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.
The total number of plant species in the world is estimated at 270,000. Approximately 1,000 to 2,000 species of plants are edible by humans. About 100 to 200 species of plants play an important role in world commerce, and about 15 species provide the majority of food crops.