Why photosynthesis can't take place in such an environment , where there is no oxygen..? And exactly why trees need oxygen for photosynthesis ?

Photosynthesis is a chemical process that leads to the release of oxygen as a by-product and is necessary for cellular respiration  

Explanation:

The plants produce or make oxygen form the wastes of starch, sunlight carbon dioxide, and water. In order to burn the sugar that is stored in the plants, it needs to have sunlight which requires oxygen. As the break down of sugar C6 H12 O6 to CO2 and H2O needs O2 to do it. Which is called cellular respiration. This is also stated by the first law of thermodynamics that energy cannot be created and neither destroyed but transforms itself. Photosynthesis is the chemical process that is used to convert this chemical energy which makes carbon and water into glucose and oxygen is released as a by-product.