[robwwilliams]

The networks and growth processes can be wonderfully complex and even social among the root systems of neighboring trees — often mediated by extensive second-order fungal networks.

The trees support the fungi (sugars) and the fungi enable exchange of nutrients and tree-to-tree hormonal signaling of environmental health state. This is useful in enabling speedier responses to pathogens and insects infestation and sharing nutrients across trees (“altruism”). Usually this sharing and communication network is among members of the same species and often the same kinship. I do not know the primary literature on this area, but this is a good introduction:

The Hidden Life of Trees by Wohlleben, Peter