Geography (slope aspects and soil characteristics) and Climate (rainfall, evaporation, and temperatures).
If a tree can grow someplace, it will, since trees outcompete herbs and shrubs.
What does a tree need to grow? WATER. LOTS and LOTS of water. Over a hundred gallons of water per day for big trees.
Why? To transport nutrients to the top of the tree through its vascular system. Evaporation of water at the top pulls the entire column of water and nutrients along by the force of attraction of water molecules. In order to get 1 drop of water to a leaf on the top of a tree, 99 drops of water have to be evaporated.
If a tree can't grow someplace, but a tall shrub can, it will.
http://tchester.org/srp/plants/communities/10a2.html
Comments and feedback: Tom Chester
Last update: 6 November 2005.