Plants are important for life on earth. They take the sun’s energy and store it in chemicals such as carbohydrates (sugars and starches). Then animals get the energy from plants by eating them, and so on. As animals can’t get energy directly from the sun, life on earth would die out pretty quickly if there was no plant/tree life.
Plants also convert carbon dioxide back into oxygen, so CO2 levels would rise, causing further warming of the planet.