DONNER SUMMIT - About a dozen volunteers safely lowered a tranquilized black bear Sunday morning from picturesque Rainbow Bridge near Donner Summit to the floor of a granite-strewn ravine.The concerted act of mercy occurred against a backdrop of escalating encounters between humans and bears in the region this year, many resulting in the death of bears in car crashes or their capture by basin residents tired of bruin break-ins.On Sunday, though, humans turned out to save the life of a 250-pound bear that literally clung for its life through the night.According to the Truckee BEAR League's Dave Baker, a medium-size …