There are many factors that influence the number of different kinds of plants in a biome. why is elevation, or height, a factor that influences what kinds of plants can live in any certain biome on earth

The higher you go up the less oxygen there is 

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.