Late-Flowering Species are Sensitive to Warming

Phenological advancement is one of the most prevalent responses of vegetation to warming. The prevailing view is that that early flowering species are particularly sensitive warming, with greater phenological advancement per °C warming relative to later flowering species. However, we observed a three to four month advancement of late flowering species to the extreme warmth of 2012, which motivated us to ask quantitatively whether late flowering species are indeed less sensitive to warming. We focused on responses to inter-annual variation in mean monthly, seasonal, and annual temperatures, using species for which we have ≥10 observations of first flowering near Manhattan, KS between 1891 and 2012 (n = 259). As many other studies have found, early flowering species advanced flowering with warmer temperatures during the year preceding flowering, while late flowering species appeared insensitive to warming during the year preceding flowering. At the seasonal time scale, however, both early and late flowering species responded similarly to spring warming, while late flowering species delayed flowering in response to summer warmth. This divergent response created the illusion that later flowering species were insensitive to temperatures during the year preceding flowering. When absolute values of sensitivities to temperature were summed across months, late flowering species exhibited greater sensitivity to temperature than early flowering species. Late-flowering species were also the most sensitive to variation in precipitation and advanced flowering when summers were wet. We therefore speculate that flowering for late species is delayed by warm, dry summers because both warmth and dryness exacerbate plant water stress and delay growth. Furthermore, warm spring temperatures might allow both early and late flowering species to more quickly attain size or developmental requirements for flowering. Advanced flowering in 2012 provides an extreme example of some typically late species reaching these requirements prior to summer. Based on this analysis, we urge caution in analyses of plant sensitivity to warming. Our findings cast doubt on whether late species are indeed less sensitive to warming or generally delay flowering in response to warming and highlight the need to better incorporate the timing of climate variability when predicting how climate change will affect the timing of life events.