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Page last updated at 12:19 GMT, Monday, 14 September 2009 13:19 UK
Autumn-time brings falling leaves

Jim Cokill
Director, Durham Wildlife Trust

Sycamore leaves
Sycamore leaves start their final act of the season

Continuing his series of articles on the County Durham and Wearside countryside, Jim Cokill reveals what's happening with trees and leaves around this time of year.

Autumn is all but here, but unfortunately, this year, we've had very little summer preceding it.

The obvious changes between the seasons are apparent, with the temperature dropping, particularly at night, and the days drawing in.

These shortening days trigger the classic sign of autumn - leaves falling from the trees.

Less light triggers a hormonal response by the tree that reduces the production of chlorophyll, the pigment that makes leaves green.

Therefore, other pigments produced by the tree become visible and the yellow pigment carotene dominates, hence the leaves take on a yellow appearance.

If the weather in autumn is warm and sunny, especially if there are cold nights too, another pigment, anthocyanin, is also produced and that is what gives the red colouration to autumn foliage.

So this year, it may be unlikely that we'll see a dramatic display of yellow, gold and red.

Leaving leaves

Autumn oak leaves
The cycle of a leaf is a fascinating process

At the same time as hormonal changes are producing changes in leaf colour, they are also triggering the process that causes the leaves to fall from the trees.

In the leaf stem, a layer of cells develops that reduce the flow of sap to the leaf.

These cells, called the abscission layer, then dissolve causing the physical separation of the leaf from the tree.

But why shed leaves?

The processes that draw water and nutrients through the tree, rely in part on sunlight and evaporation from the surface of the leaf.

In winter, with water potentially frozen in the soil and less sunlight being available, the tree suffers from drought conditions and the leaves become surplus to requirements.

That autumn leaf-kicking time is almost upon us!



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