In recent years, humans have been burning more and more fossil fuels. Fossil fuels are oil, natural gas, and coal. One of the extra products created by burning fossil fuels is ozone. Since the level of ozone is rising, the scientists wanted to know how this affects how trees grow. Tree growth starts in leaves. Therefore, scientists started by looking at the leaves. They looked at how many leaves were produced and the amount of nutrients in the leaves. Topics: Active Forest Management, Carbon, Pollution, Wilderness, Carbon, Fossil Fuels, Leaf Litter, Nitrogen.