treeguy123, I have not observed the flutter/shimmer effect but it has probably evolved to help the species survive. For instance, very large leaves - banana and similar - would be completely broken off in a high wind. The leaf tears or shreds in the familiar fashion to avoid this lethal event. The torn edges seal themselves and the previously simple leaf now has "become" a compound leaf. Maybe the point (petiole?)of 'rotation' is a vulnerable area and the shimmer prevents defoliation

I'd doubt it helps against defoliation; if anything, the fluttering leaves are more liable to damage by hitting each other. Certainly poplar leaves are often fairly ragged by late summer.

My guess (but it's only a guess) is that the leaf motion improves gas exchange rates and so increases carbon dioxide absorbtion.

Resin

Thanks both of you.
There are at least 3 interesting and good sounding theories I've read when google searching on why they flutter:
1. Helps reduce and dissipate strong wind thru the tree canopy in high winds Link (8th paragraph down)

2. Helps get sunlight evenly in to the canopy so all leaves can photosynthesize well. Link (click Quaking Aspen)

3 Helps Co2 exchange, but shown by these links, it may or may not Link
Link

I might try to set up an experiment sometime to see if fixed poplar leaves have more drag in the wind than fluttering ones. This might prove fluttering leaves reduce drag for the tree in high wind gust events.

If anybody has more information or studies on Poplar leaf flutter function(s) post it, it would be great to read more about this. Thanks.