Phenological responsiveness and fecundity decline near the southern range limit of Trientalis borealis (Primulaceae)

In boreal species, populations at a southern range edge are at risk of being extirpated due to rising temperatures from climate change and potentially from demographic factors that affect small populations. The ability to track climate through phenological responsiveness can affect survival and reproductive success, with implications for long-term population persistence. This study examines phenological responsiveness and fecundity in populations of an understory plant species, Trientalis borealis, in the Midwestern USA using herbarium specimens and field observations. Date of flowering was well predicted by the mean May temperature. We detected a shift in the mean flowering date for populations north of latitude 45 degrees N of an average of 1.4 days earlier per decade. We did not find shifts in mean flowering date for populations south of 45 degrees N. Trientalis borealis exhibits phenological responsiveness differently in different parts of its range. In populations north of latitude 45 degrees N, T. borealis advances its date of flowering, on average, by approximately 3 days for every 1 degrees C increase in mean May temperature. In populations south of 45 degrees N, T. borealis advances its flowering date less than 1 day for every 1 degrees C increase in mean May temperature. Flowering and seed set increased along a latitudinal gradient away from the range edge. If population growth is limited by seed production in this species, this suggests that populations near the southern range limit are not performing well, perhaps due to direct effects of warming, a lack of phenological responsiveness that exacerbates exposure to warming, or a lack of genetic diversity.