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I'm sorry very to hear about your situation.
If there is sufficient evidence that the landowner knew or should have known of the unreasonable risk, the landowner might be liable for damage caused by a tree. If there is any evidence offered, the jury would decide if the evidence was sufficient to prove notice of the defective condition of the tree/trees.
A court may consider the defense that a falling tree was an act of God. In the legal sense, an act of God is an extraordinary manifestation of the forces of nature without the intervention of man. An act is not considered an act of God, in the legal sense, if it could have been prevented by the exercise of reasonable diligence or ordinary care.
It appears from prior court cases that if a healthy tree falls into your yard because of an act of God, the disposal of that fallen tree is now your mother's responsibility. It wouldn't be the park owner's or the other owner's liability.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable.
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Thank you, ***** ***** luck to you!