Perhaps while in the forest you’ve seen tree stumps that look something like this.
Evidence the roots of this former tree are connected/grafted to nearby living trees.
This was a Douglas Fir (Psudotsuga menziesii) that was cut down some time ago. Instead of the stump dying and rotting away, which would normally happen with this species, it has healed over the “wound”. What made this possible is that the roots of this tree fused or grafted onto roots of the same species growing nearby years ago. This group of Doug Firs is now acting like a single organism, similar to the way a single tree will wound over where a branch has been cut.
Douglas Fir stump healing over the cut portion.
There’s another even more amazing network growing through the forests- mushrooms. Their roots (called mycelium) connect to tree roots and other plant roots to form symbiotic relationships where vital nutrients are shared between species- but that’s a whole other blog entry.