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Some trees have adapted to cold winter air by losing their leaves. What do you think would happen to trees if their leaves did not fall off before a cold winter?
The entire tree would freeze and die. In warm weather, the roots take in water and send it up to the leaves, where it is used for photosynthesis. The leaves use the water to make food that gets transported throughout the plant. If the leaves did not fall off, then water in the leaves might freeze and kill the leaves, stems, and maybe even the trunk and the roots. When the leaves fall off, the trees live on stored food until spring.