The impact of acid rain on trees ranges from minimal to severe, depending on the region of the country and on intensity and type of air pollutant. Acid rain, acid fog and acid vapour damage the surfaces of leaves and needles, reduce a tree's ability to withstand cold, and inhibit plant germination and reproduction. Consequently, tree vitality and regenerative capability are reduced. .
How else does acid rain affect forests?.
Prolonged exposure to acid rain causes forest soils to lose valuable nutrients. It also increases the concentration of aluminum in the soil, which interferes with the uptake of nutrients by the trees. Lack of nutrients causes trees to grow more slowly or to stop growing altogether. More visible damage, such as defoliation, may show up later. Trees exposed to acid rain may also have more difficulty withstanding other stresses, such as drought, disease, insect pests and cold weather. .
The ability of forests to withstand acidification depends on the ability of the forest soils to neutralize the acids. This is determined by much the same geological conditions that affect the acidification of lakes. Consequently, the threat to forests is largest in those areas where lakes are also seriously threatened - in central Ontario, southern Quebec, and the Atlantic provinces. These areas receive about twice the level of acid rain that forests can tolerate without long-term damage.
Are these effects reversible?.
Acid rain induces irreversible changes to forest soils and their fertility in parts of Ontario, Quebec and the Atlantic provinces, as well as in the northeastern United States. For now, forests in affected areas "where acid rain exceeds the critical loads "are using the pool of minerals accumulated during pre-industrial times although some monitoring sites are already deficient and visual damage has appeared. The loss of nutrients in forest soils may threaten the long-term sustainability of forests in areas with sensitive soils.