China also began suffering from mounting overpopulation during this period. Population growth was stagnant for the first half of the 17th century due to civil wars and epidemics, but prosperity and internal stability gradually reversed this trend. The introduction of new crops from the Americas such as the potato and peanut allowed an improved food supply as well, so that the total population of China during the 18th century ballooned from 100 million to 300 million people. Soon all available farmland was used up, forcing peasants to work ever-smaller and more intensely worked plots. The Qianlong Emperor once bemoaned the country's situation by remarking "The population continues to grow, but the land does not." The only remaining part of the empire that had arable farmland was Manchuria, where the provinces of Jilin and Heilongjiang had been walled off as a Manchu homeland. The emperor decreed for the first time that Han Chinese civilians were forbidden to settle. Mongols were forbidden by the Qing from crossing the borders of their banners, even into other Mongol Banners and from crossing into neidi (the Han Chinese 18 provinces) and were given serious punishments if they did in order to keep the Mongols divided against each other to benefit the Qing.