Furthermore, acid rain covers large areas and therefore can explain the large scale areas of decline observed in Europe and eastern North America. The effects of acid rain on the forest ecosystem can be examined at several levels. Acid rain may remove essential nutrients from the soil as hydrogen ions in rainfall moving through the soil replace similarly charged elements, such as calcium and magnesium, on the soil particles. By reducing soil pH, acid rain may lead to metal toxicity as these metals become mobilized in the soil solution. Since plants cannot preferentially exclude specific elements, those elements most common in the soil solution will end up in the plants.
Many trees have beneficial associations between their roots and fungal hyphae called mycorrhiza.
This mycorrhizal association provides the fungus with a home in the plants roots and provides the plant with a greater uptake of nutrients. Finally, at the forest ecosystem level, acid rain leads to imbalances in nutrient cycling primarily by reducing the rate of leaf litter decomposition. This reduced rate of decomposition means that fewer nutrients are made available for plant growth each year. A number of research projects demonstrate the fact that acid rain impacts upon many parts of the forest ecosystem. Plant feeding insects may be at risk of accumulating toxic metals via increased plant uptake of these elements under acidified conditions.