Leaf strategy

Circa 2002
Ever wonder what happens in the Fall to make tree leaves turn the brilliant colors -- red, yellow, and gold?  There are several processes that take place to produce these colorful changes.

Chlorophyll 
Chlorophyll is the pigment that gives leaves their green color. It serves as a photo receptor, or light receiver, for the tree. Each molecule of chlorophyll contains four atoms of nitrogen. Because the tree cannot afford to lose this important element, the nitrogen is moved to the tree's twigs in the fall when the chlorophyll is broken down.

Inside the leaf
The pigments that give leaves their green, yellow and red colors are found in elongated cells called palisade cells.  In the fall, chlorophyll is broken down and nutrients are moved into the tree through the veins in the leaves.  This allows the yellow and red pigments to show through. 
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When autumn leaves start to fall the tree moves nitrogen and starches from the broken-down chlorophyll into the tree, exposing the other colors.  The tree then begins a process that removes the leaf.  It begins by adjusting its hormone ethylene, which allows cells at the base of the leaf to pull apart.  The spent leaf then falls to the ground. Replacing leaves each spring allows the tree to replace damaged leaves with a new set of " light receivers." 

The red colors in leaves are created by anthocyanin. Some trees have no anthocyanin and turn bright yellow in autumn. Yellow leaves get their color through pigments called carotenoids.

Don't forget the roots 
As photosynthesis slows in the reduced light and heat of autumn, leaf pores contract and less water vapor escapes ino the air.  As the ground gradually cools, roots keep absorbing moisture until the soil temperature reaches 40 degrees. Because the release of water through the top growth has slowed, there's a good supply of moisture in the system. This makes fall an ideal time for planting conifers, since there's little danger of transplanting's major cause of failure: desiccation.