a. Stratification: The vertical distribution of different species occupying different levels in an ecosystem is called stratification. Trees occupy the topmost vertical layer of a forest, shrubs occupy the second layer and herbs and grasses occupy the bottom-most or base layers. b. Gross primary productivity: GPP is the net amount of energy that is produced by the main energy producers of the ecosystem in a particular area during the specific time. GPP tends to state the total productivity. c. Leaching: Natural process by which water-soluble substances (such as calcium, fertilizers, pesticides) are washed out from soil or wastes is called leaching. These leached out chemicals cause pollution of surface and sub-surface water. d. Humification: The process of the formation of humus is called humification.The creation of humus in the soil after interaction with bacteria which has broken down dead and dying biological matter into humus.