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The causes of tree invasion into meadows in Lassen Volcanic National Park were investigated by dating invasive trees and associating age structure patterns with regional variations in fire history, grazing, and climate change Massive invasion was associated primarily with cessation of livestock grazing and burning between 1905 and 1933, but climatic changes may also have contributed. Maintenance of meadows may require continued use of unnatural processes due to uncertainties about pre-European fire regimes and other resource considerations. [ABSTRACT FROM AUTHOR]