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Asked by to Amelia, Izzy, Sarah on 19 Mar 2014. This question was also asked by .
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Amelia Frizell-Armitage answered on 19 Mar 2014:
This is a really popular question! If you scroll through past questions you’ll see loads of answers to similar questions that can help you. I’ll give you a short summary here.
Leaves are expensive for a tree to keep – they need a lot of energy and resources put in to keep them running and repair them if they get damaged. In spring and summer this isn’t an issue because the leaves are like the energy factory of the tree – they produce energy from photosynthesis and usually make more energy than they need. However, when there is less light available over winter the leaves can’t produce as much energy as they can’t photosynthesise for so many hour in the day. This means that the leaves are probably using more energy than they are making, so it doesn’t balance in terms of cost for the tree to keep the leaves.
Leaves turn brown to fall off because the tree cuts the leaf off from its circulatory system, and as the leaf dies one of the first things to go is the chlorophyll. Chlorophyll is what makes a leaf green. Once this is gone the colour changes.
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Isabel Webb answered on 19 Mar 2014:
Plants don’t get enough sunlight in the winter and so they lose their leaves to save energy.
The first step is to a stop making chlorophyll which is a green chemical used in photosynthesis. Losing green reveals the brown colour which was there all along but hidden. Next the water supply is cut off and the leaf dries out an dies. Then the plant lets it fall off.
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