Acid rain 22 United States 16 Rain forests 14 Rain water harvesting 10 Pollution 8 Acid Rain 7 Ecology 7 Biodiversity 6 Climate Change Education and Research 6 Forests 6 History 6 Biological Resources (Plant) 5 Climate Change Adaptation 5 Environmental aspects 5 Forests and Forest Trees (Biology and Ecology) 5 Global warming 5 Green house effect 5 Literature 5 Ozone depletion 5 Rain forest ecology 5 Tropical rain forests 5 Air pollution 4 Deforestation 4 Deserts 4 English literature 4 English poetry 4 English poetry criticism-- Literature 4 Forest ecology 4 Horticulture 4 Literature English 4