"Although emperor after emperor refused entry to almost all human beings from Europe and the Americas, they could not keep out other species. Key players were American crops, especially sweet potatoes and maize;* their unexpected arrival, the agricultural historian Song Junling wrote in 2007, was "one of the most revolutionary events" in imperial China's history. The nation's agriculture, based on rice, had long been concentrated in river valleys, especially those of the Yangzi and Huang He (Yellow) rivers. Sweet potatoes and maize could be grown in the dry uplands. Farmers moved in numbers to these areas, which had previously been lightly settled. The result was a wave of deforestation, followed by waves of erosion and floods, which caused many deaths. The regime, already straining under many problems, was further destabilized--to Europe's benefit."