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. When soils become too acidic, the health of hard-wood trees declines. Sensitive soils are often shallow, sandy soils, which are most common on the Canadian Shield in central and northern Ontario. Other soils containing more clay are better able to neutralize acid. They are more often found in southern Ontario.