TIMING, MAGNITUDE, AND IMPACT OF ACIDIC DEPOSITION ON SENSITIVE FOREST SITES

Adverse effects of acidic deposition on forest health are most likely to occur in forests which develop a thick raw or "mor" humus layer in which the effective cation exchange capacity is highly sensitive to acid input. A study of the trend of exchangeable Ca and Mg ions in sensitive humus layers over the past six decades indicated that a downward shift in equilibrium has occurred that is consistent with theories of ion mobilization and coincident in time with increasing acidic deposition in the mid-1900's. Independent records of a base cation mobilization in wood supports the view of a change in the root zone in sensitive forest sites and in lake water chemistry. Induced nutrient deficiency in declining forest stands was indicated by a high Al:Ca ratio in fine root tip tissue as a marker of altered uptake efficiency which leads to increased vulnerability to biotic factors and greater sensitivity to abiotic stresses. Evidence indicates that sensitive sites were damaged by acidic deposition 20 to 40 yr ago, long before the problem of "acid rain" was recognized.