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.
One large tree can provide a day's supply of oxygen for up to four people. Each year, one person uses wood and paper products equivalent to a 100 foot tree 18 inches in diameter. Over 5,000 products are made from trees. Trees are included in most religions. Some hold certain trees sacred; other use trees to help teach beliefs.