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.
Nadkarni looked up the world's human population as of Dec. 31 and found that on that day, we numbered 6,456,789,877 (again, very more or less). Punching the figures into her calculator, she figured that the world supports 61 trees per person.