Fascinating. But why? If the "dead" tree can't contribute what is the (evolutionary) advantage of keeping it alive? As natural selection goes, how does being so "generous" benefit the living trees?
Perhaps it's the same advantage of living in a society like ants or humans. By keeping each other alive we stand stronger against other forces of nature and/or other species of trees or plants.
You get to keep the dead trees root system and remnant trunk for nutrient abortion and water and nutrient storage.
Perhaps the stump helps the remaining trees in the area by suppressing competitive species growth.
The remaining trees also get to take advantage of the extra canopy space that the fallen tree leaves behind.
The extra erosion resistance of having a nearby root system is probably significant.