Intraspecific variation in flowering phenology affects seed germinability in the forest herb Primula elatior

Background and aims - Phenological responses to environmental cues are known to be phylogenetically conserved across species, but the adaptive nature of phenological responses to the local environment within the populations of individual species needs further exploration. In temperate forests, the canopy green up timing is expected to act selectively upon the growth and flowering phenology of understorey plants. Methods - Here we quantified the variation in the flowering phenology of a forest herb growing in the understorey of early versus late leafing tree species and explored the effect of this variation on the germinability of its seeds. The flowering of individual plants was recorded in six populations at twelve time points in early spring. Germinability was tested in a lab germination experiment. The individual plants were visited again in a second growing season to quantify the variation in phenology between years. Key results - Variability between plants was found within populations rather than between populations or forest stands. Phenology was consistent across growing seasons with individuals flowering either early or late in both studied years (which was not simply due to differences in plant size). Early flowering individuals had a fitness advantage in the sense that they produced higher proportions of germinable seeds. Conclusions - We quantified phenological variation between the individuals of an understorey plant species relative to the canopy phenology and related this variation to fitness. While some interesting patterns emerged, fitness components other than seed germinability and the plant life stages following germination should be studied as well to understand the evolutionary significance of phenology in forest understorey plants.

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