Make the most of what you find. You can utilize a fallen trees root formation to make quick structures. Look for a tree root formation which you can crawl under. It can be roofed over with smaller fallen trees with branches and evergreen boughs on top to help keep you dry.
Alternatively, If its a nice flat root system sticking up into the air, it can be used as part of a shelter wall. This flat base of the fallen tree can also function as a reflector for a fire.