Oorsprong en opbouw van de Salix pentandra-populatie op Schiermonnikoog

The Salix pentandra population on the Dutch Wadden island of Schiermonnikoog has been the subject of this study. Chiefly age class composition and sex ratio have been investigated, as well as the distribution of age and sex over the total range. A hypothesis concerning the origin of S. pentandra on the island is proposed. In about 1920 the first two S. pentandra specimens reached Schiermonnikoog, probably as seed. These were both females, so no sexual reproduction took place at all. Not until 1940 a male specimen came to the island and from 1946 onwards the population started to grow exponentially. At the moment the population of this willow counts c. 900 specimens on the island, mainly growing in the central, moist, fresh dune valleys. The oldest specimen now is c. 60 years, and c. 30% of the whole population is between 1 and 10 years old. Most younger specimens are found in the eastern part of the area, in dune valleys which have recently developed. The average sex ratio is 7♀ /4♂, which holds for most age groups as well as for most parts of the population. Solely in the most eastern part of the area with some brackish influence more males were counted than females. Males are usually somewhat bigger and taller than females of the same age.