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Horses sleep under the tree in order to escape from the cold at night because the atmosphere near the tree is warmer. Trees consume energy from sunlight during the day for the process of photosynthesis. During this process, trees absorb carbon dioxide and emit oxygen.

At night, respiration takes place, and as a result, carbon dioxide is emitted into the atmosphere. This carbon dioxide is a greenhouse gas and it is responsible for the warm climate in the atmosphere.

During the daytime, infrared radiation is absorbed by the earth, which emits this radiation at night. Carbon dioxide is capable of trapping this infrared radiation. In turn, carbon dioxide reradiates the heat into the atmosphere. So, the horse will experience heat under the tree even though it is a cold climate outside.

Thus, the trees become responsible for the heat on cold nights due to the emission of carbon dioxide.