Agricultural production has traditionally been for subsistence and is based on a system of shifting cultivation in the high islands. Staple crops include taros, sweet potatoes, bananas, cassavas, and breadfruit. Yams are grown on Pohnpei, Kosrae, Yap, and Fais islands. Other vegetables, such as cucumbers, eggplant, head cabbage, Chinese cabbage, bell peppers, green onions, and tomatoes, are also produced. Other fruits include mangoes, papayas, pandanus, pineapples, lemons, and limes, with oranges and tangerines also produced on Kosrae. The ubiquitous coconut palm is used for a wide range of subsistence purposes, and copra is the main cash crop and the nation's leading export. Crop production in 1999 included (in thousands of tons): coconuts, 140; copra, 18; cassava, 12; and bananas, 2. Black and white peppers were introduced to Micronesia in 1938, but pepper growing only began in Pohnpei (the FSM's most important pepper-producing island) in 1960. Rich volcanic soil and heavy rainfall make gourmet Pohnpei peppers highly regarded.