Sexual Dimorphism and between-Year Variation in Flowering, Fruit Set and Pollinator Behaviour in a Boreal Willow

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Abstract
Sexual dimorphism in floral characters, flower visitor behaviour and fruit and seed set were studied in the boreal dioecious willow Salix myrsinifolia-phylicifolia. Males and females produced approximately the same number of catkins per individual, but males produced more flowers per catkin. Females had a higher concentration of sugars in the nectar than males, but the standing crop of nectar, measured as μg organic carbon per flower, did not differ between the sexes. Females had a hexoserich nectar and males a sucrose-dominated nectar. Bumblebees discriminated between male and female willows, but their preference changed during the day and differed between bumblebees with different tongue length. The relative abundance of different species of bumblebees varied considerably between years. Hand-pollination in two populations significantly increased both fruit and seed set per catkin. The production of flowers in female plants showed a large between-year variation. Both the average fruit set and seed set per fruit were below 50% during the period 1981-1985. Fruit set showed greater variation than did seed set per fruit. We suggest that the early time of flowering and the strongly female-biased sex ratios commonly observed in populations of this willow (60-80% females), increase the risk of pollen limitation. The production of many more flowers than fruits may represent a bet-hedging strategy to match the uncertainty in the conditions for fruit maturation, although it may also be of value as it potentially allows the plant to selectively mature fruits of a high genetic quality.