Trees are a major part of the natural system that cycles carbon dioxide (CO2) and oxygen. As a major part of that system, increases in air pollutants have impacts on trees. However, in some cases, increasing levels of some pollutants can have a positive effect on the growth of certain species of trees. Whether the pollutant is negative depends on the pollutant and the type of tree.
Carbon Dioxide (CO2)
Increases of CO2, which was recently declared a pollutant by the U.S. Environmental Protection Agency, may have positive effects on tree growth, according to BBC reports aired on the National Geographic Channel. As atmospheric levels of have increased, tree growth and size in some areas have increased. The BBC reported that some forests have experienced a 20 percent increase in growth that mirrors increasing CO2 levels.
National Aeronautics & Space Administration (NASA) studies show that increasing levels of atmospheric ozone result on increases in birch tree growth ranging from 20 to 26 percent. However, some species of trees are harmed by high ozone levels. According to Kenneth Stoltz, Deputy Director of Forest Health Programs at the Southern Research Station of the United States Department of Agriculture (USDA) Forest Service, high ozone levels can harm pines and other connifers, including discoloration of the needles that can result in lower efficiencies in photosynthesis and a resulting decrease in growth.
Urban airborne pollutants resulted in physiological stress and damage to trees, according to the Journal of Arboriculture. Among the effects are reduced photosynthesis due to chemical changes in chlorophyll content and biochemical changes resulting in reduced gas permeability of plant structures. These changes resulted in reduced tree growth directly attributable to air pollution in urban areas.