Until relatively recently, most scientists thought rain was caused by mineral particles in the air which were just the right size for water (or ice) to condense around them. Research is beginning to show that a major factor in rain creation could be bacteria associated with plants. http://en.wikipedia.org/wiki/Bioprecipitation
If this is true then it is yet another way in which forests create rain. A forest has a far greater plant surface area than farmland or desert.