An irrigation line break caused a dawn redwood (9' tall) and a crabapple (9') tall to lose their foliage in the heat of August (90F+) period. Intense watering for two weeks brought almost all the foliage back on the crabapple, and did a fairly good job on the dawn (meta sequoia). The dawn usually has some trouble each August (we live 60 miles outside of Sacramento and August has lots of 90F+ days). I generally spray it lightly for days during such a tough stretch, and each year it comes back fine.
I was talking to a gardener about this and she offered that trees have something like 1 main leaf and an auxiliary leaf, if the main dies the aux takes over. Sometimes they have a few extra auxs. Does this translate into a tree having two, maybe three lives if hit by a lack of water?