A delicate balance between beneficial and harmful soil fungi associated with tree roots guides the diverse mixture of tree species that thrive in subtropical forests, according to a new study.

Scientists had previously hypothesized that forest diversity resulted from a "rare-species advantage" driven by pathogenic, or harmful, fungi. The more trees there were in an area, the more quickly such fungi would accumulate on tree roots, the common model claimed. This would create a deadlier environment for new seedlings.

Now, new research suggests that beneficial fungi are closely involved as well. A Chinese-led team recently reported in Science that symbiotic fungi, which associate tightly with roots and help trees by exchanging nutrients, alter the rate that pathogenic fungi accumulate in the soil. This fungal give-and-take is critical in determining which trees grow most successfully in forest plots, the team showed.

"Species differ in their sensitivity to having their own kind around," said Nathan Swenson, a forest ecologist at the University of Maryland in College Park, Maryland, and a co-author of the study. "That sensitivity itself is related to the types of positive associations they have with fungi in the soil."

To unveil these partnerships, a team led by Lei Chen, a forest ecologist at the Institute of Botany of the Chinese Academy of Sciences, studied a subtropical forest plot in Gutianshan National Nature Reserve in Zhejiang Province, China. The researchers collected and analyzed soils from the roots of 322 trees, representing 34 species. They isolated DNA from the samples and used genetic sequencing to identify the species of fungi near the roots of each tree.

In addition to soil collection, the researchers tracked the number and sizes of seedlings, saplings and adult trees in the plot over nearly a decade, logging more than 25,000 measurements.

The subtropical plot measured 0.24 square kilometers (0.093 square miles). "This is a relatively small area that’s like half of a golf course," said Daniel Johnson, a forest ecologist at the University of Florida in Gainesville, Florida, who was not part of the study. "But what they did was look at the entire community of the forest and quantified how the fungi were associating with the trees. They’ve done something on a scale that no one’s ever done before."

The researchers found that larger trees had a higher concentration of pathogenic fungi than smaller trees. This finding agrees with earlier hypotheses for the rare-species advantage, which posited that older trees would create a deadlier environment for younger seedlings and saplings nearby.

The amount of pathogenic fungi was lower around trees that interacted with helpful ectomycorrhizal fungi, which coat the surfaces of roots and exchange nutrients via the soil. In contrast, pathogenic fungi were more common around trees that interacted with a different type of symbiotic fungi, arbuscular mycorrhizal fungi, that tunnel into tree roots and exchange nutrients directly with the roots, bypassing the soil.

The researchers developed mathematical models for which trees survived over a decade in their plot. Tree species with more ectomycorrhizal fungi also had higher-than-average survival rates; this corresponds with a weaker rare-species advantage. On the other hand, tree species with more arbuscular mycorrhizal fungi had lower-than-average survival rates; this corresponds with a stronger rare-species advantage.

These findings support a model where symbiotic fungi regulate how pathogenic fungi build up around tree roots. The researchers hypothesize that the root-coating fungi might form a protective barrier that keeps out harmful fungal invaders. In contrast, the root-tunneling fungi create tiny entry points that pathogenic fungi can use to get in.
This work extends previous studies, based on tropical forests, that only included pathogenic fungi in a mechanism for the rare-species advantage, said Chen. "There are tropical forests, subtropical forests, and temperate forests," he said. "So actually, we ecologists want to know a general pattern that can explain all forests." (Mongabay)

View overlooking the forest in Gutianshan National Nature Reserve. Photo courtesy of Lei Chen 

Researchers taking a break between experiments in Gutianshan National Nature Reserve. Photo courtesy of Lei Chen.

A canopy crane in the forest in Gutianshan National Nature Reserve. Photo courtesy of Lei Chen.