Deforestation is the cutting/removal of trees either naturally or by anthropogenic activities. Since roots of trees holds soil firm, in their absence soil is vulnerable to erosion by any calamity or human influence.

Plants evolve oxygen by the intake of greenhouse gases like carbon-di-oxide, hence when trees are cut greenhouse gasses increases and cause global warming.

Also forests are the house of biodiversity since they support them by providing everything they need naturally. Hence deforestation is the main reason behind all of them.