Seasonal changes in the fruits of Viburnum opulus, a fleshy-fruited temperate-zone shrub

Certain temperate-zone plant species retain their fleshy fruits through the fall and winter. The production of "persistent fruits" has been interpreted as an adaptive reproductive strategy because plants can offer "low-investment" fruits when competition with other plants for seed dispersers is low. In theory fruits are defended chemically against fall pathogens, and palatability improves seasonally with cold-induced chemical changes. We studied the ecology of persistent fruits in Viburnum opulus (Caprifoliaceae), a fleshy-fruited temperate-zone shrub. Viburnum opulus fruits remained uneaten through the fall until most were removed, apparently by avian seed predators. Some shrubs retained fruits through May. Birds in the field and laboratory preferred all other fruit species tested over V. opulus. Palatability of V. opulus fruits to captive American Robins declined rather than improved throughout the winter, contrary to expectation. Tannins and pH remained constant, while sucrose concentration doubled over a 6-month period. Seeds regurgitated by robins in the spring germinated at similar rates as seeds processed the previous fall. Although V. opulus showed some of the theoretically expected adaptations for delayed seed dispersal (chemical defense, germination following spring dispersal), it did not show others (seasonal improvement in palatability). Plants with persistent fruits may depend on years of severe weather and food scarcity for seed dispersal.