Why trees leafs turn green in summer? And yellow in fall?

Question

How they do it?
Why?
And why leafs start to fall in fall and winter? How do they grow again in spring and summer?

Answer 1

They don't "turn" green they grow that way.
The yellow pigment is already in the leaf but is masked by the chlorophyll whicn is darker and in greatewr amounts. When the weather turns the green pigments break down first leaving the yellows, oranges and maybe reds (in maples)

Answer 2

Leaves are green because of chlorophyll, the green pigment that enables them to capture light energy and use it in the building of all sorts of organic molecules. (And chlorophyll is green because it mostly absorbs red and blue light, and reflects the remainder - green)

Now under certain climatic conditions (periodic drought, cold and dark winters etc) it pays off for the tree to shed its leaves and shut off most of its metabolism (because it couldn't supply the necessary water, or because light energy is so scarce that it's not worth maintaining a foliage and growing mindlessly). Leaves fall as part of an energy (and possibly life-)saving mechanism. I don't know what exactly triggers this, but I'd guess under temperate conditions it's the length of day and temperature in some combination. Leaves fall because of the tree switching off most metabolism and material supply to the foliage; and they lose their green colour because chlorophyll is degraded. The yellow (or red) colour is, I'd guess, due to other, simpler and chemically more stable pigments (eg. carotenoids, which are mostly yellowish AFAIK) present in the leaf, which stay there longer than the larger and more delicate molecules of chlorophyll. Eventually all pigments decompose, and all that remains is brown, dried and dead organic matter, waiting to be eaten up by microorganisms.

In the spring it could again be something like temperature and the amount of light that pushes the "start growth" button for trees. The plant has reserves stored in the form of, eg, starch, which can provide the energy for re-growth until the leaves start to function again.

BTW, the chlorophyll is NOT in the "circulatory system" (by that I guess the answerer meant transport structures like xylem vessels) of the tree. It's in the leaves' mesophyll tissues, and it's not the circulatory system that gives "food" to the leaves. It provides them water and ions. The leaves make the "food" by photosynthesis (for which water is essential!), and then hand it over to a transport system (not the one that carries water!) which distributes it all over the plant.

Answer 3

Sunlight in the summer is stronger and helps produce chloropyll in the leaves, which absobs every color of ligt except for green, and that is why leaves appear green, ithey reflect green light. In the autumn, the sunlight is weaker due to the tilt of earths axis, and the leaves can't absorb enough light, so it begins to cease the production of chlorophyll and in turn reflects the next best color-red or yellow or brow, until death which is all dried up and boring to look at.

Answer 4

Okay long story short it has to do with the moisture. The "food"the plant eats is sent through its circulatroy system. This contains chlorophyl. This is waht gives it it's green colour. As the temperature gets colder the tree's circulatory system becomes more centralized. They moisture and food are no longer sent to the leaves. So they lose their colour dry out and eventually drop off.

Answer 5

to do with production of chlorophyll, makes it green. In the winter months this isn't produced leading to a more yellow colour.

Answer 6

it's like apples when they turn brown
its oxydization and the air.
the leaves turn brown in fall, because they die.
spring is the rebirth and regrowth.
its just a chemical reaction.
they grow again because the tree is still alive.
thats like when strands of your hair fall out,
another pore opens up and starts to grow.

Answer 7

it is caused by chlorophyl in the production of sugar