Mycorrhizal fungi, or mycorrhizae, live inside or attached to plant roots. The two types of organisms help each other to survive; that is, they are symbiotic. Fungi help plants to uptake soil nutrients in exchange for sugars produced by the plants. In forests, mycorrhizae form long strands called hyphae that run between trees, acting as connectors.  This giant underground transportation network is called the “common mycorrhizal network” or CMN.  The CMN uses chemical communication to exchange nutrients between trees on an “as-needed” basis.  Besides nutrients, the CMN also helps trees get water that their own roots would not be able to reach.  

When deep soils are moist and shallow soils are dry, trees pull water upward through their tap roots, which are deep in the soil, up to the shallow roots. Evaporation through the leaves and from the soil surface act like a drinking straw, pulling water through the plant and soil.  Mycorrhizal fungi can grab that water coming up from the deep tap root of one tree, send it along the highways of the CMN, and deposit it in the roots of a distant tree. With the mycorrhizae’s help, trees with roots too shallow to suck up water from deep soil can still get the water they need.  Whether trees are stressed from drought or are just a tiny seedling competing for water in a forest of giant adults, mycorrhizae help them survive.  Mycorrhizae can move water between trees of the same species, and between trees of different species.  By creating cooperation between species, the invisible underground network shapes the community of forest trees that we see above ground.

We do not yet know exactly how this microscopic highway in the soil works to move water around.  We do know that the active area of the fungus is the rhizomorph.  This is a special structure where nutrient and water exchange between the fungus and its host plants happens.  Both fungi and plants have proteins called aquaporins in their cell membranes. Aquaporins act like gates to allow water in or out of the cell. If both the rhizomorph aquaporins and the plant cell’s aquaporins are open at the same time, water can move between them.  There remains many mysteries to be uncovered about how symbiotic fungi and plants interact to distribute water in a forest.  

Climate change is presenting us with challenges related to water, food, and nutrient availability.  Some places may have shortages, while others have excess. The relationship between forest trees and the fungi that help them share resources based on need might help us solve these problems.

Learn more about mycorrhizal networks in this video by NOVA’s Gross Science.