So it is here in Ningxiang , a 10-hour drive from the factories on the southern coast , that clues can be found to a problem once thought inconceivable : The world 's most populous nation , which has powered its stunning economic rise with a cheap and supposedly bottomless pool of migrant labor , is experiencing shortages of about two million workers in Guangdong and Fujian , the two provinces at the heart of China 's export-driven economy .