Levels of air pollutants are highest in Ontario's southwest, and lower in the north and northwest. But the hardwood trees in the survey's southern plots tended to be in better health than those in the north. The zones are divided according to their soil, landscape, tree species and climate. The northern zones had a higher average DI than those in the south. Climate is one reason for this difference.
Hardwood trees grow more slowly and are more prone to climate effects in the north.
Another reason for higher decline indexes in the north is the quality of the soil. Acidic deposition, often called acid rain, includes the acidic particles in rain, snow, and airborne dust that come from air pollution. All this acid eventually falls on the forest, and then to the forest soil. Some soils are more sensitive than others to the acid dropped by air pollution. These sensitive soils cannot neutralize the acid.