"The loss of trees and other vegetation can cause climate change, desertification, soil erosion, fewer crops, flooding, increased greenhouse gases in the atmosphere, and lots of other  problems for Indigenous people that affects their daily lives. Deforestation happens for many  different reasons."

Source: https://pachamama.org/effects-of-deforestation

Deforestation causes floods. When trees get cut they do not absorb water from rain or overflow and this can cause floods.

Trees store carbon dioxide from our atmosphere and release oxygen. They are the opposite to humans because humans breathe in oxygen and breathe out carbon dioxide, but trees breathe in carbon dioxide and breathe out oxygen. This is a cycle that happens so if one of these go extinct the other does too. See how we rely on trees? 

The carbon dioxide that trees store gets released when trees get cut down. It has caused 10% of climate change so far, so if we want to stop climate change, we need to stop deforestation first.

Desertification is when a piece of land becomes like a desert with cracks because it is so dry. 

Soil erosion is when the roots of a tree become weak when trees are cut down and the top layer of soil goes away to reveal a less rich soil.

In deforested areas, there is less water in the air to be returned to the soil. This then causes drier soil and the inability to grow crops.