Trees are very important they take in carbon dixode during the day turning it into carbohydrate to grow and releasing oxygen.
They provide food for humans and animals and even insects. They replensih the soil with leaves and braches that decompose and turn into compost. They keep air around them cooler in summer with shade and releasing water that cools the air around them. Cities with trees in certain areas are cooler becasue they shade ground so the sun does not warm it or concrete or asphalt.
The one problem now is you need Emerald ash borer treatment diy to save all the ash trees now just like elm tree sand some other ones.
Trees are very important and Vital. It is the biggest plants on the planet. They give us oxygen, stabilize the soil, stores carbon and give life to the worlds all Animals.