How did conditions on Earth change during the Carboniferous and Devonian periods and how did those changes affect plants?

During the Devonian period, Earth experienced high levels of atmospheric oxygen which supported the development of the first forests. In the Carboniferous period, these forests led to high levels of carbon dioxide absorption, which decreased the greenhouse effect, leading to global cooling. These changes allowed the evolution of large, complex plant life such as ferns, lycopods, and seed plants.