Suzanne Simard, forest ecologist at the University of British Columbia, and her colleagues have made the major discovery that trees and plants really do communicate and interact with each other. She discovered an underground web of fungi connecting the trees and plants of an ecosystem. This symbiosis enables the purposeful sharing of resources, consequently helping the whole system of trees and plants to flourish.

Simard was lead to the discovery by the observation of webs of bright white and yellow fungal threads in the forest floor. Many of these fungi were mycorrhizal, meaning they have a beneficial, symbiotic relationship with a a host plant, in this case tree roots. Microscopic experimentation revealed that the fungi actually moves carbon, water and nutrients between trees, depending upon their needs.

http://www.ecology.com/2012/10/08/trees-communicate/