Fungi could be the driving force behind rare-species advantage

A new study has unearthed what could be the root cause of rare-species advantage in trees.

Scientists are theorising how complex interactions between soil fungi and tree roots impact the phenomenon whereby individual trees are more likely to thrive when only a few other trees of that same species exist in proximity.

Scientists have long agreed that rare-species advantage helps maintain forest diversity, by preventing any one species of tree from dominating. However, the reasons behind it have never been fully understood.

Research out of the University of Maryland and the Chinese Academy of Sciences looked at beneficial and pathogenic fungi in the soil, in combination with tree survival rates and the number of same-species neighbours, to identify some definite patterns.

The team observed whether the type of beneficial soil fungi living around tree roots in a subtropical forest in China affected how quickly the trees accumulated harmful, pathogenic fungi as they grew.

They found the beneficial ectomycorrhizal fungi, which surround the outside of tree roots, protected trees from being infiltrated by harmful pathogenic fungi. At the same time, they discovered the arbuscular mycorrhizal fungi that bores into roots may create a pathway for harmful fungi to enter the tree.

The rate of accumulation of pathogenic fungi was found to impact how well the trees survived when growing near other trees of the same species. This is thought to be a consequence of trees with quicker rates of pathogen uptake accumulating higher pathogen loads as they mature, and thus becoming more infectious to seedlings and smaller trees of the same species in close proximity.

Furthermore, because trees fortified with ectomycorrhizal fungi seemed to be better protected from infection even when close to infected neighbours of the same species, they tended to fare better in denser groups.

Tree species associated with ectomycorrhizal fungi thus had slower pathogenic fungi accumulation rates and a weaker rare-species advantage. Conversely, trees associated with arbuscular mycorrhizal fungi had higher pathogenic fungi accumulation rates, and therefore stronger rare-species advantage. Variations in the level of rare-species advantage between species and locations could therefore be explained by the type of beneficial fungi in the soil.

Professor Nathan Swenson, at the University of Maryland’s Department of Biology, said the significant role of fungal pathogens has become increasingly evident in recent years.

“This is the first time anyone has drilled down into that role to look at the rates trees accumulate pathogenic fungi, together with the relationships trees have with beneficial fungi," Professor Swenson said.

“The two are so strongly correlated, and the relationship between those two factors is such a strong predictor of how species density affects survival. We didn't expect such a strong correlation."