Flowering phenology, climate change, and assortative mating in Plantago lanceolata

Background/Question/Methods

Climate change is a topic of increasing concern for the stability of plant populations and communities; average spring temperature has increased over the past century, changing time of first flower in many species.  Temperature change may also alter flowering duration and shape of the flowering curve, affecting mating opportunities at the individual and population level.  Within populations, early-flowering plants may mate more often with other early-flowering plants, and late-flowering with late.  The strength of the phenotypic correlation between mates is measured as assortative mating (“AM”); when phenotypes are positively correlated AM approaches 1, while under random mating AM is 0.  AM coupled with moderate to high heritability (as in many flowering traits) can increase the additive variance of a trait, which could in turn affect a population’s ability to respond to selection.  This has implications for the ability of plant populations to adapt to climate change, as a change in temperature could affect AM, in turn increasing (or decreasing) additive variance in such a way as to affect the population’s ability to respond to further temperature increase.

As a first step in examining the effects of increased temperature on AM, in 2009 we measured the magnitude of potential assortative mating (“PAM”) in Plantago lanceolata (Plantaginaceae) by establishing 32 artificial populations of 22 plants each, censusing open flowers every 2-3 days from late-May through late-July, and using this data to calculate mating opportunities for each mother-father pair based on frequency and degree of flowering overlap.  We then combined this measure of phenology overlap with the phenotypic similarity of each mother-father pair to calculate PAM for each artificial population.
Results/Conclusions

We found that, of the traits examined (leaf number, day of first flower, flower number at peak flowering, and days from first to peak flowering), all but leaf number were significantly different from zero.  As expected, PAM was highest for day of first flower (mean ± StD = 0.26 ± 0.09).  In future field seasons we will find a reliable method of increasing temperature in the field and, using this, examine the effect of temperature on the strength of assortative mating.  We expect to find that assortative mating by day of first flower increases under warmer conditions, though effects on overall phenology and correlated traits is harder to predict.