Hi, Rasha Assem, and 'Happy Eid Ul-Fitr,
If a tree has deep roots, it (doesn't- won't) fall over in strong winds.
Is it a fact or something that is possible to happen in the future?
Both answers are possible. Although I see that 'doesn't' works better here, I always teach students what they have in their books for the sake of their exams. In fact, this is another old question that I discussed with the editor of Longman Website in 2011. She said:
"Although I do think it is generally true that a tree with deep roots doesn't fall down in a strong wind, I don't think it is necessary to express this with a zero conditional. I could be talking about a particular coming storm. Again, deictic words are not always used in real life. It's making a prediction about a general truth. Compare this to what was taught in Prep 1, Unit 8 with sentences like "If you put wood in water, it'll float." It is a general truth, yet here it is making a prediction. Since there is no context here but an isolated sentence, that is possible."