It turns out that trees not only communicate through fungal networks in their roots, they also pass nutrients around from tree to tree, even between species!
The fungi (many species) grow in contact with the roots of the tree, enjoying the steady source of carbohydrates that the tree has produced, and in exchange giving the tree advantages such as an increased surface area to absorb water, an ability to absorb some minerals like Phosphorus and Nitrogen, and more. Some species of fungus actually attract springtails (tiny insects) in order to kill them and absorb nitrogen from them. One study found that Eastern White Pines can get as much as 25% of their nitrogen from springtails (courtesy of these fungi).
Once nutrients and water are in the fungal network they can be transported around the forest underground. Carbohydrates, nitrogen, water, etc. gathered at one tree can end up above ground in another tree many yards away.
In this way the older and larger trees – Mother Trees – can support the growth of many smaller trees around them, and help young trees get established. Not that any conscious decision-making is involved, but it shows that the tree’s “experiences” are a lot richer than we thought. Amazing stuff, and a whole new perspective on the forest.
There are links to the scientific papers through Suzanne Simard’s web page here: http://farpoint.forestry.ubc.ca/FP/search/Faculty_View.aspx?FAC_ID=3198