Three years ago, I worked in Southern Appalachia as a field researcher. My job was to identify plants and help carry out an ant seed dispersal study. That work has since then been published, and although I have moved on with other career goals, that summer in the mountains has imprinted itself on my psyche forever. The warm and wet ecosystems in the Southern Appalachia are nearly tropical, almost reminiscent of cloud forests I’ve traveled to in Central America. I couldn’t believe my eyes when I embarked on my first solo hike in this region. Tree diversity alone was staggering, let alone all of the cool species of fungi I found along the way. The most abundant fungal encounter was a Marasmioid species with long radiating rhizomorphs.
Rhizomorphs are incredibly useful structures that allow diploid fungal tissues to quickly spread throughout the forest. Many species that produce rhizomorphs synthesize the pigment melanin to aid in protection from the sun and other opportunistic fungi and bacteria. The melanized rhizomorphs can traverse large distances faster than subterraneous hyphae since they are unimpeded by soil debris. Additionally, these organisms have a competitive edge as fallen plant material can become intercepted by these aboveground structures. These fungi can then start breaking down these fallen plant resources before they even reach the ground.
This is again reminiscent of some fungi I have found in Central America. In tropical rainforests, competition is so fierce for resources that in some saprotrophic species, these aboveground rhizomorphs have become even more complex; highly branched structures that resemble fungal nets for catching debris. Its wild to me, that you can find these intricate similarities in ecosystems nearly 2,000 miles apart. These analogous fungal structures have evolved in fungi in these distant ecosystems because of the generally low levels of available nutrients.
It is always a shock to my students when I tell them that tropical rainforests are actually very nutrient poor. Most of the nutrients in these systems is incorporated into living tissue, leaving the rainforest floor largely depleted of the already limited supply of nitrogen and phosphorus. Additionally, the enormous quantities of rain in both locations washes away much of the nutrients that does end up on the forests floor. This is why as soon as a leaf or twig drops from the canopy above in either location, organisms specialized in decomposition get right to work and compete fiercely with each other. Instead of fighting for these fallen debris, fungi that catch twigs and leaves before they hit the ground have the substrate to themselves.
Nearly two years ago I wrote an article about aerial hyphae in the tropics. There, I talk about how these aboveground structures greatly enhance insect diversity. These fungal nets increase the area for insects that require leaf litter to carry out their life cycle. I just found another study that was published over 20 years ago that revealed another important ecological interaction involving these marasmoid rhizomorphs. Kent McFarland and Christopher Rimmer carried out an intriguing study in 1996 that looked into bird nest material in spruce-fir communities in Northeastern United States.
By analyzing 54 nests from 10 different bird species, these researches found that 85% of the nests had the rhizomorphs of the horsehair fungus (Marasmius androsaceus) incorporated as building material. These researchers clearly show that in these northern ecosystems, these rhizomorph forming fungi are integral to bird success. Melanized marasmioid rhizomorphs are hardy enough to offer structural support, yet flexible enough to be contorted into a suitable nesting structure.
I always love discovering something new about nature, but I love it even more when I learn new information from an old publication. To me, science is like music; I am always trying to stay in the loop of the current good stuff, but always take a look in the past for inspiration. It’s crazy that this bird study came out when I was in kindergarten, while I was learning how to write my name. This just goes to show that a simple survey study can be used and appreciated well into the future. This also shows us that there are countless, wide-reaching fungal interactions out there that we haven’t even discovered yet. What other ecological interactions could rhizomorph forming fungi be a part of?