The removal of trees (de = remove, forest = trees).
The cutting of woods and trees.
The destruction of forested areas.
The excess CO2 in the atmosphere will increase global warming. (APEX)
The burning of plastics is known to produce carenogens (cancer causing dioxins). Some plastics, especially those used in older computers and some older electronics contain brominated flame retardants, of which the health side effects are unknown. Plastics are further manufactured from Oil (long chain polymers), hence when burnt, give off carbon dioxide. A known side effect of this (other than global warming) is the acidification of rain, which during the 1980's caused mass deforestaton in skandinavia.
There are several reasons. Cutting down areas of forest removes wildlife habitats, leaving animals with no shelter and nowhere to find food in the case of some herbivores. This can result in the death of some animals and therefore a loss of biodiversity. Many ingredients in medicines come from plant species in forests. Deforestation removes these plants and so loses key pharmaceutical ingredients. Perhaps even some undiscovered future medicines. Trees and plants use carbon dioxide in photosynthesis to produce oxygen. This is particularly useful in today's industrial world where more and more harmful CO2 is being put into the atmosphere. Forests act as a "carbon sink" removing the harmful CO2 from the atmosphere and giving out oxygen which can be used by other forms of life. Deforestation reduces this, leaving more CO" in the atmosphere which then contributes to global warming.