The problems with "sun" coffee

Coffee (Coffea arabica) is an understory tree or shrub which naturally grows in shade. 

In order to increase the yield of coffee shrubs and individual farms, there was a movement over the last 20 years towards the "technification" of coffee farming, which replaced traditional shade grown farming with sun cultivation.  Over 2.5 million acres of forests in Central America were destroyed to make way for monocultures of sun grown coffee.  This caused an immediate loss in biodiversity, both in the many types of trees and plants that were eliminated, as well as the animals that depended on them.  Ecosystems in tropical areas, where coffee is grown, are particularly complex, and this loss is devastating.  (For more on the degrees of sun or shade under which coffee is grown, see "What is shade grown coffee?")

Other than the loss of biodiversity that is a result of fewer trees and reduced complexity, there are other negative environmental impacts of growing coffee in full sun.

• In shade plantations, dead leaves from the overstory trees provide nutrients to the coffee shrubs as they decay.  In sun plantations, these nutrients are not available, so fertilizers must be used, especially nitrogren (since many traditional overstory trees are nitrogen-fixing legumes), which is essential to coffee growth.
  
• There are fewer weeds in shade plantations, because fallen leaves from overstory trees in shade plantations act as a natural mulch.  Herbicides are needed to control weeds in sun plantations.
• Soils in sun plantations are more exposed to the elements, particularly drenching rains typical of tropical areas.  This means more erosion of topsoil, and the leaching of chemical fertilizers, pesticides, and herbicides into local watersheds. Erosion and water pollution are serious consequences of growing coffee on sun plantations.
• Coffee plants in sun plantations grow faster and age more quickly than those grown in shade, and therefore must be replaced more often, specifically at about  6 versus 30 year intervals, respectively.

The Natural Resources Defense Council has a thorough examination of the environmental dimensions of coffee production, as does the World Wildlife Fund.