Answer:
One way to rewrite the question is:
What is the reason that there is a greater presence of young cacti under tall trees?
Explanation:
The reason for rewriting questions is that more accurate information than what you are requesting is needed for the reader to better interpret and get a logical answer.
I rewrote it this way because doing so does not lose the intention of asking why, however, it is a better way to see the question, it is clearer about the required information and it also asks for a direct explanation.
