Some kinds of fungi establish symbiotic relationships with plant roots. Erosion destroys the roots and changes the remaining spores. Can we bring them back? If so, what kinds?
Can we improve beneficial fungi (arbuscular mycorrhizal fungi, or AMF) in the soil?
Tropical biologists have known for a long time that most tropical trees grow better and are healthier if their roots are colonized by mycorrhizal fungi. Sometimes the relationship is obligatory and the tree cannot grow without its symbiotic partner. These relationships may be most crucial in infertile soils, because the fungal tendrils (hyphae) that reach out into the soil bring nutrients to the plant, greatly increasing what the roots can do on their own. In turn, the fungi receive sugars from the plant and use them in their own growth.
Most tropical plants have a relationship with arbuscular mycorrhizal fungi (AMF), which form structures inside plant cells. Left photo above shows dark blue round vesicles (sacs) inside a root tip of one of our trees. The next photo shows small square vesicles and a fungal tendril reaching out from the right side of the root. AMF reproduce by microscopic spores in the soil (third photo). Pines and some other groups contain a completely different group of fungi, ectomycorrhizae. The fungus grows on the outside of the roots. Their reproductive structures are mushrooms that emerge from the forest floor. The fourth photo shows a mushroom at the base of one of our pines.
We studied the impact of erosion on spore availability, what kind of AMF our target trees contained, whether the source of spores affected the degree of benefit to our seedlings, and how pine ectomycorrhizae might affect the AMF needed by our other trees.
Arbuscular Mycorrhizal Fungi soil census: Healthy soils contain spores of beneficial fungi (AMF) which germinate and establish symbioses with tree roots, helping them to collect nutrients such as phosphorous from the soil. We did a baseline census of the kinds of fungi across a range of our soils from the most deeply eroded areas to the least. The worst soils had few spores, and they were dominated by a single type, Acaulospora. The best soils, located in a small patch of secondary forest on the farm, did not have the highest spore quantity but had a high diversity of number of types of spores.
Arbuscular Mycorrhizal Fungi root census : Although the soil census was the first step, the most important fungi are those that are active inside roots. Active types might differ from the types represented by spores in the soil. We used molecular techniques to determine the types of AMF inside roots of the 12 best-performing tree species. We found that the two species of the legume Inga that we have studied had the most different kinds of AMF. This diversity might explain why these are the only successful legume species that we have found so far, so successful that they can act as nurse trees for seedlings planted nearby.
Mycorrhizal sourcesWe found the surprising result that pines had AMF as well as ectomycorrhizae, and AMF are found in abundance where the pines are established. Therefore, pines might serve as facilitators for other trees. We don't think we need to worry that establishing pines on a farm will render the land unsuitable for AMF and the trees that depend on them.
Our work is published in two papers in scientific journals and a PhD dissertation at UCI by Dr. Riley Pratt (right).