Latitudinal clines in the phenology of floral display associated with adaptive evolution during a biological invasion.

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Flowering phenology strongly influences reproductive success in plants. Days to first flower is easy to quantify and widely used to characterize phenology, but reproductive fitness depends on the full schedule of flower production over time. We investigated flowering schedules in relation to the onset and duration of flowering and tested for latitudinal clines in schedule shape associated with rapid evolution and range expansion of an invasive plant. We examined floral display traits among 13 populations of Lythrum salicaria, sampled along a 10-degree latitudinal gradient in eastern North America. We grew these collections in a common garden field experiment at a mid-latitude site and quantified variation in flowering schedule shape using principal coordinate analysis (PCoA) and quantitative metrics analogous to central moments of probability distributions (i.e., mean, variance, skew, and kurtosis). Consistent with earlier evidence for adaptation to shorter growing seasons, we found that populations from higher latitudes had earlier start and mean flowering day, on average, when compared to populations from southern latitudes. Flowering skew increased with latitude, whereas kurtosis decreased, consistent with a bet-hedging strategy in biotic environments with more herbivores and greater competition for pollinators. Heritable clines in flowering schedules are consistent with adaptive evolution in response to a predicted shift toward weaker biotic interactions and less variable but more stressful abiotic environments at higher latitudes, potentially contributing to rapid evolution and range expansion of this invasive species.