The pilot reported that both he and the passenger walked the runway and removed debris from it, including sticks and rocks, prior to takeoff. The runway had softened due to snow and water runoff and the pilot said he experienced slight sinking conditions, but it was not muddy or sloppy. The pilot elected to depart to the south in order to takeoff on a downhill slope. The pilot performed a soft field takeoff by pulling the yoke back to reduce friction from the nose wheel. About 40 miles per hour (mph), the airplane hit several ruts and slowed down. He initiated rotation at 60 mph, but could not gain sufficient altitude prior to hitting a tree at the departure end of the runway, causing substantial damage. The pilot reported that there were no mechanical malfunctions or failures of the airplane.