What does acid rain do to trees?

The impact of acid rain on trees ranges from minimal to severe, depending on the region of the country and on the acidity of the rain. 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.

Acid rain also depletes supplies of important nutrients (e.g. calcium and magnesium) from soils. The loss of these nutrients is known to reduce the health and growth of trees (see below).

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. Forests in upland areas may also experience damage from acid fog that often forms at higher elevations.

Are these effects reversible?

Acid rain has caused severe depletion of nutrients in forest soils in parts of Ontario, Quebec and the Atlantic provinces, as well as in the northeastern United States. While this may be reversible, it would take many years - in some areas hundreds of years - for soil nutrients to be replenished to former levels through natural processes such as weathering, even if acid rain were eliminated completely. For now, forests in affected areas - where acid rain exceeds the critical loads - are using the pool of minerals accumulated during post-ice age times, although some monitoring sites are already deficient in minerals and visual damage to forests has appeared. The loss of nutrients in forest soils may threaten the long-term sustainability of forests in areas with sensitive soils. If current levels of acid rain continue into the future, the growth and productivity of ~ 50% of Canada's eastern boreal forests will be negatively affected.

Click to enlarge

The maximum amount of acid deposition that a region can receive without damage to its ecosystems is known as its critical load. It depends essentially on the acid-rain neutralizing capacity of the water, rocks, and soils. This map, of the upland forest soil Steady-state critical load exceedances for southeastern Canada (eq/ha/yr), shows areas of eastern Canada where the levels of acid deposition exceed the capacity of the soils to neutralize the acid without harming the long-term sustainability of the environment. The Steady-state exceedance calculations assume that the forests are not harvested.

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