Apartment rental rates in Southern California will climb 3.5 percent to 6 percent this year, according to a forecast released today by the University of Southern California Lusk Center for Real Estate. "The new supply of apartments has not kept up with demand from newcomers attracted to jobs in our diverse economy and the Southern California lifestyle," said Delores Conway, director of the Casden Forecast. Apartment occupancies are at all-time highs in Los Angeles, Orange, Riverside and San Bernardino counties because of a steady stream of young professionals and Latin and Asian immigrants who cannot afford the average home along with relocated executives who choose not to get locked into a mortgage, she also said, and the region's steadily growing and diversified economy sets the stage for increased job formation and more demand for apartments in the coming year.