The gas commonly released by trees during photosynthesis is **oxygen**. During this process, trees and other green plants convert carbon dioxide and water into glucose and oxygen using sunlight as energy. Specifically, chlorophyll in the leaves captures light energy, facilitating the conversion of carbon dioxide and water into sugar and releasing oxygen as a byproduct. This oxygen is essential for most living organisms on Earth, as it is required for cellular respiration. Photosynthesis is crucial for maintaining atmospheric oxygen levels and supporting life, playing a fundamental role in the planet's ecosystem.