Proximate and Ultimate Causes of Fruit Ripening Asynchrony in Vertebrate-Dispersed Plants in Southeastern Michigan.

Abstract

Asynchronous fruit ripening within individual bird-dispersed plants has been proposed to be an adaptation to avoid satiation of dispersal agents. In particular, this hypothesis has been proposed to account for asynchronous ripening in summer-fruiting species at mid-latitudes in temperate North America. I investigated patterns of ripening synchrony among fleshy-fruited species at the George Reserve in southeastern Michigan, explored proximate mechanisms causing asynchronous ripening, and tested the ultimate (evolutionary) hypothesis that asynchronous ripening avoids satiation. Ripening was not more asynchronous for six summer-fruiting species than for six fall-fruiting species. The best single predictor of ripening synchrony for these species was plant family: two species in the Ericaceae had much more asynchronous fruiting than species in three other families. Flowering was also most asynchronous in the two ericaceous species, but flowering synchrony was not correlated with ripening synchrony among seven other species. Ripening synchrony did not correlate with flowering synchrony among individual plants of a synchronous ripener, Prunus virginiana, nor those of an asynchronous ripener, Amelanchier arborea. Flowering was much more synchronous than ripening of all 32 plants (seven species) tested, and accounted for only a small part of the variance in ripening date for individuals of A. arborea, Ilex verticillata, and Vaccinium corymbosum. These results indicate that ripening synchrony is not merely a consequence of flowering synchrony, but can respond independently to natural selection. Patterns among species in ripening synchrony could not be explained by any one common proximate factor. Shading delayed fruit ripening in the synchronous P. virginiana but not in the asynchronous A. arborea. In neither species was ripening synchrony affected by removal of ripening fruit. Within A. arborea and V. corymbosum plants ripening date was highly negatively correlated with seed number per fruit, but species with variable seed number did not have more asynchronous ripening on average than those with constant seed number. Seed number per fruit in A. arborea was not affected by either addition of xenogamous pollen or manipulation of resources during fruit initiation. To test whether more synchronous fruit ripening satiates summer frugivores, I manipulated fruit displays of A. arborea trees and compared removal of ripe fruit from these trees to paired controls. Fruit removal was slower from the manipulated, synchronous, trees in some replicates but faster in others. Patchy fruit removal indicated that frugivores, primarily Bombycilla cedrorum, were temporally unpredictable. Asynchronous ripening may allow plants whose ripe fruits cannot persist to ensure seed dispersal in the face of this unpredictability.