The vibrant greens of summer have given way to the rich colors of autumn--browns, reds, oranges, and golds. The keys to changing leaf colors are long cool nights and short sunny days.
Leaves are nature's food factory but as the autumn sunlight decreases and nights become cooler, leaves stop producing food for the tree. The tree will rest and live off the food it stored during the summer. This causes a leaf's chlorophyll (the pigment that give leaves their green color) to break down. The green chlorophyll will disappear from the leaf and its yellow and orange colors will become more prominent.
The red and purple colors (maple trees) occur when glucose (sugar used for energy and growth) is trapped in the leaf's sap cells after food production stops.
The brown color of oak leaves is created from waste left in the leaves at the end of the summer season.
Once a leaf has changed color, the tree begins the process of shedding it by releasing more of the plant hormone ethylene. This allows cells at the base of the leaf stem to pull apart and fall to the ground.
Stems, twigs, and buds can survive in extreme cold but leaf tissue is too tender and would freeze during the cold months. The trees must toughen up and protect their leaves (evergreens) or dispose of them (maples, oaks, elms, etc...) to ensure the tree's continued survival.
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