INTRODUCTION HISTORY AND INVASION PATTERNS OF AMMOPHILA-ARENARIA ON THE NORTH COAST OF CALIFORNIA

European benchgrass (Ammophila arenaria [L.] Link), introduced to stab ilize sand, is fully naturalized in central and northern California an d has supplanted populations of native dune plants in many areas, incl uding the North Spit of Humboldt Bay. We interpreted air photos of the North Spit for the presence of Ammophila arenaria at three points in time: 1939/1942, 1962, and 1989. To quantify the spread and to detect invasion patterns of Ammophila, we then compiled maps using a geograph ic information system. We documented introduction dates and locations to improve the accuracy of our photo interpretation. Interpretation of the three photo series revealed invasion and expansion of Ammophila i n both foredunes and inland dunes. The most dramatic increase in cover was found in the foredunes, whereas documented and presumed plantings were found mostly on inland dunes. Adjusting for intentional eradicat ion, Ammophila cover on the North Spit was found to have increased bet ween 1939 and 1989 by 574%. The square root of the area occupied by Am mophila on the North Spit increased linearly with time, conforming to biological invasion theory.