Ohia trees are dying rapidly, Why?

[Digitalis]

One of the reasons we chose the OL lot we did was because of all the awesome ohia. Questions:

How far from the trunk do the roots spread, generally?

If there are invasives growing right alongside the ohia (we have lots of strawberry guava and princess flower) what is the best approach- to uproot the nasty and risk messing with the ohia's root system, to cut down the invasive but leave the stump in place, or just leave it all together?

Do ohia like mulch? (Their roots seem very exposed.)

What are some good understory plants we could be planting?

Mahalo!