Trees give off oxygen during the process of photosynthesis, while they take in carbon dioxide. Photosynthesis occurs when plants use sunlight, water, and carbon dioxide to create glucose (sugar) and oxygen.

Trees only release carbon dioxide during cellular respiration, which is the process by which plants use oxygen to break down glucose and produce energy. They do this continually but at lower rates relative to photosynthesis.

So, trees give off oxygen during the day when there is sunlight, and they release a mix of oxygen and carbon dioxide at night or in dark conditions when there is no sunlight. During the day, the rate of photosynthesis exceeds the rate of cellular respiration, resulting in a net release of oxygen. While at night, the rate of cellular respiration exceeds the rate of photosynthesis, releasing a net amount of carbon dioxide.