For Utah Powder:
1. The air moves across the Great Salt Lake and creates a localized Lake Effect snow that lslams into the front side of the Wasatch and its box canyons dumping prodigous amounts of snow...that is why Snowbird, Alta, Solitude and Brighten receive 500+ inches of snow a year and the resorts on the back side of the Wasatch front average 300+ inches per year.
2. Because of the dryness of the desert, the water content of the snow is minimal, thus falling as powder.
3. As for trees...the only impact they have is containing any wind blown snow providing deeper powder and holding the snow longer.