1. What factor controls the amount of humus you find in an area?
For this question i think the factor is precipitation. Is that correct?
2. Why do vegetation change with increasing elevation?
please help. thanks in advance.
http://www.google.com/search?hl=en&rls=com.microsoft%3Aen-us%3AIE-SearchBox&rlz=1I7SUNA&q=humus+precipitation&btnG=Search
There may be information in here that can help.
In response to your second question, temperature and water availability would be two factors.
I hope this helps a little more. Thanks for asking.