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A history of the introduction and spread of Ammophila arenaria on the North Spit of Humboldt Bay, California
European beachgrass (Ammophila arenaria (L.) Link) is fully naturalized in central and northern California and has supplanted native populations of dune vegetation in many areas. Managers at The Nature Conservancy's Lanphere Christensen Dunes Preserve in northern California are concerned by its rapid expansion on the North Spit of Humboldt Bay where the preserve is located. Air photos of the North Spit were interpreted for the presence of Ammophila arenaria at three points in time: 1939/42, 1962, and 1989. Maps were compiled using a geographic information system with the goals of quantifying the spread of Ammophila on the North Spit and detecting invasion patterns. Historic research was conducted to document introduction dates and locations and to improve interpretation of the historic photographs. Interpretation of the three photo series revealed invasion and expansion of Ammophila in both foredunes and inland dunes. The most dramatic increase in cover was found in the foredunes, while documented and presumed plantings were found on inland dunes only. During the 1939-1989 period, excluding areas of purposeful eradication, Ammophila cover on the North Spit increased by 574%.