The evolution of altruistic behaviours is unclear. There is no single universally accepted reason. The most common argument is probably that it was selected for in the same way your genes would have a survival advantage even if you die while helping your cousin or sibling or just fellow human survive. Because they're almost genetically identical to you, their survival is almost the same as your survival, from the gene's perspective.
[1] and [2] are introductions to the mycorrhizal networks and how they work on a chemical and biological basis, [3] and [4] are really nicely presented talks about it, by renominated researchers.
It's so ridiculous that they even found evidence that the connections between seedlings and their mother trees are stronger and more reliant than to others; which seems to suggest that they recognize their own offspring once it grows up.
There's also a huge organization that focusses on research of mapping those vast networks, called SPUN (society for protecting underground networks) [5]
On a darwinistic level I'd argue that once there probably was a stage in evolution when plants could not survive on their own in harsh weather conditions, the ones that survived were able to communicate and exchange "goods" for their own benefits; like exchanging sugar against nitrogen in case you had too much of it that you wouldn't need anyways.
(I'm no biologist so don't take my advice on anything on the topic, I am only building algae-fungi based bio reactors to try to find a way to bind carbon dioxide and put it into proteins as a food source as a hobby project)
> Dying trees literally distribute their resources to healthy ones,
I'm struggling to imagine how such genes could possibly reach fixation.
Want to explain?