Contrasting dispersal phenologies in two fleshy-fruited congeneric shrubs, Rhus aromatica Ait. and Rhus glabra L. (Anacardiaceae)

The avifrugivore availability hypothesis predicts that summer-fruiting species will have an extended fruiting season and slow fruit removal, and the foliar flag hypothesis predicts that fruit dispersal in autumn-fruiting species coincides with fall bird migration. Phenology of fruit removal from plants of the early summer-fruiting shrub Rhus aromatica Ait. (var. aromatica) and of the later summer-fruiting shrub Rhus glabra L. (Anacardiaceae) was studied primarily to test the avifrugivore availability and foliar flag hypotheses of fruit dispersal, respectively. Fruits of R. aromatica ripened synchronously in early June, and 77-89% of them were gone from the plants by the early July, thus failing to support the avifrugivore availability hypothesis. The pattern of rapid fruit removal in this species, which was consistent throughout a 3-year period, is in contrast with previous reports of dispersal until September and even the next summer. Display size (at both infructescence and clump levels) had no effect on removal rate in R. aromatica. Fruits of R. glabra matured in August, and about 20% of them remained undispersed the following spring. In both species, fruit removal was mostly due to dispersers rather than to natural fruit fall. No correlations were found between fruit characteristics (pulp weight, pulp-to-seed ratio, moisture content of pulp) and probability of removal during the dispersal season in either species. Browsing by mammals, most likely white-tailed deer, was responsible for the rapid removal of fruits from R. aromatica plants during the 1st week after they turned mature-red; thereafter, most fruit removal was by birds. Two removal peaks were found for R. glabra fruits: a small one in September and a large one in winter to early spring. These peaks correspond to the fall migration season for migratory birds and the scarcity of food for winter-resident birds, in the study area, respectively. Thus, the dispersal pattern of R. glabra fruits does not support the foliar flag hypothesis.