By Steve Gorman (Reuters) -A wildfire threatening some of the world’s oldest giant sequoia trees in California’s Yosemite National Park expanded five-fold over the weekend as smoke prompted air quality alerts throughout the park and obscured views of its famed scenery. As of Sunday, the blaze had scorched nearly 1,600 acres (648 hectares) of timber and brush at the southern end of the park, up from 250 acres on Friday, a day after fire was first reported by visitors on the Washburn Trail of the Mariposa Grove of Giant Sequoias. National Park Service personnel immediately closed the Mariposa Gr…