The pilot was advised during the preflight weather briefing that VFR flight was not recommended. The pilot indicated, during the briefing, that he was going to find an instructor or somebody with an instrument rating to fly him to the intended destination. About two hours later the pilot departed alone; he reported that he planned to either turn around or go to an alternate airport, if the weather en route deteriorated. According to the pilot, he proceeded en route by following a highway. He lost sight of the ground, due to fog, so he turned around, but as he did so, the nose of the airplane pitched down, and as he recovered, he saw trees in the path of the airplane. The pilot stated, 'I decided it was best to put it in the trees.' The pilot said the reason the nose pitched down was from winds around the nearby mountains, possibly causing wind shear, and which his already slower than normal airspeed, it caused him to lose altitude.