Dead Italian Alder

I have watefront property with two Italian Alders--one is healthy, the other inexplicably dead. The dead one is actually a year or so older than the healthy one, but both are at least 25 feet tall with 4" or larger trunks. Both have easy access to the water, and I was told that this species is good for retaining the levee. Neither tree one has not been abused by over-pruning or construction. The dead one is adjacent to a fig and New Zealand flax. The healthy one is not. That's all I know, but I would like to know how this sizeable tree could have died before I plant another in the same place.
Also, I have a Magnolia that is maybe 50 feet tall with 2 foot dia. trunk. The tree is dropping more leaves than usual, has a larger number small dead branches, and didn't flower this year, at least not yet. The tree has suffered from having patios and lawn around it for all it's life and is otherwise a very robust and healthy tree. But, considering it's size and location in the center of the yard, the loss of this tree would be tragic. If I can apply fertilizer or some improvement to the roots, I would like to know what can be done to make sure the tree has the vigorous growth it's been known for in the past.
Thanks in advance for your help...
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