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Traditionally it has been viewed as a part of nature 's renewable energy cycle , harvested once a year at the break of spring when birch trees awaken after winter dormancy and their sap begins to flow from tree root to tree tip .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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