Since trees are autotrophs, they must produce their own energy through photosynthesis. Photosynthesis requires light, water, carbon dioxide and chlorophyll.

The chlorophyll in the plant absorbs light from the sun. The energy from the sun then splits water molecules in the tree into hydrogen and oxygen. The hydrogen combines with the carbon dioxide that is absorbed from the air to produce a simple sugar that the plant uses for energy. Oxygen molecules are left as byproducts and are released into the air.

Trees must absorb carbon dioxide to photosynthesize and produce the energy needed for survival.

All life forms are carbon based and store energy as a carbon based starch or sugar.  Trees take in carbon dioxide and chloraphyll molecules use the sun's energy and hydrogens from water to make sugars which can be turned into starches for storage.  Their waste product from this process is oxygen.