Complex sentence: The population did not recover until subsequent migrations into Sichuan between the sixteenth and nineteenth centuries from the north (after the formation of Mandarin there), which supplanted the earlier varieties of Chinese in Sichuan.
Split into simpler sentences: The population did not recover until it was replenished by subsequent migrations of Mandarin speakers from the north between the sixteenth and nineteenth centuries. The newcomers' dialect largely supplanted the earlier varieties of Chinese in Sichuan.