Effects of Acidic Precipitation on Trees.- I. Introduction.- II. Effects of Acidic Precipitation on Tree Reproduction.- III. Effects of Acidic Precipitation on Vegetative Components.- IV. Effects of Acidic Precipitation on Tree Mineral Nutrition.- V. Effects of Acidic Precipitation on Tree Resistance to Secondary Stresses.- VI. Tree Mortality, Forest Decline, and the Forest Dieback Phenomenon.- VII. Future Research Needs.- VIII. Summary.- References.- Effects of Acidic Precipitation on Crops.- I. Introduction.- II. Wet Deposition and Foliage.- III. Direct Effects on Vegetation.- IV. Factors Affecting Vegetation Responses.- V. Limitations to Experimental Approaches.- VI. Summary and Conclusions.- References.- Effects of Acidic Precipitation on Soil Productivity.- I. Introduction.- II. Potential Soil Effects.- III. Linkages and Effects.- IV. Evolving Concepts on Soil Effects.- V. Research Needs.- References.- Effects of Acidic Precipitation on Stream Ecosystems.- I. Introduction.- II. Effects of Acidification on Populations and Communities.- III. Effects of Acidification on Biological Processes.- IV. Future Research Needs.- References.- Effects of Acidic Precipitation on Lake Ecosystems.- I. Introduction.- II. Discussion.- III. Summary Recommendations.- References.- Effects of Acidic Precipitation on Forest Ecosystems in North America.- I. Introduction.- II. Forest Ecosystems: An Overview.- III. Forest Ecosystem Interaction with Regional-Scale Atmospheric Deposition.- IV. North American Forest Ecosystem Decline.- V. Global Scale Air Pollutant Stress on Forest Ecosystems.- VI. Future Assessment of Forest Ecosystem Stress Related to Atmospheric Contamination.- VII. Conclusions.- References.- Effects of Acidic Precipitation on Forest Ecosystems in Europe.- I. Introduction.- II. Assessment of Deposition.- III. Rates of Deposition.- IV. Input-Output Relations of Forest Ecosystems as a Measure of the Deviation from Steady State.- V. Changes in Soil Acidity and Present Status of Forest Soil Acidification.- VI. Causes of the Present Soil Acidification.- VII Effects of Acidic Deposition on Litter Decomposition.- VIII. Direct Effects of Acidic Deposition on Leaves.- IX. Effects of Soil Acidification on Trees.- X. Heavy Metals: Input-Output Relations and Possible Effects.- XI. Cause-and-Effect Relationships in Tree Damage on the Basis of Case Studies.- XII. Future Research Needs.- References.- Effects of Acidic Precipitation on the Biota of Freshwater Lakes.- I. Introduction.- II. Physical and Chemical Changes Related to Acidification with Potential Importance to Biota of Softwater Lakes.- III. Descriptive Studies of Changes in Aquatic Biota Related to Acidification.- IV. Effects on Community and Ecosystem Function from Acid-Related Biotic Changes and Indirect Effects of Acidification.- V. Conclusions.- References.- Effects of Acidic Precipitation on Soil Microorganisms.- I. Introduction.- II. Effects on Soil Microorganisms.- III. Organic Matter Decomposition.- IV. Soil Respiration and Soil Enzymatic Activities.- V. Nitrogen Cycle.- VI. Mycorrhizae Associations.- VII. Heavy Metal Effects.- VIII. Future Research Needs.- IX. Conclusion.- References.- Aluminum Toxicity and Tolerance in Plants.- I. Introduction.- II. Expression of Aluminum Phytotoxicity.- III. Mechanisms of Aluminum Phytotoxicity.- IV. Mechanisms of Aluminum Tolerance.- V. Future Research Directions and Concluding Remarks.- References.