Quaking aspen (Populous tremuloides)
South end of Ridgeway Dorm complex
Pride of the southern Rocky Mountains, this tree forms groves of tall, slender spires covered with smooth greenish-white bark, topped by a billowing expanse of leaves that tremble in response to the slightest breeze. They blanket entire mountain sides at middle elevations, and in the fall - when every tree in the grove simultaneously turns golden yellow - they implant an indelible memory. Excellent trout streams flow past their lower slopes. They are a reliable source of firewood. Beavers gnaw their trunks and drop them on your cabin, before you arrive in early summer.
So much for personal nostalgia. The range of Populus tremuloides extends far beyond the southern Rocky Mountains - to Alaska, in fact, and much of the terrain in between. (At extreme latitudes the trees are much shorter, but they are upright, and they still tremble). The observation that an entire hillside turns yellow simultaneously is explained by the fact that up to hundreds of neighboring trees constitute a single organism; quaking aspens reproduce mainly by suckering, so each tree in the grove is connected to a single root system. This makes the quaking aspen (grove) one of the world's largest organisms. It is also one of the oldest; some root systems are thousands of years old. Quaking aspen is an important lumber tree (largely for pulp), and an excellent source of food for all sorts of creatures, from moose to voles. The "trembling" behavior of its leaves is attributable to the highly flattened shape of their supporting twigs.
And so much for facts. In Cripple Creek. Colorado, at the turn of the 20th century it was common knowledge that the tree trembled because it had supplied the wood for the Crucifixion. Google verifies that this legend is not confined to the Colorado gold fields. Alas, there are no poplars in Palestine.