Question

why tree give carbon dioxide

Answer 1

Photosynthesis is the plant's way of making food for themselves. They do this over the period of daylight, using carbon dioxide water and oxygen to create simple sugars (and oxygen of course). Since it has no use for this oxygen, it disposes it by releasing it in the atmosphere. 

However, plants need to live too. They made that food for this very reason. Therefore, when photosynthesis is not going on (at night-time especially), the plant uses the food it has created to sustain itself. As with any other metabolic processes, the final products will be carbon dioxide and water which is exhaled by the plant
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