Question

why are the trees absent in tibetan plateau

Answer 1

Tibet's elevation and lack of precipitation produces an alpine tundra environment, which reduces species diversity. 

Answer 2

There is less precipitation in tibetian plateau. It makes it diverse and the lack of evalation there is less trees seen in the environment. There are alpine trees and it receives less rainfall and as it is situated in the heart of the country and there is no seas and oceans near it rather it is far away which prevents evaporation in that place .