How trees, fungi and pathogens shape the forest

The Research column will be a weekly feature on the scientific opportunities on campus written by staff writer Diler Haji.

Biologists have been working to reveal just how much diversity tropical forests hold. (Keith Pittaway/Creative Commons Flickr)

 

Say you were hiking in a Connecticut forest. The next day, you’re trudging through the Everglades and the day after that you’re hacking away with a machete in the rainforest. If you counted how many different plant species you came across in each expedition, you’d quickly realize there is much more diversity as you get closer to the tropics.  

This global trend has become a point of interest for countless biologists. Fungi living in the soil may hold the answer to why.

“One of the reasons why we see all of this diversity in tropical forests is because of our natural enemies, herbivores or pathogens,” said Valerie Milici, a first-year Ph.D. candidate at the University of Connecticut in the ecology and evolutionary biology department, specifically studying plant-fungal interactions.

Many of these natural enemies are pathogenic fungi that kill the roots of particular plants. However, there exists another class of fungi with which plants have partnered in what’s called a mutualistic relationship. They’re fittingly called mutualistic fungi.

Unlike pathogenic fungi, mutualist fungi provide plants with essential nutrients like phosphorous and nitrogen. The plant returns the favor with sugars it creates using the energy of the sun – what we call as photosynthesis.  

Another benefit to the plant is protection.

Certain mutualistic fungi are great at protecting plant roots. One of the best is the ectomychorizal fungi (EM), which form what Milici calls a “net” around the plant roots, thus protecting plants from pathogenic invaders. Arbuscular mychorizal (AM) fungi are also mutualists. Unlike the EM form, they pierce into the roots and branch out between and within the root's cells. This makes AM fungi much less effective in protecting plant roots from pathogens compared to EM fungi.

Both AM and EM fungi are the most abundant, diverse and worldwide fungal mutualists that exist, so it is not surprising that research shows their critical role in our forests’ structure.

"It’s possible that by reducing the risk of pathogens, EM fungi let members of the same species grow nearer to each other,” Milici said. "When more of the same plants are clustered together, you get less diversity. If one gets infected, then your natural enemy suddenly has a buffet.”

This theory was confirmed in “Plant-soil feedbacks and mycorrhizal type influence temperate forest population dynamics,” a recent study of AM and EM fungi in the northeastern United States published in Science. When trees of the same species were transplanted with EM fungi, they grew much better than trees of the same species that were transplanted with AM fungi. In fact, many more seedlings survived with AM fungi only when they were planted near trees of different species.

On the global scale, it’s well known that AM fungi are most diverse in the tropics, while EM fungi are most diverse in the temperate north. Milici points out that there are pockets of EM fungi in the tropics as well, but those pockets are usually monocultures — populations of tress from the same species.

So, if you ever wondered why there are fewer tree species in the northeast, the answer may have a lot to do with how plants interact with fungi in the soil. But that is not where the story ends. Researchers are also working to understand how fungi influence how plants interact with animals, like pollinators, through interactions with plants.

Milici points to a bar graph from a 2007 paper published in Ecology on tropical plants, titled “Ecological implications of anti-pathogen effects of tropical fungal endophytes and mycorrhizae.” She explains that the researchers coated plant leaves with pathogens to figure out whether AM fungi protected the plant aboveground. Plants infected with AM fungi lost 25 percent of their leaves, but plants without AM fungi lost 80 percent of their leaves. Even when the researchers loaded the soil with phosphorous, rendering the AM fungi useless to the plant, the trend didn’t change.

Clearly, AM fungi are just as important in the tropics as EM fungi are to the temperate north.

“It’s something that’s transferable through the body of the plant,” Milici said, referring to what may be a systemic hormone transferred from the fungi to the trees.  


Diler Haji is a staff writer for The Daily Campus. He can be reached via email at diler.haji@uconn.edu.