Conditions that promote karst development are well-jointed, dense limestone near the surface; a moderate to heavy rainfall; and good groundwater circulation. Limestone (calcium carbonate) dissolves relatively easily in slightly acidic water, which occurs widely in nature.
Crowther said first he had to figure out how many trees are on Earth and that number was far more than anyone expected: 3.04 trillion trees, according to a study published Wednesday in the journal Nature. 14 Photos Trees Around The World.
1 There are close to 350 million Real Christmas Trees currently growing on Christmas Tree farms in the U.S. alone, all planted by farmers.  North American Real Christmas Trees are grown in all 50 states and Canada.