Background/Question/Methods A critical limitation in the forecasting of plant phenological responses to climate change is that we lack knowledge of whether populations of the same species—particularly those whose ranges span multiple continents—have similar phenological sensitivity to warming temperatures. Phenological sensitivity has the potential to vary spatially for many reasons, including local adaptation, drift, and plasticity. We investigate whether the phenological timing and climate sensitivity of an invasive plant, garlic mustard (Alliaria petiolata), differed between its native range in Eurasia and introduced range in North America. We define sensitivity as the number of days a plant delays or advances its phenology per unit change in a climate factor. We used 4,890 research-grade iNaturalist observations of A. petiolata from 2017 – 2019. For each observation image, we recorded the plant’s reproductive phenological stage (budding, flowering, or fruiting), the julian date, and the geographic coordinates. We paired each observation with gridded climatic data from the global Climatic Research Unit (CRU) Time Series. We averaged monthly winter (December – February) temperature and precipitation values for each observation, then used a linear model to examine the effects of mean winter temperature, precipitation (log transformed), and continent of collection (Eurasia or North America) on phenological date. Results/Conclusions The phenological timing of A. petiolata was earlier in its introduced range than the home range for the flowering (2.99 days ± 0.64 s.e.) and fruiting (3.19 days ± 0.64 s.e) stages. In addition, A. petiolata was 2 times more sensitive to changes in winter temperature in the introduced range, advancing its reproductive phenology by -1.37 days/ °C (±0.08 s.e.) in Eurasia and -2.68 days / °C (±0.09 s.e.) in North America. By contrast, A. petiolata was less sensitive to changes in winter precipitation in the introduced range, delaying its reproductive phenology by +0.51 days / 10% increase in precipitation in Eurasia and +0.13 days / 10% increase in precipitation in North America. We found the phenology and climate sensitivity of A. petiolata differed between the plant’s native and introduced range, suggesting that populations within each continent will continue to diverge phenologically as the climate warms. We predict that increases in winter temperature will advance the reproductive phenology of A. petiolata to a greater degree within in its introduced range than its native range. However, these effects may also be counteracted by regional changes in rainfall due to climate change, which is predicted to differentially effect North America and Eurasia.
