Trees are needed for life as we know it. Trees create oxygen which we breathe. Take away that and we will have issues. Trees absorb carbon dioxide and potentially harmful gasses, such as sulfur dioxide, carbon monoxide, from the air and release oxygen. Without this the gases will be in the atmosphere and the results could be catastrophic.
One tree supplies enough oxygen for 4 people to breath per day which is fairly impressive and didn't know that.
Trees have been a good resource as building materials for centuries and finding an alternative will take some doing. they provide a habitat for various species of birds and animals which help the eco system thrive.