Legal Question in Personal Injury in California

Either resolve it through an insurance claim or file a lawsuit for the value damaged, and hope for the best.

You don't provide enough information to answer, particularly where the water came from, e.g., rain or broken pipes, and whether the manager knew or should have known water was going to get into your unit. Storage places don't become automatically fully responsible for your stuff just because you rent space and put it there. They have to have done something wrong, or failed to do something they were obligated to do. They aren't obligated to keep trees from falling onto your unit, unless the were responsible for caring for the tree and did something wrong in that regard. They have an implied duty to provide you a water-tight unit, but only if they know something has happened that makes it no longer water-tight and do nothing about it. If the tree fell in a rain storm and the rain immediately got into your unit, and they were not responsible for the tree falling, you are probably out of luck. This is why it's so important to have your own insurance on stuff in storage.

Who's tree was it? Why did the tree fall? Was the tree old and ready to fall? Was the tree on an adjoining property, or on premises? After the tree fell, did the landlord immediately have the structure inspected for damage? If so, was there damages found? If so, how long did the landlord take to repair the damage, or was it repaired at all? Maybe they found no damage because the inspection was faulty. Maybe they never inspected or repaired the structure? Did it rain during the month when the tree fell? What was the source of water?

These are some basic question which must be answered before you can establish liability.

Good luck.