WHEN DOES OUTCROSSING OCCUR IN A MASS-FLOWERING PLANT?

Abstract

The flowering pattern of many animalpollinated woody plants may be divided into three phases: a) an initial phase in which a low but increasing number of flowers is produced per day, b) a short peak phase in which most of the total flower production occurs, and c) a final phase in which a low but decreasing number of flowers is produced per day. Recently, evolutionary ecologists have focused much attention on the effects of this mass-flowering pattern on pollinator attraction and movement (Janzen, 1967, 1971a; Gentry, 1974; Heinrich, 1975; Stiles, 1975; Carpenter, 1976; Frankie et al., 1976; Augspurger, 1980). These studies show that many species of floral visitors preferentially forage on individual plants that produce large numbers of flowers per day. Presumably, these individuals are both conspicuous and highly rewarding to pollinators (Heinrich and Raven, 1972; Gentry, 1974). In terms of pollinator movement, however, the floral rewards may be so clumped during the peak phase of the flowering pattern that there is little incentive for the pollinators to forage on different individuals (Heinrich and Raven, 1972; Stiles, 1975; Carpenter, 1976; Frankie et al., 1976). In contrast, the low levels of daily flower production that characterize the initial and final phases of the flowering pattern may be insufficient to attract pollinators from alternative floral resources. Consequently this mass-flowering pattern poses a dilemma, especially for self-incompatible species (see Heinrich and Raven, 1972; Heinrich, 1975; Carpenter, 1976; Frankie et al., 1976; Gentry, 1978). This study seeks to answer two questions: During what phase of the flowering pattern does outcrossing occur and what factors promote outcrossing? Specifically, this study documents the flowering pattern of Catalpa speciosa (Warder ex Barney) Engelm. (Bignoniaceae) and examines the effect of different floral abundances and changes in floral abundances through time on the proportion of flowers that are selfand cross-pollinated on an individual plant.