Acid rain and forest damage

Sulfur dioxide is toxic to plants, inhibiting growth at concentrations of 0.1 p.p.m. This direct effect of sulfur combustion occurs only in the vicinity of the emission source. Most of the SO2 released is eventually oxidized in the atmosphere where the indirect effects of acid rain, through pH changes, influence the availability of nutrients in the soil. 

It is particularly difficult to measure experimentally the concentrations and total amounts of ions in the vicinity of the roots that are available to, and can be taken up by, trees. Micro-site variations of a particular species can even be significant between individual, neighbouring plants. An alternative method that avoids the difficulties that arise in making meaningful analyses of groundwater is to measure the quantities of various ions taken into a growing tree. This is done by determining the increase in the amounts of ions present in the leaves. This may be correlated with soil chemistry, to some extent, to identify nutrient deficiencies. 

Magnesium is an essential constituent of the green pigment, chlorophyll, which is used by plants to manufacture sugars from carbon dioxide, water and sunlight. 

When the supply of magnesium is insufficient the leaves turn yellow, and the vital activity of the plant is restricted. Some internal redistribution can maintain the greatest activity of chlorophyll production at the most effective sites but the plant cannot thrive and extreme deprivation of magnesium results in death. There is evidence to show that, in some species, aluminium in the groundwater opposes the uptake of magnesium and calcium. Acid rain appears to increase the concentrations of aluminium ions in solution, by reactions of the types mentioned under mineral weathering above. Acidification can also result in the diminution of the concentrations available of other required nutrients, such as Na, K and Ca, by 

---