Plant phenological response to climate varies across species. Using a combination of herbarium records and historic climate data, we explored how plant phenology has been influenced by historic temperature variation from 1900- 2014 and assessed the differential effect of climate on phenological change between native and non-native species. This study will add to a growing body of research investigating how native and nonnative species phenology responds to climate and is one of the first to analyze these effects in southwestern Pennsylvania. Specifically, we worked with a subset of 11 plant species from the Carnegie Museum of Natural History’s herbarium, which were comprised of 6 native and 5 non-native forbs common to old fields in Pennsylvania. We scored 1,266 specimens for their phenological phase (i.e., vegetative, budding, fruiting, or flowering). For our final analysis, we used only used specimens in the flowering (n=460) and fruiting (n=464) phenophases (>50% open flowers and mature fruit, respectively). These were combined with historic temperature records from weather stations within southwestern Pennsylvania, sourced from Climate Data Online. We used linear mixed-effect models to compare how mean monthly temperatures in the 3 months prior to a specimen’s collection date correlated with fruiting or flowering date.
Results/Conclusions
Between 1900 and 2014, mean annual spring temperatures (February – May) ranged from 4.4°C to 10.1°C. We found that increases in temperature were significantly correlated with an advancement in fruiting and flowering date for native forb species but not nonnative forbs. From the lowest to highest temperature quartile, there was a 7.02 (± 3.68 s.e.) day advancement in flowering and 15.97 (± 4.08 s.e.) day advancement of fruiting amongst native species. Nonnative forbs did not exhibit a phenological response to temperature, which could suggest that these species do are less plastic in this trait. On average, the nonnative species included in this study flowered 39.85 (± 3.09 s.e.) days earlier and fruited 42.71 (± 2.84 s.e.) days earlier in the growing season than the native species. This early phenology may explain nonnatives’ lack of plasticity to temperature variation. We hypothesize that there are physiological constraints associated with early-season phenology, such as frost susceptibility and nutrient limitation during leaf-out, that constrains nonnatives’ response to temperature during the later phases of reproduction. The findings of this analysis suggest that native and nonnative species will have differential responses to future climate warming.