Okay, being the geek that I am, I must admit a friend and I did some tests. God just typing this makes me feel like an over-the-top way too hardcore airsofter.
We weren't trying to do a legitimate "this is best for woodland" test, we just grabbed 4 camos at random and decided to see how they'd fare. This test was obviously not to the calibre of governmental research, but it suited our purposes. We based it on sight when stationary, and the how long it took an observer to detect motion when the wearer of a pattern moved, and found the M81 to be a so-so pattern for general woodland use, but conditions had to be just right. However, this was in the spring, when there'd be the most green, so that may have impacted the tests in one way or another.
We only tested one distance (75 yards) so like I said, it wasn't very professionally conducted. We picked a happy medium between "too close" and "too far" given the conditions (and our lack of optics for longer distances.)
From 75 yards, the M81 worked best when there was significant underbrush/low vegetation, accompanied with a proper amount of canopy cover to help the black and brown be used to full effect. Even then, it was hit and miss: Too much vegetation, brown and black made visable contrast; too much shadow and the green & tan created an exposing contrast. As I said, it was hit or miss. Depending on position, the M81 either worked or it didn't, there wasn't very much of a middle ground.
Interestingly enough, the very simple 2-colour Soviet "berezka" pattern concealed the wearer better than M81 in both open vegetation, and underbrush/vegetation with canopy cover.
We tested two other patterns also -- Czech "Clown" (Mlok) and MultiCam. Both fared decently; we were impressed with how well the Mlok actually did, despite its asinine appearance.
Our procedure wasn't very scientific or professional, but it was still fun.