these were successful, as "the lands of Huai were fully planted, sericulture was widespread, and the [Southern Tang] state grew rich and strong." Mulberry trees (桑樹) were planted in large numbers in the Huai and Yangtze River basins. In May 939 farmers that had cultivated 3,000 or more in the past three years were given fifty bolts of silk from Li Bian's treasury. Payment of 20,000 copper cash was awarded to those that reclaimed at least 80 mu (≈1,040 acres) of land. All new fields they cultivated were not taxed for 5 years. These measures have been credited with increasing agricultural production across the realm. During the reign of Li Jing he authorized several military ventures that according to He Jainming incurred heavy financial burdens that inhibited further economic development of the Southern Tang realm. Tributes were paid to the Later Zhou once they defeated the Southern Tang in 958. After the Song dynasty was established in 960 these payments became exorbitant. Tribute to the Song incurred a downturn of the Southern Tang economy. After forcible incorporation into the Song Empire the former southern kingdoms became the focal point of the Chinese economy. Throughout their brief period of independence they had "laid the groundwork for the great economic surge that followed." Coinage Copper mining in the north was heavily disrupted by the constant warfare of the Five Dynasties and Ten Kingdoms era. In consequence bronze coins became scarce. The situation became dire in northern China. In 955 the Later Zhou Emperor Chai Rong banned the possession of bronze utensils upon pain of death. The Southern Tang and their neighbors minted coinage from clay, iron, or lead. These coins for domestic circulation (despite having minimal value intrinsically) and to prevent the export of precious bronze coinage to neighboring states. Li Bian possibly cast a coin with the inscription Daqi Tongbao (大唐通寶) while the state still had its original name of Qi. During the reign of Li Jing several cash coins were created of different commercial value. Datang Tongbao (大唐通寶) and Tangguo Tongbao (唐國通寶) coins maintained the same inscription, size, and weight but used different fonts of Chinese calligraphy Yongtong Quanhuo (永通泉貨) coins were initially valued at 10 copper coins but counterfeits were widely circulated leading to distrust of them. Copper coins were still preferred by the populous, which caused a rapid depreciation of the newer coins. Kaiyuan Tongbao (開元通寶) coins that