Vegetation can attenuate UHF. It isn't too bad on 2 meters. Vegetation type and moisture content also have an effect. The vegetation here doesn't have a high moisture content, and once you get above about 2,000 ft in elevation, the vegetation is dominated by pines, incense cedar, and fir. Conifers don't appear to have as much of an impact as broadleaf vegetation, but I could be wrong. 6 meters does OK over hilly terrain as long as you aren't at the bottom of a deep or narrow canyon. HF is even better. Over here most of the highways run parallel to and nearly at the top of most canyons. That, combines with the numerous repeaters on mountaintops, make VHF coverage OK in most places. The canyons along the west slope of the Sierra Nevada are also open towards a west or southwesterly direction, which provides good access to quite a few repeaters in the coast ranges.