Question

why do leaves fall down from tree when they turn into yellow

Answer 1

Yellow colors that were always present in leaves become unmasked as the chlorophyll is broken down and absorbed. Called carotenoids, these are theyellow pigments that give treeslike birch, beech and tulip their bright fall colors. ... (Carotenoids play a similar role in the human body, as it turns out.

Answer 2

Leaves turn yellow because of the moisture stress, which can be from either over watering or under watering. 
Yellow colors that were always present in leaves become unmasked as the chlorophyll is broken down and absorbed. 
During that process, the trees lose a lot of water – so much water that when winter arrives, the trees are no longer able to get enough water to replace it. And so now we know. Leaves fall—or are pushed—off trees so that the tree can survive the winter and grow new leaves in the spring.

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