Rapid shifts in phenology during biological invasions can increase survival and reproduction of invasive species, contributing to their spread and impact. However, it is not clear how quickly invasive populations can tune their phenology in response to local environmental cues. Global herbarium specimens represent spatial and temporal snapshots of phenology that could be analyzed for environmental tuning in the form of correlations between phenological development and local climatic factors. In this study, we combined inflorescence measurements of herbarium specimens with historical weather data to investigate modern and historic clines in flowering phenology throughout the North American distribution of the invasive plant Lythrum salicaria (purple loosestrife).
Lythrum salicaria was introduced to eastern North America from Europe over 200 years ago, but spread slowly until approximately 60 years ago when it rapidly expanded northward and westward. Field surveys and experimental manipulations have demonstrated rapid evolution of locally adaptive clines in flowering time along a latitudinal gradient in eastern North America. We characterized inflorescence morphology (i.e. buds, flowers, fruits) of 1,854 herbarium specimens and interpolated the number of growing days and growing-degree days by inverse distance weighing of nearby NOAA weather station data.
We calculated a flowering time index (FTI) from phenology measurements which corrected for variation in collection date and local growing conditions. FTI from eastern North America herbarium specimens was strongly correlated with genetic differences in time to first flower from the start of growing season as measured in 4 common garden environments. Population differences in flowering time clines have been consistently observed in multiple common garden experiments, indicating significant genetic variation with little genotype-by-environment interactions. This suggests that genetic variation in flowering time can be modelled in specimens from natural history collections, at least in Lythrum salicaria. We also identified latitudinal clines in phenology of Midwestern, Western, and West Coast populations of North America. The strength of these clines decreased from east to west, and in eastern North America, the cline was weaker in specimens collected <1960 compared to more contemporary collections. These results are consistent with gradual evolution of latitudinal clines during invasion. A similar approach could be used to study phenological variation of other species through time and across large geographical areas.