Mycorrhizae are fungi that form mutually beneficial associations with plant roots. The mutualism works like this: The mycorrhiza grows in and around the plant’s root tissue, and its hyphae, or thread-like vegetative parts, serve plants by branching out in the soil and absorbing nutrients that are of importance to the plant. Since the hyphae are often so much smaller and more abundant than the plant roots, they create a network that drastically increases the plant’s absorptive area. In return, the plant provides food for the fungus, most often in the form of carbohydrates the plant produces during photosynthesis.
Still with me? It gets better.
Nina Wurzburger (now of Princeton) and Ronald Hendrick of the University of Georgia collected data on nitrogen (N) levels in the soil and in the roots of plants in a southern Appalachian forest, including rhododendrons, hardwood trees and herbs. Rhododendron leaves are rich in tannins, and when their leaves drop and decay, they enrich the surrounding soil with tannin complexes. The researchers found that when these tannin complexes were present, there was a lot more N in the soil. Further, rhododendron roots were found to absorb more N than surrounding trees or shrubs.
The researchers think that leaves dropped by rhododendrons have properties that package N in ways that make the nutrient more accessible to the rhododendron than to surrounding plants. So when the leaf decays into the soil, it makes the soil less amenable for N uptake in other plants. Further, the mycorrhizae that live on rhododendron roots have specialized ways to absorb N within these complexes, which might include the shape and length of the roots and other specialized ways to extract N from organic matter.
This tightly-knit feedback loop might help explain why rhododendron has expanded so broadly in the southern Appalachians and has had detrimental effects on forest diversity. As the authors conclude:
“Feedback processes between plant litter chemistry and mycorrhizal roots and fungi may be highly complex and occur at fine spatial scales, but they have the potential to drive patterns in N cycling and productivity in terrestrial ecosystems.”
Wurzburger, N., & Hendrick, R. (2009). Plant litter chemistry and mycorrhizal roots promote a nitrogen feedback in a temperate forest Journal of Ecology, 97 (3), 528-536 DOI: 10.1111/j.1365-2745.2009.01487.x