Pollen limitation and cleistogamy in subalpine Viola praemorsa

Abstract

Early-flowering species may be especially susceptible to occasional pollen limitation and, therefore, may benefit from a mixed-mating strategy that provides reproductive assurance. We studied cleistogamous (CL) and chasmogamous (CH) fruit set of spring-flowering Viola praemorsa Dougl. ex Lindl. along an elevational gradient in the Rocky Mountains, testing whether pollen limitation or allocation to CL reproduction covaried with timing of flowering onset, within and across sites. Contrary to predictions, we found no pollen limitation of reproduction at any site, and variation among sites in the pattern of allocation to cleistogamy was not related to growing season length. Differences in reproductive strategy between early- and late-flowering plants within sites were attributable to differences in plant size, with relative allocation to cleistogamy increasing with size. This pattern has been found in some other cleistogamous species, and may indicate a cost of large CH floral displays, perhaps associated with geitonogamy or herbivory. We found no experimental evidence for resource reallocation in response to CH reproductive output, although a weak negative relationship between CH and CL fruit set across a larger sample of unmanipulated plants suggests such a trade-off. The significance of cleistogamy may be clarified by studying how pollinator visitation, self-pollination, and herbivore damage vary temporally and with floral display size.