The introduction of exotic Eucalyptus plantation declines the genetic diversity causing tremendous threat to natural ecosystem like “Ecological desert”. The great Eucalyptus debate and the ongoing controversy regarding the replacement of indigenous tree species with extensive Eucalyptus species and its consequences focus attention to environment conscious people. So the present investigation was undertaken to make a comprehensive study on the ecology of exotic tree Eucalyptus citriodora Hook and the leading indigenous tree Gamari (Gmelina arborea Roxb.), in relation to three important biotic components like ground floor vegetation, microbial association, mesobiotal association and three abiotic components like leaf litter decomposition, physico-chemical characteristics of soil, soil system and clay mineralogy throughout the eastern Brahmaputra valley. In order to able to conserve the environment congenital to Human inhabitation sound understanding of manmade ecosystem is essential. The results encountered in this investigation are presented in this book. The book is indeed very much beneficial for Ecologists, foresters, environmentalists and students working in this area.