FRUIT RIPENING ASYNCHRONY IS RELATED TO VARIABLE SEED NUMBER IN AMELANCHIER AND VACCINIUM

Two vertebrate-dispersed woody plants, Vaccinium corymbosum and Amelanchier arborea, were studied in southeastern Michigan to determine the proximate causes of their within-plant fruit ripening asynchrony. Individual flowers were followed from opening (A. arborea) or fruit initiation (V. corymbosum) to fruit ripening. Fifty-nine to 100% ofthe variance in fruit-ripening dates within V. corymbosum plants was due to variance in the development time; little was due to variance in initiation dates or covariance between these two terms. Similarly, 98% of the variance in fruit-ripening dates in A. arborea was due to variance in the number of days from flower opening to fruit ripening. Fruit developmental time in V. corymbosum and flower-fruit interval in A. arborea were significantly correlated with seed number: fruits with more seeds developed faster. I hypothesize that in both species ripening asynchrony is largely a consequence of variability in seed number. ASYNCHRONY in the ripening of fleshy fruits on an individual plant occurs in many species with animal-dispersed seeds (Frankie, Baker and Opler, 1974; Thompson and Willson, 1979; Hilty, 1980; Opler, Frankie and Baker, 1980; Howe and Smallwood, 1982). Several recent hypotheses suggest this ripening asynchrony is an adaptation for increased reproductive success. According to one hypothesis, the increased duration of the fruit display resulting from ripening asynchrony may increase either the number and diversity of animals taking fIuits (frugivores) (Smythe, 1970; McDonnell et al., 1984) or the diversity of weather conditions to which seeds are exposed following dispersal (Stapanian, 1982). A second hypothesis is that asynchronous ripening reduces the time ripe fruits remain on the plant before dispersal, exposed to predation and decay, when dispersers are not abundant. This scarcity of dispersers may be due either to seasonal changes in abundance (e.g., migration) (Thompson and Willson, 1979; Herrera, 1982) or to specialization by the plant for dispersal by frugivores ' Received for publication 9 April 1985; revision accepted 17 July 1985. I thank the Museum of Zoology, University of Michigan, for permission to use the Edwin S. George Reserve and for an ESGR scholarship. I received additional support from Rackham Block Grants from the University of Michigan and a National Science Foundation Graduate Fellowship. C. Bartley, J. Dudley, S. Fogle, and S. Waldman helped with field work and/or data compilation. J. Fry and G. Estabrook helped refine the variance-partitioning model. A. Stephenson, D. Levey, S. Hermann, and S. Vander Kloet kindly shared unpublished data. J. Bronstein, G. Estabrook, D. Goldberg, and B. Rathcke provided valuable comments on earlier drafts of this manuscript. with low population density (i.e., specialist or obligate frugivores, McKey, 1975; Howe and Estabrook, 1977). Third, ripening asynchrony may reduce the number of ripe fruits available at any one time, and this may decrease the time a frugivore forages on the plant, therefore increasing the proportion of ingested seeds dispersed away from the parent. A fourth hypothesis applies to species in which partially ripe fruits have colors distinct from ripe fruits; in these species asynchronous ripening may enhance the attractiveness of the fruit display by increasing the time it is bicolored (Stiles, 1982; Willson and Thompson, 1982). Finally, for species with indeterminate flowering (e.g., Phytolacca americana, McDonnell et al., 1984), asynchronous fruiting may enable a plant to more effectively exploit a growing season with an unpredictable termination. While the ultimate (evolutionary) cause of within-plant ripening asynchrony has received much attention but remains unresolved, the proximate (mechanistic) cause ofthis phenomenon has received little attention to date. In this paper I present data bearing on three proximate mechanisms that could cause ripening asynchrony: asynchrony in flowering or fruit initiation, variability among fruits in the time from initiation to ripening (developmental time), and correlation between fruit initiation date and developmental time (i.e., early initiated fruits develop faster than later initiated fruits). Variability in fruit developmental time in turn might be due to characteristics of the fruits themselves, for example, the number of seeds in the fruit. In this paper I present a model that uses