a professor at the University of Maryland, Baltimore County (UMBC).

Tree species diversity 

The study delves into the roles of two major classes of mutualistic fungi: ectomycorrhizal and arbuscular mycorrhizal. While both types may improve juvenile survival, ectomycorrhizal fungi, forming a protective sheath around plant roots, seem to have a more significant impact, particularly at higher latitudes. 

On the other hand, arbuscular fungi, more common near the equator, may offer less protection against pathogens and less specialization with particular tree species, potentially encouraging diverse tree growth nearby.

“We found initial evidence for both the diversity-promoting effects of arbuscular fungi and the diversity-reducing effects of ectomycorrhizal fungi,” explained Baker, suggesting these mechanisms could influence global biodiversity patterns in tree species.

ForestGEO network 

The research also underscores the role of the Smithsonian Institution’s Forest Global Earth Observatory (ForestGEO) Network in facilitating this study. The network comprises forest plots globally, where researchers commit to a detailed inventory of trees every five years. The current study utilized data from 43 of the network’s 77 plots, including one at UMBC’s main campus.

UMBC’s ForestGEO plots, unique in their urban, temperate setting, were established in 2012 and boast a high species diversity, contributing valuable data to the study. The plots’ diverse microbial life has implications for understanding urban forest ecology, a focus of Baker’s recent research.

Future research 

Looking ahead, Delavaux envisions leveraging the extensive tree census data and generating additional microbial genetic sequencing data from 30 plots to directly link the microbiome to plant community structure. 

“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,” said Baker, highlighting the burgeoning interest in understanding how microbes drive global biodiversity patterns. Delavaux’s enthusiasm for future research reflects a growing recognition of the intricate connections between microbial life and global ecological processes.

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