Answer: Because of very low temperature, thin soil and a short growing season at high altitudes.
- Above 4500 m the ground remains covered with snow and ice for most of the year, so trees cannot grow.
- The temperature falls rapidly with height and the air is thin and dry, which does not allow tall trees to survive.
- High velocity winds, steep rocky slopes and thin soil prevent the growth of vegetation.
- Hence only alpine grasses, mosses and small shrubs grow in summer, and the highest parts are permanently bare.