Why do aspen leaves flutter? It's in both their common name (quaking aspen) and scientific name (Populus tremuloides). The leaves are undeniably built for it; the leaf stalks are flattened, allowing the leaves to twist and bend from side to side in the slightest breeze (which thermals reliably provide each and every sunny afternoon in the mountains). Sure, poets would claim there's something pleasant about taking a nap beneath their glowing foliage while they rustle and sparkle in the clear mountain air. But natural selection awards little value to deciduous trees for giving bipedal mammals a relaxing siesta. Why, then, do the scientists think they flutter?
Photosynthesis, like any other chemical reaction, proceeds optimally in certain ranges of conditions there's an optimal temperature, humidity, wavelength of light, abundance of oxygen and carbon dioxide, and so on. Heat for example: chemical reactions slow down as the temperature drops, yet if you raise the temperature too much undesirable reactions begin to occur. Such as oxidation fire in its extreme, but even at lower temperatures, oxygen is a highly reactive and destructive element that will attack many delicate structures in the biochemical machinery of the chloroplast.
Simplistically, one would expect the more light the better, but as with all else in life, there is such a thing as too much of a good thing. Overexposure to the sun can, of course, cause UV damage, but even if the UV is blocked (most leaves contain some quantity of carotene or other similar colored pigments that help protect them from UV... and cause them to turn color in the fall when the delicate chlorophyll fades) too much visible light can cause something called photoinhibition. Above a certain point the chlorophyll becomes saturated and can no longer safely use all the photons bombarding it. These excess photons can damage a number of sensitive compounds used in both photosystem I and photosystem II in the Calvin Cycle. In the worst case the damage can even be permanent (causing, among other things, bleach-spots on house plants left in direct sunlight too long). [1]
So the fluttering leaves have two important (non-aesthetic) effects: at the top of the tree where the high-elevation radiation is at its greatest, it effectively reduces the illumination on the leaf by presenting an oblique angle to the sun. At the same time, this allows light to penetrate into the lower reaches which would normally be deeply shaded. Thus the upper leaves receive less light, reducing photoinhibition, and the lower leaves receive more light, increasing the rate of photosynthesis throughout the tree.