[atulatul]

Mostly, it is due to weather and the direction of sunlight (and due to pests, etc.). Also leaves at the top, those on the outside get more exposed to sunlight and the plant allocates these leaves more chlorophyll for synthesis (hence green). Comparatively, leaves on the inside do not get much sunlight and serve no purpose, so to say. So the plant/ tree sucks the chlorophyll back from these leaves. Now such leaves which do not take part in synthesis can expose the plant to too cold/ too hot weather due to their surfaces area. So the plant/tree sheds them. Of course too severe weather will impact all leaves so in winter etc trees are bare.

If interested, I highly recommend (not a pure science book) Hidden Life Of Trees https://www.amazon.com/Hidden-Life-Trees-Communicate_Discove...