Question

why do trees grow faster in summer

Answer 1

In most other places, plants grow more in summer either because it's too cold in winter (northern/southern climates) or because the summer is when the rains come (tropical climates). Answer 2: ... Plants depend on sunlight to grow. So in the Northern Hemisphere during the summer, plants grow more.

Answer 2

because plants depend on sunlight to grow
and the process of photosynthesis needs sunlight to prepare their food

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