Why Does The Quaking Aspen Quake?

Populus tremuloides, or quaking or trembling aspen, is found in the mountains of northern and western North America.
The unusual ability of aspen leaves to twist and bend in the wind is due to their flattened petioles, or leaf stems. When the leaves can twist, it helps protect the trees from severe winds, perhaps by helping dissipate energy more uniformly throughout the canopy.
It also improves the rate of photosynthesis by reducing the exposure of the outer leaves to extreme sunlight by presenting them at an oblique angle to the sun throughout the day, while at the same time allowing more light through to the shaded lower leaves. Other plant species besides aspen turn their leaves for this reason.
It is a beautiful sight to see aspen leaves all "trembling" in the wind, particularly in autumn, when they turn a vivid golden color.
Aspens are cloned, growing in large colonies derived from a single seedling and spreading by means of root suckers. Individual trees only live 40 to 150 years above ground, but the root system of the colony may live for many thousands of years, sending up new trunks as the older ones die off.
A large aspen colony in Utah, dubbed "Pando," is reputedly more than 80,000 years old, making it possibly the oldest living colony.

It is flattened petioles that are considered to be the reason behind the phenomenon of trembling of the quaking aspen. Though other species also have the fattened petioles, they are flattened partially along the length while in the quaking aspen these are flattened along the whole length of the petiole and these are flattened from side to side.  

The quaking aspen are found in North America and mostly you can find them in the cooler areas and in these areas also it is of limited habitation as it does not tolerate permafrost. In Canada you can find these trees to be almost omnipresent and in the United States they are mostly confined to Nebraska and Indiana. You can also find them in Mexico and in that country they are confined to the Guanajuato area.